General characteristics of individuals included in the study.

General characteristics of individuals included in the study.

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In frequently burned ecosystems, many plants persist by repeated resprouting from basal or belowground buds. This strategy requires that plants reach a balance between biomass loss and recovery, which depends on the shape of the relationship between pre- and post-fire size. Previous analyses of this relationship, however, have focused on the size o...

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Savanna models that are based on recurrent disturbances such as fire result in nonequilibrium savannas, but these models rarely incorporate vegetation feedbacks on fire frequency or include more than two states (grasses and trees). We develop a disturbance model that includes vegetation‐fire feedbacks, using a system of differential equations to re...

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... Escape height has been reported for tree populations in several mesic savannas (e.g. Bond et al., 2012;Freeman et al., 2017;Nguyen et al., 2019;Pilon & Durigan, 2017;Prior et al., 2010;Schafer & Just, 2014;Werner, 2012;Werner & Franklin, 2010). The term 'resistance height' used in this paper is equivalent to the term 'escape height' used in some papers (Balfour & Midgley, 2006;Bond et al., 2012). ...
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1. In mesic savannas worldwide, trees experience frequent fires, almost all set by humans. Management fires are set to reduce or enhance tree cover. Success depends greatly on responses of sub‐adult trees to such fires. To date, the number of successive years that sub‐adult trees can resprout nor the number of years that they must resist being top‐killed by successive fires, nor the requisite height, have been reported. 2. In a six‐year experimental field study in Guinean savannas of West Africa, we monitored annually the heights and responses of 1,765 permanently tagged sub‐adult trees under annual fires set in three different periods of the long dry season: early‐dry season (EDS), mid‐dry season (MDS) and late‐dry season (LDS). Annual MDS fires are the common local management protocols of Guinean savannas, although EDS fires are common in some of the savannas. 3. Results showed that overall, the proportion of sub‐adults that resisted being top‐killed differed across fire seasons. Further, resisting one fire gave a better chance of resisting the next. Only sub‐adults that were able to resist direct damage for three successive EDS and MDS fires reached sufficient height to be recruited to the adult stage. Resistance height (avoiding topkill) was ∼1 m for EDS and ∼2 m for both MDS and LDS fires. Recruitment height (threshold for transition to adult stage) was ∼3 m for EDS and ∼ 3.3 m for MDS fires. No height was great enough for sub‐adult trees to be recruited to adult stages in LDS fire. Synthesis and applications: The results of this novel field study showed clearly that successive early‐ and mi‐dry season fires can enhance tree density and that successive late‐dry season fires alone reduce tree density in Guinean savannas due to the effects of successive fires on sub‐adult trees. The results suggest that a planned regime of these seasons of fire could be used to maintain the desired tree density in Guinean savannas and may inform fire management in other mesic savannas where goals are to increase or decrease tree densities. It also provides relevant information for comparative studies on the mechanisms of recruitment of sub‐adult trees to an adult stage in all mesic savannas, a process that ultimately determines savanna physiognomy.
... The larger the size of the tree prior to topkill, the greater the potential access to resources to drive resprouting and stem growth. It has in fact been well established that growth following resprouting is positively correlated with pre-disturbance stem size (Grady & Hoffmann, 2012;Holdo, 2006b;Schafer & Just, 2014) and that this relationship can hold vegetation in a persistent, stable fire trap (Grady & Hoffman, 2012). It is unclear, however, how post-disturbance growth rates and their dependence on pre-disturbance size vary across species and the consequences of this interspecific variation for escape. ...
... We show that there is considerable variation in growth following resprouting: our estimates project an almost five-fold difference in the expected time needed to recover pre-disturbance size (starting from a pre-disturbance basal area of 20 cm 2 ) between the slowest and fastest grower. Supporting previous work (Bonfil et al. 2004;Grady & Hoffmann, 2012;Schafer & Just, 2014), we found a strong dependence of post-disturbance growth on pre-disturbance size (even two years after the original disturbance event), but there was no evidence for variation in the magnitude of this dependence across species. Fast-growing species might be expected to exhibit a stronger relationship with predisturbance size than slow-growing species, for example, assuming that fast-growing species are more likely to invest in growth and escape from future disturbances rather than in storage and the ability to tolerate disturbance. ...
