Fire Trap Research Articles

Fire traps in the wet subtropics: New perspectives from Hong Kong

Abstract Fires undermine efforts to restore degraded forests in the wet tropics and subtropics. Grasslands and shrublands established after fires are more fire susceptible than forests and tend to be set alight more often, creating a positive feedback loop that curbs succession (i.e. create a fire trap). Understanding the factors that underpin the strength of these fire traps could transform restoration programmes by identifying the steps needed to escape them. Fire traps are notoriously challenging to quantify because multiple factors influence fire occurrence and vegetation recovery. Here we used multi‐decadal satellite imagery from Landsat to create a 34‐year time series of burn areas and vegetation dynamics in wet subtropical Hong Kong. These dynamic maps were then used to characterise (1) the influence of successional stage on fire occurrence, having accounted for topographical and ignition source imbalances using neighbourhood analyses and entropy balancing (EBAL) weights, and (2) recovery time to the next successional stage by survival analysis with EBAL weights. Our analyses revealed that fire regimes in the wet subtropics are defined by strong fire‐vegetation feedbacks. Grasslands and shrublands were 20 and 9 times more susceptible to fires than forests in similar topographic positions. Human activities compounded these differences by disproportionally introducing more ignition sources to grasslands (2.3 times) and shrublands (2.0 times) than to forests. Burnt shrublands recovered to forests faster (19 years) than grasslands (40 years). Proximity to forest patches had strong positive effects on recovery rates, highlighting the importance of seed sources. Post‐fire recovery was faster on wetter northwest‐facing sites and valleys. Overall, topography strongly influenced recovery processes but hardly affected fire occurrence. Synthesis and applications. Our study provides the first quantification of fire trap processes in the wet subtropics, which provides new opportunities for evidence‐based fire suppression and post‐fire restoration. Our results suggest that (1) fire traps could be mitigated by fire suppression programmes as the traps are currently exacerbated by ignition source imbalance; (2) establishing green fire breaks represents an effective fire suppression measure in the wet subtropics; and (3) active restoration could target areas where models predict sluggish post‐fire natural regeneration.

Escaping the fire trap: Does frequent, landscape-scale burning inhibit tree recruitment in a temperate broadleaf ecosystem?

Frequent prescribed burning is commonly used to restore and maintain open forest ecosystems in temperate broadleaf regions of the eastern United States. Repeated surface fires reduce the abundance of small woody stems through top-kill or mortality, with implications for age structure and ecosystem development. In this study, we quantified tree recruitment patterns over two decades across two landscape-scale studies in Missouri: the Missouri Ozark Forest Ecosystem Project (a control, with no burning) and the Chilton Creek Management Area (burned on 2–3 year fire return interval). Through time, frequent burning reduced the rate of ingrowth (i.e., new trees entering the overstory size class, defined as ≥11.4 cm diameter at breast height (DBH)), and the effect of burning on the ingrowth rate was consistent across site types (summits, exposed backslopes, protected backslopes, or upland waterways). Two decades of frequent burning reduced the abundance of scarlet oak (Quercus coccinea) in the ingrowth population. In the unburned control, the proportional abundance of mesic species such as red maple (Acer rubrum) and blackgum (Nyssa sylvatica) increased during the study period; frequent fire controlled the population of red maple but resulted in a similar increase in blackgum abundance in the ingrowth population of the burned area. Our results indicate that frequent burning affects the tree recruitment process in multiple ways: 1) reducing the source population for recruitment by reducing midstory abundance; 2) increasing probability of mortality or top-kill during the recruitment period. Fire-free periods may be necessary to allow eventual replacement of canopy trees in temperate broadleaf open forest ecosystems.

