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On the relationship between fire regime and vegetation structure in the tropics
New Phytologist
Abstract
We assessed data from 11 experiments examining the effects of the timing and/or frequency of fire on tropical forest and/or savanna vegetation structure over one decade or more. The initial ‘control treatment’ in many such cases consisted of previously cleared land. This is as opposed to natural vegetation subject to some sort of endogenous fire regime before the imposition of fire treatments.
Effects of fire on fractional foliar cover are up to 10‐fold greater when clearing pre‐treatments are imposed. Moreover, because many of the ‘classic’ fire trials were initialised with applied management questions in mind, most have also used burning regimes much more frequent and/or severe than those occurring in the absence of human activity.
Once these factors are taken into account, our modelling analysis shows that nonanthropogenic fire regimes serve to reduce canopy vegetative cover to a much lower extent than has previously been argued to be the case.
These results call into question the notion that fire effects on tropical vegetation can be of a sufficient magnitude to maintain open‐type savanna ecosystems under climatic/soil regimes otherwise sufficient to give rise to a more luxurious forest‐type vegetation cover.
... Field-based 13 and empirical analyses using remotely sensed products identify the coincidence of burnt area with the reduced occurrence ("missing") of intermediate (50-60%) tree covers 14 15-18 (Supplementary Fig. 2) as evidence of a substantial impact of fire on tree cover. On the other hand, recent field analyses of fire impacts in 65 relation to soil and climate 19,20 suggest that fire could have a smaller effect on tropics-wide tree cover than suggested up to now. Critical evaluations of the most used global woody cover product have also questioned the intermediate tree cover gap [21][22][23][24] . ...
... Tree cover is reduced only slightly less than the annual average area burnt -on a par with the impact of agricultural land use per unit area ( Supplementary Fig. 9), so our results do not preclude a significant impact of fire on cover if the same areas within a given grid cell are burning each year, which would explain tree cover changes found in fire exclusion experiments 45 . In mesic tropical 235 systems (savanna and dry forest), fire effects on tree cover can be substantial if vegetation experiences frequent fires, particularly later in the dry season 19 . These regions show a higher impact (Table 1; Fig. 2). ...
... Fire influence on tree cover 19 355 Fig. 5. Reduction in tree cover due to a 1% increase in burnt area for (top) 90% and (bottom) 10% uncertainty range. Note that the colour scales of the two maps differ. ...
Explaining tropical tree cover distribution in areas of intermediate rainfall is challenging, with fire’s role in limiting tree cover particularly controversial. We use a novel Bayesian approach to provide observational constraints on the strength of the influence of humans, fire, rainfall seasonality, heat stress, and wind throw on tropical tree cover. Rainfall has the largest relative impact on tree cover (11.6-39.6%), followed by direct human pressures (29.8-36.8%), heat stress (10.5-23.3%) and rainfall seasonality (6.3-22.8%). Fire has a smaller impact (0.2-3.2%) than other stresses, increasing to 0.3-5.2% when excluding human influence. However, we found a potential vulnerability of eastern Amazon and Indonesian forests to fire, with up to 2% forest loss for a 1% increase in burnt area. Our results suggest that vegetation models should focus on fire development for emerging fire regimes in tropical forests and revisit the linkages between rainfall, non-fire disturbances, land use and broad-scale vegetation distributions.
... Fire may be a key determinant of global savanna and forest distributions Staver et al., 2011b), but the importance of fire for stabilizing savannas is debated (see, e.g., Good et al., 2016;Veenendaal et al., 2018) and its effects on herbaceous vegetation across biomes are unelucidated. Frequent fires may allow savannas to persist where climate and soils might otherwise allow forests to dominate (Bond et al., 2005;Sankaran et al., 2005); these fire regimes may sharpen transitions between savannas and forests (Lloyd et al., 2008;Veenendaal et al., 2018). ...
... Fire may be a key determinant of global savanna and forest distributions Staver et al., 2011b), but the importance of fire for stabilizing savannas is debated (see, e.g., Good et al., 2016;Veenendaal et al., 2018) and its effects on herbaceous vegetation across biomes are unelucidated. Frequent fires may allow savannas to persist where climate and soils might otherwise allow forests to dominate (Bond et al., 2005;Sankaran et al., 2005); these fire regimes may sharpen transitions between savannas and forests (Lloyd et al., 2008;Veenendaal et al., 2018). Vegetation influences fire in turn, and fire-vegetation feedbacks thus potentially maintain savannas and forests as alternative stable states (Beckage et al., 2011;Staver et al., 2011a). ...
... Meanwhile, tree cover in forests inhibits herbaceous vegetation accumulation, thereby limiting fire spread and maintaining the closed canopy (see, e.g., Nowacki & Abrams, 2008). However, others propose that differences in fire regime are not necessary to explain savannaforest distributions and that climate and soils primarily determine the vegetation structure of these two biomes (Good et al., 2016;Veenendaal et al., 2018). Evaluating vegetation dynamics in response to fire across a range of environmental conditions is therefore essential for both understanding vegetation distributions and developing sustainable ecosystem management plans. ...
Fire-vegetation feedbacks potentially maintain global savanna and forest distributions. Accordingly, vegetation in savanna and forest biomes should have differential responses to fire, but fire response data for herbaceous vegetation has yet to be synthesized across sites. Here, we examined herbaceous vegetation responses to experimental fire at 30 sites spanning four continents. Across a variety of metrics, herbaceous vegetation increased in abundance where fire was applied, with larger responses to fire in wetter and in cooler and/or less seasonal systems. Compared to forests, savannas were associated with a 4.8 (±0.4) times larger difference in burned versus unburned herbaceous vegetation abundance. In particular, grass cover decreased with fire exclusion in savannas, largely via decreases in C4 grass cover, whereas changes in fire frequency had a relatively weak effect on grass cover in forests. These differential responses underscore the importance of fire for maintaining the vegetation structure of savannas and forests.
... Mesic savannas represent alternative stable states to forest (Hirota et al., 2011;Staver et al., 2011a) in areas of intermediate rainfall (between 700 and 1900 mm/year for Africa; Aleman et al., 2020;Staver et al., 2011a), where savannas are maintained thanks to regular disturbances, notably by fire (Langevelde et al., 2003;Veenendaal et al., 2018;Venter et al., 2018). The MDNP being in the intermediate rainfall area, the observed forest progression might either be the result of a reduction of fire disturbances or to global climate change inducing locally wetter climatic conditions. ...
... In most cases, no fire occurrence was recorded during at least five years for pixels of woody savanna that transited to forest between the last two consecutive monitoring periods. Fire-free intervals facilitate tree recruitment and growth and allow trees to approach canopy closure which suppresses fire by excluding grasses (Bond and Midgley, 2000;Hirota et al., 2011;Staver et al., 2011b;Veenendaal et al., 2018Veenendaal et al., , 2015. In LLNP savannas newly protected from fire can sufficiently thicken up over a 15 year period to reach a structure comparable to a colonizing forest (Jeffery et al., 2014). ...
... Our results suggest that humid savannas seem to be the most unstable/prone to forest transition under a low fire frequency as the latter is not sufficient to prevent woody biomass build-up. Inversely, increased woody biomass and cover is known to depress grass production (Hoffmann et al., 2012;Veenendaal et al., 2018) that is the main fire fuel after drying-up, thereby indirectly limiting fire intensity and propagation (Lehmann et al., 2011). The shift towards woody savanna and associated ineffective fire regimes seems here to occur for AGB values around 27 Mg.ha ...
Woody encroachment and forest progression are widespread in forest-savanna transitional areas in Central Africa. Quantifying these dynamics and understanding their drivers at relevant spatial scales has long been a challenge. Recent progress in open access imagery sources with improved spatial, spectral and temporal resolution combined with cloud computing resources, and the advent of relatively cheap solutions to deploy laser sensors in the field, have transformed this domain of study. We present a study case in the Mpem & Djim National Park (MDNP), a 1,000 km² protected area in the Centre region of Cameroon. Using open source algorithms in Google Earth Engine (GEE), we characterized vegetation dynamics and the fire regime based on Landsat multispectral imagery archive (1975–2020). Current species assemblages were estimated from Sentinel 2 imagery and the open source biodivMapR package, using spectral dissimilarity. Vegetation structure (aboveground biomass; AGB) was characterized using Unmanned Aerial vehicle (UAV) LiDAR scanning data sampled over the study area. Savanna vegetation, which was initially dominant in the MDNP, lost about 50% of its initial cover in <50 years in favor of forest at an average rate of ca. 0.63%.year⁻¹ (6 km².year⁻¹). Species assemblage computed from spectral dissimilarity in forest vegetation followed a successional gradient consistent with forest age. AGB accumulation rate was 3.2 Mg.ha⁻¹.year⁻¹ after 42 years of forest encroachment. In savannas, two modes could be identified along the gradient of spectral species assemblage, corresponding to distinct AGB levels, where woody savannas with low fire frequency store 40% more AGB than open grassy savannas with high fire frequency. A fire occurrence every five year was found to be the fire regime threshold below which woody savannas start to dominate over grassy ones. A fire frequency below that threshold opens the way to young forest transitions. These results have implications for carbon sequestration and biodiversity conservation policies. Maintaining savanna ecosystems in the region would require active management actions to limit woody encroachment and forest progression, in contradiction with global reforestation goals.
... A fire-free interval of sufficient duration is therefore required to ensure woody saplings to reach an adult state with a critical size at which they are no longer susceptible to topkill (Bond and Midgley, 2000). At the individual level the point at which a tree has grown and is much less likely to be topkilled by subsequent fires can be referred to as a fire-resistance threshold and once a stem has surpassed this threshold it can continue to grow and extent his crown area leading to a gradual canopy closure (Veenendaal et al., 2015(Veenendaal et al., , 2018. The savannah gradually reaches a fire-suppression threshold due to its canopy closure that provides moist conditions with a low-light understory environment. ...
... Under sufficient rainfall, savannas are considered as unstable systems (Langevelde et al., 2003;Sankaran et al., 2005) where regular disturbances such as fire are absolutely required to maintain tree-grass coexistence in the system by buffering against transition to a closed-canopy state (Veenendaal et al., 2018;Venter et al., 2018). Under very high rainfall comparable to the MDNP situation, even fires may not be sufficient to ensure savanna stability (Jeffery et al., 2014;Djeumen et al., 2021). ...
... On average, no fire occurrence was recorded for savanna pixels that transited to forest between two consecutive monitoring periods. Fire-free intervals facilitate tree recruitment and allow trees to approach canopy closure which suppress fire by excluding grasses (Bond and Midgley, 2000;Veenendaal et al., 2015Veenendaal et al., , 2018 , 1996;Youta, 1998;Youta et al., 2003;Ibanez et al., 2013). Cardoso et al. (2020) evidenced the presence of an ecotone community in the Loppé National Park that occupies a narrow belt between savanna and forest and stabilises the forestsavanna mosaic even when the savanna is burned regularly. ...
Understanding the effects of global change (combining anthropic and climatic pressures) on biome distribution needs innovative approaches allowing to address the large spatial scales involved and the scarcity of available ground data. Characterizing vegetation dynamics at landscape to regional scale requires both a high level of spatial detail (resolution), generally obtained through precise field measurements, and a sufficient coverage of the land surface (extent) provided by satellite images. The difficulty usually lies between these two scales as both signal saturation from satellite data and ground sampling limitations contribute to inaccurate extrapolations. Airborne laser scanning (ALS) data has revolutionized the trade-off between spatial detail and landscape coverage as it gives accurate information of the vegetation’s structure over large areas which can be used to calibrate satellite data. Also recent satellite data of improved spectral and spatial resolutions (Sentinel 2) allow for detailed characterizations of compositional gradients in the vegetation, notably in terms of the abundance of broad functional/optical plant types. Another major obstacle comes from the lack of a temporal perspective on dynamics and disturbances. Growing satellite imagery archives over several decades (45 years; Landsat) and available computing facilities such as Google Earth Engine (GEE) provide new possibilities to track long term successional trajectories and detect significant disturbances (i.e. fire) at a fine spatial detail (30m) and relate them to the current structure and composition of the vegetation. With these game changing tools our objective was to track long-term dynamics of forest-savanna ecotone in the Guineo-Congolian transition area of the Central Region of Cameroon with induced changes in the vegetatio structure and composition within two contrasted scenarios of anthropogenic pressures: 1) the Nachtigal area which is targeted for the dam construction and subject to intense agricultural activities and 2) the Mpem et Djim National Park (MDNP) which has no management plan. The maximum likelihood classification of the Spot 6/7 image aided with the information from the canopy height derived from ALS data discriminated the vegetation types within the Nachtigal area with good accuracy (96.5%). Using field plots data in upscaling aboveground biomass (AGB) form field plots estimates to the satellite estimates with model-based approaches lead to a systematic overestimation in AGB density estimates and a root mean squared prediction error (RMSPE) of up to 65 Mg.ha−1 (90%), whereas calibration with ALS data (AGBALS) lead to low bias and a drop of ~30% in RMSPE (down to 43 Mg.ha−1, 58%) with little effect of the satellite sensor used. However, these results also confirm that, whatever the spectral indices used and attention paid to sensor quality and pre-processing, the signal is not sufficient to warrant accurate pixel wise predictions, because of large relative RMSPE, especially above (200–250 Mg.ha−1). The design-based approach, for which average AGB density values were attributed to mapped land cover classes, proved to be a simple and reliable alternative (for landscape to region level estimations), when trained with dense ALS samples. AGB and species diversity measured within 74 field inventory plots (distributed along a savanna to forest successional gradient) were higher for the vegetation located in the MDNP compared to their pairs in the Nachtigal area. The automated unsupervised long-term (45 years) land cover change monitoring from Landsat image archives based on GEE captured a consistent and regular pattern of forest progression into savanna at an average rate of 1% (ca. 6 km².year-1). No fire occurrence was captured for savanna that transited to forest within five years of monitoring. Distinct assemblages of spectral species are apparent in forest vegetation which is consistent with the age of transition. As forest gets older AGBALS recovers at a rate of 4.3 Mg.ha-1.year-1 in young forest stands (< 20 years) compared to 3.2 Mg.ha-1.year-1 recorded for older forest successions (≥ 20 years). In savannas, two modes could be identified along the gradient of spectral species assemblage, corresponding to distinct AGBALS levels, where woody savannas with low fire frequency store 50% more carbon than open grassy savannas with high fire frequency. At least two fire occurrences in five years is found to be the fire regime threshold below which woody savannas start to dominate over grassy ones. Four distinct plant communities were found distributed along a fire frequency gradient. However the presence of fire-sensitive pioneer forest species in all scenarios of fire frequencies (from low to high fire frequencies) would suggest that the limiting effect of fire on woody vegetation is not sufficient to hinder woody encroachment in the area bringing therefore sufficient humidity required for the establishment of pioneer forest saplings within open savannas. These results have implications for carbon sequestration and biodiversity conservation policies. The maintenance of the savanna ecosystem in the region would require active management actions, and contradicts reforestation goals (REDD+, Bonn challenge, etc.).
... In their comment on Veenendaal et al. (2018), Laris & Jacobs (2021; in this issue of New Phytologist, pp. 11-13) question the appropriateness of fire experiments to simulate effects of fire on tropical vegetation cover as well as objecting to our use of the word 'natural' to describe nonanthropogenic fire regimes. ...
... Tropical forests usually have long fire return intervals, varying from 10 yr to decadal or even centennial/ millennial intervals (Hall & Swaine, 1981;Hope & Tulip, 1994;Cochrane et al., 1999;Cochrane, 2003;Archibald et al., 2010). In short: a lot can be said about what constitutes a typical fire regime, but as specifically stated in Veenendaal et al. (2018), we used the 4-yr interval as a demonstration of a reasonable norm for fire return times in the absence of significant human intervention and at a scale where fire-vegetation interaction processes play out. Here we would also like to note thatcontrary to the claims of Laris and Jacobsthere was no misquotation here as Furley et al. (2008) actually wrote that 'natural fires typically break out in the late dry season with a mean fire return interval (1941-96) of 4.5 years'. ...
... In terms of the importance of our supposed neglect of anthropic regimes, we also would like to note that we stated in our paper that: 'Indeed, perhaps we should not think of nonanthropic fire regimes as being in any way representative of the current fire regime of the savanna lands with, in particular in Africa, human influences on fire patterns now being the dominating effect for all but the most strictly protected areas.' Indeed, lest Laris and Jacobs's comment leads to any confusion pertaining to this issue, we point out that throughout Veenendaal et al. (2018) we on many occasions clearly contrasted the effect of anthropic and nonanthropic fire regimes on tropical vegetation structure. However, that is not what our paper was specifically about. ...
