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Rate of forest recovery after fire exclusion on anthropogenic savannas in the Democratic Republic of Congo
Abstract
Deforestation in the tropics is often followed by the creation of anthropogenic savannas used for animal hus-bandry. By discontinuing burning regimes, forests may recolonize the savanna and carbon stocks may recover. However, little is known about the success and speed of tropical forest recovery, while such information is vital for a better quantification of efforts to reduce emissions from deforestation and forest degradation (REDD+) as well as supporting Forest Landscape Restoration (FLR) practices. Therefore, we designed a forest regeneration experiment within a savanna patch in the Mayombe hills (Democratic Republic of Congo), by discontinuing the annual burning regime in an 88 ha exclosure since 2005. 101 permanent inventory plots (40.4 ha) were installed in 2010 and remeasured in 2014. Tree species were classified as savanna or forest specialists. We estimate a forest specialist encroachment rate of 9 stems ha −1 yr −1 and a savanna specialist disappearance rate of 16 stems ha −1 yr −1. Average diameter of forest specialists did not change due to an increasing influx of recruits, while average diameter of savanna trees increased due to decreasing recruitment. Carbon stored by forest specialists increased from 3.12 to 5.60 Mg C ha −1 , suggesting a forest carbon recovery rate of 0.62 Mg C ha −1 yr −1. Using the average carbon stock of 19 nearby mature rainforest plots as a reference, we estimate a total forest carbon recovery time of at least 150 years. The Manzonzi exclosure may potentially become an important reference experiment to quantify REDD+ schemes in Central Africa. Furthermore, this natural regeneration experiment demonstrates how carbon sequestration and biodiversity conservation can go hand-in-hand. However, more censuses are needed to better quantify the long-term carbon recovery trajectory within the protected area.
... In the DRC, deforestation and forest degradation are mainly the result of the expansion of slash-and-burn agriculture (the main form of agriculture in the country) (Ickowitz et al. 2015;Katembera et al. 2015) combined with fuelwood extraction, mining, logging and urbanization (Paluku 2005;Duveiller et al. 2008;Sikuzani et al. 2017;MECNT 2012;Megevand 2013;Deklerck et al. 2019). Although mainly practiced on a small scale, shifting cultivation can range from a few hectares to hundreds of hectares (Harris et al. 2017;Potapov et al. 2012;de Wasseige et al. 2014). ...
... Indigenous communities have been managing landscapes with fire over several millennia (Peltier et al. 2014;Klimaszewski-Patterson et al. 2018). Fire is widely used as a lowcost and effective tool to manage and transform land for agriculture and grazing, to improve hunting conditions, to clear dense vegetation and to control pests (Kim, Sexton, and Townshend 2015;FAO 2020;Deklerck et al. 2019). Among the various categories of fires, forest fire, mainly of human origin (voluntary or accidental), is used for domestic activities such as the opening fields for agriculture, honey harvesting, brush clearing, livestock and hunting (FAO 2010;Posner, Maercklein, and Overton 2009;Zhao et al. 2021;Tyukavina et al. 2022). ...
... The inhabitants of the Mayombe area around the LBR depend mainly on the forests for hunting, agriculture and fuelwood collection. They also depend on the savannas for grazing livestock and growing certain types of crops (Deklerck et al. 2019). ...
Human-induced fire is one of the most important determinants of forest cover and change in tropical and subtropical regions of the world. Yet its impact on forest cover and forest cover change remains unclear, as fires in Africa generally do not spread over very large area. This is particularly the case in the Democratic Republic of Congo (DRC), a region of the world that is still poorly investigated. Here, we propose to study the effect of human-induced fire on land use and land cover change in a protected area of the DRC, i.e. the Luki Biosphere Reserve (LBR). We investigate tree cover changes in and around the reserve between 2002 and 2019 using Landsat 7 ETM+, Landsat 8 OLI/TIRS and MODIS MCD12Q1 images and quantify human induced fires using MODIS MCD64A1 images. The study combines land use and land cover (LULC) change detection analysis of four images, two acquired in 2002 and two acquired in 2019, with multi-temporal assessment of annual burnt area acquired between 2002 and 2019 from MODIS MCD64A1 to assess the role of fire in LULC changes and the sensitivity of different LULC types to fire. The results show a dynamic conversion of primary forest to secondary forest over about 16% of the area, the evolution of savanna to secondary forest over 9.6% (Landsat image) and the replacement of secondary forest by savanna over 8.1% (MODIS image) of the total area of Luki Reserve. Of the total area undergoing land use change, 34.1% (Landsat image) and 35.7% (MODIS image) were caused by fire, which however did not cause a significant LULC change. For the LULC types that experienced fire events, the least stable type was primary forest, which had the lowest stability rate (34.2% and 23% for Landsat and MODIS image analysis, respectively) compared to others. This result illustrates the importance of fire as a driver of primary forest loss and degradation in the region. Despite the high exposure of savannas to fire events, they were not significantly destabilized by fire (stability rates of 86.3 and 97% for Landsat and MODIS analysis, respectively). Future analyses should focus on discriminating between different fire types to better understand the complex relationship between fire and ecosystem conditions.
... In Central Africa, a significant challenge to predict the effect of global change on biome distributions is to bring out innovative approaches that offset the scarcity of spatially and temporally detailed landscape-scale information. Previous efforts have been either samplebased or employed coarse spatial resolution data (Youta-Happi, 1998;Youta-Happi et al., 2003;Cuni-Sanchez et al., 2016;Deklerck et al., 2019) owing to the challenges of maintaining sampling over long periods combined with the inherent constraints of field data collection which do not consider the whole variability of the landscape. Remote sensing (RS) therefore has a great potential for use in mapping biodiversity (Broadbent et al., 2008;Hill, 2013;Féret and Boissieu, 2020), biomass (Bastin et al., 2014;Kumar et al., 2015;Pandit et al., 2018;Forkuor et al., 2019) and periodical phenomena i.e. fires (Nangendo et al., 2006;Miller and Thode, 2007;Escuin et al., 2008;Sunderman and Weisberg, 2011;Chen et al., 2017) with continuous spatial coverage over large geographic areas. ...
... Spectral species diversity appeared structured along a forest-age gradient. This reflects the successional gradient of floristic assemblages where fast growing pioneers species with low aboveground biomass yet strong photosynthetic activity dominate recent transitions and are then gradually replaced by long-lived species in old regenerating forests (Fairhead and Leach, 1994;Youta et al., 2003;Ibanez et al., 2013;Cuni-Sanchez et al., 2016;Deklerck et al., 2019). ...
... ;Cuni- Sanchez et al., 2016;Aleman et al., 2017;Devine et al., 2017;Axelsson and Hanan, 2018;Deklerck et al., 2019) with significant impacts on the global carbon budget of the continent(Poulter et al., 2014). ...
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.).
... Tropical forest is subject to both natural and anthropogenic fire (Mitchard, 2018). Forest fires cause large carbon emission and may have long-term effects, such as changes in age composition, degradation and succession (Brando et al., 2019;Deklerck et al., 2019;Pellegrini et al., 2017;Pugh et al., 2019). Fire disturbance may change the response of forest TBC to climatic factors because of its strong effects on forest carbon cycle (Pellegrini et al., 2017;Pugh et al., 2019). ...
... Many processes could be impacted by the fire regime which is controlled partly by climate (Mitchard, 2018). For example, fire driven by extreme high temperature and low rainfall may promote tree mortality rate, cause large CO 2 emission from biomass, induce forest edge effects and determine succession of species (Brando et al., 2019;Deklerck et al., 2019;Pellegrini et al., 2017;Pugh et al., 2019). We applied a fire mask in our analyses since we only aimed to reveal the direct contribution of climatic factors on τ and NPP without being affected by fire disturbance. ...