Modeling the biomass allocation of tree resprout in a fire-prone savanna

Young savanna trees can quickly grow back from belowground storage structures after topkill. This capacity is a tolerance trait that confers persistence at the plant individual level, enabling them to survive diverse disturbance regimes.We simulated the growth of a single resprouting stem (without and with fire) of a deciduous tree species that allocates its photo-assimilates during the vegetation season to reserves, belowground and aboveground parts (leaves and stem). As savannas grow under highly seasonal climates, the model considers that trees are leafless during the dry season and following growth is only possible thanks to reserves. Stem architecture constrains the leaf biomass to be proportional to stem length rather than biomass. We compared the success of different allocation strategies, with and without fire and according to the seasonality. To do so, the height of the resprouting stem after 50 yrs and the time to reach 2 m were modeled for three species of a humid savanna.The viable and faster growth strategies are those for which allocation to belowground parts is <40%. There is very little sensitivity to allocation to reserves since successful growth is observed for allocation to reserves between 0.5% and 85% of photo-assimilates. In the literature and in our results, there is little impact of fire on the stem height or the time needed to escape the fire trap. Our model suggests that (1) allocation to leaves is determinant as leaves are the primary source of assimilates that can then be turned into fire-resistant structures (reserves and roots) and (2) fire only weakly slows down the plant growth compared to dry season..

Interspecific variation in post‐disturbance growth responses of a savanna tree community and its implications for escaping the fire trap

AbstractVegetation 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.

Tree height is more important than bark thickness, leaf habit or habitat preference to survive fire in the cerrado of south-east Brazil

Although cerrado trees have evolved with fire for millions of years, it is not well-understood which tree attributes are more important to survive fire in the Brazilian savanna. To address this issue, we used pre- and post-fire data on 367 cerrado trees (113 native species) planted in an arboretum in south-east Brazil and then left unburnt until 2019, when a prescribed burn was applied. Tree size (height and diameter) had been measured in 2017. Four months after the fire, we assessed tree size, relative bark thickness (bark-to-diameter ratio), leaf habit (evergreen or deciduous) and habitat preference (savanna specialist or generalist). These were the predictor variables used in generalised linear models exploring tree survival and resprouting type. Most trees survived fire: 59% resprouted epicormically, 25% resprouted basally, 6% had root suckers, and only four trees died. Basal and epicormic resprouting were related to tree size: small trees (diameter ≤ 5 cm) resprouted basally more frequently, whereas tall trees (height ≥ 3.7 m) resprouted epicormically more frequently. Our results suggest that rapid growth is more important than bark thickness, leaf habit or habitat preference to escape the fire trap, because it allows cerrado trees to reach a fire-resistant height more quickly.

Does rapid utilization of elevated nutrient availability allow eucalypts to dominate in the tropical savannas of Australia?

Northern Australia's savannas are among the most fire‐prone biomes on Earth and are dominated by eucalypts (Eucalyptus and Corymbia spp.). It is not clear what processes allow this group to dominate under such extreme fire frequencies and whether a superior ability to compete for nutrients and water might play a role. There is evidence that eucalypts are adapted to frequent fires; juvenile eucalypts escape the fire trap by growing rapidly in height between fires. However, non‐eucalypts are less able to escape the fire trap and tend to have stand structures strongly skewed toward suppressed juveniles. The mechanisms that drive these contrasting fire responses are not well understood. Here, we describe the results of a controlled glasshouse seedling experiment that evaluated the relative importance of nutrient and water availability in determining height growth and biomass growth of two eucalypt and one noneucalypt tree species, common in northern Australian savannas. We demonstrate that growth of eucalypt seedlings is particularly responsive to nutrient addition. Eucalypt seedlings are able to rapidly utilize soil nutrients and accumulate biomass at a much greater rate than noneucalypt seedlings. We suggest that a seasonal spike in nutrient availability creates a nutrient‐rich microsite that allows eucalypt seedlings to rapidly gain height and biomass, increasing their likelihood of establishing successfully and reaching a fire‐resistant size. Our results extend our understanding of how eucalypts dominate northern Australian savannas under extremely high fire frequencies.

Better lucky than good: How savanna trees escape the fire trap in a variable world.

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 modeling 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.