In their comment on Veenendaal et al, (2018) Laris and Jacobs question the appropriateness of fire experiments to simulate effects of fire on tropical vegetation cover as well as objecting to our use of the word “natural” to describe non‐anthropogenic fire regimes. They also challenge some of the conclusions we drew as regards the likelihood of fire‐mediated feedbacks causing alternate stable states (ASS) in forest‐savanna transitions.
... Some researches argue that the two biomes represent alternative stable states modulated by fire-mediated feedbacks and their existence in one state depends upon perturbation (e.g. Fire and severe drought) and history of the community (Hoffmann et al., 2012a;Veenendaal et al., 2018). In the alternative stable states, return to the previous states may not be possible in the short term. ...
... Climate and soil factors are key determinants of vegetation structure (Lloyd et al. 2008, Lloyd & Veenendaal, 2016Veenendaal et al., 2015;Veenendaal et al., 2018) Edaphic properties influence the distribution and structure of tropical woody vegetation types (Coetzee et al., 1976;Dowling et al., 1986;Goodland & Pollard, 1973;Trapnell, et al., 1950) with soil depth and/or texture and consequently soil water storage an important driver of species composition and stand Fixed or mixed structure San Jose & Farinas, 1983;Swaine 1996;Veenendaal et al., 1996b). Although in savannas, soil nutrients may be sufficient to potentially sustain forest (Bond, 2010;Gray & Bond, 2015), forest soils are generally of a higher nutrient status than savanna soils under similar climate conditions (Lloyd et al., 2008;Quesada et al., 2012;Ruggiero et al., 2002). ...
... But interestingly, from this study and others, what actually seems to be happening is a longer-term persistence or slow change (at least at decadal timescales) of intermediate vegetation types in many places (Cuni-Sanchez et al., 2016;Goetze et al., 2006;). Notwithstanding this, in vegetation types where edaphic conditions are not sufficient to permit canopy closure, there is a strong correlation with fire frequency (Table 2), which is, itself, likely to have influenced canopy cover (for a quantitative analysis of this process see Veenendaal et al., 2018). Nevertheless, using the mean fire frequency estimates available here, less than 25% of the data set variation in CAI could be explained by variation in fire frequency (OLS: R 2 = 0.21, P < 0.005), this being much less than the variation accountable for by the soil properties of Table 3. ...
... In the seasonal tropics, fire commonly occurs in the dry season, irrespective of distinctions made between early versus late season fires, when microclimate and fuel characteristics are suitable to sustain fires of varying intensity (Hoffmann et al., 2012b;Prior et al., 2010;Sow et al., 2013;Veenendaal et al., 2018). Consequently, in pyrogenic savannas, fire effects and those of dry season drought are naturally entangled, confounding the influences of the two factors on tree seedling survival. ...
... This effect, which represents the natural occurrence of fire and dry season drought, was stronger than the separate effects of fire and irrigation, consistent with our prediction. This may perhaps reflect the influence of soil moisture on survival and regrowth of resprouting plants (Pausas et al., 2016;Pratt et al., 2014) The interactive effect was also stronger for forest species than for transition species with the implication being that encroachment of forest species in savannas that are moist enough is only possible under a diminished fire regime (Louppe et al., 1995;Mitchard & Flintrop, 2013;Veenendaal et al., 2018). Otherwise, transition species (rather than forest species) may initiate forest encroachment in savannas (Cardoso et al., 2016;Hennenberg et al., 2005) and also explains the existence of dry forest communities with tree species typically adapted to regular fire (Swaine, 1992;Ametsitsi et al., 2020). ...
Background: In the seasonal tropics, fire generally occurs in the dry season. Consequently, the effects of fire and dry season drought on seedling establishment of different tree functional types are correlated. Therefore, factors that are more important for tree recruitment in forest-savanna ecotones are still poorly understood. Methods: We studied seedling establishment success of seven tropical tree species (of forest and forest-savanna transition origins) in a common garden experiment using combinations of dry season (irrigation vs no-irrigation) and fire (burning vs no-burning). Results: We found that burning caused a significant decline in survival of forest species, but not of forest-savanna transition species. The combination of burning without dry season irrigation (which typifies dry season fire) had the largest overall effect on survival, mass and height of plants. The separate effect of burning was larger than that of dry season drought. Seedling size at the onset of the dry season significantly predicted root starch content and, hence, dry season survival probability of forest-savanna transition species, but not of forest species. Dry season irrigation increased post-fire survival of forest-savanna transition species, but not of forest species, possibly linked to differences in resprout capacity and root starch reserves. Conclusion: Our work shows that fire and dry season drought produce a stronger negative effect together, than their separate effects on seedling establishment particularly of forest species. This provides a mechanistic explanation for the existence of forest-savanna transition species in fire-prone humid savannas. Implications for Conservation: The mosaic of forest, transitional and savanna vegetation within the forest-savanna ecotone reflects the influences of fire and dry season drought. Any changes in these elements will influence vegetation dynamics within the forest-savanna ecotone.
... Field-based 13 and empirical analyses using remotely sensed products identify the coincidence of burnt area with the reduced occurrence ("missing") of intermediate (50-60%) tree covers [14][15][16][17][18] as evidence of a substantial impact of fire on tree cover. On the other hand, recent field analyses of fire impacts in relation to soil and climate 19,20 suggest that fire could have a smaller effect on tropics-wide tree cover than suggested up to now. Critical evaluations of the most used global woody cover product have also questioned the intermediate tree cover gap [21][22][23][24] . ...
... Therefore, our results do not preclude a substantial impact of fire on cover if the same areas within a given grid cell are burning each year, which would also explain tree cover changes found in fire exclusion experiments 57 . In mesic tropical systems (savanna and dry forest), fire effects on tree cover can be substantial if vegetation experiences frequent fires, particularly later in the dry season 19 . These regions show a higher impact (Table 1; Fig. 4). ...
Explaining tropical tree cover distribution in areas of intermediate rainfall is challenging, with fire’s role in limiting tree cover particularly controversial. We use a novel Bayesian approach to provide observational constraints on the strength of the influence of humans, fire, rainfall seasonality, heat stress, and wind throw on tropical tree cover. Rainfall has the largest relative impact on tree cover (11.6–39.6%), followed by direct human pressures (29.8–36.8%), heat stress (10.5–23.3%) and rainfall seasonality (6.3–22.8%). Fire has a smaller impact (0.2–3.2%) than other stresses, increasing to 0.3–5.2% when excluding human influence. However, we found a potential vulnerability of eastern Amazon and Indonesian forests to fire, with up to 2% forest loss for a 1% increase in burnt area. Our results suggest that vegetation models should focus on fire development for emerging fire regimes in tropical forests and revisit the linkages between rainfall, non-fire disturbances, land use and broad-scale vegetation distributions.
... Research on other Cerrado formations has demonstrated that at regional or local scales several factors, such as the presence of soil aggregates, nutrients, amount of exchangeable aluminum, water availability, and soil drainage, can directly or indirectly influence plant diversity and habitat heterogeneity (Ribeiro et al. 2021;Lira-Martins et al. 2022). In addition, soil texture (Lira-Martins et al. 2022), water table depth (Ribeiro et al. 2021), topographical relief (Oliveira-Filho et al. 1989), fire frequency (Veenendaal et al. 2018), and anthropogenic disturbance (Ribeiro and Walter 2008;Veenendaal et al. 2018) also affect the floristic composition of plant communities. ...
... Research on other Cerrado formations has demonstrated that at regional or local scales several factors, such as the presence of soil aggregates, nutrients, amount of exchangeable aluminum, water availability, and soil drainage, can directly or indirectly influence plant diversity and habitat heterogeneity (Ribeiro et al. 2021;Lira-Martins et al. 2022). In addition, soil texture (Lira-Martins et al. 2022), water table depth (Ribeiro et al. 2021), topographical relief (Oliveira-Filho et al. 1989), fire frequency (Veenendaal et al. 2018), and anthropogenic disturbance (Ribeiro and Walter 2008;Veenendaal et al. 2018) also affect the floristic composition of plant communities. ...
Background and aims Understanding the correlation between soil properties, species composition and diversity in veredas (Brazilian savanna palm swamps) can provide insights for managing this unique and endangered environment.
Methods We evaluated the relationships between the distribution and composition of herbaceous, subshrub, and shrub species and soil physicochemical properties of 21 vereda sites (315 10-m transects) in Central Brazil.
Results Our results showed high floristic alpha and beta diversity in the studied sites and significant differences in soil properties of the veredas. The proportion of exclusive species in the veredas ranged from 4 to 38%, indicating that the plant species distribution in the veredas is mosaic-like. Vereda soils were acidic, with high levels of aluminum, organic matter, and sand, but low levels of phosphorus, magnesium, and calcium. Soil phosphorus, pH, organic matter, cation saturation, and sand proportion were important in understanding veredas’ species composition (86% of the variation) and species richness (63%). Phosphorus and pH were positively correlated with species richness, whereas organic matter was negatively correlated. Organic matter, cation saturation, and sand were negatively correlated with compositional similarity, but phosphorus was positively correlated. Veredas with high floristic diversity had low fertility soils.
Conclusion The studied veredas showed significant differences in soil properties, with some variables being key drivers in assembling significantly diverse herbaceous-shrub communities in these wetland islands surrounded by a dry matrix of the Cerrado savanna.
... High grass growth during the rainy season enhances the fuel load in the dry season. This results in high intensity fires with burning temperatures between 791 and 896˚C as observed by [29] in the study area [80] also observed that fire return intervals are largest for tall savanna woodlands and dry forests. Thus, there is a possibility that fires in these areas may increase as conditions (heavy rainfall that promotes good grass growth and the hash Harmattan conditions) for fire ignition become more conducive [81]. ...
... It was noted that fire regimes in the Guinea savanna are characterised by land use patterns and practices linked to livelihood and socio-cultural activities, and which are in turn determined by the season of burning, severity, frequency and size. The season of burning was rated very important, which confirms why most burning is done in the dry season to prepare farm lands for cropping in the rainy season [80]. This finding agrees with other studies which found vegetation burning in other tropical savannas [39] to occur mostly in the dry season; the active fire season in the Sudano Guinean savanna. ...
Understanding people’s practices, knowledge and perceptions of the use of fire and fire regimes can inform fire management plans that could contribute to savanna conservation and sustainable management. We investigated the frequency of fire use, control and perceptions of fire regimes for selected livelihood and socio-cultural activities in six districts in the Guinea savanna of Ghana. The six districts were selected to have a good representation of fire prone areas in the region based on fire frequency data obtained from the CSIR Meraka Institute, South Africa. A multiple regression analysis showed that people’s use of fire for the selected socio-cultural activities from district, occupation, gender, age and ethnic group significantly predicted fire use for the activities R² = 0.043, F (5,498) = 5.43, p < 0.000. Age and occupation added significantly to the use of fire. The study revealed that the majority of respondents (83%) across the study districts used fire once a year for at least one of the following activities: land preparation, weed/pest control, burning postharvest stubble, bush clearing around homesteads, firebreaks, charcoal burning and hunting. The study also showed a higher frequency of fire use for land preparation for cropping than for the other activities. Less than a fifth of the respondents (17%) indicated that they do not use fire for any of the selected activities. The majority of respondents (65%) mentioned that they controlled their use of fire to prevent destruction to property or injuring humans. The study revealed a higher frequency of fire use in the dry season for land preparation for cropping. However, respondents rated season of burning as the most important attribute, with little attention to the other attributes of a fire regime, contrary to what is theoretically recognized. Understanding traditional fire use practices in terms of how to regulate the mix of frequency, intensity/severity, season, size and type of fire for these and other socio-cultural purposes could help to mitigate and/or manage bushfires in West African savannas and enhance savanna conservation and management. Hence, the need to better understand people’s knowledge and perceptions of fire regimes in fire assisted socio-cultural practices in West Africa.
... The structure, composition and function of savannas are dependent on the interplay of climatic and edaphic factors (N'Datchoh et al., 2015;Veenendaal et al., 2018). However, humans have predominantly influenced savannas through traditional land use practices which resonate with social and cultural belief systems in different places around the world (Jurisch et al., 2012;Trauernicht et al. 2013Trauernicht et al. , 2015Fache and Moizo 2015;Sluyter and Duvall 2016). ...
... Several studies have indicated the extensive anthropogenic use of fire in pursuit of the day to day socio-cultural and livelihood activities by most rural people in the African savannas (e.g. Bassett et al., 2003;Archibald 2016;Knowles et al., 2016;N'Datchoh et al., 2015;Veenendaal et al., 2018). These activities include hunting, land preparation for crop production (slash and burn), pasture management, weed/grass control and honey harvesting (Lopez-Merino et al., 2009;Dayamba et al., 2010;Dwomoh and Wimberly 2017). ...
The West African savanna experiences a prolonged dry season with Harmattan winds which facilitate large and persistent biomass burning from November to April. The fires are mostly caused by humans, mainly in pursuit of the day to day livelihood activities. We examined how fire influences the population structure and abundance of two economically important woody species Vitellaria paradoxa (Shea tree) and Anogeissus leiocarpa (African Birch) in six land use types in the Guinea savanna, Ghana. We calculated the stand basal area, mean densities of juveniles and adult trees, Lorey's mean height of adult trees and Simpson's index of dominance. Eight diameter size classes of each species were analysed by comparing their observed distributions to a three-parameter Weibull distribution across the land use types. A total of 3366 individuals of A. leiocarpa (n = 1,846) and V. paradoxa (n = 1,520) were enumerated. The basal area of A. leiocarpa and V. paradoxa in sacred groves (16.9 m 2 ha − 1) and unburnt woodlands (20.6 m 2 ha − 1) was higher than the estimates in the other land use types. High mean densities of A. leiocarpa and V. paradoxa were found in sacred groves (22.7 ± 29.7 stems ha − 1) and fallows (15.3 ± 2.2 stems ha − 1), respectively. Mean density of juveniles of both A. leiocarpa (248.0 ± 89.1 stems ha − 1) and V. paradoxa (68.0 ± 29.7 stems ha − 1) were higher in unburnt woodlands than in the other land use types. A. leiocarpa was absent in fallows and burnt crop fields. An inverse J-shaped distribution was found in sacred groves for both species. The absence of A. leiocarpa in burnt crop fields and the decrease of some size classes of V. paradoxa in both burnt and unburnt crop fields indicate the need for sustainable conservation of both species. Furthermore, the inverse J-shape distribution found in sacred groves for both A. leiocarpa and V. paradoxa implies that these species thrive best with minimal anthropogenic disturbances. Although species conservation is achieved through conventional protection, traditional or cultural conservation practices which avoid the indiscriminate use of fire should be highly promoted to ensure sustainable conservation of species.
... In their article, 'On the relationship between fire regime and vegetation structure in the tropics', Veenendaal et al. (2018) present a useful critique of the savanna fire experiments while simultaneously using the results of these experiments to develop a model of the effects of frequent fire on savanna woody cover. They then use the results of the model to support their argument that a 'natural' fire regime (as opposed to an anthropogenic one) is not sufficient to cause savannization (the conversion of dry forest to open savanna). ...
... Unfortunately, Veenendaal et al. (2018) appear so focused on comparing their modeling results to a nonexistent natural regime that they perpetuate a false natural/anthropogenic fire dichotomy and thus reproduce Aubr eville's bias, although in a more subtle fashion. They too imply that the results of the experiments represent a more damaging anthropogenic regime as opposed to a hypothetically less damaging natural one. ...
... It is an important step in understanding the impact of fire regimes on tree structural diversity and vegetation dynamics within ecosystem management and conservation efforts (Veenendaal et al., 2018). Fire regimes, aside from determining the survival and resilience of native tree species, will impact broader ecological functioning in these fire-prone systems. ...
Changes in fire regimes, climate change and land use practices threaten tree structural diversity and vegetation structure across spatial and temporal scales. Therefore, this study investigated the impacts of fire on the species dynamics, land cover changes and the vulnerability of tree ecosystem services in the northern savanna regions of Ghana, encompassing the Northern, Upper East, Upper West, Savannah, and North East regions. The study analyzed data collected between 2001 and 2022 using a combination of remotely sensed satellite data (MODIS NDVI, Sentinel-2 images) and field observation. Species composition was assessed through vegetation inventories, and statistical models, including regression analysis and multivariate techniques, were applied to determine fire-induced changes in tree structural diversity and land use patterns on 30m x 30m plots of land (30 in total), across ten (10) communities. Land cover classification and change detection were performed using supervised classification in QGIS. The results showed a clear-cut reduction in forest cover and increased shrubby savannah and agroforestry types driven by recurring fires and agricultural land conversion. Fire-prone areas, particularly rangelands and vegetation zones near settlements, accounted for over 70% of recorded fires, making them the most frequently affected by wildfires. The dominance of fire-adapted species, such as Vitellaria paradoxa (14%) and Parkia biglobosa (11%) of the total species recorded, was observed, whereas fire-sensitive species declined in areas with high fire frequency. The study also provided insights into the vulnerabilities of significant ecosystem services and products, such as water bodies, forests and farmlands that are ferociously threatened by fire. The findings stipulate the need to implement more comprehensive and complicated approaches to fire management, integrating human activities and fire and ecosystem services preservation within savannah ecosystems.