Tropical forests store about 70% of the total living biomass on land and yet very little is known about changes in this vital carbon reservoir. Changes in their biomass stock, determined by changes in carbon input (i.e., net primary production [NPP]) and carbon turnover time (τ), are critical to the global carbon sink. In this study, we calculated transient τ in tropical forest biomass using satellite-based biomass and moderate-resolution imaging spectroradiometer (MODIS) NPP and analyzed the trends of τ and NPP from 2001 to 2012. Results show that τ and NPP generally have opposite trends across the tropics. Increasing NPP and decreasing τ (“N+T−”) mainly distribute in central Africa and the northeast region of South America, while decreasing NPP and increasing τ (“N−T+”) prevail in Southeast Asia and western Amazon forests. Most of the N+T− tropical forest areas are associated with mean annual precipitation (MAP) below 2,000 mm·y⁻¹ and most N−T+ tropical forests with MAP above 2,000 mm·y⁻¹. The τ and NPP trends in the N+T− region are statistically associated with radiation, precipitation and vapor pressure deficit (VPD), while the τ and NPP trends in the N−T+ region are mainly associated with temperature and VPD. Our results inherit the uncertainties from the satellite-based datasets and largely depend on the carbon use efficiency from MODIS. We thus systematically assessed the robustness of the findings. Our study reveals regional patterns and potential drivers of biomass turnover time and NPP changes and provides valuable insights into the tropical forest carbon dynamics. © 2020. The Authors. Earth's Future published by Wiley Periodicals LLC on behalf of American Geophysical Union.
... In Central Africa, savanna ecosystems and their floral diversity in relation to fire remain underexplored; studies focus on carbon stocks and biomass (Batsa Mouwembe et al. 2017;Ifo et al. 2018;Nieto-Quintano et al. 2018), forest recovery (Deklerck et al. 2019), or the forest-savanna interface (Cardoso et al. 2018). In general, research on African savanna forb floras in relation to fire constitutes a significant knowledge gap, with most studies focusing on grasses and trees, and dry savannas (Siebert and Dreber 2019). ...
Background and aims – Old-growth savannas in Africa are impacted by fire, have endemic and geoxylic suffrutices, and are understudied. This paper explores the Parc National des Plateaux Batéké (PNPB) in Gabon and the impact of fire on its flora to understand if it is an old-growth savanna. It presents 1) a vascular plant checklist, including endemic species and geoxylic suffrutices and 2) an analysis of the impact of fire on the savanna herbaceous flora, followed by recommendations for fire management to promote plant diversity.
Material and methods – 1,914 botanical collections from 2001–2019 collected by the authors and others were extracted from two herbaria databases in 2021 to create the checklist. The impact of fire was explored through a three season plot-based inventory of plant species (notably forbs and geoxylic suffrutices) in five annually, dry-season burned study areas located at 600 m in elevation. A two-factor ANOVA was conducted across two burn treatments and three season treatments.
Key results – The area has a vascular flora of 615 taxa. Seven species are endemic to the Plateaux Batéké forest-savanna mosaic. Seventeen species are fire-dependent geoxylic suffrutices, attesting to the ancient origins of these savannas. Burning promotes fire-dependent species.
Conclusion – The PNPB aims to create a culturally-adapted fire management plan. The combination of customary fire and fire-adapted species in the savanna creates a unique ancient forest-savanna mosaic in Central Africa that merits protection while recognising the role that the Batéké-Alima people have in shaping and governing this landscape.
... Spectral species composition appeared structured along a forest-age gradient. This reflects the successional gradient of floristic assemblages where fast growing pioneer species with low aboveground biomass yet strong photosynthetic activity dominate recent transitions and are then gradually replaced by long-lived species in old regenerating forests (Cuni-Sanchez et al., 2016;Deklerck et al., 2019;Ibanez et al., 2013;Youta-Happi et al., 2003). The fact that spectral variability increased with forest ages in our study area apparently contradicts findings by (Réjou-Méchain et al., 2014) in another context of Central African forests, where floristic variability assessed from field inventory data was found to be higher at the initial stages of succession. ...
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.
... Forest fire was induced by humans as a means for land clearing for agriculture and for wild animal hunting. In the tropics, human-induced fires are the main cause of large-scale deforestation and forest degradation in Brazil [55,56], Indonesia [57,58], and the Democratic Republic of the Congo [59]. ...
Understanding the drivers of deforestation and forest degradation and the agents of such drivers is important for introducing appropriate policy interventions. Here, we identified drivers and agents of drivers through the analysis of local perceptions using questionnaire surveys, focus group discussions, and field observations. The Likert scale technique was employed for designing the questionnaire with scores ranging from 1 (strongly disagree) to 5 (strongly agree). We found nine direct drivers of forest deforestation and forest degradation, namely illegal logging (4.53 ± 0.60, ± is for standard deviation), commercial wood production (4.20 ± 0.71), land clearing for commercial agriculture (4.19 ± 1.15), charcoal production (3.60 ± 1.12), land clearing for subsistence agriculture (3.54 ± 0.75), new settlement and land migration (3.43 ± 0.81), natural disasters (3.31 ± 0.96), human-induced forest fires (3.25 ± 0.96), and fuelwood for domestic consumption (3.21 ± 0.77). We also found four main indirect drivers, namely lack of law enforcement, demand for timber, land tenure right, and population growth. Our analysis indicates that wood furniture makers, medium and large-scale agricultural investors, charcoal makers, land migrants, firewood collectors, and subsistent farmers were the agents of these drivers. Through focus group discussions, 12 activities were agreed upon and could be introduced to reduce these drivers. In addition to enforcing the laws, creating income-generating opportunities for locals along with the provision of environmental education could ensure long-term reduction of these drivers. The REDD+ project could be an option for creating local income opportunities, while reducing deforestation and forest degradation.
... The dry season is well marked with a season that goes from May to September, as well as a small dry season of 2 or 3 weeks in February (Quinif, 1986). According to the Köppen Classification, this phytogeographical region of Congolese Mayombe forest belongs to the AW4 system (Deklerck, 2019). Average annual rainfall is between 1200 -1400 mm (Sys, 1960). ...
Tropical forests cover a large part of the earth's surface. It is increasingly facing anthropogenic pressure that is changing its structure. The effect of current climate changes are already perceptible at the level of species and communities. Therefore, it is important to understand the functioning of the forest and the ecology of species in order to project the future of the forest. Functional traits varying along environmental gradients are considered as good indicators of ecosystem functioning and species response to environmental change. The objective of this PhD dissertation is to assess and explain dynamics and resilience capacity of the Central African tropical moist forest, based on a selection of species traits. We considered reproductive phenology, cambial and leaf phenology and series of growth rings. We performed case studies in the Luki Biosphere Reserve in the extreme south of the moist forest belt, but still with a floristic composition dominated by common forest species. Firstly, we analyzed seasonality and synchronicity of the reproductive phenology of tree species using archived data collected every 10 days from 1947 to 1958 in the Luki Man and Biosphere Reserve (Chapter 2). The results show that the reproductive phenology of the Mayombe species is seasonal and follows the rainfall pattern. Flowering is generally observed in December-February at the time of the rainfall decrease during the short dry season. A significant relationship between flowering and diameter was only observed for 13 out of 87 species. Secondly, data from high resolution dendrometers and time-lapse cameras, cambial pinning experiments, tree-ring width series and historical phenology data were used to monitor leaf and cambial phenology. We tested growth synchronicity between Prioria balsamifera trees and also their correlation with climatic variables (Chapter 3). The results obtained show that P. balsamifera has an annual leaf shedding with the peak at the end of the dry season and the beginning of the rainy season. The new leaves appear at the time when the old ones are still on the tree. A Gompertz function was fitted and showed that cambial activity starts at the beginning of the rainy season. Synchronicity between series of tree-ring widths was absent between P. balsamifera trees although they undergo the same environmental conditions. This variable environmental responses among individuals of the same population can be understood as an aspect of intraspecific biodiversity contributing to the resilicence capacity of the species. Thirdly, we compared different approaches for measuring tree growth. Therefore, we used data from permanent plots, a 66-years cambial pinning experiment, and census measurements made annually on individual trees (Chapter 4). For the different measurement approaches, cambial pinning gives the most accurate measurements and is recommended for understory species that are generally small in diameter. At the community scale, it is advised to use classical repeated diameter measurements in permanent plots on fixed measurement points with a 5-year census interval. To conclude, although located in the extreme south of the Congo basin and of the Mayombe Forest in Atlantic central Africa, and experiencing a relatively dry (~ 1200 mm rainfall) and a seasonal climate, the forest in Luki shows a structure, functioning and composition that do not deviate from the other forests studied elsewhere in central Africa. The cloudy dry season preventing evapotranspiration might allow the presence of moist forest tree species at the drier end of the rainfall gradient.