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Vegetation states in savannas are highly sensitive to tree growth rates, which determine whether individual trees can “escape” periodic disturbances. Resprouting trees have lopsided shoot:root ratios and are often multi‐stemmed, and these variables can modify post‐disturbance growth rates and therefore the probability of escape. To date, few studies have systematically examined the implications of interspecific variation in these factors for escape. We conducted a two‐year field experiment across 16 tree species in a South African lowveld savanna to quantify growth metrics following topkill. We examined the dependence of growth on pre‐disturbance stem size and the relationship between growth rate and the tendency of trees to produce a few large vs. many small resprouts following disturbance. We found that resprout growth was strongly influenced by pre‐disturbance size, but the strength of this relationship did not vary across species. In contrast, our results showed that fast‐growing species tended to allocate resources toward a few dominant stems, while slow‐growing species allocated new biomass towards many smaller stems. Tree species that produced a few large stems also tended to produce individual stems that were tall and thin, further suggesting that the “few large vs. many small” axis is linked to intrinsic species attributes. These findings have implications for understanding how interspecific variation in savanna tree communities may influence their ability to escape disturbance traps.
... For each of the 290 located individuals, we recorded the sex and maximum stem height (cm). We did not include number of stems in the current analyses as maximum stem height has been shown to be sufficient for characterizing growth in this system (Schafer and Just 2014). As L. subcoriacea exhibits limited clonality, we defined an individual as all stems separated by \ 0.5 m. ...
... A previous study of resprouting shrubs across wetland ecotones on Fort Bragg indicated that small-sized individuals recovered most or all of their pre-burn size 1-year post-fire (Grady and Hoffmann 2012), but the pre-burn measurement collection in that study differed from our collection methods. A separate study of resprouting found that none of the six tree species investigated recovered their aboveground biomass 1 year after physical removal of stems (Schafer and Just 2014). While we did not measure stems of other species surrounding L. subcoriacea individuals, our anecdotal observations of the response of those species suggested that their post-fire recovery was similar to L. subcoriacea, with neighboring species not outcompeting or overtopping regenerating L. subcoriacea individuals. ...
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Understanding demographic vital rates and the factors that affect those rates are key components of successful conservation strategies for many threatened and endangered rare plant species. Lindera subcoriacea is a rare dioecious shrub that occupies isolated wetland habitats in a small number of locations in the southeastern United States. The species faces a number of threats to its continued persistence, including habitat destruction, invasive species, and population isolation. From 2011 to 2019, we collected demographic information from 290 L. subcoriacea individuals within 28 populations on Fort Bragg, North Carolina and used the data to estimate demographic vital rates in unburned populations and after being exposed to prescribed fire. We then constructed population matrices and estimated population growth rates under a 3-, 5-, and 10-year return interval. Results indicated that L. subcoriacea individuals have high survivorship in both burned and unburned populations, seed production was reduced 1- and 2-year post-fire, seed production was highly uneven across individuals, seedling recruitment was extremely low, and simulated population growth rates were only above 1.0 under the 10-year fire return interval. Taken together, these results indicate that (1) L. subcoriacea populations are persisting with population growth rates close to one, (2) the short-term impacts of fire on the overall population growth rate of L. subcoriacea, while only 2–3% may determine long-term population viability, and (3) extremely uneven seed production and limited recruitment of seedlings into larger size classes make L. subcoriacea populations vulnerable to stochastic demographic processes.
... Experiments on the effects of disturbances on resprouting commonly include clipping aboveground plant parts (e.g., Cruz et al. 2003;Schafer and Just 2014;Martinez-Vilalta et al. 2016). If the main effect of repeated loss of aboveground biomass is carbohydrate starvation, then clipping and burning should have similar effects on resprouting (Hmielowski et al. 2014;Michielsen et al. 2017). ...
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Background Resprouting is an effective strategy for persistence of perennial plants after disturbances such as fire. However, can disturbances be so frequent that they limit resprouting? We examined the effects of fire and mowing frequency on eight species of resprouting shrubs in Florida scrub, USA, using a factorial field experiment. We burned or mowed plots at four disturbance return intervals (DRI): either annually, biennially, every three years, or once in six years (with all plots being treated in the sixth year to control for time since disturbance). We analyzed plant growth responses (height, aboveground biomass, number of stems) based on sampling pre treatment, and six months, one year, two years, and four years post-treatment. We also measured non-structural carbohydrates (NSC) and soil properties to evaluate these factors as potential drivers of resprouting responses. Results Fire temperatures were hot (mean maxima 414 to 698 °C among burn days), typical of larger fires in Florida scrub. Plant biomass and heights were affected by DRI (being suppressed by frequent disturbance, especially initially) and varied among species with palmettos recovering biomass faster, and species within the same genus generally showing similar responses. Biomass recovery in mown versus burned treatments showed comparable effects of DRI and similar trajectories over time. Numbers of stems were affected by DRI, disturbance type, and species, and increased after disturbances, especially with less frequent disturbances and mowing, and subsequently declined over time. NSC concentrations varied among species and over time and were positively related to biomass. One year post disturbance, soil moisture and organic matter content were higher in mown plots, while pH was higher in burned plots. Given the slightly lower elevation of the mown plots, we interpreted these differences as site effects. Soil properties were not affected by DRI and did not affect biomass responses. Conclusions Although very frequent disturbances reduced shrub growth responses, the magnitude of plant responses was modest and the effects temporary. Because resprouting shrubs in Florida scrub appear resilient to a range of disturbance return intervals, frequent fire or mowing can be used effectively in restorations.