... The line-plotted method was used for the vegetation analysis (Valbuena et al. 2012;Threlfall et al. 2016;Veenendaal et al. 2018). Two line transects, with a minimum distance between plots of 300 m, were placed in each ecosystem-type (Robiansyah 2018). ...
Lense ON, Wanma JF, Kesaulija FF, Mansyur FI, Rachim AK, Krey K, Wanma B, Kesaulija R, Simanjorang D, Simbiak F. 2024. Diversity of flora and fauna in various forest ecosystem types of South Sorong District, Southwest Papua Province, Indonesia. Biodiversitas 25: 3884-3898. The distribution of unspoiled natural conditions, uniqueness, and high biodiversity are not limited to conservation areas but also to production and protected forest areas and other land use areas in Southwest Papua Province, Indonesia. These forest ecosystems have high flora, fauna, and areas that are priority animal pockets whose home ranges reach and enter essential ecosystem areas. However, the need for such diverse information has been an issue in this region. Hence, a diversity study of vegetation, mammals, birds, reptiles, amphibians, and butterflies in various ecosystem types is needed. The six types of ecosystems, namely the alluvial lowland forest of Nakna, the lowland karst forest of Boldon, the lowland limestone forest of Wara, the alluvial swamp forest of Konda, the peat swamp forest of Nakna, and the mangrove forest of Konda, were targeted. The line-plotted method was used for vegetation. Direct observation on a 1,500 m line transect, the point count method, and the Visual Encounter Survey were employed to collect data from mammals, birds, reptiles, amphibians, and butterflies. Results indicated that the lowland limestone forest of Nakna and the lowland karst forest of Boldon consisted of the highest number of trees (532 and 373 trees ha-1, respectively), species (56 and 52 species ha-1, respectively) per unit area. Unique and endemic orchid species, including Dendrobium transversilobum J.J.Sm. and Bulbophyllum septemtrionale (J.J.Sm.) J.J.Sm., also exist in this region, adding to the wonder of the region's biodiversity. The research also discovered a total of 9 species of mammals, 52 species from 25 families of birds, 39 species of reptiles and amphibians, of which species Litoria sanguinolenta (Van Kampen, 1909) is a new record to the region and 58 species of day butterflies from the Papilonoidea superfamily. Several key species of flora and fauna in this region require special attention as they are in alert status, which may require intervention to maintain their existence in nature so that they do not disappear or even become extinct.
... Fire decreases tree cover, which subsequently favors the lightdependent grassy layer, promoting fuel for fire spread and maintaining an open-canopy state (Bernardino et al., 2021). However, the role of fire (either natural or anthropogenic) as a determinant of forest-savanna transition is not fully understood (Staal & Flores, 2015;Veenendaal et al., 2018), particularly in mesic climates (Archibald et al., 2019). ...
The forest - savanna transition is the most widespread ecotone in the tropical regions and with important ecological, climatic, and biogeochemical implications at local to global scales. However, the processes and mechanisms that control this transition vary among regions and remain not fully understood in all of them. In general, this transition is influenced by interactions between vegetation and environmental factors such as climate, soil properties, fire, and herbivory. However, the importance of these effects can vary substantially across continents, which can result in different responses to environmental change. For this reason, more regional studies are needed to describe and understand the factors and interactions that
control forest - savanna transition in different regions. Using remotely-sensed data, we examined the relationship between the tropical forest-savanna transition and several environmental factors in northern South America, in the Llanos ecoregion. We used several vegetation structure metrics, as well as multiple precipitation statistics, soil properties, and a fire regime descriptor. In addition, we developed a statistical analysis on the interactive effects of soil silt content, fire frequency as well as three dry season precipitation variability components (season length, wet day frequency, and precipitation intensity) on the forest -savanna transition, using tree cover, canopy cover and PAVDmax as indicator variables that
differentiate forest from savanna. Our results show that savannas in the Llanos region occur in mean annual precipitation (MAP) levels in which forest would be predicted based on previously proposed thresholds to other savanna regions. Our results also highlight that the MAP range in which both forest and savanna can occur in our study area correspond, almost exclusively, to forest in other South American regions and globally. Although both forest and savanna can also occur in a large interval of intermediate values of dry season precipitation variability (PV) components, forest dominates in areas with higher precipitation frequency and
intensity than savanna. Savanna tends to occur in pixels where fires are present, while fires are absent in forest. However, a large proportion of pixels classified as savanna pixels have no fires in the analysis period, even those that occur in the same climatic or edaphic space of forest. Finally, our analysis shows that fire frequency and dry season precipitation are the most important variables to predict the forest-savanna transition. This highlights the role of fire regime and water availability in determining the limits between forest and the second largest
area of savanna in South America. Further, our results support the importance of refining our understanding of the factors, relationships, and mechanisms that control forest-savanna transition at regional scales, as a requirement to assess the effects of environmental change on tropical forest and savanna distribution
... Our review of the evidence from these studies (Notes S1) reveals that they do not clearly support the AES proposition (Table 1), that is, we concur with previous reviews (Veenendaal et al., 2018;Bond, 2019;Pausas & Bond, 2020) that no study convincingly fulfils the Petraitis (2013) criteria. The studies were mostly not designed to test the AES hypothesis, so it is unfair to be critical of their design. ...
Ecologists are being challenged to predict how ecosystems will respond to climate changes. According to the Multi‐Colored World (MCW) hypothesis, climate impacts may not manifest because consumers such as fire and herbivory can override the influence of climate on ecosystem state. One MCW interpretation is that climate determinism fails because alternative ecosystem states (AES) are possible at some locations in climate space. We evaluated theoretical and empirical evidence for the proposition that forest and savanna are AES in Africa. We found that maps which infer where AES zones are located were contradictory. Moreover, data from longitudinal and experimental studies provide inconclusive evidence for AES. That is, although the forest‐savanna AES proposition is theoretically sound, the existing evidence is not yet convincing. We conclude by making the case that the AES proposition has such fundamental consequences for designing management actions to mitigate and adapt to climate change in the savanna‐forest domain that it needs a more robust evidence base before it is used to prescribe management actions.
... It has been identified as the main driver of overall ecosystem productivity (Migliavacca et al., 2021), and linked to numerous ecological dynamics. It has been linked to biophysical processes, such as fire regime (Just et al., 2016;Veenendaal et al., 2018) or hydrology (Wu et al., 2022;Zhu et al., 2012), as well as ecosystem functional traits and energetics (Campos-Silva & Piratelli, 2020;Migliavacca et al., 2021; Understanding the global biogeography of the interplay between diverse species, and particularly between autotrophs (typically plants) and heterotrophs (herbivores, carnivores, mixed feeders) is at the heart of ecology. It is also critical to understand the consequence of anthropogenic activity and mitigate its negative impacts. ...
Background Vegetation structure is increasingly recognized as a key variable to explain ecosystems states and dynamics. New Remote Sensing tools are available to complement labor intensive field investigations and consider the global biogeography of this parameter. Objectives We propose to model the processes explaining the interaction between vegetation structure and animal community assembly globally, while requiring minimal computing power, based on the most fundamentals assumptions. Methods We integrate spaceborne (GEDI: Global Ecosystem Dynamics Investigation) and ground based (TLS: Terrestrial Laser Scanning) Lidar data in the Madingley general ecosystem model. We compare the outcome of this integration to previous version and to the TetraDensity estimate of animal biomass and Elton traits database for arboreality. Results Animal biomass density simulated by Madingley is closer to global estimates when integrating vegetation structure. The strength of this effect increases with higher cohort body mass and varies with local environmental conditions and stochastic processes. Simulated proportion of arboreality across cohorts is consistently higher than observations. This is consistent with the divergence of biases between model and database. Conclusions Our results concur with our hypotheses about the role of vegetation structure on animal community assembly, as it reduces total animal biomass abundance. However, assessing the accuracy of its relative weight is challenging. While we have global products about arboreality and animal biomass density, they represent modern day ecosystem state, including anthropogenic activity, while Madingley simulates potential ecosystem optimum. Therefore, we call for further research in this field and for challenging modelling attempts to compare with.
... Ceci induit une complexité entre les différents facteurs écologiques. C'est cela que Veenendaal et al. (2018) Par exemple, il sera donc important d'accentuer des études sur le feu et les paramètres du sol. Aussi avec ce dérèglement climatique dont faire face la planète terre, il serait judicieux de mener des études en lien avec le climat dans les aires fréquemment brûlées. ...
The practice of fire can be a real source of change in biodiversity structures. The aim of this systematic review is to summarize recent
literature on the research topics of wildland fire in Africa. For this, the Google Scholar, Web of Science and SCOPUS scientific article
databases were consulted from 1960 to 2022. Satellite images of fire in Africa via "NASA FIRMS Fire Archive" were also downloaded.
Descriptive statistics and maps were produced using Excel spreadsheet and QGIS software respectively. T-tests were performed to
identify significant differences in the number of studies over the years in R software.The results show that West Africa (45.76%) and
Southern Africa (38.41%) have investigated more vegetation fires. As for West Africa, Burkina-Faso (53.08%) paid more attention to
this theme. Topics related to the ecology of wildfire (31.46%) and wildfire in relation to biomass (30.33%) were the most studied in
Africa. One of the perspectives arising from this study is based on satellite images (remote sensing), which are underdeveloped in fire
studies in Africa, despite the fact that they constitute real sources for identifying and predicting the future effects of the repetitive fires
experienced by African ecosystems. So, in the future, more emphasis needs to be placed on these themes.
Keywords: fire, repetition, savannah, biodiversity, Modis
... For instance, various studies have demonstrated that human activities are drivers of shifts in vegetation states (Rietkerk et al 2004), and also of land area burnt (Archibald et al 2009). In more mesic areas, Veenendaal et al (2018) found fire maintains open savanna grasslands under deliberate human-mediated, high-frequency, late season fire regimes. It must be acknowledged that vegetation and fire patterns are also shaped by terrain that can alter the spread of fire (Bowman 2000, Ondei et al 2017), as well as creating environmental gradients that affect the distribution of forests such as rain shadows, although global analyses fire and forest and savanna distributions typically do not consider terrain variables (Hirota et al 2011, Staver et al 2011. ...
Tropical forest and savanna biomes are pivotal in the functioning of the Earth system. Both are biodiverse and under increasing threat due to land clearing and anthropogenic climate change, and play important roles in the global carbon cycle, through maintenance of a large carbon pool in tropical forests, and exchange in savannas through extensive landscape fires. Reliable mapping of tropical forest and savanna is essential to understand how the current distribution of these vegetation types is controlled by climate land clearing and fire.. Using Google Maps satellite imagery, we manually classified 24,239 random points as forest, savanna, or anthropogenic landscapes within the tropics and applied this novel dataset to defining the climatic zone where forest and savanna exist as alternative states. Because fire and climate are correlated, we developed separate geospatial models to rank the importance of climate, topography, and human influence on vegetation present. This modelling confirmed that those areas with more fires had lower probabilities of tropical forest, that forest was most likely in areas with high mean annual rainfall with little seasonal variation in precipitation, and that anthropogenic factors disrupt this environmental predictability. We also identified areas where tropical forest and savanna both co-occur, but these were relatively uncommon. These relationships suggest that future drier climates projected under anthropogenic climate change, combined with clearing and burning that have reduced tropical forest extent to a subset of its theoretical distribution, will lead to irreversible loss of tropical forests. Our modelling provides global mapping that can be used track further changes to distribution of tropical forests.
... Gallery forests are narrow and surrounded by flammable savannas, from where such fires may spread into the forest (Ribeiro & Walter, 2008;Kellman and Meave, 1997). Nonetheless, gallery forests may suffer mild burns and still keep high tree cover under a long fire return interval, likely because fire events may not be strong enough to open the forest canopy (Veenendaal et al., 2018). Strong widespread fires, however, may severely impact the ecosystem, killing most trees and destroying the organic soil layer (Flores et al., 2021). ...
Coexisting vegetation types in tropical landscapes can respond in contrasting ways to rainfall, despite being in the same climatic envelope. Understanding such heterogeneity in vegetation-rainfall interactions is key for predicting how ecosystems might respond to future environmental changes. Here we test whether temporal coupling between vegetation greenness and rainfall is a good indicator of ecosystem state in the landscape. For this, we study a well-preserved landscape of the Brazilian Cerrado that is formed by mosaics of contrasting ecosystems, including savannas, dry forests and gallery forests. First, we correlate the time-series of rainfall and vegetation greenness to quantify their coupling for each vegetation type. We then compare vegetation-rainfall coupling with other state variables, such as local-scale vegetation structural and functional traits, as well as differences in environmental conditions in which these vegetation types exist. Coexisting vegetation types are set in contrasting local-scale environmental conditions and have distinct responsiveness to rainfall. Commonly used structural and functional state variables, such as tree cover and tree height, do not depict such marked differences between the vegetation types, particularly for gallery and dry forests. Dry forests have the strongest coupling and decrease their greenness during dry seasons, reflecting vegetation deciduousness on nutrient-richer soils. In contrast, gallery forests increase their greenness during the dry season, when direct radiation peaks, likely due to perennial access to groundwater. Savannas are less responsive to rainfall and have a more stable greenness throughout the year. Our findings suggest that heterogeneity in local abiotic conditions contribute to determining both vegetation distribution and ecosystem states in these tropical savanna landscapes. Changes in these conditions as a result of climate and land-use changes will likely alter the distribution of vegetation types in the future. Our functional metric may thus be useful for assessing future responses of tropical ecosystems to changes in precipitation.
... For instance, various studies have demonstrated that human activities are drivers of shifts in vegetation states (Rietkerk et al. 2004), and also of land area burnt (Archibald et al. 2009). In more mesic areas, fire can only maintain open savanna grasslands under deliberate human-mediated, high-frequency, late season fire regimes (Veenendaal et al. 2018). It must be acknowledged that vegetation and fire patterns are also shaped by terrain that can alter the spread of fire (Bowman 2000;Ondei et al. 2017), as well as creating environmental gradients that affect the distribution of forests such as rain shadows, although global analyses fire and forest and savanna distributions typically do not consider terrain variables (Hirota et al. 2011;Staver et al. 2011). ...
Tropical forest and savanna biomes are pivotal in the functioning of the Earth system. Tropical forests are one of the largest terrestrial biosphere carbon pools, whereas savannas exchange carbon between the biosphere and atmosphere via frequent and extensive landscape fires. Both are biodiverse and under increasing threat due to land clearing and anthropogenic climate change. Reliable mapping of tropical forest and savanna is essential to provide understanding of how anthropogenic impacts are affecting the extent of these biomes. Using Google Maps satellite imagery, we manually classified 24,239 random points as forest, savanna, or anthropogenic landscapes within the tropics. Because fire and climate are correlated, we developed separate geospatial models to rank the importance of climate, topography, and human influence on vegetation present. This modelling confirmed that those areas with more fires had lower probabilities of tropical forest, that forest was most likely in areas with high mean annual rainfall with little seasonal variation in precipitation, and that anthropogenic factors disrupt this environmental predictability. We found there were environments where tropical forest and savanna were equally probable are geographically restricted. These relationships suggest that future drier climates projected under anthropogenic climate change, combined with clearing and burning that have reduced tropical forest extent to a subset of its theoretical distribution, will lead to irreversible loss of tropical forests. Our modelling provides global mapping that can be used track further changes to distribution of rainforests.
... habitat types associated with small mammal trap positions. Another use could be determining the percentage of canopy cover in small forest patches in studies investigating forest regeneration (Rolo, Olivier, and van Aarde, 2017) or the effects of fire (Veenendaal et al., 2018). Here higher altitude flights would be sufficient as ground-truthing can identify tree species, whilst orthophotographs can be used to quantify canopy cover. ...