Tropical ecosystems adapted to high water availability may be highly impacted by climatic changes that increase soil and atmospheric moisture deficits. Many tropical regions are experiencing significant changes in climatic conditions, which may induce strong shifts in taxonomic, functional and phylogenetic diversity of forest communities. However, it remains unclear if and to what extent tropical forests are shifting in these facets of diversity along climatic gradients in response to climate change. Here, we show that changes in climate affected all three facets of diversity in West Africa in recent decades. Taxonomic and functional diversity increased in wetter forests but tended to decrease in forests with drier climate. Phylogenetic diversity showed a large decrease along a wet-dry climatic gradient. Notably, we find that all three facets of diversity tended to be higher in wetter forests. Drier forests showed functional, taxonomic and phylogenetic homogenization. Understanding how different facets of diversity respond to a changing environment across climatic gradients is essential for effective long-term conservation of tropical forest ecosystems. Different aspects of biodiversity may not necessarily converge in their response to climate change. Here, the authors investigate 25-year shifts in taxonomic, functional and phylogenetic diversity of tropical forests along a spatial climate gradient in West Africa, showing that drier forests are less stable than wetter forests.
Tropical forests are global centres of biodiversity and carbon storage. Many tropical countries aspire to protect forest to fulfil biodiversity and climate mitigation policy targets, but the conservation strategies needed to achieve these two functions depend critically on the tropical forest tree diversity-carbon storage relationship. Assessing this relationship is challenging due to the scarcity of inventories where carbon stocks in aboveground biomass and species identifications have been simultaneously and robustly quantified. Here, we compile a unique pan-tropical dataset of 360 plots located in structurally intact old-growth closed-canopy forest, surveyed using standardised methods, allowing a multi-scale evaluation of diversity-carbon relationships in tropical forests. Diversity-carbon relationships among all plots at 1 ha scale across the tropics are absent, and within continents are either weak (Asia) or absent (Amazonia, Africa). A weak positive relationship is detectable within 1 ha plots, indicating that diversity effects in tropical forests may be scale dependent. The absence of clear diversity-carbon relationships at scales relevant to conservation planning means that carbon-centred conservation strategies alone would inevitably miss many high diversity ecosystems. As tropical forests can have any combination of tree diversity and carbon stocks both will require explicit consideration when optimising policies to manage tropical carbon and biodiversity.
The forest, savanna, and grassland biomes, and the transitions between them, are expected to
undergo major changes in the future, due to global climate change. Dynamic Global Vegetation
Models (DGVMs) are very useful to understand vegetation dynamics under present climate, and to
predict its changes under future conditions. However, several DGVMs display high uncertainty in
predicting vegetation in tropical areas. Here we perform a comparative analysis of three different
DGVMs (JSBACH, LPJ-GUESS-SPITFIRE and aDGVM) with regard to their representation of the ecological
mechanisms and feedbacks that determine the forest, savanna and grassland biomes, in an attempt to
bridge the knowledge gap between ecology and global modelling. Model outcomes, obtained including
different mechanisms, are compared to observed tree cover along a mean annual precipitation
gradient in Africa. Through these comparisons, and by drawing on the large number of recent
studies that have delivered new insights into the ecology of tropical ecosystems in general, and
of savannas in particular, we identify two main mechanisms that need an improved representation in
the DGVMs. The first mechanism includes water limitation to tree growth, and tree-grass
competition for water, which are key factors in determining savanna presence in arid and semi-arid
areas. The second is a grass-fire feedback, which maintains both forest and savanna occurrences in
mesic areas. Grasses constitute the majority of the fuel load, and at the same time benefit from
the openness of the landscape after fires, since they recover faster than trees. Additionally,
these two mechanisms are better represented when the models also include tree life stages (adults
and seedlings), and distinguish between fire-prone and shade-tolerant savanna trees, and
fire-resistant and shade-intolerant forest trees. Including these basic elements could improve the
predictive ability of the DGVMs, not only under current climate conditions but also and especially
under future scenarios.
Key message
Understanding species-specific response as well as wedging and zero xylem growth is vital for tree-ring analysis of tropical understory trees.
Abstract
Knowledge on intra-annual xylem growth remains understudied in tropical regions, especially for understory species. However, it is important to disentangle seasonal tree response in this complex environment. The aim is to assess intra-annual wood formation and its variability in selected understory tree species of a semi-deciduous tropical forest. The cambium of four species from the Luki reserve of the Mayombe (DR Congo) was monthly marked at the stem base via the pinning method. To assess ring anomalies on the stem disks, digitization of the last 5–10 rings was performed along the circumference. Relative growth was determined based on X-ray CT volumes of the pinning zone, as well as on sanded surfaces and microsections. Stem disks allowed to visualize ring anomalies and growth variations. Intra-annual growth was successfully derived via X-ray CT and could be fitted with a Gompertz function. A species-specific response is observed, although there is circumferential variability. However, the most remarkable result is that many of the trees in the data set had no xylem formation at the stem base, throughout the entire season, thus forming missing rings. Intra-annual variability in growth illustrates the different responses of species and individual trees to environmental drivers. Phenology might explain the differences, although site and competition should be considered as well. A large number of trees show no xylem growth at all, apart from wound-induced local growth, causing missing rings which have important implications for the tree-ring analysis in tropical regions.
Atmospheric carbon dioxide records indicate that the land surface has acted as a strong global carbon sink over recent decades1, 2, with a substantial fraction of this sink probably located in the tropics3, particularly in the Amazon4. Nevertheless, it is unclear how the terrestrial carbon sink will evolve as climate and atmospheric composition continue to change. Here we analyse the historical evolution of the biomass dynamics of the Amazon rainforest over three decades using a distributed network of 321 plots. While this analysis confirms that Amazon forests have acted as a long-term net biomass sink, we find a long-term decreasing trend of carbon accumulation. Rates of net increase in above-ground biomass declined by one-third during the past decade compared to the 1990s. This is a consequence of growth rate increases levelling off recently, while biomass mortality persistently increased throughout, leading to a shortening of carbon residence times. Potential drivers for the mortality increase include greater climate variability, and feedbacks of faster growth on mortality, resulting in shortened tree longevity5. The observed decline of the Amazon sink diverges markedly from the recent increase in terrestrial carbon uptake at the global scale1, 2, and is contrary to expectations based on models6.
(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$^{-1}$ ($\geqslant$ 30 cm girth) on the protected plot, forty-two trees ha$^{-1}$ on the early burnt plot and twenty trees ha$^{-1}$ on the late burnt plot. Corresponding figures for basal area are 3.43, 0.51, 0.24 m$^2$ ha$^{-1}$. (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$^{-2}$ on the protected plot, and 260 g m$^{-2}$ and 144 g m$^{-2}$ 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).
Terrestrial carbon stock mapping is important for the successful implementation of climate change mitigation policies. Its accuracy depends on the availability of reliable allometric models to infer oven-dry aboveground biomass of trees from census data. The degree of uncertainty associated with previously published pantropical aboveground biomass allometries is large. We analyzed a global database of directly harvested trees at 58 sites, spanning a wide range of climatic conditions and vegetation types (4004 trees ≥ 5 cm trunk diameter). When trunk diameter, total tree height, and wood specific gravity were included in the aboveground biomass model as co-variates, a single model was found to hold across tropical vegetation types, with no detectable effect of region or environmental factors. The mean percent bias and variance of this model was only slightly higher than that of locally fitted models. Wood specific gravity was an important predictor of aboveground biomass, especially when including a much broader range of vegetation types than previous studies. The generic tree diameter-height relationship depended linearly on a bioclimatic stress variable E, which compounds indices of temperature variability, precipitation variability and drought intensity. For cases in which total tree height is unavailable for aboveground biomass estimation, a pantropical model incorporating wood density, trunk diameter and the variable E outperformed previously published models without height. However, to minimize bias, the development of locally derived diameter-height relationships is advised whenever possible. Both new allometric models should contribute to improve the accuracy of biomass assessment protocols in tropical vegetation types, and to advancing our understanding of architectural and evolutionary constraints on woody plant development.This article is protected by copyright. All rights reserved.