Tree resprout dynamics following fire depend on herbivory by wild ungulate herbivores

Abstract 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. 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 (&lt;1 m) to adult tree (&gt;1 m) height classes may be further influenced by plant defences, such as symbiotic ants. 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 (a) herbivory by different combinations of cattle, meso‐wildlife (15–1,000 kg) and megaherbivores (&gt;1,000 kg) and (b) the presence of acacia–ant mutualists that confer tree defences. We sampled height, stem length and ant occupancy of resprouts exposed to different herbivore combinations before and after controlled burns. Resprout height of saplings that were short prior to fire (&lt;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. Synthesis. We provide experimental evidence for two potential pathways through which large herbivores exert control over sapling escape from the fire trap: (a) post‐fire meso‐wildlife browsing of short (&lt;1 m) resprouts and (b) elephant browsing of the largest size class of resprouts, which would otherwise be most likely to escape the fire trap.

Transplant Experiments Point to Fire Regime as Limiting Savanna Tree Distribution

Plant species range shifts are predicted to occur in response to climate change. The predictions are often based on the assumption that climate is the primary factor limiting the distribution of species. However the distribution of grassy biomes in Africa cannot be predicted by climate alone, instead interactions between vegetation, climate and disturbance structure the ecosystems. To test if climatic variables, as predicted by an environmental niche model, determine the distribution limits of two common savanna tree species we established a transplant experiment at a range of latitudes and altitudes much broader than the distribution limits of our study species. We planted seedlings of two common savanna trees, Senegalia nigrescens and Colophospermum mopane, at eight paired high and low elevation sites across an 850 km latitudinal gradient in South African savannas. At each site seedlings were planted in both grassy and cleared plots. After two years of growth, rainfall, temperature and location inside or outside their distribution range did not explain species success. Grass competition was the only variable that significantly affected plant growth rates across all sites, but grass competition alone could not explain the distribution limit. Species distributions were best predicted when maximum tree growth rates were considered in relation to local fire return intervals. The probability of sapling escape from the fire trap was the most likely determinant of distribution limits of these two species. As trees grew and survived 100s of kilometers south of their current range limits we conclude that climate alone does not explain the current distribution of these trees, and that climate change adaptation strategies for savanna environments based only on climatic envelope modelling will be inappropriate.

Defining the fire trap: Extension of the persistence equilibrium model in mesic savannas

AbstractMesic savannas are dominated by trees that are strong resprouters caught in a frequent fire trap. Persistence within this fire trap has been described by a resprout curve of SizeNext ~ f(Pre‐fire size), defined by the Michaelis‐Menten function. A key feature of this resprout curve is a stable persistence equilibrium that represents the size of individual plants upon which a population will converge over successive inter‐fire time steps under a given fire regime. Here, we contend that such a resprout curve does not adequately describe resprout tree dynamics in frequently burnt mesic savannas because it is constrained to an asymptote. We propose a new framework for modelling the resprout curve, which recognizes that local environmental stochasticity and growth patterns can interact to change the growth response function entirely, and thus more readily reflect the range of feasible resprout responses. Importantly, we define an unstable equilibrium representing the size above which individuals have escaped the fire trap and explore mechanisms that can shift an individual from persistence to escape. Through a case study from northern Australia, we confirm that our framework provides a simple yet practical approach to defining these critical aspects of savanna tree growth dynamics: persistence and escape.

Post-fire resprouting strategies of rainforest and savanna saplings along the rainforest–savanna boundary in the Australian monsoon tropics