The application of emerging technologies in ecological research, specifically the use of radiocarbon dating and drone surveys for the investigation of crocodile life history and population dynamics
... The results show that there are too few trees compared to observations for the western parts of Brazil. The simulations with fire improve tree cover in savanna, which is consistent with the findings of Staver et al. (2011) andLasslop et al. (2016); however, there is still ongoing discussion around how much impact fire really has on tree cover in the savanna compared to other dry disturbances such as wind throw, heat stress and rainfall distribution (Veenendaal et al., 2018;Brovkin et al., 2009). ...
Global studies of climate change impacts that use future climate model projections also require projections of land surface changes. Simulated land surface performance in Earth system models is often affected by the atmospheric models' climate biases, leading to errors in land surface projections. Here we run the Joint UK Land Environment Simulator Earth System configuration (JULES-ES) land surface model with the Inter-Sectoral Impact Model Intercomparison Project second-phase future projections (ISIMIP2b) bias-corrected climate model data from four global climate models (GCMs). The bias correction reduces the impact of the climate biases present in individual models. We evaluate the performance of JULES-ES against present-day observations to demonstrate its usefulness for providing required information for impacts such as fire and river flow. We include a standard JULES-ES configuration without fire as a contribution to ISIMIP2b and JULES-ES with fire as a potential future development. Simulations for gross primary productivity (GPP), evapotranspiration (ET) and albedo compare well against observations. Including fire improves the simulations, especially for ET and albedo and vegetation distribution, with some degradation in shrub cover and river flow. This configuration represents some of the most current Earth system science for land surface modelling. The suite associated with this configuration provides a basis for past and future phases of ISIMIP, providing a simulation set-up, postprocessing and initial evaluation, using the International Land Model Benchmarking (ILAMB) project. This suite ensures that it is as straightforward, reproducible and transparent as possible to follow the protocols and participate fully in ISIMIP using JULES.
... This remarkable increase in the frequency of man-made fires contrasts with the historical fire regime (Setterfield 1997;Bowman et al. 2011). Although the Cerrado is resilient to fire disturbances, the imposition of this new anthropic regime has serious impacts on its plant communities (da Silva and Bates 2002;Hoffmann et al. 2009;Veenendaal et al. 2018), yet to be fully evaluated. ...
In the last 20 years, fire frequency in South America's largest savanna, the Cerrado, has increased by 41%. Although resprouting represents the main post-fire regeneration mechanism that allows plant species to persist in the Cerrado, under the new scenario of high fire incidence, small individual trees may not be able to regrow, while large individual trees may be subject to meristematic apical death or “topkill”. In this study, we evaluated the resistance and resilience of a tree species with a wide geographic distribution in the Cerrado, Stryphnodendron adstringens (Fabaceae) to non-prescribed fire and its resprouting dynamics. Seventy out of the 72 individuals of S. adstringens studied suffered topkill, indicating a low resistance to fire. To monitor the development and dynamics of resprouts for 17 months, we randomly selected 54 individual trees. Altogether, 143 resprouts were recorded in these 54 individuals during 3, 6, 10, and 17 months after fire, being 90% in the first 3 months. Larger individual trees had a larger number of resprouts and of larger sizes. Resprout mortality (14%) was higher in the 17th month post-fire. Multiple logistic regressions revealed that the survival probability of resprouts to the 17th month increased with their size but decreased with their number. We conclude that S. adstringens has low resistance but high resilience to, at least, a single fire. Also, there is a trade-off between the production of resprouts and their chance of survival. Thus, frequent fires can reduce the recruitment and persistence of the species.
... Conversely, fire is a very important agent of vegetation change [43] and is an intrinsic part of the landscape [44][45][46]. The extent and time of recovery of the burnt ecosystems is unknown, as it depends on multiple factors [47,48]. The study participants are quite confident that the burnt ecosystems will recover completely within a timespan of roughly 30 years, and estimate that it will take ten more years to restart tapping in the areas that are currently totally burnt. ...
Resin tapping has long been an important traditional economic activity in less advantaged areas that serves as a source for income to local populations as well as an important management tool intricately linked with ecosystem services. In Greece, the majority of the remaining resin tappers are located in communities close to Pinus halepensis stands in the northern part of Evia Island. However, on 3 August 2021, a wildfire burned more than 50,000 Ha in the region, bringing the remaining resin tappers to a standstill. This paper aims to examine the views of 89 resin tappers in N. Evia and their perspectives on their profession as expressed during personal interviews. According to the participants’ replies, resin production is expected to bounce back to pre-fire levels in 40 years. A considerable portion of those interviewed (39.3%) were forced to work in other professions near their communities during the period that followed the fire. They were very dissatisfied by the fire management and initial mitigation actions, and fairly satisfied by the ecosystem recovery works that followed. However, they have high expectations for employment in a seven-year long government-funded support program which will allow them to continue living in their communities, and in this context they are willing to make a professional shift to forest operations work. In this context, the role of state authorities is expected to be crucial. The provision of vocational training and financial incentives could be decisive in preventing them from transitioning to alternative production sectors.
... Specifically, vegetation-fire feedbacks have been proposed as a driver of alternate stable states in tropical forests and humid savannas (Langevelde et al., 2003;Hirota et al., 2011;Staver et al., 2011;Higgins & Scheiter, 2012;Staver & Levin, 2012;Accatino & De Michele, 2013Dantas et al., 2016;D'Onofrio et al., 2018), boreal (Johnstone et al., 2010;Rogers et al., 2015;Abis & Brovkin, 2019) and temperate forests (Kitzberger et al., 2012(Kitzberger et al., , 2016Tepley et al., 2016). However, other potential drivers of alternate stable states, mostly related to edaphic conditions, also have been proposed (Fletcher et al., 2014;Bowman & Perry, 2017;Veenendaal et al., 2018). The hypothesis that in the Mediterranean Basin forests and open shrublands are alternative stable states is motivated by observations of succession that has stalled in shrublands (Baeza et al., 2006;Ac acio et al., 2009;Santana et al., 2010;Ac acio & Holmgren, 2014) and of loss of resilience in oak and pine forests after repeated fires (Diaz-Delgado et al., 2002;Mayor et al., 2016b). ...
... Ramos-Neto & Pivello, 2000). Fourth, experimental work has also considered the influence of seasonal timing on fire impacts (Smit et al., 2010;Veenendaal et al., 2018), a factor that we did not consider here, although most burned area in the Cerrado occurs in the mid to late dry season, a period often underrepresented in experimental work given the higher likelihood of fires escaping into neighboring areas (Vernooij et al., 2021). ...
Fires mediate grass and tree competition and alter vegetation structure in savanna ecosystems, with important implications for regional carbon, water, and energy fluxes. However, direct observations of how fire frequency influences vegetation structure and post‐fire recovery have been limited to small experimental field studies. Here, we combined lidar‐derived canopy height and canopy cover from NASA's Ice, Cloud, and land Elevation Satellite‐2 with over two decades of burned area data from the Moderate Resolution Imaging Spectroradiometer sensors to provide the first biome‐wide estimates of post‐fire changes in canopy structure for major vegetation types in the Cerrado (Brazil). Mean canopy height decreased with increasing burn frequency for all natural cover types, with the greatest decline observed for forests and savannas. The ability to separate changes in fractional canopy cover from height growth using lidar data highlighted the long‐time scales of vegetation recovery in forests and savannas after fire. For forests in medium and high precipitation areas, canopy cover returned to unburned values within 5 years following fire, whereas mean canopy height remained below unburned values, even in the oldest fires (14–20 years). Recovery times increased with decreasing rainfall, with average values of both fractional cover and canopy height below unburned areas after 14–20 years for savannas. We observed gradual recovery of vegetation height and cover over decades, even in mesic or wet savanna regions like the Cerrado. Infrequent fire activity, particularly in areas with greater land management, influences ecosystem structure across the biome, with important consequences for biodiversity conservation.
... The fires are strongly associated with Topic 1, characterised by Cyperaceae and Poaceae, suggesting that fuel type may have been important in determining the fire regime, although fuel load and moisture typically also play a role in fire ignitions (McLauchlan et al., 2020). Fire is thought to be important in maintaining open savannah ecosystems where climatic and soil conditions would be sufficient to support forest cover, and fire may have played a role in delaying succession to swamp forest at CEN-17.4 (Veenendaal et al., 2018;Aleman et al., 2019). ...
... The results show that there are too few trees compared to observations for western parts of Brazil. The simulations with fire on improve tree cover in savanna, which is consistent with the findings of Lasslop et al. (2016) and Staver et al. (2011), however there is still ongoing discussion around how much impact fire really has on tree cover in the savanna compared to other dry disturbances such as wind throw, heat 320 stress, rainfall distribution (Veenendaal et al., 2018;Brovkin et al., 2009). https://doi.org/10.5194/egusphere-2022-1196 ...
Global studies of climate change impacts that use future climate model projections also require projections of land surface changes. Simulated land surface performance in Earth System models is often affected by the atmospheric models’ climate biases, leading to errors in land surface projections. Here we run the JULES-ES land surface model with ISIMIP2b bias-corrected climate model data from 4 global climate models (GCMs). The bias correction reduces the impact of the climate biases present in individual models. We evaluate JULES-ES performance against present-day observations to demonstrate its usefulness for providing required information for impacts such as fire and river flow. We simulate a historical and two future scenarios; a mitigation scenario RCP2.6 and RCP6.0, which has very little mitigation. We include a standard JULES-ES configuration without fire as a contribution to ISIMIP2b and JULES-ES with fire as a potential future development. Simulations for gross primary productivity (GPP), evapotranspiration (ET) and albedo compare well against observations. Including fire improves the simulations, especially for ET and albedo and vegetation distribution, with some degradation in shrub cover and river flow. This configuration represents some of the most current earth system science for land surface modelling. The suite associated with this configuration provides a basis for past and future phases of ISIMIP, providing a simulation setup, postprocessing and initial evaluation using ILAMB. This suite ensures that it is as straightforward, reproducible and transparent as possible to follow the protocols and participate fully in ISIMIP using JULES.
... and dependent on, the dry season fire regime. Without fire, savanna communities exhibit profound changes in composition (Andersen et al., 2012) as they transition to a predominance of tree cover with attendant losses in understory structural and compositional diversity (Lipsett-Moore et al., 2018; though see Veenendaal et al., 2018). ...
Savanna burning programs across northern Australia generate millions of dollars per year for Indigenous communities through carbon and other greenhouse gas (GHG) markets. In catalyzing Indigenous knowledge and workforce to mitigate destructive wildfires, these programs are considered a success story on a range of social, ecological and economic measures. Scaling-up to temperate ecosystems requires a focus on applying the architecture and governance of these programs, and accounting for fundamental differences in context. We examine the opportunities and challenges in applying the architecture of savanna burning to an IFM program in central British Columbia, Canada (the Chilcotin). The Chilcotin project involves Yunesit’in and Xeni Gwet’in First Nations, and we draw from eight key elements of the Australian savanna burning model to identify a project area that includes Aboriginal title and reserve lands. The area encompasses Interior Douglas Fir (IDF) and Sub-Boreal Pine—Spruce (SBPS) biogeoclimatic zones, or dry forest and grassland ecosystems where low intensity fires are applied by community members to remove forest fuels, with the goal of mitigating wildfires and associated GHG emissions. The multi-decadal intervals between contemporary fires in the Chilcotin region make it challenging to accurately document historical fire location, scale and intensity, and thus to establish an emissions baseline. If this issue can be resolved, the British Columbia Forest Carbon Offset Protocol version 2 (FCOPv2) offers promise for developing verified carbon credits for three reasons: first, carbon (CO2), nitrous oxide (N2O), and methane (CH4), the three main GHG emissions from Indigenous fire management, are included in the protocol; second, credits under FCOPv2 are eligible for either compliance or voluntary markets, offering diversification; and third, a range of activities are eligible under the standard, including fire management and timber harvesting, which offers flexibility in terms of management practices. The Chilcotin project is likely to generate substantial co-benefits related to cultural, health and wellbeing, and livelihood values among First Nations participants. The Australian experience suggests that getting governance right, and building community ownership through “bottom-up” governance, is critical to the success of these programs. From the Australian model, community-based planning, like the Healthy Country Planning approach, can be a positive step to take, engaging community in goal setting for the program to guide and take ownership of its direction.
... At first glance, the growing popularity of the fire-herbivore paradigm can appear to be supplanting the idea that climate and soils matter at all (e.g. Veenendaal et al., 2018). But rather than viewing these two paradigms as mutually exclusive, we suggest that recent work to understand the role of fire and herbivores in shaping grassland and forest distributions does not replace, but adds nuance, specificity, and mechanistic detail, where the climate-soil paradigm falls short. ...
Recent advances in ecology and biogeography demonstrate the importance of fire and large herbivores – and challenge the primacy of climate – to our understanding of the distribution, stability, and antiquity of forests and grasslands. Among grassland ecologists, particularly those working in savannas of the seasonally dry tropics, an emerging fire–herbivore paradigm is generally accepted to explain grass dominance in climates and on soils that would otherwise permit development of closed‐canopy forests. By contrast, adherents of the climate–soil paradigm, particularly foresters working in the humid tropics or temperate latitudes, tend to view fire and herbivores as disturbances, often human‐caused, which damage forests and reset succession. Towards integration of these two paradigms, we developed a series of conceptual models to explain the existence of an extensive temperate forest–grassland mosaic that occurs within a 4.7 million km2 belt spanning from central Europe through eastern Asia. The Eurasian forest‐steppe is reminiscent of many regions globally where forests and grasslands occur side‐by‐side with stark boundaries. Our conceptual models illustrate that if mean climate was the only factor, forests should dominate in humid continental regions and grasslands should prevail in semi‐arid regions, but that extensive mosaics would not occur. By contrast, conceptual models that also integrate climate variability, soils, topography, herbivores, and fire depict how these factors collectively expand suitable conditions for forests and grasslands, such that grasslands may occur in more humid regions and forests in more arid regions than predicted by mean climate alone. Furthermore, boundaries between forests and grasslands are reinforced by vegetation–fire, vegetation–herbivore, and vegetation–microclimate feedbacks, which limit tree establishment in grasslands and promote tree survival in forests. Such feedbacks suggest that forests and grasslands of the Eurasian forest‐steppe are governed by ecological dynamics that are similar to those hypothesised to maintain boundaries between tropical forests and savannas. Unfortunately, the grasslands of the Eurasian forest‐steppe are sometimes misinterpreted as deforested or otherwise degraded vegetation. In fact, the grasslands of this region provide valuable ecosystem services, support a high diversity of plants and animals, and offer critical habitat for endangered large herbivores. We suggest that a better understanding of the fundamental ecological controls that permit forest–grassland coexistence could help us prioritise conservation and restoration of the Eurasian forest‐steppe for biodiversity, climate adaptation, and pastoral livelihoods. Currently, these goals are being undermined by tree‐planting campaigns that view the open grasslands as opportunities for afforestation. Improved understanding of the interactive roles of climate variability, soils, topography, fire, and herbivores will help scientists and policymakers recognise the antiquity of the grasslands of the Eurasian forest‐steppe.
... Although fire can favour the establishment of a grassy vegetation at higher soil nutrient status, the suppression of this disturbance should allow for a higher woody biomass accumulation (Fig. 7). Given increasing tree cover will naturally suppress fire (Hoffmann et al. 2012), fire disturbance in high nutrient soils generally has an anthropogenic source (Veenendaal et al. 2018). ...
Background
The Cerrado of central Brazil—the world’s largest Neotropical savanna – is comprised of a mosaic of highly heterogeneous vegetation growing on an extremely diverse geologic and geomorphologic background. Geomorphic processes under stable tectonic and climatic conditions facilitated the development of diverse edaphic properties, which interact with disturbance events to form unique vegetation types.
Scope
In this review, we detail how the geophysical environment affects soil formation and evaluate the mechanisms through which edaphic conditions control vegetation structure, floristic diversity and functional diversity.
Conclusion
The influence of geomorphic processes on edaphic properties has a marked impact on the ecology and evolution of plant communities. Species exhibit morphological and physiological adaptations that optimise their successful establishment in particular soil conditions. Furthermore, fire disturbance alters these soil-vegetation associations further regulating the structural nature of these communities. Therefore, we propose an integrative view where edaphic, chemical and physical properties act as modulators of vegetation stands, and these conditions interact with the fire regime. The knowledge of plant edaphic niches, their functional traits related to resource acquisition and use, as well as the interaction of edaphic properties and disturbance regimes is paramount to research planning, conservation, and successful restoration of the full diversity of Cerrado vegetation types.