Aims
The effects of fire ensure that large areas of the seasonal tropics are maintained as savannas. The advance of forests into these areas depends on shifts in species composition and the presence of sufficient nutrients. Predicting such transitions, however, is difficult due to a poor understanding of the nutrient stocks required for different combinations of species to resist and suppress fires.
Methods
We compare the amounts of nutrients required by congeneric savanna and forest trees to reach two thresholds of establishment and maintenance: that of fire resistance, after which individual trees are large enough to survive fires, and that of fire suppression, after which the collective tree canopy is dense enough to minimize understory growth, thereby arresting the spread of fire. We further calculate the arboreal and soil nutrient stocks of savannas, to determine if these are sufficient to support the expansion of forests following initial establishment.
Results
Forest species require a larger nutrient supply to resist fires than savanna species, which are better able to reach a fire-resistant size under nutrient limitation. However, forest species require a lower nutrient supply to attain closed canopies and suppress fires; therefore, the ingression of forest trees into savannas facilitates the transition to forest. Savannas have sufficient N, K, and Mg, but require additional P and Ca to build high-biomass forests and allow full forest expansion following establishment.
Conclusions
Tradeoffs between nutrient requirements and adaptations to fire reinforce savanna and forest as alternate stable states, explaining the long-term persistence of vegetation mosaics in the seasonal tropics. Low-fertility limits the advance of forests into savannas, but the ingression of forest species favors the formation of non-flammable states, increasing fertility and promoting forest expansion.
Tropical tree height-diameter ( H:D ) relationships may vary by forest type and region making large-scale estimates of above-ground biomass subject to bias if they ignore these differences in stem allometry. We have therefore developed a new global tropical forest database consisting of 39 955 concurrent H and D measurements encompassing 283 sites in 22 tropical countries. Utilising this database, our objectives were:
1. to determine if H:D relationships differ by geographic region and forest type (wet to dry forests, including zones of tension where forest and savanna overlap).
2. to ascertain if the H:D relationship is modulated by climate and/or forest structural characteristics (e.g. stand-level basal area, A ).
3. to develop H:D allometric equations and evaluate biases to reduce error in future local-to-global estimates of tropical forest biomass.
Annual precipitation coefficient of variation ( P V), dry season length ( S D), and mean annual air temperature ( T A) emerged as key drivers of variation in H:D relationships at the pantropical and region scales. Vegetation structure also played a role with trees in forests of a high A being, on average, taller at any given D</ i>. After the effects of environment and forest structure are taken into account, two main regional groups can be identified. Forests in Asia, Africa and the Guyana Shield all have, on average, similar H:D relationships, but with trees in the forests of much of the Amazon Basin and tropical Australia typically being shorter at any given D than their counterparts elsewhere. The region-environment-structure model with the lowest Akaike's information criterion and lowest deviation estimated stand-level H across all plots to within amedian −2.7 to 0.9% of the true value. Some of the plot-to-plot variability in H:D relationships not accounted for by this model could be attributed to variations in soil physical conditions. Other things being equal, trees tend to be more slender in the absence of soil physical constraints, especially at smaller D . Pantropical and continental-level models provided less robust estimates of H , especially when the roles of climate and stand structure in modulating H:D allometry were not simultaneously taken into account.
Deforestation contributes 6-17% of global anthropogenic CO2
emissions to the atmosphere. Large uncertainties in emission estimates
arise from inadequate data on the carbon density of forests and the
regional rates of deforestation. Consequently there is an urgent need
for improved data sets that characterize the global distribution of
aboveground biomass, especially in the tropics. Here we use multi-sensor
satellite data to estimate aboveground live woody vegetation carbon
density for pan-tropical ecosystems with unprecedented accuracy and
spatial resolution. Results indicate that the total amount of carbon
held in tropical woody vegetation is 228.7PgC, which is 21% higher than
the amount reported in the Global Forest Resources Assessment 2010 (ref.
). At the national level, Brazil and Indonesia contain 35% of the total
carbon stored in tropical forests and produce the largest emissions from
forest loss. Combining estimates of aboveground carbon stocks with
regional deforestation rates we estimate the total net emission of
carbon from tropical deforestation and land use to be
1.0PgCyr-1 over the period 2000-2010--based on the carbon
bookkeeping model. These new data sets of aboveground carbon stocks will
enable tropical nations to meet their emissions reporting requirements
(that is, United Nations Framework Convention on Climate Change Tier 3)
with greater accuracy.
Tropical forest loss and degradation are proceeding at unprecedented rates, eroding biological diversity and prospects for sustainable economic development of agricultural and forest resources. There is increasing evidence that forest plantations can play a key role in harmonising long-term forest ecosystem rehabilitation or restoration goals with near-term socio-economic development objectives. Recent studies have shown that plantations can facilitate or "catalyse," forest succession and biodiversity enrichment in their understories on sites where persistent ecological barriers to succession would otherwise preclude recolonisation by native forest species. These studies suggest that the catalytic effect of plantations is due to changes in understorey microclimatic conditions, increased vegetation structural complexity, and development of litter and humus layers that occur during the early years of plantation growth. These changes lead to increased seed inputs from neighbouring native forests by seed dispersing wildlife attracted to the plantations, suppression of grasses or other light-demanding species that normally prevent tree seed germination or seedling survival, and improved light, temperature and moisture conditions for seedling growth. Wildlife, particularly birds and bats, are critical "allies" in the restoration process, responsible for seed dispersal for the overwhelming majority of tree, shrub and liana species present in moist tropical forests. Understanding the habitat preferences and behaviour of these restoration facilitators, including their relationship to vegetation structure and composition, can help us to better design restoration treatments (including tree species selection) that will lead to rapid increases in floristic diversity and overall improvements in the value of these forests as wildlife habitat. In this paper, the results of recent studies conducted since 1995 in several countries in Latin America, Africa and the Asia-Pacific region on the phenomenon of plantation-catalysed native forest restoration will be summarised and their potential application in wildlife conservation programmes discussed.
Policies to reduce emissions from deforestation would benefit from clearly derived, spatially explicit, statistically bounded
estimates of carbon emissions. Existing efforts derive carbon impacts of land-use change using broad assumptions, unreliable
data, or both. We improve on this approach using satellite observations of gross forest cover loss and a map of forest carbon
stocks to estimate gross carbon emissions across tropical regions between 2000 and 2005 as 0.81 petagram of carbon per year,
with a 90% prediction interval of 0.57 to 1.22 petagrams of carbon per year. This estimate is 25 to 50% of recently published
estimates. By systematically matching areas of forest loss with their carbon stocks before clearing, these results serve as
a more accurate benchmark for monitoring global progress on reducing emissions from deforestation.
Aim (1) To estimate the local and global magnitude of carbon fluxes between savanna and the atmosphere, and to suggest the significance of savannas in the global carbon cycle. (2) To suggest the extent to which protection of savannas could contribute to a global carbon sequestration initiative.Location Tropical savanna ecosystems in Africa, Australia, India and South America.Methods A literature search was carried out using the ISI Web of Knowledge, and a compilation of extra data was obtained from other literature, including national reports accessed through the personal collections of the authors. Savanna is here defined as any tropical ecosystem containing grasses, including woodland and grassland types. From these data it was possible to estimate the fluxes of carbon dioxide between the entire savanna biome on a global scale.Results Tropical savannas can be remarkably productive, with a net primary productivity that ranges from 1 to 12 t C ha−1 year−1. The lower values are found in the arid and semi-arid savannas occurring in extensive regions of Africa, Australia and South America. The global average of the cases reviewed here was 7.2 t C ha−1 year−1. The carbon sequestration rate (net ecosystem productivity) may average 0.14 t C ha−1 year−1 or 0.39 Gt C year−1. If savannas were to be protected from fire and grazing, most of them would accumulate substantial carbon and the sink would be larger. Savannas are under anthropogenic pressure, but this has been much less publicized than deforestation in the rain forest biome. The rate of loss is not well established, but may exceed 1% per year, approximately twice as fast as that of rain forests. Globally, this is likely to constitute a flux to the atmosphere that is at least as large as that arising from deforestation of the rain forest.Main conclusions The current rate of loss impacts appreciably on the global carbon balance. There is considerable scope for using many of the savannas as sites for carbon sequestration, by simply protecting them from burning and grazing, and permitting them to increase in stature and carbon content over periods of several decades.