... Because the resprout category also includes surviving trees, many resprouts were large in diameter. Resprouting from roots or stems is a survival tactic that allows a tree to persist after disturbance (Clarke et al., 2013;Schafer and Just, 2014). Resprouting trees can survive even severe damage (Schafer and Just, 2014). ...
... Resprouting from roots or stems is a survival tactic that allows a tree to persist after disturbance (Clarke et al., 2013;Schafer and Just, 2014). Resprouting trees can survive even severe damage (Schafer and Just, 2014). In the present study, some trees were cut down to stumps during the flood debris removal process and were still able to resprout. ...
... Deforestation and changes in land use intensity are common activities that could negatively impact native forest biota [3][4][5][6]. New seed input [7], buried seeds in the soil [8,9], resprouting [10], and seedling survival rates determine the development of subsequent vegetation after disturbances [11]. Thus, in pristine natural forests, continuous seed input and storage could ensure plant community regeneration following a disturbance [12]. ...
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Human activity negatively affects the sustainability of forest ecosystems globally. Disturbed forests may or may not recover by themselves in a certain period of time. However, it is still unclear as to what parameters can be used to reasonably predict the potential for self-recovery of human-disturbed forests. Here, we combined seed rain, soil seed bank, and seed emergence experiments to evaluate the potential for self-recovery of a highly disturbed, tropical, mixed deciduous forest in northeastern Thailand. Our results show a limited potential for self-recovery of this forest due to low seedling input and storage and an extremely high mortality rate during the drought period. There were 15 tree species of seedlings present during the regeneration period in comparison with a total number of 56 tree species in current standing vegetation. During the dry season, only four tree seedling species survived, and the highest mortality rate reached 83.87%. We also found that the correspondence between the combined number of species and composition of plant communities obtained from seed rain, soil seed bank, and seedling emergence experiments and the standing vegetation was poor. We clearly show the temporal dynamics of the seed rain and seedling communities, which are driven by different plant reproductive phenology and dispersal mechanisms, and drought coupled with mortality. We conclude that this highly disturbed forest needs a management plan and could not recover by itself in a short period of time. We recommend the use of external seed and seedling supplies and the maintenance of soil water content (i.e., shading) during periods of drought in order to help increase seedling abundances and species richness, and to reduce the mortality rate.
... Key factors at larger scales are historical disturbance regimes, disturbance type and severity, and environmental characteristics, among others (Kruger et al., 1997;Bellingham and Sparrow, 2000;Del Tredici, 2001;Bond and Midgley, 2001;Bond and Midgley, 2003;Pausas and Keeley, 2014). At a smaller scale, growth form Vesk, 2006;Zizka et al., 2014) and individual plant size (Burrows, 1985;Hodgkinson, 1998;Keeley, 2006;Schafer and Just, 2014) are also key to explain plant response to disturbance, mainly because there is a trade-off between reserve storage and the production of new tissues (Bond and Midgley, 2001;Schwilk and Ackerly, 2005;Vesk, 2006). For example, it's been reported that shrubs in semiarid environments tend to produce a greater number of thinner resprouts than trees, and a greater total volume of resprouts per unit area, so their canopies can rapidly achieve the reproductive size; whereas trees allocate reserves to a smaller number of thicker resprouts to grow tall and escape the flame zone (Hoffmann and Solbrig, 2003;Zizka et al., 2014). ...
... Further measurements will help confirm the performance of the proposed RCIs for said species. Our results indicate that despite the variability in the number and diameter of resprouts and trunks between and within species, the allometric relationship between their respective estimated composite diameters (ECRD and ECTD) is closely linked to the relationship between the diameter of the main resprout and the main trunk, as has been shown for a limited number of tree species by Schafer and Just (2014). ...