In tropical areas where climatic conditions support both rainforests and savannas, fire is considered one of the main factors determining their distribution, particularly in environments where growth rates are limited by water availability. The observed expansion of some rainforests into savannas suggests that rainforest saplings could have traits that enable them to survive in the savanna environment, including recovering from infrequent fires. We applied the Clarke (New Phytol 197:19–35, 2013) buds-protection-resources framework to the rainforest–savanna system of the North Kimberley (Western Australia), to compare the resprouting response of five savanna species saplings burnt by an ambient early dry season fire with seven rainforest species saplings burnt using an experimental treatment that mimicked a savanna fire. Most plants survived the fire, although plant mortality was higher for rainforest (19 %) than savanna (2 %) individuals, as was stem mortality (37 vs. 12 %). All rainforest and savanna species expressed aerial resprouting; two of the savanna species and two of the rainforest species did not express basal resprouting. After 1 year, most savanna individuals had more and longer shoots than the rainforest saplings and had regained their original height, while rainforest plants were on average 43 % shorter than their pre-fire height. These results suggest that, although rainforest species are less able to escape the ‘fire trap’ than savanna species, they are able to recover from a low-intensity fire.

Caught in a human disturbance trap: Responses of tropical savanna trees to increasing land-use pressure

Increasing land-use pressure threatens the persistence of tree populations in West Africa’s savannas. We do not fully understand yet why tree species respond differently to human disturbances, hampering the design of appropriate management strategies. To identify typical response pathways, we compared tree populations in three land-use types with increasing levels of human disturbance (protected forest, fallow and field). We analyzed size-class distributions (SCDs) of species and plant functional types, and compared the performance of juvenile and adult age-classes. Biomass was derived from biometric measurements via allometric equations. Higher land-use pressure increased juvenile proportions of plant functional types, but divergent responses were found for species: Juvenile proportions on fields were either very low (0%) or, in most cases, very high (>96%), leading to SCDs with significant negative slopes. While negative slopes are commonly interpreted as indicating populations with sufficient recruitment, they could also indicate growth suppression, particularly if size-classes are missing between juveniles and adults. This ‘juveniles get trapped’ pathway is well-recognized for near-natural savannas with a high incidence of fire (‘fire trap’) or wild browsers (‘browse trap’), and is attributed to a high resprouting ability of trees. As resprouting constitutes a pre-adaptation to human disturbances, the trap concept should be extended accordingly. Species dominating in highly disturbed environments are either characterized by trait combinations allowing them to persist in a ‘human disturbance trap’, or they are actively protected. Our results advocate for designing management strategies that take into account how species’ age-classes respond to disturbances. As the escape of juveniles to mature vegetation is a main demographic bottleneck for most Sudanian savanna species, it is essential to create escape opportunities.

Does fire limit tree biomass in Australian savannas?

Processes allowing coexistence of trees and grasses in tropical savannas have long intrigued ecologists. Early theories focused on climatic controls, but a conceptual model has emerged suggesting that savanna trees are subject to a fire-mediated recruitment bottleneck, with frequent fires preventing recruitment of saplings into the tree layer and maintaining biomass well below its climate-determined upper bound. We propose that this conceptual model has been overemphasised in northern Australia, where tree abundance is more strongly controlled by water availability. The dominant trees, eucalypts, have a remarkable capacity to grow through the ‘fire trap’ to reach fire-resistant sizes. This fire tolerance makes eucalypts relatively unresponsive to management-imposed reductions in fire frequency and intensity. Other trees in these savannas are typically more fire sensitive and respond positively to such management. There are suggestions that savanna fire management could lead to increases in woody biomass, but we contend that if tree biomass is strongly limited by water availability, then potential increases in tree biomass are relatively limited, at least in relation to the dominant eucalypt component. There is potential to increase the biomass of the more fire-sensitive non-eucalypts, but the upper bound of non-eucalypt tree biomass in these eucalypt-dominated systems remains poorly understood.