... Yet another possibility is that the bimodality results from artificial discontinuities in the data, introduced by its generative machine-learning algorithm [6,7]. Therefore, the firemediated alternative stable state hypothesis is still contested, with some left unconvinced and in favour of other causal mechanisms that distinguish between savanna and forest [23]. Taking into consideration the possible al-ternative explanations for bimodality, further tests of bistablity using remote-sensed data would have to rely on more specific indicators than bimodality. ...
It is thought that tropical forests can exist as an alternative stable state to savanna. Therefore, perturbation by climate change or human impact may lead to crossing of a tipping point beyond which there is rapid large-scale forest dieback that is not easily reversed. Empirical evidence for bistability due to fire-vegetation feedbacks relies on tree cover bimodality in satellite-observed data, but this may also be explained by spatial heterogeneity, or by biases in the data. Theoretical evidence for bistability does not consider the interaction of fire with the vegetation landscape. Focusing on landscapes consisting of tropical forest and grassland, we show that the microscopic rules of fire and vegetation spread lead to an emergent relation between macroscopic forest structure and dynamics. This relation defines a landscape-scale balance equation of forest area change in which the forest perimeter determines the nonlinearity in forest growth while the forest perimeter weighted by adjacent grassland area determines the nonlinearity in forest loss. We demonstrate that our equation enables analysis of resilience or abrupt shifts for a given landscape, using only the landscape state at a single point in time and measurable parameters of the underlying microscopic spatial processes, thereby avoiding the problems associated with reliance on bimodality.
... Fire promotes significant changes in vegetation and plays key roles in plant physiology and community structure (Veenendaal et al., 2018). The post-fire plant regeneration process can take decades. ...
Aim of the study: To evaluate the effects of a mega-fire on the arbuscular mycorrhizal fungi (AMF) community and parameters in soils under Cerrado vegetation.
Study area: Chapada dos Veadeiros National Park, Goiás, Brazil. This site suffered the biggest fire in its history on October 10, 2017, with an affected area of 66,000 ha.
Material and methods: We analyzed AMF spore density, roots’ mycorrhizal colonization rate, easily extractable glomaline (EEG), as well as the AMF genera present. These parameters were evaluated in burned and unburned areas of five common phytophysiognomies of the region.
Main results: Fire presence immediately influenced the mycorrhizal community parameters in Cerrado soils, which tended to increase afterwards. The presence of AMF genera did not differ between burned and unburned areas, with Acaulospora, Claroideglomus, Diversispora, Glomus, Funneliformis, Sclerocystis, and Gigaspora being present. The recovery of AMF community conditions in the Cerrado after fire events could also be observed in the parameters evaluated, as the values of density, roots’ mycorrhizal colonization rate, and EEGe were similar in the burned and unburned areas.
Research highlights: AMF diversity, and especially their community parameters, show great recovery after fire events, since they are crucial in processes like nutrient cycling and soil aggregation.
... For these studies, flights at 50 m altitudes or below can be used to quantify (in square metres) habitat types associated with small mammal trap positions. Another use could be determining the percentage of canopy cover in small forest patches in studies investigating forest regeneration (Rolo, Olivier & van Aarde, 2017) or the effects of fire (Veenendaal et al., 2018). Here higher altitude flights would be sufficient as ground-truthing can identify tree species, whilst orthophotographs can be used to quantify canopy cover. ...
Low-cost uncrewed aerial vehicles (UAVs) have become ubiquitous, and advanced UAV
systems are affordable for many field ecologists and wildlife managers. Many hobbyist
UAVs have been applied to ecological studies, but proprietary software limits their widespread application with little quantification with regards to their accuracy and efficiency in
the creation of maps through photogrammetry. Our study addressed these concerns by
evaluating a combination of an entry-level UAV and open-source photogrammetry drone
mapping software as a low-budget mapping solution for ecologists. Geometrically accurate
orthophotograph maps were created from flights at altitudes below 70 m with and without
differential global positioning system (d-GPS) ground control points. Object measurement
errors were constrained below 30 mm for altitudes up to 70 m, and errors fell below 10 mm at
30 m altitudes with d-GPS points and below 20 mm without the use of d-GPS ground control
points. Our analyses provide guidelines that parameterize the requirements for the mapping
of smaller areas. Ecological surveys that do not require <50 mm accuracy can benefit from
the methods described here, and many ecological studies that are presently using costly
software and UAV platforms could save when adopting this approach.
... We measured leaf maximum gas exchange rates and leaf nutrient concentrations in a total of 60 ground layer species (graminoids, herbs, subshrubs and palms) three years after an accidental fire in the burned area, and did parallel measurements in an unburned typical savanna site (IBGE 2020). Since fire decreases canopy cover and increases soil fertility Veenendaal et al. 2018), generally providing higher resource availability, we expected to detect these effects even after a medium-term period (3 years) since the last disturbance event. Therefore, we expected that all species, regardless of growth form, would be capable to sustain higher nutrient concentration in their leaves and depict a higher capacity to assimilate carbon in more disturbed and open environments i.e., the burned site, compared to more stable and closed environments (i.e., 20 years without fire). ...
Fire is one of the most important factors driving community assembly and ecosystem functioning in tropical savannas. However, few studies have evaluated the physiological responses of ground layer plant communities to fire disturbance. Here we used different fire regimes to investigate possible changes in leaf maximum gas exchange (Amax and gs) and leaf nutritional content (N, P, K, Ca and Mg) among different plant growth forms in savanna ground layer communities. We compared responses of ground layer plant communities under two different fire regimes: (1) no recent fire occurrence; and (2) two recurrent fire events in the last 20 years. We estimated canopy cover, soil chemical properties and species abundance on burned and unburned plots in order to calculate abundance-weighted species average trait values for gas exchange and leaf nutrient content. We found that burned plots exhibited lower canopy cover and soil organic matter content, and an overall higher soil macronutrients availability compared to unburned plots. These environmental differences clearly influenced the ground layer plant communities, which depicted higher Amax and gs in burned areas regardless of growth form. We found no significant differences among leaf nutrient traits, except for a lower Mg concentration in the burned site species. Our results support the hypothesis that distinct fire regimes select for a different set of leaf functional traits, with fire occurrence acting as an important driver increasing the maximum photosynthetic rate on the ground layer. While nutrient use seems not to be affected by medium-term recurrent fires, physiological plasticity on carbon and water use processes in response to changes in resource availability can promote the persistence of savanna species under frequent fire.
... Intraspecific variation, such as that caused by phenotypic plasticity, may result in even stronger gradients in CWM trait values if the trait plasticity aligns with community-level patterns (Derroire et al., 2018). Generalists, when grown in forests, tend to have trait values more similar to forest species (Maracahipes et al., 2018), which should cause nonlinearities in CWM trends for some highly plastic traits such as crown depth or height, analogous to the stand etiolation process suggested by Veenendaal et al. (2018) as a source of discontinuity in canopy cover. Grime (1977) suggests a general pattern of succession beginning with ruderal herbs, moving to a more competitive and stress-tolerant strategy, and ending with a highly stress-tolerant climax guild; other models of forest succession invoke similar shifts, for example, from pioneer to shade-tolerant species (Bazzaz & Pickett, 1980). ...
Under fire suppression, many tropical savannas transform into forests. Forest expansion entails changes in environmental variables and plant community structure. We hypothesized that forest expansion into savanna results in a shift in community‐weighted mean functional traits from stress tolerance to competitiveness, with generalist species having trait values intermediate between those of specialists of savanna and forest habitats.
We studied 30 plots distributed over three savanna–forest boundaries undergoing forest expansion in the Brazilian Cerrado, capturing a gradient from open savanna to recently formed forest. We measured functional traits of 116 woody species of savanna specialist, generalist and forest specialist functional groups and quantified changes in species composition and mean traits across the basal area gradient.
We identified two main axes of species traits. The first separated forest and generalist species from savanna specialists, with the latter possessing traits associated with resistance to disturbance and stress— such as thick leaves, thick bark, slower height growth and lower shade tolerance. Our second trait axis separated shrubs and understorey trees from pioneer species. Generalist species’ traits did not differ substantially from forest species, nor did they tend to have a typical pioneer strategy.
Community‐weighted trait means changed linearly with forest development. There was a steady increase in traits associated with competitive dominance rather than stress tolerance and fire resistance, indicating a wholesale shift in the selective environment. Several of these patterns—for example, increasing height and decreasing light requirements—are common in old‐field succession. In contrast to old‐field succession, we found that SLA increased, leaf thickness decreased and wood density stayed constant.
The assembly of forests appears to be shaped by environmental filters that contribute to a functional trajectory distinct from most other studied ecosystems. Our results highlight the importance of the functional composition of the early community and of the early colonizers of the open environment. Differences between savanna and forest specialists reflect the selective effects of the contrasting environments, while the traits of generalists—and their interaction with environmental filters—drive the dynamics of forest expansion.
A free Plain Language Summary can be found within the Supporting Information of this article.
... Understanding the dynamics of fire and vegetation is particularly valuable for informing land management strategies and predicting ecosystem response to climate change (Ekblom and Gillson 2010;Breman et al. 2012;Veenendaal et al. 2018). Charcoal morphological analysis based on records of past environmental change preserved in lake sediments is a useful method for determining grass versus non-grass vegetation and is therefore capable of contributing to this understanding. ...
Fire is inextricably linked to the vegetation that provides the fuel load. For palaeofire records to contribute meaningfully to the reconstruction of past landscape fire history, it is helpful to identify the vegetation that has been burnt, for example, grassy versus woody vegetation in tropical savannas. The morphological characteristics of charcoal particles can provide useful information on source vegetation type, and the aspect ratio of charcoal particles has been proposed to identify the contribution of grasses to environmental records. Stable carbon isotope analysis of pyrogenic carbon can also chemically identify the proportion of C3 and C4 biomass in charcoal samples but has yet to be widely applied alongside charcoal morphological analysis. Using carbon isotope analysis we demonstrate that C3 sedges contribute elongate charcoal to a fire record where C4 grasses are absent. These results challenge the widespread assumption that elongate charcoal is primarily or exclusively derived from grass, as most experimental studies demonstrating this relationship were conducted in environments where graminoids (grass-like forms) did not significantly contribute to available fuels. In turn, this complicates the simple interpretation of elongate aspect ratios for charcoal in fire records as direct proxies for the proportion of grasses in an environment, beyond differentiating temperate forests from grasslands. Minimal work to date has been done on separating charcoal derived from different graminoid types and future studies would benefit from the ability to differentiate graminoids including Poaceae and Cyperaceae in fire records. These results highlight the benefits of a multi-proxy approach to the interpretation of fire records in tropical savannas.
... While the role of the RSC in these grass-dominated ecosystems is thus thought to be small, the significance of the RSC in areas with more woody fuel may be higher (Bertschi et al., 2003;Christian et al., 2007;Hao et al., 1991). With prescribed-fire management, dead organic matter and woody carbon stocks may increase over time (Oliveras et al., 2013;Pivello, 2011;Veenendaal et al., 2018). For long-term emission abatement potential, it is therefore important to understand how these changes in fuel composition affect EFs. ...
Landscape fires, often referred to as biomass burning (BB), emit substantial amounts of (greenhouse) gases and aerosols into the atmosphere each year. Frequently burning savannas, mostly in Africa, Australia, and South America are responsible for over 60 % of total BB carbon emissions. Compared to many other sources of emissions, fires have a strong seasonality. Previous research has identified the mitigation potential of prescribed fires in savanna ecosystems; by burning cured fuels early in the dry season when landscape conditions still provide moist buffers against fire spread, fires are in general smaller, patchier, and less intense. While it is widely accepted that burned area (BA) and the total carbon consumed are lower when fires are ignited early in the dry season, little is known about the intraseasonal variability of emission factors (EFs). This is important because potentially, higher EFs in the early dry season (EDS) could offset some of the carbon benefits of EDS burning. Also, a better understanding of EF intraseasonal variability may improve large-scale BB assessments, which to date rely on temporally static EFs. We used a sampling system mounted on an unmanned aerial vehicle (UAV) to sample BB smoke in the Estação Ecológica Serra Geral do Tocantins in the Brazilian states of Tocantins and Bahia. The protected area contains all major Cerrado vegetation types found in Brazil, and EDS burning has been implemented since 2014. Over 800 smoke samples were collected and analysed during the EDS of 2018 and late dry season (LDS) of 2017 and 2018. The samples were analysed using cavity ring-down spectroscopy, and the carbon balance method was used to estimate CO2, CO, CH4, and N2O EFs. Observed EF averages and standard deviations were 1651 (±50) g kg-1 for CO2, 57.9 (±28.2) g kg-1 for CO, 0.97 (±0.82) g kg-1 for CH4, and 0.096 (±0.174) g kg-1 for N2O. Averaged over all measured fire prone Cerrado types, the modified combustion efficiency (MCE) was slightly higher in the LDS (0.961 versus 0.956), and the CO and CH4 were 10 % and 2.3 % lower in the LDS compared to the EDS. However, these differences were not statistically significant using a two-tailed t test with unequal variance at a 90 % significance level. The seasonal effect was larger in more wood-dominated vegetation types. N2O EFs showed a more complex seasonal dependency, with opposite intraseasonal trends for savannas that were dominated by grasses versus those with abundant shrubs. We found that the N2O EF for the open Cerrado was less than half the EF suggested by literature compilations for savannas. This may indicate a substantial overestimation of the contribution of fires in the N2O budget. Overall, our data imply that in this region, seasonal variability in greenhouse gas emission factors may offset only a small fraction of the carbon mitigation gains in fire abatement programmes.
... No doubt fire plays a role in the native Cerrado by transforming the vegetation structure, species composition and dynamics [81][82][83], and is one of the most important determinant factors, especially when used to maintain the diversity of phyto-physiognomies and biodiversity, or used by humans as a management tool to open areas for agriculture and for keeping native pasture green at the end of the dry season. ...
The Cerrado is the richest savanna and is undergoing one of the planet’s most rapid land transformations for pasture and agriculture; around 45% of the biome has been deforested. Agriculture is of strategic importance to Brazil, but it also modifies ecosystems and jeopardizes habitats and biodiversity. Well-managed agricultural lands can have a favorable impact on environmental conservation. In this paper, we reviewed our current knowledge about water ecology and fire management to show that an ecosystem services perspective can bring about a conciliation of agriculture production with conservation by supporting effective land use decision-making and the optimization of public policy. The landscape/watershed scale seems to be the most relevant for decision-making on how to achieve production and conservation results. This scale appears to be an appropriate level for engaging with stakeholders. Fire frequency and timing (season) combination are determinant of individuals’ survivorship. The combination determines vegetation recovery, and it is important to maintain high biodiversity, especially for the herbaceous layer, but it is a limitation to woody vegetation recovery. A pragmatic and conciliatory land use agenda must be based on scientific knowledge and support innovative decision-making solutions for policy-makers and stakeholders, particularly farmers and donors.
Global warming significantly threatens species in the Cerrado, the world's largest savannah. Therefore, understanding how plants respond to temperature change, particularly in relation to leaf-level photosynthetic capacity, is crucial to understanding the future of Cerrado vegetation. Here, we determined the optimum temperature of the maximum rate of RuBP-carboxylation and maximum electron transport rate (TOptV and TOptJ, respectively) of 12 tree species in two opposite borders (northeastern and southeastern) of the Cerrado with distinct temperature regimes. We focused on four widespread species found in both sites, four restricted to the northeast, and four to the southeast. We compared TOptV and TOptJ between regions and between widespread species (co-occurring in both sites) and species restricted to each ecoregion. Additionally, we also explored the relationship between TOptV and TOptJ with leaf nitrogen (N), phosphorus (P), and potassium (K). As a result, we found that TOptV and TOptJ values were similar across species, regardless of the study region or species distribution range. The similarity of TOpt values among species suggests that photosynthetic performance is optimized to current temperatures. Additionally, we also observed that the TOptV and TOptJ were similar to the local maximum ambient temperatures. Therefore, if these species do not have enough plasticity, the increasing temperature predicted for this region may reduce their photosynthetic performance. Finally, the studied species exhibited general relationships between the TOptV and TOptJ and foliar key nutrients, particularly with P, suggesting the nutrient availability has an important role on thermal acclimation of leaves. These findings offer valuable insights into physiological and ecological mechanisms in photosynthesis performance present at the Cerrado species.