Policies to reduce global warming by offering credits for carbon sequestration have neglected the effects of forest management on biodiversity. I review properties of forest ecosystems and management options for enhancing the resistance and resilience of forests to climate change. Although forests, as a class, have proved resilient to past changes in climate, today's fragmented and degraded forests are more vulnerable. Adaptation of species to climate change can occur through phenotypic plasticity, evolution, or migration to suitable sites, with the latter probably the most common response in the past. Among the land-use and management practices likely to maintain forest biodiversity and ecological functions during climate change are (1) representing forest types across environmental gradients in reserves; (2) protecting climatic refugia at multiple scales; (3) protecting primary forests; (4) avoiding fragmentation and providing connectivity, especially parallel to climatic gradients; (5) providing buffer zones for adjustment of reserve boundaries; (6) practicing low-intensity forestry and preventing conversion of natural forests to plantations; ( 7) maintaining natural fire regimes; (8) maintaining diverse gene pools; and (9) identifying and protecting functional groups and keystone species. Good forest management in a time of rapidly changing climate differs little from good forest management under more static conditions, but there is increased emphasis on protecting climatic refugia and providing connectivity.
One way of mitigating global climate change is protecting and enhancing biosphere carbon stocks. The success of mitigation initiatives depends on the long-term net balance between carbon gains and losses. The biodiversity of ecological communities, including composition and variability of traits of plants and soil organisms, can alter this balance in several ways. This influence can be direct, through determining the magnitude, turnover rate, and longevity of carbon stocks in soil and vegetation. It can also be indirect through influencing the value and therefore the protection that societies give to ecosystems and their carbon stocks. Biodiversity of forested ecosystems has important consequences for long-term carbon
storage, and thus warrants incorporation into the design,
implementation, and regulatory framework of mitigation
initiatives.
The terrestrial carbon sink has been large in recent decades, but its size and location remain uncertain. Using forest inventory
data and long-term ecosystem carbon studies, we estimate a total forest sink of 2.4 ± 0.4 petagrams of carbon per year (Pg
C year–1) globally for 1990 to 2007. We also estimate a source of 1.3 ± 0.7 Pg C year–1 from tropical land-use change, consisting of a gross tropical deforestation emission of 2.9 ± 0.5 Pg C year–1 partially compensated by a carbon sink in tropical forest regrowth of 1.6 ± 0.5 Pg C year–1. Together, the fluxes comprise a net global forest sink of 1.1 ± 0.8 Pg C year–1, with tropical estimates having the largest uncertainties. Our total forest sink estimate is equivalent in magnitude to the
terrestrial sink deduced from fossil fuel emissions and land-use change sources minus ocean and atmospheric sinks.
Savannas are known as ecosystems with tree cover below climate-defined equilibrium values. However, a predictive framework for understanding constraints on tree cover is lacking. We present (a) a spatially extensive analysis of tree cover and fire distribution in sub-Saharan Africa, and (b) a model, based on empirical results, demonstrating that savanna and forest may be alternative stable states in parts of Africa, with implications for understanding savanna distributions. Tree cover does not increase continuously with rainfall, but rather is constrained to low (<50%, "savanna") or high tree cover (>75%, "forest"). Intermediate tree cover rarely occurs. Fire, which prevents trees from establishing, differentiates high and low tree cover, especially in areas with rainfall between 1000 mm and 2000 mm. Fire is less important at low rainfall (<1000 mm), where rainfall limits tree cover, and at high rainfall (>2000 mm), where fire is rare. This pattern suggests that complex interactions between climate and disturbance produce emergent alternative states in tree cover. The relationship between tree cover and fire was incorporated into a dynamic model including grass, savanna tree saplings, and savanna trees. Only recruitment from sapling to adult tree varied depending on the amount of grass in the system. Based on our empirical analysis and previous work, fires spread only at tree cover of 40% or less, producing a sigmoidal fire probability distribution as a function of grass cover and therefore a sigmoidal sapling to tree recruitment function. This model demonstrates that, given relatively conservative and empirically supported assumptions about the establishment of trees in savannas, alternative stable states for the same set of environmental conditions (i.e., model parameters) are possible via a fire feedback mechanism. Integrating alternative stable state dynamics into models of biome distributions could improve our ability to predict changes in biome distributions and in carbon storage under climate and global change scenarios.
Although fire is recognised as an important determinant of the structure and function of South African savannas, there are few studies of long-term impacts. Controlled burning blocks of contrasting fire season and frequency have been maintained throughout the Kruger National Park for almost 50 years. This paper reports on a quantitative study of the Satara plots to determine the long-term impacts of fire frequency on woody community structure and soil nutrients. Increasing fire frequency significantly decreased woody plant basal area, biomass, density, height, and mean stem circumference. The number of stems per plant and the proportion of regenerative stems increased with increasing fire frequency. Effects on species richness of woody plants were inconsistent. There were no significant differences attributable to fire frequency for any of the soil variables except organic matter and magnesium. Organic carbon was highest in the fire exclusion treatment and lowest in soils from plots burnt triennially. Magnesium levels were greatest in the annually burnt soils and least in the triennial plots.
Isolated savannas enclosed by forest are especially abundant in the eastern part of the Congolese Mayombe. They are about 3000 years old, and were more extensive some centuries ago. The boundary between forest and savanna is very abrupt, as a consequence of the numerous savanna fires lit by hunters. Floristic composition and vegetation structure data, organic carbon ratios, delta 14C and delta 13C measurements presented here show that forest is spreading over savanna at the present time and suggest that the rate of forest encroachment is currently between 14 and 75 m per century, and more probably about 20-50 m per century. As most savannas are less than 1 km across, such rates mean, assuming there are no changes in environmental conditions, that enclosed savannas could completely disappear in the Mayombe in about 1000-2000 years. (Résumé d'auteur)
The response of terrestrial vegetation to a globally changing environment is central to predictions of future levels of atmospheric carbon dioxide. The role of tropical forests is critical because they are carbon-dense and highly productive. Inventory plots across Amazonia show that old-growth forests have increased in carbon storage over recent decades, but the response of one-third of the world's tropical forests in Africa is largely unknown owing to an absence of spatially extensive observation networks. Here we report data from a ten-country network of long-term monitoring plots in African tropical forests. We find that across 79 plots (163 ha) above-ground carbon storage in live trees increased by 0.63 Mg C ha(-1) yr(-1) between 1968 and 2007 (95% confidence interval (CI), 0.22-0.94; mean interval, 1987-96). Extrapolation to unmeasured forest components (live roots, small trees, necromass) and scaling to the continent implies a total increase in carbon storage in African tropical forest trees of 0.34 Pg C yr(-1) (CI, 0.15-0.43). These reported changes in carbon storage are similar to those reported for Amazonian forests per unit area, providing evidence that increasing carbon storage in old-growth forests is a pan-tropical phenomenon. Indeed, combining all standardized inventory data from this study and from tropical America and Asia together yields a comparable figure of 0.49 Mg C ha(-1) yr(-1) (n = 156; 562 ha; CI, 0.29-0.66; mean interval, 1987-97). This indicates a carbon sink of 1.3 Pg C yr(-1) (CI, 0.8-1.6) across all tropical forests during recent decades. Taxon-specific analyses of African inventory and other data suggest that widespread changes in resource availability, such as increasing atmospheric carbon dioxide concentrations, may be the cause of the increase in carbon stocks, as some theory and models predict.
Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than approximately 650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered 'stable' systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of approximately 650 mm, savannas are 'unstable' systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics.
Forests out of balance
Are tropical forests a net source or net sink of atmospheric carbon dioxide? As fundamental a question as that is, there still is no agreement about the answer, with different studies suggesting that it is anything from a sizable sink to a modest source. Baccini et al. used 12 years of MODIS satellite data to determine how the aboveground carbon density of woody, live vegetation has changed throughout the entire tropics on an annual basis. They find that the tropics are a net carbon source, with losses owing to deforestation and reductions in carbon density within standing forests being double that of gains resulting from forest growth.
Science , this issue p. 230
We report above-ground biomass (AGB), basal area, stem density and wood mass density estimates from 260 sample plots (mean size: 1.2 ha) in intact closed-canopy tropical forests across 12 African countries. Mean AGB is 395.7 Mg dry mass ha?1 (95% CI: 14.3), substantially higher than Amazonian values, with the Congo Basin and contiguous forest region attaining AGB values (429 Mg ha?1) similar to those of Bornean forests, and significantly greater than East or West African forests. AGB therefore appears generally higher in palaeo- compared with neotropical forests. However, mean stem density is low (426 ± 11 stems ha?1 greater than or equal to 100 mm diameter) compared with both Amazonian and Bornean forests (cf. approx. 600) and is the signature structural feature of African tropical forests. While spatial autocorrelation complicates analyses, AGB shows a positive relationship with rainfall in the driest nine months of the year, and an opposite association with the wettest three months of the year; a negative relationship with temperature; positive relationship with clay-rich soils; and negative relationships with C : N ratio (suggesting a positive soil phosphorus–AGB relationship), and soil fertility computed as the sum of base cations. The results indicate that AGB is mediated by both climate and soils, and suggest that the AGB of African closed-canopy tropical forests may be particularly sensitive to future precipitation and temperature changes.
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.
The impact of Holocene drought events on the presumably stable Central African rainforest remains largely unexplored, in particular the significance of fire. High-quality sedimentary archives are scarce and palynological records mostly integrate over large regional scales subject to different fire regimes. Here we demonstrate a direct temporal link between Holocene droughts, palaeofire and vegetation change within present-day Central African rainforest, using records of identified charcoal fragments extracted from soil in the southern Mayumbe forest (Democratic Republic of Congo). We find three distinct periods of local palaeofire occurrence: 7.8 - 6.8 ka BP, 2.3 - 1.5 ka BP, 0.8 ka BP - present. These periods are linked to well-known Holocene drought anomalies: the 8.2 ka BP event, the 3rd millennium BP rainforest crisis and the Medieval Climate Anomaly. During and after these Holocene droughts the Central African rainforest landscape was characterised by a fragmented pattern with fire-prone open patches. Some fires occurred during the drought anomalies although most fires seem to lag behind them, which suggests that the open patches remained fire-prone after the actual climate anomalies. Charcoal identifications indicate that mature rainforest patches did persist through the Early to Mid-Holocene climatic transition, the subsequent Holocene Thermal Optimum and the third millennium BP rainforest crisis, until 0.8 ka BP. However, disturbance and fragmentation were probably more prominent near the boundary of the southern Mayumbe forest. Furthermore, the dominance of pioneer and woodland savanna taxa in younger charcoal assemblages indicates that rainforest regeneration was hampered by increasingly severe drought conditions after 0.8 ka BP. These results support the notion of a dynamic forest ecosystem at multi-century time scales across the Central African rainforest.This article is protected by copyright. All rights reserved.
Charcoal was sampled in four soil profiles at the Mayumbe forest boundary (DRC). Five fire events were recorded and 44 charcoal types were identified. One stratified profile yielded charcoal assemblages around 530 cal yr BP and > 43.5 cal ka BP in age. The oldest assemblage precedes the period of recorded anthropogenic burning, illustrating occasional long-term absence of fire but also natural wildfire occurrences within tropical rainforest. No other charcoal assemblages older than 2500 cal yr BP were recorded, perhaps due to bioturbation and colluvial reworking. The recorded paleofires were possibly associated with short-lived climate anomalies. Progressively dry climatic conditions since ca. 4000 cal yr BP onward did not promote paleofire occurrence until increasing seasonality affected vegetation at the end of the third millennium BP, as illustrated by a fire occurring in mature rainforest that persisted until around 2050 cal yr BP. During a drought episode coinciding with the ‘Medieval Climate Anomaly’, mature rainforest was locally replaced by woodland savanna. Charcoal remains from pioneer forest indicate that fire hampered forest regeneration after climatic drought episodes. The presence of pottery shards and oil-palm endocarps associated with two relatively recent paleofires suggests that the effects of climate variability were amplified by human activities.
In closed oak-hickory forest, tree stem density declined markedly following the burn. Tree basal area and density decreased from 17.5 m 2/ha and 630 trees/ha in the preburn sample to 12.0 m 2/ha and 310 trees/ha 5 yr later. On forest-prairie edge, tree basal area and density increased slightly during the same time period from 3.0 m 2/ha and 117 trees/ha to 5.2 m 2/ha and 172 trees/ha. Closed canopy forests in fire susceptible areas accumulate fuels to levels that encourage fires of sufficient intensity to destabilize forest systems and convert them to open forests or savannahs. On the forest-prairie edge, amounts and patterns of fuel accumulation, and species response to burning, are such that fire can be considered to be a factor promoting stability. -from Authors
AimFire protection gradually changes the density of woody plants in numerous savannas around the world. In this study changes of structure in two tropical savanna areas with contrasting history of fire protection in the central Brazilian Cerrado is documented.LocationVegetation was sampled with line intercept transects in two adjacent sites in Brasilia, Federal District. These transects were located within a nature reserve protected from fire since 1972 and within an adjacent reserve area that burns every 2 years.Methods
Five savanna physiognomies, from a low forest (‘cerradão’) to an open savanna (‘campo sujo’), were sampled in both sites.ResultsFire protection increased the abundance of woody plants and favoured fire-sensitive species. With some exceptions, shrubs tended to be less affected by fire than trees. Species distribution was affected by a complex interaction of fire and physiognomy. Fire had the strongest effect on ‘campo sujo’ savanna, and a less significant effect on the intermediate physiognomies.Main conclusionsProtection permits the establishment of fire sensitive species. A long enough protection against fire could lead to the appearance of more wooded physiognomies in the Cerrado.
The savannas of Southern and East Africa have five major characteristics that together distinguish them from other ecosystems:
1.
Both the herbaceous and woody layers contribute significantly to primary production, and they generally occur in an irregular mosaic.
2.
The vegetation is spatially very heterogeneous.
3.
They support, in their natural state, a high biomass of large ungulates.
4.
Rainfall is very variable and, as a consequence, so is primary production.
5.
Fires occur at irregular intervals, the frequency decreasing with aridity, from virtually annual in some of the wetter savannas (>750 mm rainfall per annum) to nil at the driest extremes. In the moister savannas, the vegetation would be closed woodland in the absence of fire.
The tropical forest of Central Africa is the second largest and probably the best preserved stretch of rain forest on Earth, yet the least known. Increasing demographic growth and economic interests are major threats to this ecosystem. Accurate knowledge on community dynamics and on the ecology of tree species growing in this forest is thus urgently needed to underpin conservation and management practices.
The Reserve of Luki at the extreme West of the Democratic Republic of Congo was a privileged site to study this ecosystem, moreover concealing spectacular biological collections and datasets. From the community level to the minute anatomy of wood this dissertation gives an overview of the ecology of trees of the Central African rain forest from 1948 until today. First, the history of community dynamics in a 200 ha forest plot was studied to highlight tendencies in the variations of species diversity and biomass content. Then the biological rhythms of five selected tree species and functional groups of species were examined to get a better understanding of natural ecosystem processes in relation with climate variations.