Article
Qualitative measures of resprouting capacity often fail to capture inter-and intra-species variation, whereas available quantitative methods can be complex and time-consuming, hindering broad-scale comparative studies. Here, we propose two quantitative indices that can be applied in a standard way in different regions. We sampled 1046 plants of 20 dominant species (6 shrubs, 7 trees and 7 tree/shrubs) from the seasonally dry forests of the arid Chaco, central-western Argentina. Sampling was conducted in burned field sites one growing season after fire. For each sampled plant we measured the diameter of the main burned trunk (MTD) and main resprout (MRD), and the number of burned trunks (TN) and resprouts (RN); we then built estimated composite diameters for trunks and resprouts (ECTD and ECRD) and we calculated two alternative Resprouting Capacity Indices: RCI 1 (ECRD/ECTD); and RCI 2 (MRD/MTD). The indices were validated against a measure of Resprouting Vigour (RV) that included detailed measurement of all trunks and resprouts for a subset of sampled plants. In all cases, variables indicative of fire severity were measured and included in the analyses. The RCIs and RV were highly related, both at the species and growth form levels. Fire severity had no significant effect on these relationships, but growth form affected RCI 2. All species were capable of resprouting, showing considerable inter-species variation for the two proposed RCIs. Species rank differed considerably between RCIs and survival-only estimations. RCI 1 was higher in tree/shrubs (i.e. species regarded as trees or shrubs) and trees than in shrubs. All species showed decreasing resprouting capacity with increasing ECTD. Our results support the use of the proposed RCIs as a robust tool to assess resprouting capacity, providing more details than survival-based assessments. Choosing one or the other implies a trade-off between accuracy and simplicity, and may depend on the scale and objective of the study, and resprouting patterns of studied species. Species, growth form and individual plant size are relevant in explaining post-fire resprouting capacity and survival.
... In this approach, the occurrence of fire is therefore uninfluenced by time since last fire, which is possible only if flammability is regained quickly after fire, owing to the rapid accumulation of fine fuels. This is commonly true in longleaf pine savannas because they regain flammability quickly after fire and can burn annually (Stambaugh et al. 2011, Schafer andJust 2014). However, a constant probability of burning is nevertheless an oversimplification because of changing fuel loads with increasing time since fire. ...
Article
Fire controls tree cover in many savannas by suppressing saplings through repeated topkill and resprouting, causing a demographic bottleneck. Tree cover can increase dramatically if even a small fraction of saplings escape this fire trap, so modelling and management of savanna vegetation should account for occasional individuals that escape the fire trap because they are “better” (i.e. they grow faster than average) or because they are “lucky” (they experience an occasional longer‐than‐average interval without fire or a below‐average fire severity). We quantified variation in growth rates and topkill probability in Quercus laevis (turkey oak) in longleaf pine savanna to estimate the percentage of stems expected to escape the fire trap due to variability in 1) growth rate, 2) fire severity, and 3) fire interval. For trees growing at the mean rate and exposed to the mean fire severity and the mean fire interval, no saplings are expected to become adults under typical fire frequencies. Introducing variability in any of these factors, however, allows some individuals to escape the fire trap. A variable fire interval had the greatest influence, allowing 8% of stems to become adults within a century. In contrast, introducing variation in fire severity and growth rate should allow 2.8% and 0.3% of stems to become adults, respectively. Thus, most trees that escape the fire trap do so because of luck. By chance, they experience long fire‐free intervals and/or a low‐severity fire when they are not yet large enough to resist an average fire. Fewer stems escape the fire trap by being unusually fast‐growing individuals. It is important to quantify these sources of variation and their consequences to improve understanding, prediction, and management of vegetation dynamics of fire‐maintained savannas. Here we also present a new approach to quantifying variation in fire severity utilizing a latent‐variable model of logistic regression.
... Survival of these trees was generally consistent with those from an earlier study where 58 permanently marked sub-adult eucalypts were followed for six years . Where a resprouting sub-adult tree initially produced multiple stems after the complete loss of above ground biomass, the maximum height of those stems was sufficient to accurately assess recovery and persistence of the individual (Werner and Franklin 2010, P. A. Werner, unpublished data), as was the case in a study of six woody species in a pine savanna in North Carolina (Schafer and Just 2014). ...