The Cerrado biome encompasses different vegetation types, ranging from savanna-like vegetation to forest-like vegetation, represented by a vegetational continuum from Cerrado Típico , Cerrado Denso and Cerradão , respectively. Nevertheless, there are still uncertainties on whether these different vegetation types do not only differ in their vegetation structure, but also in their species compositions. Based on vegetation surveys from 167 plots in the central Brazilian Cerrado , we addressed two questions: i) How variable is the vegetation structure and species between different Cerrado vegetation types? Second, ii) how strongly are vegetation structure and species composition linked? To answer these questions, we performed hierarchical clustering for species composition and vegetation structure. Our results showed that for species composition only 18% of the variance was explained by hierarchical clustering, while for vegetation structure 82% of variance was explained. Additionally, there was a significant difference in the structure metrics between clusters, showing that it is possible to clearly identify different Cerrado vegetation types based on vegetation structures, but not by species composition. Finally, we suggest that trait plasticity in Cerrado trees should drive structural differences among vegetation types, which could be the focus of future studies.
Woody encroachment (WE) and agricultural expansion are widespread in tropical savannas, where they threaten biodiversity and ecosystem function. In Africa's largest savanna, the miombo woodlands, cropland expansion is expected to cause extensive habitat loss over the next 30 years. Meanwhile, widespread WE is altering the remaining untransformed vegetation. Quantifying the extent of both processes in the Angolan miombo woodlands (~570,000 km ² ) has been challenging due to limited infrastructure, a history of conflict, and widespread landmines. Here, we analyze spectral satellite imagery to investigate the extent of WE and cropland expansion in the Angolan miombo woodlands since 1990. We asses WE using two complementary metrics: multi‐decade canopy greenness trends and conversion from grassland to woodland. We also examine whether WE trends are driven by landscape fragmentation and decreasing fire frequency. We found that from 1990 to 2020, 34.1% of the Angolan miombo woodlands experienced significant WE or was converted to cropland, while open grassy vegetation declined by 62%. WE advanced rapidly even in areas experiencing extraordinarily high burn frequencies and was not adequately explained by changing temperature or precipitation. WE was concentrated far from the agricultural frontier, in remote areas with low population densities. These results challenge the hypothesis that human‐altered fire regimes are the primary driver of WE in mesic savannas. The results will help decision‐makers conserve the miombo woodlands' biodiversity and ecosystem services, by highlighting that strategies to slow habitat loss must address WE and cropland expansion together.
The forest–savanna transition is the most widespread ecotone in the tropics, with important ecological, climatic, and biogeochemical implications at local to global scales. However, the factors and mechanisms that control this transition vary among continents and regions. Here, we analyzed which factors best explain the transition in northern South America (Llanos ecoregion and northwestern Amazon), where common thresholds on typical environmental factors (for example, mean annual precipitation (MAP), wet season precipitation) fail to predict it. For instance, savannas in the Llanos occur at MAP levels (> 1500 mm) which are typical of forests in other tropical regions. We examined the transition’s climate features, soils, and disturbance (fire frequency) spaces using remotely sensed data. We used logistic generalized linear models to assess the effect of seasonal (season length) and intra-seasonal (daily precipitation frequency and intensity) precipitation metrics during the dry season, soil silt content, and fire frequency, on the transition using canopy cover, tree cover, and the maximum Plant Area Volume Density as vegetation structure descriptor variables. Fire frequency and precipitation frequency were the most important variables explaining the transition. Although most fires occur in savannas, we found that a significant percentage of savanna pixels (46%) had no fires. This study indicates that the transition should be characterized regionally in response to biogeographic differences (for example, climatic space) among regions and continents. Our results highlight the importance of fire frequency and intra-seasonal precipitation in determining the transition in northern South America. Furthermore, future studies should consider regional differences in the climatic space of forest and savanna to improve projections of global change impacts on these highly diverse ecosystems.
One of the foundational premises of ecology is that climate determines ecosystems. This has been challenged by alternative ecosystem state models, which illustrate that internal ecosystem dynamics acting on the initial ecosystem state can overwhelm the influence of climate, and by observations suggesting that climate cannot reliably discriminate forest and savanna ecosystem types. Using a novel phytoclimatic transform, which estimates the ability of climate to support different types of plants, we show that climatic suitability for evergreen trees and C4 grasses are sufficient to discriminate between forest and savanna in Africa. Our findings reassert the dominant influence of climate on ecosystems and suggest that the role of feedbacks causing alternative ecosystem states is less prevalent than has been suggested.
Fire is a widespread phenomenon that plays an important role in Earth's ecosystems. This study investigated the global spatiotemporal patterns of burned areas, daytime and nighttime fire counts, and fire radiative power (FRP) from 2001 to 2020. The month with the largest burned area, daytime fire count, and FRP presented a bimodal distribution worldwide, with dual peaks in early spring (April) and summer (July and August), while the month with the largest nighttime fire count and FRP showed a unimodal distribution, with a peak in July. Although the burned area showed decline at the global scale, a significant increase occurred in temperate and boreal forest regions, where nighttime fire occurrence and intensity have consistently increased in recent years. The relationships among burned area, fire count, and FRP were further quantified in 12 typical fire-prone regions. The burned area and fire count exhibited a humped relationship with FRP in most tropical regions, whereas the burned area and fire count constantly increased when the FRP was below approximately 220 MW in temperate and boreal forest regions. Meanwhile, the burned area and FRP generally increased with the fire count in most fire-prone regions, indicating an increased risk of more intense and larger fires as the fire count increased. The spatiotemporal dynamics of burned areas for different land cover types were also explored in this study. The results suggest that the burned areas in forest, grassland, and cropland showed dual peaks in April and from July to September while the burned areas in shrubland, bareland, and wetlands usually peaked in July or August. Significant increases in forest burned area were observed in temperate and boreal forest regions, especially in the western U.S. and Siberia, whereas significant increases in cropland burned area were found in India and northeastern China.
Anthropogenically elevated CO 2 (eCO 2 ) concentrations have been suggested to increase woody cover within tropical ecosystems through fertilization. The effect of eCO 2 is built into Earth system models, although testing the relationship over long periods remains challenging. Here, we explore the relative importance of six drivers of vegetation change in western Africa over the past ~500,000 years (moisture availability, fire activity, mammalian herbivore density, temperature, temperature seasonality, CO 2 ) by coupling past environmental change data from Lake Bosumtwi (Ghana) with global data. We found that moisture availability and fire activity were the most important factors in determining woody cover, whereas the effect of CO 2 was small. Our findings suggest that the role of eCO 2 effects on tropical vegetation in predictive models must be reconsidered.
Recent extreme wildfire seasons in several regions have been associated with exceptionally hot, dry conditions, made more probable by climate change. Much research has focused on extreme fire weather and its drivers, but natural wildfire regimes – and their interactions with human activities – are far from being comprehensively understood. There is a lack of clarity about the ‘causes’ of wildfire, and about how ecosystems could be managed for the co-existence of wildfire and people. We present evidence supporting an ecosystem-centred framework for improved understanding and modelling of wildfire. Wildfire has a long geological history and is a pervasive natural process in contemporary plant communities. In some biomes, wildfire would be more frequent without human settlement; in others they would be unchanged or less frequent. A world without fire would have greater forest cover, especially in present-day savannas. Many species would be missing, because fire regimes have co-evolved with plant traits that resist, adapt to or promote wildfire. Certain plant traits are favoured by different fire frequencies, and may be missing in ecosystems that are normally fire-free. For example, post-fire resprouting is more common among woody plants in high-frequency fire regimes than where fire is infrequent. The impact of habitat fragmentation on wildfire crucially depends on whether the ecosystem is fire-adapted. In normally fire-free ecosystems, fragmentation facilitates wildfire starts and is detrimental to biodiversity. In fire-adapted ecosystems, fragmentation inhibits fires from spreading and fire suppression is detrimental to biodiversity. This interpretation explains observed, counterintuitive patterns of spatial correlation between wildfire and potential ignition sources. Lightning correlates positively with burnt area only in open ecosystems with frequent fire. Human population correlates positively with burnt area only in densely forested regions. Models for vegetation-fire interactions must be informed by insights from fire ecology to make credible future projections in a changing climate.
A bimodal distribution of tropical tree cover at intermediate precipitation levels has been presented as evidence of fire-induced bistability. Here we subdivide satellite vegetation data into those from human-unaffected areas and those from regions close to human-cultivated zones. Bimodality is found to be almost absent in the unaffected regions, whereas it is significantly enhanced close to cultivated zones. Assuming higher logging rates closer to cultivated zones and spatial diffusion of fire, our spatiotemporal mathematical model reproduces these patterns. Given a gradient of climatic and edaphic factors, rather than bistability there is a predictable spatial boundary, a Maxwell point, that separates regions where forest and savanna states are naturally selected. While bimodality can hence be explained by anthropogenic edge effects and natural spatial heterogeneity, a narrow range of bimodality remaining in the human-unaffected data indicates that there is still bistability, although on smaller scales than claimed previously.
Sampling along a precipitation gradient in tropical South
America extending from ca. 0.8 to 2.0 m a−1, savanna soils had
consistently lower exchangeable cation concentrations and higher
C / N ratios than nearby forest plots. These soil
differences were also reflected in canopy averaged leaf traits with
savanna trees typically having higher leaf mass per unit area but
lower mass-based nitrogen (Nm) and potassium
(Km). Both Nm and
Km also increased with declining mean annual
precipitation (PA), but most area-based leaf traits
such as leaf photosynthetic capacity showed no systematic variation
with PA or vegetation type. Despite this invariance,
when taken in conjunction with other measures such as mean canopy
height, area-based soil exchangeable potassium content, [K]sa ,
proved to be an excellent predictor of several photosynthetic
properties (including 13C isotope
discrimination). Moreover, when considered in a multivariate context
with PA and soil plant available water storage
capacity (θP) as covariates, [K]sa
also proved to be an excellent predictor of stand-level canopy area,
providing drastically improved fits as compared to models
considering just PA and/or θP. Neither calcium, nor magnesium, nor soil pH could substitute for
potassium when tested as alternative model predictors
(ΔAIC > 10). Nor for any model could simple soil
texture metrics such as sand or clay content substitute for either
[K]sa or θP. Taken in conjunction
with recent work in Africa and the forests of the Amazon Basin, this
suggests – in combination with some newly conceptualised
interacting effects of PA and θP
also presented here – a critical role for potassium as a modulator
of tropical vegetation structure and function.
Amazon forests are a key but poorly understood component of the global carbon cycle. If, as anticipated, they dry this century, they might accelerate climate change through carbon losses and changed surface energy balances. We used records from multiple long-term monitoring plots across Amazonia to assess forest responses to the intense 2005 drought, a possible analog of future events. Affected forest lost biomass, reversing a large long-term carbon sink, with the greatest impacts observed where the dry season was unusually intense. Relative to pre-2005 conditions, forest subjected to a 100-millimeter increase in water deficit lost 5.3 megagrams of aboveground biomass of carbon per hectare. The drought had a total biomass carbon impact of 1.2 to 1.6 petagrams (1.2 × 1015 to 1.6 × 1015 grams). Amazon forests therefore appear vulnerable to increasing moisture stress, with the potential for large carbon losses to exert feedback on climate change.
The Amazon Basin has experienced more variable climate over the last decade, with a severe and widespread drought in 2005 causing large basin-wide losses of biomass. A drought of similar climatological magnitude occurred again in 2010; however, there has been no basin-wide ground-based evaluation of effects on vegetation. We examine to what extent the 2010 drought affected forest dynamics using ground-based observations of mortality and growth utilizing data from an extensive forest plot network. We find that during the 2010 drought interval, forests did not gain biomass (net change: −0.43 Mg ha-1, CI: −1.11, 0.19, n = 97), regardless of whether forests experienced precipitation deficit anomalies. This loss contrasted with a long-term biomass sink during the baseline pre-2010 drought period (1998 − pre-2010) of 1.33 Mg ha-1 yr-1 (CI: 0.90, 1.74, p < 0.01). The resulting net impact of the 2010 drought (i.e., reversal of the baseline net sink) was −1.95 Mg ha-1 yr-1 (CI:−2.77, −1.18; p < 0.001). This net biomass impact was driven by an increase in biomass mortality (1.45 Mg ha-1 yr-1 CI: 0.66, 2.25, p < 0.001), and a decline in biomass productivity (−0.50 Mg ha-1 yr-1, CI:−0.78, −0.31; p < 0.001). Surprisingly, the magnitude of the losses through tree mortality was unrelated to estimated local precipitation anomalies, and was independent of estimated local pre-2010 drought history. Thus, there was no evidence that pre-2010 droughts compounded the effects of the 2010 drought. We detected a systematic basin-wide impact of drought on tree growth rates across Amazonia, with this suppression of productivity driven by moisture deficits in 2010, an impact which was not apparent during the 2005 event [Phillips et al., 2009]. Based on these ground data, both live biomass in trees and corresponding estimates of live biomass in roots, we estimate that intact forests in Amazonia were carbon neutral in 2010 (−0.07 PgC yr-1 CI:−0.42, 0.23), consistent with results from an independent analysis of airborne estimates of land-atmospheric fluxes during 2010 [Gatti et al., 2014]. Relative to the long-term mean, the 2010 drought resulted in a reduction in biomass carbon uptake of 1.1 PgC, compared to 1.6 PgC for the 2005 event [Phillips et al. 2009]. Continue reading full article Ancillary Supporting Information PDF Info The Amazon Basin has experienced more variable climate over the last decade, with a severe and widespread drought in 2005 causing large basin-wide losses of biomass. A drought of similar climatological magnitude occurred again in 2010; however, there has been no basin-wide ground-based evaluation of effects on vegetation. We examine to what extent the 2010 drought affected forest dynamics using ground-based observations of mortality and growth utilizing data from an extensive forest plot network. We find that during the 2010 drought interval, forests did not gain biomass (net change: −0.43 Mg ha-1, CI: −1.11, 0.19, n = 97), regardless of whether forests experienced precipitation deficit anomalies. This loss contrasted with a long-term biomass sink during the baseline pre-2010 drought period (1998 − pre-2010) of 1.33 Mg ha-1 yr-1 (CI: 0.90, 1.74, p < 0.01). The resulting net impact of the 2010 drought (i.e., reversal of the baseline net sink) was −1.95 Mg ha-1 yr-1 (CI:−2.77, −1.18; p < 0.001). This net biomass impact was driven by an increase in biomass mortality (1.45 Mg ha-1 yr-1 CI: 0.66, 2.25, p < 0.001), and a decline in biomass productivity (−0.50 Mg ha-1 yr-1, CI:−0.78, −0.31; p < 0.001). Surprisingly, the magnitude of the losses through tree mortality was unrelated to estimated local precipitation anomalies, and was independent of estimated local pre-2010 drought history. Thus, there was no evidence that pre-2010 droughts compounded the effects of the 2010 drought. We detected a systematic basin-wide impact of drought on tree growth rates across Amazonia, with this suppression of productivity driven by moisture deficits in 2010, an impact which was not apparent during the 2005 event [Phillips et al., 2009]. Based on these ground data, both live biomass in trees and corresponding estimates of live biomass in roots, we estimate that intact forests in Amazonia were carbon neutral in 2010 (−0.07 PgC yr-1 CI:−0.42, 0.23), consistent with results from an independent analysis of airborne estimates of land-atmospheric fluxes during 2010 [Gatti et al., 2014]. Relative to the long-term mean, the 2010 drought resulted in a reduction in biomass carbon uptake of 1.1 PgC, compared to 1.6 PgC for the 2005 event [Phillips et al. 2009].
Contributing to recent discussions regarding the evidence for and against forest and savanna representing alternative steady states (ASS) across much of the tropical lands, we here address several issues raised by Staal and Flores (2015) in a recent commentary published in this journal (Biogeosciences, 12, 5563–5566, 2015). Our analysis shows that – in what could alternatively be titled "A Tale of Five Fallacies" – substantial errors in reasoning exhibited by both Staal and Flores (2015) and the ASS community in general in terms of arguments invoked as providing support for ASS as a key factor determining tropical vegetation distributions. Specifically we: (1) demonstrate that bimodal distributions of canopy cover need not necessarily be associated with ASS ("fallacy of confirmation bias"); (2) show that models suggesting the mathematical feasibility of ASS can never be taken as any sort of proof as to their actual existence ("fallacy of misplaced concreteness"); (3) conclude that studies failing to make climate-independent associations between soil properties and forest/savanna distributions (and thereby concluding that ASS must be present) have inevitably failed to measure the right things. Or in many cases, not even made the required measurements ("fallacy of suppressed evidence"). Moreover, we also find: (4) assertions that ASS associated concepts such as detrimental effects of fire on soil chemical properties and positive effects of forest tree species on soil fertility are totally without foundation ("fallacy of wishful thinking") and (5) a strong tendency amongst ASS advocates to take results from temperate ecosystems and incorrectly assume that this provides support for the existence of ASS in the tropical forest and savanna lands ("fallacy of hasty generalisation"). We conclude all arguments presented to date in support of the widespread existence of ASS in the tropical regions to be flawed. As an alternative, we suggest that forest-savanna transitions may be better understood as reflecting the effects of soil physical and chemical properties on tropical vegetation structure and function with fire-effected feedbacks simply serving to reinforce these patterns through a "sharpening switch" mechanism.