Long-term inventory data revealed 58 years of forest dynamics after an initial transformation thinning treatment and continuous forest use until today. Perturbations maintained at a moderate level by the protected status of the Reserve seem to have favoured species diversity and biomass sequestration in this forest. Besides, most tree species were found to have annual rhythms of leaf and reproductive phenology but in a wide array of patterns, from synchronous annual peaks to continuity. Direct and indirect associations with intra-annual and supra-annual climate variations suggest that changes in environmental conditions will affect the phenological rhythms of tropical trees. Dendrochronological analyses proved annual ring formation for the five study tree species and positive correlation between growth and rainfall. For the three understory species radial growth was found to associate with precipitation during the rainy season but in a different month for each species. For canopy species strong heterogeneity of growth patterns was found, between species and between individuals of the same species. A more detailed study of radial wood growth by use of cambial marking experiments showed that individual sensibilities to the type of substrate and fine plasticity of cambial activity in response to environmental changes are possible causes for this growth variability. Cambial dormancy in tropical trees may not be strict like in trees of temperate regions, but highly plastic and triggered by endogenous factors as well as climate variations.
This heterogeneity of responses to environmental changes between species and between individuals of the same species growing in the same site supports the idea that plurality is a key concept in species-rich rain forests. As a consequence, studying the diverse components of this heterogeneous mix remains extremely challenging and requires repeated efforts on the long run. Protecting this natural resource that is so far from being understood is therefore of utmost importance.
Climate change is leading to the development of land-based mitigation and adaptation strategies that are likely to have substantial impacts on global biodiversity. Of these, approaches to maintain carbon within existing natural ecosystems could have particularly large benefits for biodiversity. However, the geographical distributions of terrestrial carbon stocks and biodiversity differ. Using conservation planning analyses for the New World and Britain, we conclude that a carbon-only strategy would not be effective at conserving biodiversity, as have previous studies. Nonetheless, we find that a combined carbon-biodiversity strategy could simultaneously protect 90% of carbon stocks (relative to a carbon-only conservation strategy) and > 90% of the biodiversity (relative to a biodiversity-only strategy) in both regions. This combined approach encapsulates the principle of complementarity, whereby locations that contain different sets of species are prioritised, and hence disproportionately safeguard localised species that are not protected effectively by carbon-only strategies. It is efficient because localised species are concentrated into small parts of the terrestrial land surface, whereas carbon is somewhat more evenly distributed; and carbon stocks protected in one location are equivalent to those protected elsewhere. Efficient compromises can only be achieved when biodiversity and carbon are incorporated together within a spatial planning process.
Aim To describe patterns of tree cover in savannas over a climatic gradient and a range of spatial scales and test if there are identifiable climate-related mean structures, if tree cover always increases with water availability and if there is a continuous trend or a stepwise trend in tree cover.
Location Central Tropical Africa.
Methods We compared a new analysis of satellite tree cover data with botanical, phytogeographical and environmental data.
Results Along the climatic transect, six vegetation structures were distinguished according to their average tree cover, which can co-occur as mosaics. The resulting abrupt shifts in tree cover were not correlated to any shifts in either environmental variables or in tree species distributions.
Main conclusions A strong contrast appears between fine-scale variability in tree cover and coarse-scale structural states that are stable over several degrees of latitude. While climate parameters and species pools display a continuous evolution along the climatic gradient, these stable structural states have discontinuous transitions, resulting in regions containing mosaics of alternative stable states. Soils appear to have little effect inside the climatic stable state domains but a strong action on the location of the transitions. This indicates that savannas are patch dynamics systems, prone to feedbacks stabilizing their coarse-scale structure over wide ranges of environmental conditions.
In 1933/34 eight coppice plots were established in Brachystegia-Julbemardia (miombo) woodland at Ndola in the Copperbelt area of Zambia by the Forestry Department. These plots have been maintained under fire protection, annual early or late dry season burning since 1934/35. Before establishment stems over 20.3 cm girth at breast height were enumerated. Three of the eight plots (one fire protected and two annually early burnt) were enumerated in 1982, 48–49 years after establishment. In addition, a coppice plot at Chitwi, 16 km southwest of the Ndola plots, cleared in 1972 and left to regenerate naturally was enumerated in May 1982 and August 1986 to assess woody plant growth.
The density of stems over 20.0 cm girth in the 13-year-old coppice at Chitwi was 2.5 times that in an adjacent shelterbelt woodland. The stem density in the fire protected plot at Ndola in 1982 was 86% of the pre-felling density while in one of the early burnt plots it was 95% of the pre-felling density. The protected plot had the lowest species diversity after 49 years, largely because of the loss of 11 understorey species that were present before felling.
There were no significant differences in stem mean girth at breast height (gbh) of canopy species in the Ndola plots under fire protection and early burning regimes. Mean annual gbh increments of abundant species were estimated at 1.17–2.21 cm yr ⁻¹ and 0.59–1.42 cm yr ⁻¹ during 0–9 and 0–49 year age-periods, respectively. Estimated mean annual basal area increments for stems over 30 cm gbh were 0.35 m ² ha ⁻¹ for the 13-year-old coppice and 0.24–0.27 m ² ha ⁻¹ for the 49-year-old coppice. These results indicate a decrease in both gbh and basal area increment with increasing age of miombo coppice
There is a growing recognition that conservation often entails trade-offs. A focus on trade-offs can open the way to more complete consideration of the variety of positive and negative effects associated with conservation initiatives. In analyzing and working through conservation trade-offs, however, it is important to embrace the complexities inherent in the social context of conservation. In particular, it is important to recognize that the consequences of conservation activities are experienced, perceived, and understood differently from different perspectives, and that these perspectives are embedded in social systems and preexisting power relations. We illustrate the role of trade-offs in conservation and the complexities involved in understanding them with recent debates surrounding REDD (Reducing Emissions from Deforestation and Degradation), a global conservation policy designed to create incentives to reduce tropical deforestation. Often portrayed in terms of the multiple benefits it may provide: poverty alleviation, biodiversity conservation, and climate-change mitigation; REDD may involve substantial trade-offs. The gains of REDD may be associated with a reduction in incentives for industrialized countries to decrease carbon emissions; relocation of deforestation to places unaffected by REDD; increased inequality in places where people who make their livelihood from forests have insecure land tenure; loss of biological and cultural diversity that does not directly align with REDD measurement schemes; and erosion of community-based means of protecting forests. We believe it is important to acknowledge the potential trade-offs involved in conservation initiatives such as REDD and to examine these trade-offs in an open and integrative way that includes a variety of tools, methods, and points of view.
Policy makers across the tropics propose that carbon finance could provide incentives for forest frontier communities to transition away from swidden agriculture (slash-and-burn or shifting cultivation) to other systems that potentially reduce emissions and/or increase carbon sequestration. However, there is little certainty regarding the carbon outcomes of many key land-use transitions at the center of current policy debates. Our meta-analysis of over 250 studies reporting above- and below-ground carbon estimates for different land-use types indicates great uncertainty in the net total ecosystem carbon changes that can be expected from many transitions, including the replacement of various types of swidden agriculture with oil palm, rubber, or some other types of agroforestry systems. These transitions are underway throughout Southeast Asia, and are at the heart of REDD+ debates. Exceptions of unambiguous carbon outcomes are the abandonment of any type of agriculture to allow forest regeneration (a certain positive carbon outcome) and expansion of agriculture into mature forest (a certain negative carbon outcome). With respect to swiddening, our meta-analysis supports a reassessment of policies that encourage land-cover conversion away from these [especially long-fallow] systems to other more cash-crop-oriented systems producing ambiguous carbon stock changes – including oil palm and rubber. In some instances, lengthening fallow periods of an existing swidden system may produce substantial carbon benefits, as would conversion from intensely cultivated lands to high-biomass plantations and some other types of agroforestry. More field studies are needed to provide better data of above- and below-ground carbon stocks before informed recommendations or policy decisions can be made regarding which land-use regimes optimize or increase carbon sequestration. As some transitions may negatively impact other ecosystem services, food security, and local livelihoods, the entire carbon and noncarbon benefit stream should also be taken into account before prescribing transitions with ambiguous carbon benefits.