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Fire is a major disturbance driving the dynamics of the world's savannas. Almost all fires are set by humans who are increasingly altering fire timing and frequency on every continent. The world's largest protected areas of savannas are found in monsoonal northern Australia. These include relatively intact, tall, open forests where traditional indigenous fire regimes have been largely replaced in the past half century by contemporary patterns with trees experiencing fire as often as three out of five years. Eucalypt canopy trees form the basic structure of these savannas and changes to the canopy due to fire regimes cascade to affect other plants and animals. In this study, we used data from nearly three decades of field studies on the effects of fire on individual trees to define eight life‐history stages and to calculate transition rates among stages. We developed a stage‐based matrix population model that explicitly considers how fire season and understory influence growth, survival, and recruitment for each life‐history stage. Long‐term population growth rates and transient population dynamics were calculated under five different fire regimes, each in two understory types, using both deterministic and stochastic simulations of seasonal timing of fires. We found that fire was necessary for long‐term persistence of eucalypt canopy tree populations but, under annual fires, most populations did not survive. Population persistence was highly dependent on fire regime (fire season and frequency) and understory type. A stochastic model tended to yield higher population growth rates than the deterministic model with regular, periodic fires, even under the same long‐term frequency of fires. Transient population dynamics over 100 yr also depended on fire regime and understory, with implications for savanna physiognomy and management. Model predictions were tested in an independent data set from a 21‐yr longitudinal field study in Kakadu National Park. This study is a novel and integrative contribution to our understanding of fire in savanna biomes regarding the potential for long‐term persistence and transient dynamics of savanna canopy tree populations. The model is relatively simple, generalizable, and adaptable for further investigations of the population dynamics of savanna trees under fire.
... Escape from the fire trap is influenced by factors such as fire behaviour (Werner & Prior, 2013), edaphic conditions (Bond & Midgley, 2001;Grady & Hoffmann, 2012;Schafer & Just, 2014) and tree species traits (Bond, Cook, & Williams, 2012;de Dantas & Pausas, 2013). Recently, several authors have suggested that large browsing herbivores also play an important role in delaying or preventing tree escape from the fire trap by consuming plant tissues that would otherwise attain a fire-resistant height (Sankaran, Augustine, & Ratnam, 2013;Staver & Bond, 2014). ...
... We found that elephant presence diminished the positive effect of pre-fire tree height on post-fire resprout height (sensu Grady & Hoffmann, 2012;Schafer & Just, 2014), and the suppressive effects of elephant presence strengthened as pre-fire tree size increased. ...
... Resprout size often scales with pre-disturbance tree size (Grady & Hoffmann, 2012;Schafer & Just, 2014;Young & Francombe, 1991) due to factors such as root carbohydrate reserves (Schutz, Bond, & Cramer, 2009) or root depth and surface area (Nolan, Mitchell, Bradstock, & Lane, 2014). Accordingly, we found that total stem length of resprouts, a proxy for above-ground biomass, was strongly associated with pre-fire height. ...
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1.Savanna tree cover is dynamic due to disturbances such as fire and herbivory. Frequent fires can limit a key demographic transition from sapling to adult height classes in savanna trees. Saplings may be caught in a ‘fire trap’, wherein individuals repeatedly resprout following fire top‐kill events. Saplings only rarely escape the cycle by attaining a fire‐resistant height (e.g. taller than the minimum scorch height) during fire‐free intervals. 2.Large mammalian herbivores also may trap trees in shorter size classes. Browsing herbivores directly limit sapling height, while grazing herbivores such as cattle facilitate sapling growth indirectly via grass removal. Experimental studies investigating how meso‐wildlife, megaherbivores, and domestic livestock affect height of resprouts following fire are rare, but necessary for fully understanding how herbivory may reinforce (or counteract) the fire trap. In our study system, interactive fire‐herbivore effects on transitions from sapling (<1 m) to adult tree (>1 m) height classes may be further influenced by plant defenses, such as symbiotic ants. 3.We used the Kenya Long‐term Exclosure Experiment (KLEE) to investigate how post‐fire resprout size of a widespread monodominant East African tree, Acacia drepanolobium was influenced by 1) herbivory by different combinations of cattle, meso‐wildlife (15–1000 kg), and megaherbivores (>1000 kg) and 2) the presence of acacia‐ant mutualists that confer tree defenses. We sampled height, stem length, and ant occupant of resprouts exposed to different herbivore combinations before and after controlled burns. 4.Resprout height of saplings that were short prior to fire (<1 m) was reduced primarily by meso‐wildlife. Negative effects of elephants on post‐fire resprout height increased with pre‐fire tree size, suggesting that resprouts of the tallest trees (with the greatest potential to escape the fire trap cycle) were preferentially browsed and reduced in height by elephants. There were no significant cattle effects. 5.Synthesis.We provide experimental evidence for two potential pathways through which large herbivores exert control over sapling escape from the fire trap: 1) post‐fire meso‐wildlife browsing of short (<1 m) resprouts and 2) elephant browsing of the largest size class of resprouts, which would otherwise be most likely to escape the fire trap. This article is protected by copyright. All rights reserved.