Proceedings includes 24 papers that deal with the use of fire in the fields of Range Management, Wildlife and National Parks, while one on ″Ancient Fires″ , deals with evidence in the fossil record of widespread fires in the Mesozoic. Following is a list of titles and authors of the papers presented: Effects of Fire on True Prairie Grasslands. By E. F. Smith and C. E. Owensby. Use of Fire as a Management Tool on the Curtis Prairie. By Roger C. Anderson. Fire in the Black Hills Forest-Grass Ecotone. By F. Robert Gartner and Wesley W. Thompson. Fire Ecology in Ponderosa Pine-Grassland. By Harold H. Biswell. Burning and the Grasslands in California. By Harold F. Heady. Fire Effects in Southwestern Semidesert Grass-Shrub Communities. By Dwight R. Cable. Effect of Fire on Shortgrass and Mixed Prairie Species. By J. L. Launchbaugh. Fire as a Tool to Manage Tobosa Grasslands. by Henry A. Wright. Progress Report on Techniques to Broadcast Burn Dozed Juniper. By Henry A. Wright, Carlton M. Britton, Robert L. Wink, and Bob Beckham. Fire in the Southeastern Grasslands. By Richard J. Vogl.
This paper discusses the effects of an accidental late dry season fire in a 34year old fire protected plot and prescribed annual early and late dry season burning on litter and coppiced miombo woodland. Compared with early burning, the accidental fire significantly reduced tree species diversity, basal area and surface litter Annual late burning over a 36 year period has suppressed woodland development but nine species considered to be fire-tolerant have persisted. The study supports an early burning policy to reduce the risk of late fires and promote miombo woodland regeneration.
Understanding the mechanisms controlling the distribution of biomes remains a challenge. Although tropical biome distribution has traditionally been explained by climate and soil, contrasting vegetation types often occur as mosaics with sharp boundaries under very similar environmental conditions. While evidence suggests that these biomes are alternative states, empirical broad-scale support to this hypothesis is still lacking. Using community-level field data and a novel resource-niche overlap approach, we show that, for a wide range of environmental conditions, fire feedbacks maintain savannas and forests as alternative biome states in both the Neotropics and the Afrotropics. In addition, wooded grasslands and savannas occurred as alternative grassy states in the Afrotropics, depending on the relative importance of fire and herbivory feedbacks. These results are consistent with landscape scale evidence and suggest that disturbance is a general factor driving and maintaining alternative biome states and vegetation mosaics in the tropics.
With grasslands and savannas covering 20% of the world's land surface, accounting for 30-35% of worldwide Net Primary Productivity and supporting hundreds of millions of people, predicting changes in tree/grass systems is priority. Inappropriate land management and rising atmospheric CO2 levels result in increased woody cover in savannas. Although woody encroachment occurs world-wide, Africa's tourism and livestock grazing industries may be particularly vulnerable. Forecasts of responses of African wildlife and available grazing biomass to increases in woody cover are thus urgently needed. These predictions are hard to make due to non-linear responses and poorly understood feedback mechanisms between woody cover and other ecological responders, problems further amplified by the lack of long-term and large-scale datasets. We propose that a space-for-time analysis along an existing woody cover gradient overcomes some of these forecasting problems. Here we show, using an existing woody cover gradient (0-65%) across the Kruger National Park, South Africa, that increased woody cover is associated with (i) changed herbivore assemblage composition, (ii) reduced grass biomass, and (iii) reduced fire frequency. Furthermore, although increased woody cover is associated with reduced livestock production, we found indigenous herbivore biomass (excluding elephants) remains unchanged between 20-65% woody cover. This is due to a significant reorganization in the herbivore assemblage composition, mostly as a result of meso-grazers being substituted by browsers at increasing woody cover. Our results suggest that woody encroachment will have cascading consequences for Africa's grazing systems, fire regimes and iconic wildlife. These effects will pose challenges and require adaptation of livelihoods and industries dependent on conditions currently prevailing.
Through interpretations of remote-sensing data and/or theoretical propositions, the idea that
forest and savanna represent "alternative stable states" is gaining increasing
acceptance. Filling an observational gap, we present detailed stratified floristic and structural
analyses for forest and savanna stands located mostly within zones of transition (where both
vegetation types occur in close proximity) in Africa, South America and Australia. Woody plant
leaf area index variation was related to tree canopy cover in a similar way for both savanna and
forest with substantial overlap between the two vegetation types. As total woody plant canopy
cover increased, so did the relative contribution of middle and lower strata of woody vegetation. Herbaceous layer cover declined as woody cover increased. This pattern of understorey
grasses and herbs progressively replaced by shrubs as the canopy closes over was found for
both savanna and forests and on all continents. Thus, once subordinate woody canopy layers are
taken into account, a less marked transition in woody plant cover across the savanna–forest-species
discontinuum is observed compared to that inferred when trees of
a basal diameter > 0.1 m are considered in isolation. This is especially the
case for shrub-dominated savannas and in taller savannas approaching canopy closure. An increased contribution of forest species to the total
subordinate cover is also observed as savanna stand canopy closure
occurs. Despite similarities in canopy-cover characteristics, woody vegetation in Africa and
Australia attained greater heights and stored a greater amount of above-ground biomass than
in South America. Up to three times as much above-ground biomass is stored in forests compared to
savannas under equivalent climatic conditions. Savanna–forest transition zones were also found to
typically occur at higher precipitation regimes for South America than for Africa. Nevertheless, consistent across all three continents
coexistence was found to be confined to a well-defined edaphic–climate envelope with soil and climate the key determinants of the
relative location of forest and savanna stands. Moreover, when considered in conjunction with the appropriate
water availability metrics, it emerges that soil exchangeable cations exert considerable control
on woody canopy-cover extent as measured in our pan-continental (forest + savanna) data set.
Taken together these observations do not lend support to the notion of alternate stable states mediated
through fire feedbacks as the prime force shaping the distribution of the two dominant vegetation types of the tropical
lands.
The interaction between droughts and land-use fires threaten the carbon stocks, climate regulatory functions, and biodiversity
of Amazon forests, particularly in the southeast, where deforestation and land-use ignitions are high. Repeated, severe, or
combined fires and droughts result in tropical forest degradation via nonlinear dynamics and may lead to an alternate vegetation
state. Here, we discuss the major insights from the longest (more than 10 years) and largest (150-hectare) experimental burn
in Amazon forests. Despite initial forest resistance to low-intensity fires, repeated fire during drought killed the majority
of trees, reduced canopy cover by half, and favored invasive grasses—but the persistence of this novel vegetation state is
unknown. Forest edges, where drying, fire intensity and grass invasion are greatest, were most vulnerable. Crucial to advancing
fire ecology in tropical forests, we need to scale these results to understand how flammability and resilience postfire varies
across Amazon forest types.
Sampling along a precipitation gradient in tropical America extending from ca. 0.8 to 2.0 m a−1, savanna soils had consistently lower exchangeable cation concentrations and higher C/N ratios than nearby forest plots. These soil differences were also reflected in canopy averaged leaf traits with savanna trees typically having higher leaf mass per unit area but lower mass-based nitrogen (Nm) and potassium (Km). Both Nm and Km also increased with declining mean annual precipitation (PA), but most area-based leaf traits such as leaf photosynthetic capacity showed no systematic variation with PA or vegetation type. Despite this invariance, when taken in conjunction with other measures such mean canopy height, area-based soil exchangeable potassium content, [K]sa, proved to be an excellent predictor of several photosynthetic properties (including 13C isotope discrimination). Moreover, when considered in a multivariate context with PA and soil plant available water storage capacity (θP) as covariates, [K]sa also proved to be an excellent predictor of stand-level canopy area, providing drastically improved fits as compared to models considering just PA and/or θP. Neither calcium, magnesium nor soil pH could substitute for potassium when tested as alternative model predictors (ΔAIC > 10). Nor for any model could simple soil texture metrics such as sand or clay content substitute for either [K]sa or θP. Taken in conjunction with recent work in Africa and the forests of the Amazon Basin this suggests – in combination with some newly conceptualised interacting effects of PA and θP also presented here – a critical role for potassium as a modulator of tropical vegetation structure and function.
According to new ecological theories, many savannas are inherently in disequilibrium and can flip from tree-dominated to grass-dominated landscapes depending upon the disturbance regime. In particular, a shift in a fire regime to a more frequent and intensive one can radically alter the tree-to-grass ratio in a given savanna. Drawing upon the ecological buffering model we argue that savanna persistence requires a relatively stable fire regime. We hypothesize that anthropogenic burning practices perform this function by producing a regular annual spatiotemporal pattern of fire that is linked to vegetation type. We test this hypothesis using a study of two areas, one in Mali and the other Burkina Faso. We use two sources of satellite data to produce an 11-year time series of the spatiotemporal pattern of fires and an example of the annual burned area pattern these fires produce. We combine the analysis of satellite imagery with interviews of rural inhabitants who set fires to understand the logic underlying the patterns of fire. Analysis of a time series of imagery reveals a strikingly regular annual spatiotemporal pattern of burning for both study areas, which cannot be explained by the regional climatic pattern alone. We conclude that the regularity of the annual fire regime in West Africa is a human-ecological phenomenon closely linked to vegetation type and controlled by people's burning practices. We argue that the anthropogenic burning regime serves to buffer the savanna and maintain its ecological stability.
(1) Three plots in Northern Guinea savanna were enumerated and then clear-felled in 1950. Since then one plot has been completely protected, a second has been burnt annually early in the dry season, and a third plot has been burnt annually late in the dry season. (2) In 1976-77 there were 202 trees ha ( 30 cm girth) on the protected plot, forty-two trees ha on the early burnt plot and twenty trees ha on the late burnt plot. Corresponding figures for basal area are 3.43, 0.51, 0.24 m ha. (3) The basal area of grass on both the burnt plots has remained constant at about 13% since 1960 whereas the basal area of grass on the protected plot has continued to decline and was 6.3% in 1976. Grass biomass at the end of the growing season in 1976 was 182 g m on the protected plot, and 260 g m and 144 g m on the early and late burnt plots respectively. (4) There were seventy-three species of vascular plants on the protected plot in 1977, and fifty-three and forty-four respectively on the early and late burnt plots. (5) Only slight differences in the soils were observed, though the protected plot had significantly more organic matter and total nitrogen. (6) The results are compared with those of similar experiments elsewhere in Africa, and recommendations are made for improved experimental design.
The results described are of the effects of fire exclusion since 1957 on a small area of savanna dose to the forest-zone boundary, on the northern Accra Plains, Ghana. A forest thicket has developed, with forest species in intimate association with nonforest species. The forest component includes healthy regeneration of the important timber species, Milicia (= Chlorophora) excelsa (Benth. & Hook.) Berg (nomenclature follows Hutchinson & Dalziel [1954-72] except where authorities are given) (max. 114 cm gbh) and Antiaris toxicaria Lesch. (53 cm gbh). Ceiba pentandra is the largest and most abundant canopy tree, with a maximum girth of 2 m (height 22 m). Other large trees were Albizia ferruginea (137 cm gbh) and the remnant savanna trees Lonchocarpus sericeus (74 cm gbh) and the naturalized exotic Azadirachta indica (99 cm gbh).
Covering almost a quarter of Brazil, the Cerrado is the world’s most biologically rich tropical savanna. Fire is an integral part of the Cerrado but current land use and agricultural practices have been changing fire regimes, with undesirable consequences for the preservation of biodiversity. In this study, fire frequency and fire return intervals were modelled over a 12-year time series (1997–2008) for the Jalapão State Park, a protected area in the north of the Cerrado, based on burned area maps derived from Landsat imagery. Burned areas were classified using object based image analysis. Fire data were modelled with the discrete lognormal model and the estimated parameters were used to calculate fire interval, fire survival and hazard of burning distributions, for seven major land cover types. Over the study period, an area equivalent to four times the size of Jalapão State Park burned and the mean annual area burned was 34%. Median fire intervals were generally short, ranging from three to six years. Shrub savannas had the shortest fire intervals, and dense woodlands the longest.
Because fires in the Cerrado are strongly responsive to fuel age in the first three to four years following a fire, early dry season patch mosaic burning may be used to reduce the extent of area burned and the severity of fire effects.
In their recent paper, Staver and Hansen (Global Ecology and Biogeography, 2015, 24, 985–987) refute the case made by Hanan et al. (Global Ecology and Biogeography, 2014, 23, 259–263) that the use of classification and regression trees (CARTs) to predict tree cover from remotely sensed imagery (MODIS VCF) inherently introduces biases, thus making the resulting tree cover unsuitable for showing alternative stable states through tree cover frequency distribution analyses. Here we provide a new and equally fundamental argument for why the published frequency distributions should not be used for such purposes. We show that the practice of pre-average binning of tree cover values used to derive cover values to train the CART model will also introduce errors in the frequency distributions of the final product. We demonstrate that the frequency minima found at tree covers of 8–18%, 33–45% and 55–75% can be attributed to numerical biases introduced when training samples are derived from landscapes containing asymmetric tree cover distributions and/or a tree cover gradient. So it is highly likely that the CART, used to produce MODIS VCF, delivers tree cover frequency distributions that do not reflect the real world situation.
The rich ecology of tropical forests is intimately tied to their moisture status. Multi-site syntheses can provide a macro-scale view of these linkages and their susceptibility to changing climates. Here, we report pan-tropical and regional-scale analyses of tree vulnerability to drought. We assembled available data on tropical forest tree stem mortality before, during, and after recent drought events, from 119 monitoring plots in 10 countries concentrated in Amazonia and Borneo. In most sites, larger trees are disproportionately at risk. At least within Amazonia, low wood density trees are also at greater risk of drought-associated mortality, independent of size. For comparable drought intensities, trees in Borneo are more vulnerable than trees in the Amazon. There is some evidence for lagged impacts of drought, with mortality rates remaining elevated 2 yr after the meteorological event is over. These findings indicate that repeated droughts would shift the functional composition of tropical forests toward smaller, denser-wooded trees. At very high drought intensities, the linear relationship between tree mortality and moisture stress apparently breaks down, suggesting the existence of moisture stress thresholds beyond which some tropical forests would suffer catastrophic tree mortality.
Fire is a major determinant of cerrado (savanna) vegetation in Brazil, and is used as a management tool during the dry season. This has consequently induced a long history of fire studies in the region. This article reviews past and present fire ecology studies in the cerrado, and emphasizes the need for a more applied approach to future work.
Savanna ecosystems are characterized by the co-occurrence of trees and grasses. In this paper, we argue that the balance between trees and grasses is, to a large extent, determined by the indirect interactive effects of herbivory and fire. These effects are based on the positive feedback between fuel load (grass biomass) and fire intensity. An increase in the level of grazing leads to reduced fuel load, which makes fire less intense and, thus, less damaging to trees and, consequently, results in an increase in woody vegetation. The system then switches from a state with trees and grasses to a state with solely trees. Similarly, browsers may enhance the effect of fire on trees because they reduce woody biomass, thus indirectly stimulating grass growth. This consequent increase in fuel load results in more intense fire and increased decline of biomass. The system then switches from a state with solely trees to a state with trees and grasses. We maintain that the interaction between fire and herbivory provides a mechanistic explanation for observed discontinuous changes in woody and grass biomass. This is an alternative for the soil degradation mechanism, in which there is a positive feedback between the amount of grass biomass and the amount of water that infiltrates into the soil. The soil degradation mechanism predicts no discontinuous changes, such as bush encroachment, on sandy soils. Such changes, however, are frequently observed. Therefore, the interactive effects of fire and herbivory provide a more plausible explanation for the occurrence of discontinuous changes in savanna ecosystems.
In the course of its development, the tree seedling undergoes morphological changes, for example in leaf form, that are quite striking in some species. These phenomena may be considered as the outcome of changes in the physiological condition of the plant, or to put it differently, as a measure of its increasing physiological age. This is a rather vague concept, as the physiological processes causing ageing are largely unknown. Hence, there is no reason to assume that ageing processes in trees are fundamentally different from those in herbaceous plants, where similar morphological changes may be observed. There are, however, practical reasons for a separate treatment.