Summary 1 Chronosequence studies have found that shrubs and lianas are generally more abund- ant in early stages of tropical forest succession, whereas canopy trees and palms become more abundant and species-rich in older stages and mature forests. 2 We analysed changes in woody seedling communities over 5 years in four second- growth forests (initially 13-26 years after pasture abandonment) in Costa Rica. We recorded community-level changes in woody seedling density, species density, species richness and composition in six woody life-forms: canopy trees, subcanopy trees, canopy palms, understorey palms, shrubs and lianas. We evaluated these changes in relation to annualized recruitment and mortality rates for each life-form. 3 Seedling density declined in all four sites over the 5 years, whereas Shannon diversity and the proportion of rare species increased. Species richness and evenness increased in all but the oldest site. 4 Canopy palm, understorey palm and canopy tree seedlings increased in species richness and relative abundance, whereas shrub and liana relative abundance declined. Canopy trees accounted for 34-42% of all new recruits. Detrended correspondence analysis showed that species composition was initially highly distinct within each forest site and remained distinct over the 5-year period. 5 Shifts in life-form were correlated with declining light availability during succession. Across sites, median light availability at the end of the study period in 2003 was posi- tively correlated with recruitment rates of understorey palms, shrubs and lianas, and was negatively correlated with mortality rates of canopy trees and palms. 6 Observed changes among seedling communities mirrored those described in chron- osequence studies on plants in larger size classes, lending support to the assumptions of chronosequence studies in Neotropical forests. 7 The results demonstrate the importance of seedling recruitment and mortality in determining the course of succession. Convergence occurs in some community pro- perties, such as relative abundance within life-forms, but not in others, such as species composition. Finally, the results illustrate the value of studying plant community dynamics at the level of woody life-forms, especially in hyperdiverse systems such as tropical forests.
Tropical forests are biologically diverse ecosystems that play important roles in the carbon cycle and maintenance of global biodiversity. Understanding how tropical forests respond to environmental changes is important, as changes in carbon storage can modulate the rate and magnitude of climate change. Applying an ecoinformatics approach for managing long-term forest inventory plot data, where individual trees are tracked over time, facilitates regional and cross-continental forest research to evaluate changes in taxonomic composition, growth, recruitment and mortality rates, and carbon and biomass stocks. We developed ForestPlots.net as a secure, online inventory data repository and to facilitate data management of long-term tropical forest plots to promote scientific collaborations among independent researchers. The key novel features of the database are: (a) a design that efficiently deals with time-series data; (b) data management tools to assess potential errors; and (c) a query library to generate outputs (e.g. biomass and carbon stock changes over time).
Boreal mixedwoods (BMWs) are the most productive and diverse forest ecosystems in North American boreal forests. A good understanding of BMW stand dynamics is a prerequisite for sustainable management of these vital resources. In this review, we describe the patterns and processes of BMWs created by natural disturbances, examine the biotic and abiotic factors that influence these patterns and processes, and discuss forest management implications related to stand development. Based on distinct structural and developmental features, BMW stand development is characterized by four stages: stand initiation, stem exclusion, canopy transition, and gap dynamics. These four stages of stand development provide a conceptual model of complex developmental processes. However, multiple pathways are possible during BMW stand development depending on disturbances, neighbour effects, and stand condition. Boreal mixedwood management at the stand level needs to emulate the natural development process and target a specific stand structure and species composition. Alternative silvicultural techniques are available to achieve the multiple objectives of BMWs. Further considerations at various temporal and spatial scales and at the operational level are required to ensure sustainable BMW management.
Tropical moist forest is being destroyed at the rate of 11 million hecares per year, much of which degrades into fire-maintained savanna or low-productivity pasture. Greater economic and ecological benefits can be realized from much of this land if it can be converted into more productive secondary forest ecosystems. This study was conducted to deterimine if anthropogenic savannas could be converted into production forest through relatively inexpensive protection of the forest edge by plowing the soil with an agricultural tractor in highly-degraded (Ancient) and less-degraded (Nascent) savannas on the coastal plain of Gabon, Central Africa. After three years of protection, vegetation surveys revealed rapid colonization of Nascent savannas by 45 species of tree seedlings. Ancient savannas also experienced colonization by tree seedlings, but at a much lower rate. Analysis of soils determined that Nascent savannas have 5 times more calcium and magnesium and higher organic matter than Ancient savannas; indications of their less-degraded nature. Protection of the forest-edge from fire can be an effective, low-cost method of converting anthropogenic savannas into production forest through natural regeneration. The rate of conversion can be maximized by focusing on less-degraded sites to capitalize on more abundant seedling recruitment and higher ecosystem nutrient stocks, but even highly-degraded sites may be reclaimed with additional management.
The strength of carbon sink and stock was assessed in a protected savanna of the Orinoco Llanos by the harvesting plant phytomass and using allometric relationships between the dry mass and the censuses of plant height. Thus, changes in the carbon stock and the proportion in the tree/grass proportion were evaluated throughout age states. Results indicate that the carbon stock in the vegetation increased from 207 to 9215 g C m−2 whereas in the soil, it varied 6680 to 12 196 g C m−2. The carbon stock accumulation was mainly related to increases in the woody layer from 36 to 9215 g C m−2 (255-fold) and in the soil from 1341 to 12 196 g C m−2 (nine-fold), respectively. The estimated pool of carbon sequestered in the Orinoco Llanos by the restored forest in 51 years was 5.69 Pg C. The expansion and conservation of this carbon pool might remove CO2 from the atmosphere to help compensate for CO2 liberation associated with other land uses or industrial practices.
The objectives of this study were to understand the role of fires on land-cover changes, and conversely the role of vegetation cover as a controlling factor of fires. The study, which was conducted in a region at the savannah/forest transition in the southwestern part of the Central African Republic, explores the differential impact on land cover of early- and late-season fires and analyses burning regimes as a function of human use of the land. This was addressed using multivariate regression models between maps of land-cover change derived from remote sensing data, maps of burnt areas and a detailed map of ecotypes. In dense forests, burning is strongly associated with land-cover changes, while in savannahs the occurrence of (mostly) early fires does not lead to land-cover change. Fires associated with continuous and fragmented burnt patches have similar impacts on vegetation cover. Dense semi-humid forests in the study area were affected by a high level of burning due to land uses at their peripheries. The results confirm recent findings concerning human control on the timing of burning in savannahs. Early fires fragment the landscape and prevent the spatial diffusion of later damaging fires. Where no human settlements are present, late fires become more prevalent. Finally, the study measured an increase in vegetation cover in a few areas affected by very early burning. Using burnt area rather than active fire data allowed a better analysis of the spatial association between landscape attributes and burning events.
The mechanisms permitting the co-existence of tree and grass in savannas have been a source of contention for many years. The two main classes of explanations involve either competition for resources, or differential sensitivity to disturbances. Published models focus principally on one or the other of these mechanisms. Here we introduce a simple ecohydrologic model of savanna vegetation involving both competition for water, and differential sensitivity of trees and grasses to fire disturbances. We show how the co-existence of trees and grasses in savannas can be simultaneously controlled by rainfall and fire, and how the relative importance of the two factors distinguishes between dry and moist savannas. The stability map allows to predict the changes in vegetation structure along gradients of rainfall and fire disturbances realistically, and to clarify the distinction between climate- and disturbance-dependent ecosystems.
It is difficult to find references to fire in general textbooks on ecology, conservation biology or biogeography, in spite of the fact that large parts of the world burn on a regular basis, and that there is a considerable literature on the ecology of fire and its use for managing ecosystems. Fire has been burning ecosystems for hundreds of millions of years, helping to shape global biome distribution and to maintain the structure and function of fire-prone communities. Fire is also a significant evolutionary force, and is one of the first tools that humans used to re-shape their world. Here, we review the recent literature, drawing parallels between fire and herbivores as alternative consumers of vegetation. We point to the common questions, and some surprisingly different answers, that emerge from viewing fire as a globally significant consumer that is analogous to herbivory.