Understanding the origin and ecological dynamics of tropical savannas in terms of natural and human drivers of change is a hot topic that may be crucial for conservation. The case of the Gran Sabana (GS), a huge savanna island within the Amazon-Orinoco rainforests, is presented as a pilot study for the Neotropics. A vivid debate exists on whether or not forests formerly covered the GS and on the potential role of anthropogenic fires in the establishment of present-day savannas. This debate has generated a conflict between conservation ecologists defending the ancient forests hypothesis and indigenous inhabitants (Pemones), for whom the use of fire is an inalienable cultural trait. Here we discuss the latest palaeoecological findings documenting past vegetation dynamics and the shaping of present GS landscapes. At the beginning of the Younger Dryas (YD), the GS was more forested than it is today but an abrupt, hitherto irreversible, shift toward savannisation, likely caused by coupled climate-fire synergies, was recorded between the mid-YD and the Early Holocene. It is suggested that fires could have been ignited by the first South American settlers in their eastward migration from the Panama Isthmus through the so called Atlantic Route. The Pemones would have established in the GS during the Late Holocene when savannas already covered the region. A simplistic debate between either forest or savanna as the “original” GS vegetation is unrealistic and should be replaced by a more dynamic approach. The term “original” vegetation itself is misleading and should not be used.
In tropical West Africa, distribution patterns of forest islands in savannas are influenced by fires which occur regularly in the grass stratum. Along continuous forest-savanna transects in the Comoe National Park, the change in the amount and composition of non-woody phytomass was investigated from savanna to forest interior. This was correlated with the cover of vegetation strata above, soil depth, and the occurrence of seasonal surface fires. Phytomass mainly consisted of leaf litter in the forests (about 400 g m(-2) at the end of the rainy season, and about 600 g m(-2) at the end of the dry season) and of grasses in the savanna (about 900 g m(-2)). Low grass biomass appeared to be primarily the result of suppression by competing woody species and not of shallow soil. The occurrence of early dry-season fires seemed to be determined mainly by the amount of grass biomass as fuel because fires occurred in almost all savanna plots while forest sites remained unaffected. However, late dry-season fires will encounter higher amounts of leaf litter raising fire probability in forests. Due to the importance of the amount of combustible phytomass, fire probability and intensity might increase with annual precipitation in both savanna and forest.
The role of fire in determining biome distribution in South Africa has long been debated. Acocks labelled veld types that he thought were 'fire climax' as 'false'. He hypothesised that their current extent was due to extensive forest clearance by Iron Age farmers. We tested the relative importance of fire and climate in determining ecosystem characteristics by simulating potential vegetation of South Africa with and without fire using a Dynamic Global Vegetation Model (DGVM). The simulations suggest that most of the eastern half of the country could support much higher stem biomass without fire and that the vegetation would be dominated by trees instead of grasses. Fynbos regions in mesic winter rainfall areas would also become tree dominated. We collated results of long term fire exclusion studies to further test the relative importance of fire and climate. These show that grassy ecosystems with rainfall >650mm tend towards fire-sensitive forests with fire excluded. Areas below 650mm showed changes in tree density and size but no trend of changing composition to forest. We discuss recent evidence that C4 grasslands first appeared between 6 and 8M years BP, long before the appearance of modern humans. However these grassy ecosystems are among the most recently developed biomes on the planet. We briefly discuss the importance of fire in promoting their spread in the late Tertiary.
Publisher Summary This chapter discusses the positive-feedback switches in plant communities. A vegetation positive-feedback switch is a process in which a community modifies the environment, making it more suitable for that community. Positive-feedback switches operate by modifying any of several features of the environment, including water, pH, soil elements, light, temperature, wind, fire, or allelopathic toxins. The four types of switch can be distinguished as: (1) one-sided switch, where a single community modifies the environment of the patches it occupies, (2) reaction switch, where the community additionally modifies the patches it is not in, but in the opposite direction, (3) symmetric switch, where communities of both alternative states modify the same factor of their environment, but in opposite directions, and (4) two-factor switch, where the two communities both modify their environments, but in different factors. The positive-feedback switches producing four major vegetational effects (A–D): a stable vegetational mosaic may be produced in a previously uniform environment(situation A), or a vegetational gradient caused by environmental change can be intensified to give a sharp boundary (situation B). These mosaics and boundaries can occur at a wide variety of spatial scales, from landscape-scale to individual plant-scale. Switches can also sharpen or displace temporal boundaries: succession can be accelerated (situation C) or delayed (situation D). Not all of these effects can be produced by all types of switch; in particular, a one-sided (type 1) switch cannot produce a stable mosaic.
The incidence and importance of fire in the Amazon have increased substantially during the past decade, but the effects of
this disturbance force are still poorly understood. The forest fire dynamics in two regions of the eastern Amazon were studied.
Accidental fires have affected nearly 50 percent of the remaining forests and have caused more deforestation than has intentional
clearing in recent years. Forest fires create positive feedbacks in future fire susceptibility, fuel loading, and fire intensity.
Unless current land use and fire use practices are changed, fire has the potential to transform large areas of tropical forest
into scrub or savanna.
This paper is the botanical report of an expedition to Dogon Kurmi, near Jos in central Nigeria, in August 1955, with comments on the soil and fauna. Seedlings of twenty-two species of both forest and savanna type were examined to throw light on whether forest plants were invading the `derived' savanna or whether the reverse was true. The trees in the forest attain 20 m or even 40 m, in the savanna about 5 m; at Dogon Kurmi the forest and savanna are found together and intergrading. When studied in the wet season the savanna grasses stood 1-2 m high; in the dry season their dry culms are fired. Fifteen transects, from six different situations, covered both gradual and abrupt transitions from kurmi to savanna in varying soils and also pure savanna and pure stands of Uapaca togoensis. These were mapped showing old and young plants of the selected species and the extent of every tree's shade. The relative `success' of a given species in differing conditions of shade and firing was estimated using numbers and height as a criterion. The need to choose common and easily identifiable species and for complete mapping, curtailed work on the high forest; in practice also, seedlings were often not distinguishable from suckers or fired saplings. Analysis of the twenty-two species (sixteen are individually discussed) show three to be randomly dispersed by wind, and five, for various reasons, clumped. Middle-sized trees of five species are scarce; in two of these the heavy seeds which fall to the ground almost entirely fail to germinate. Five fire-tolerant species attain a larger size in open situations; of apparent shade-lovers four are limited more by fire and one by its mode of dispersal. Edaphic factors only occasionally influence the distribution of forest and savanna plants, though five plants show weak correlation with clay content and only one tolerates soils of small effective depth. Though some shrubs and climbers may protect trees against fire, Uapaca alone of such trees helps forest trees to grow under it; there is little to suggest that the forest is encroaching on the savanna and the reverse is happening in one transect.
1. The Derived savanna zone stretches across Nigeria from east to west, separating the Rain forest to the south from the fire-swept Guinea savanna to the north. 2. In western Kabba the Derived savanna comprises two major communities, Daniellia-Elaeis and Daniellia-Uapaca. 3. In eastern Kabba the characteristic Derived savanna disclimax is obscured by a mosaic of intermediate and transitional types which may indicate the nature of the original vegetation linking forest and savanna. 4. It is concluded that the Derived savanna vegetation has been induced by the activities of farmers in clearing land, thereby admitting a grassy ground layer, and consequently fire, into uninflammable or weakly inflammable forest and transitional communities.
In their recent paper, Hanan et al. (Global Ecology and Biogeography, 2014, 23, 259–263) argue that the use of classification and regression trees (CARTs) to calibrate global remote sensing datasets, including the MODIS VCF tree-cover dataset, makes these data inappropriate for analysing the frequency distribution of tree cover. While we agree with their most general point – that the use of remote sensing products should be informed and deliberate – their analysis overlooks a few key aspects of the use of CARTs in generating global tree-cover data. Firstly, while their presentation of flaws in the use of CARTs is compelling, their use of hypothetical data obscures the reasons why CARTs are a useful tool. Secondly, they do not actually examine the error distributions of the MODIS VCF tree-cover data. Such an analysis, which we perform, revealed the following: (1) the MODIS VCF product may not be useful for differentiating over small ranges of tree cover (less than c. 10%); (2) that the bimodality of low and high tree cover, with a frequency minimum at intermediate tree cover, is not attributable to bias in MODIS VCF tree-cover calibrations; and (3) that the MODIS VCF is not well-resolved below c. 20–30% tree cover, such that MODIS cannot be used with any confidence to evaluate multimodality in tree cover in that range. Further validation and calibration are likely to be helpful and, at low tree cover, necessary for improving MODIS VCF tree-cover estimates. However, the MODIS VCF – which has facilitated major steps in our ability to examine ecological phenomena at global scales – remains a useful tool for well-informed ecological analysis.
1. As a prelude to an account of studies on seasonal changes in the climate, soil and vegetation of the Olokemeji Forest Reserve, the physical features, climate, history and vegetation of the reserve are described. 2. There are two main types of typical vegetation in south-western Nigeria, both of which are present in the reserve: Moist Semi-deciduous forest, which is the climatic climax for the area, and Derived Savanna woodland, which is a biotic fire climax. The Derived Savanna and the secondary nature of the forest are due to previous cultivation. 3. A research site in the typical dry type of Moist Semi-deciduous forest had more or less complete strata at heights of 18-23 m, 10-14 m and 4-9 m and a continuous herb stratum. The floristic composition was very mixed. The site was probably last farmed during the third quarter of the nineteenth century. 4. The canopy on the research site in the Derived Savanna woodland varied from complete to open. There were incomplete strata at heights of 4-8 m and 1.5-4 m and a herb stratum largely composed of grasses. 5. Ninety-five per cent of the forest species and 30% of the savanna species were phanerophytes. Half the savanna plants were hemicryptophytes and many were therophytes. There were many geophytes on the savanna site and some on the forest site. Twenty-seven per cent of the forest species (almost entirely woody plants) and 5% of the savanna species (all herbs) were climbers. The taller forest trees were mainly deciduous and the smaller trees were increasingly evergreen. All the savanna species were deciduous. 6. The bark of the savanna trees was generally over three times as thick as that of the forest trees. 7. Most leaves from both sites had the following characters. They were mesophylls or microphylls (many savanna herb leaves were nanophylls), had a `maximum circle diameter' between 27 and 81 mm, a length/width ratio between 2 and 20, were unlobed and had entire margins. The leaves of savanna monocotyledons were exceptions in that they mostly had a `maximum circle diameter' between 3 and 9 mm and a length/width ratio between 20 and 200. Eighty per cent of the forest species, compared with 15% of the savanna plants, had drip tips and these were about twice the length of the savanna drip tips. 8. Most woody plants on both sites had a deep tap root with subsidiary roots growing horizontally and mainly 10-20 cm below the soil surface. 9. The soil was predominantly loamy, rather sandy on the forest site and rather silty and clayey on the savanna site.
Fire is considered a critical management tool in fire prone landscapes. Often studies and policies relating to fire focus on why and how the fire regime should be managed, often neglecting to subsequently evaluate management's ability to achieve these objectives over long temporal and large spatial scales. This study explores to what extent the long-term spatio-temporal fire patterns recorded in the Kruger National Park, South Africa has been influenced by management policies and to what extent it was dictated by underlying variability in the abiotic template. This was done using a spatially explicit fire-scar database from 1941 to 2006 across the 2 million ha Park. Fire extent (ha burnt per annum) 1) is correlated with rainfall cycles 2) exhibits no long-term trend and 3) is largely non-responsive to prevailing fire management policies. Rainfall, geology and distance from the closest perennial river and the interactions between these variables influence large-scale fire pattern heterogeneity: areas with higher rainfall, on basaltic substrates and far from rivers are more fire prone and have less heterogeneous fire regimes than areas with lower rainfall, on granitic substrates and closer to rivers. This study is the first to illustrate that under a range of rainfall and geological conditions, perennial rivers influence long-term, landscape-scale fire patterns well beyond the riparian zone (typically up to 15 km from the river). It was concluded that despite fire management policies which historically aimed for largely homogeneous fire return regimes, spatially and temporally heterogeneous patterns have emerged. This is primarily because of differences in rainfall, geology and distance from perennial rivers. We postulate that large-scale spatio-temporal fire pattern heterogeneity is implicit to heterogeneous savannas, even under largely homogenizing fire policies. Management should be informed by these patterns, embracing the natural heterogeneity-producing template. We therefore suggest that management actions will be better directed when operating at appropriate scales, nested within the broader implicit landscape patterns, and when focusing on fire regime parameters over which they have more influence (e.g. fire season).
Savanna ecosystems comprise 22% of the global terrestrial surface and 25% of Australia (almost 1.9 million km2) and provide significant ecosystem services through carbon and water cycles and the maintenance of biodiversity. The current structure, composition and distribution of Australian savannas have co-evolved with fire, yet remain driven by the dynamic constraints of their bioclimatic niche. Fire in Australian savannas influences both the biophysical and biogeochemical processes at multiple scales from leaf to landscape. Here we present the latest emission estimates from Australian savanna biomass burning and their contribution to global greenhouse gas budgets. We then review our understanding of the impacts of fire on ecosystem function and local surface water and heat balances, which in turn influence regional climate. We show how savanna fires are coupled to the global climate through the carbon cycle and fire regimes. We present new research that climate change is likely to alter the structure and function of savannas through shifts in moisture availability and increases in atmospheric carbon dioxide (CO2), in turn altering fire regimes with further feedbacks to climate. We explore opportunities to reduce net greenhouse gas emissions from savanna ecosystems through changes in savanna fire management.This article is protected by copyright. All rights reserved.
The effect of long-term exclusion of fire and herbivory, as mediated by aspect, on soils and the structure, composition, and diversity of vegetation was investigated in Döhne Sourveld (34 years), Piet-Retief Sourveld (45 years) and Southern Tall Grassveld (49 years). Exclusion had no effect on the soil chemistry of the surface 15 cm of soil in Dohne and Piet Retief Sourveld. Exclusion promoted the ingress of woody elements, including heath-like species, but the extent of ingress was related to water availability (rainfall, aspect), proximity of propagules, duration of protection, and accidental fires. Alien invasive woody species, in particular Acacia mearnsii established at all sites. Woody species were mostly bird- or wind dispersed, some had no known nearby population, although some Acacia species were herbivore-dispersed. The wettest grassland, Piet Retief Sourveld on a southern aspect, was transformed to a shrubland matrix (mainly Helichrysum splendidum) with tree clumps of forest precursor species (Rhus dentata, Halleria lucida, Myrsine africana, Buddleja salviifolia, Leucosidea sericea) and A. mearnsii. A species-rich grassland on the northern aspect had become dominated by tall Hyparrhenia rufa, with scattered shrubs of L. sericea and Helichrysum rogulosum. The southern aspect exclosure in Döhne Sourveld had developed an A. mearnsii - B. salviifolia savanna, with Cymbopogon excavatus, Themeda triandra and Tristachya leucothrix as common grasses. The northern aspect exclosure on Döhne Sourveld maintained a grassland character, although heath-like elements (Stoebe vulgaris, Erica aspalanthifolia) established. Protection of the Southern Tall Grassveld promoted an open savanna of exotic and indigenous Acacia and broadleaf species, an increase in life forms including the succulent Aloe maculata, and the transformation of a T. triandra - T. leucothrix sward to an Aristida junciformis sward. Ferns benefitted from exclusion of fire and herbivory. Fire and herbivory rather than soil properties would seem the primary determinants of the structure of sour grasslands.
The higher flammability of tropical savanna, compared with forest, plays a critical role in mediating vegetation‐environment feedbacks, alternate stable states, and ultimately, the distribution of these two biomes. Multiple factors contribute to this difference in flammability, including microclimate, fuel amount and fuel type. To understand this transition in flammability, we studied fuel characteristics and microclimate across eight savanna–forest boundaries in south‐central Brazil. At each boundary, the environment was monitored for one week with automated measurements of near‐surface wind speed, air temperature, relative humidity and presence of dew. Manual measurements were performed to quantify fuel amounts and fuel moisture. These data were used to parameterize the fire behaviour model BehavePlus5 in order to simulate fire behaviour over the savanna–forest boundary. There were strong gradients across the boundary in all variables with the exception of total fuel load. During the day, savannas had higher wind speed and air temperature, and lower relative humidity and fuel moisture than forests. Although fuel loads were similar in savanna and forest, savanna was characterized by lower fuel bulk density, largely because of the presence of grasses. Based on these measurements, the fire behaviour model predicted savanna fires to be faster, more intense, and with greater flame lengths, relative to forest. A sensitivity analysis indicated that the primary cause of these differences was the low fuel bulk density characteristic of grassy fuels, with lesser contributions from wind speed, fuel moisture and total fuel load. These results indicate that the dominance of grassy fuels is the primary cause of the high flammability of savanna.