Understanding fire patterns and fire drivers for setting a sustainable management policy of the New-Caledonian biodiversity hotspot

New Caledonia (NC) is a biodiversity hotspot sheltering terrestrial ecosystems of high ecological and conservation value including tropical dry forests, rainforests, and maquis. However, uncontrolled bushfires threaten this exceptional biodiversity. A science-based fire management policy could reduce the impact of unwanted fires and help facing climate change. However, to date, data on the location, extent, causal factors and spatial patterns of fires had not been collected. We compiled a 13-year-long (1999–2011) spatially-explicit fire database for NC using MODIS and Landsat data. Using boosted regression trees we disentangled the role of anthropogenic factors, physiography, weather and vegetation on fire activity. We also characterized the location of fires and the vegetation composition at the fire edges, in order to determine which ecosystems were especially vulnerable. Fire size distribution was typically asymmetric with many small fires (<10ha) and very few large fires (>500ha). Ignitions were preferentially located close to villages, cities or roads, at low elevation and linked to high values of fire weather index. Fires were larger at the end of the dry season and during El Niño events. Most fires were bushfires burning in savannas, thickets and maquis, while rainforests were rather ‘avoided’ by fire. However, bushfires generally propagated towards forests of high-conservation value, thus increasing the potential for forest edge erosion. As savanna-forest and maquis-forest mosaics are dominant in the landscape, we discuss the extent to which NC could become a ‘fire trap’ where fire cannot be easily extirpated. Based on our spatially-explicit information on fire activity, we make recommendations for a sustainable forest and fire management policy which would balance the traditional use of fire and the conservation of the most valuable ecosystems. In particular, it may help by reducing the damages of large and destructive bushfires ignited during drought peaks.

Forest Development 44 Years after Fire Exclusion in Formerly Annually Burned Oldfield Pine Woodland, Florida

Young mixed hardwood forest communities that colonized abandoned croplands during the late 19th century in northern Florida were typically winter burned annually after pine saplings could survive fire. Hardwood trees persisted as coppice that grew from root crowns within grassy undergrowth. This plant community changed little thereafter, except for the continued growth of pine trees. An 8.64-ha tract of this community was inventoried in 1966, from which fire was permanently excluded thereafter. This tract, called NB66, was reinventoried in 2010 to document maturation of the plant community and to identify the contributing causes that controlled ecological development. Hardwood coppice that was released from fire grew to form a nearly continuous canopy averaging 19.7 m high after 44 years. Many older pines died and disintegrated without causing canopy gaps. Prior to 19th century plantation agriculture, the original vegetation consisted of shortleaf pine-oak-hickory (SPOH) woodland, which intergraded with longleaf pine savanna on sandier soils. These species occur at NB66 but the forest remains dominated by native offsite species, which are aggressively reproducing. These offsite species formerly occurred in less elevated landscape positions in association with magnolia-beech and bottomland hardwood forests. Frequently burned old-field pineland adjacent to NB66 has remained stable indicating that no factors other than fire exclusion caused release of hardwoods from their “fire trap.” There was no trend toward recovery of the original SPOH community, and forest development is postulated on the basis of mesophication processes. Recruitment of new species—both woody and herbaceous—since 1966 was modest; most remain uncommon.

Native ungulates of diverse body sizes collectively regulate long‐term woody plant demography and structure of a semi‐arid savanna

Summary Large mammalian herbivores are well recognized to play important roles in regulating woody cover and biomass in savannas, but the extent to which browsing ungulates are capable of regulating woody populations in the absence of other disturbances such as fire is unclear. Moreover, the degree to which browser effects on savannas operate through effects on woody plant recruitment vs. mortality has rarely been examined. We conducted a 10‐year, replicated herbivore exclusion experiment in a semi‐arid savanna in East Africa (mean annual rainfall = 514 mm), where fires have been actively suppressed for decades. Browsers dramatically influenced recruitment, growth and mortality of all size classes of woody vegetation. A decade of herbivore exclusion resulted in a sevenfold increase in recruitment, a 2.5‐fold decrease in mortality and a threefold increase in woody biomass inside exclosures, while biomass outside exclosures remained relatively unchanged. At the plant community level, extensive browsing of seedlings by small‐bodied ungulates suppressed woody recruitment in this semi‐arid system, generating a ‘browsing trap’ comparable to the ‘fire trap’ reported for mesic systems. Browsing by large‐ and medium‐bodied ungulates reduced both growth and survival of individuals in larger size classes. At the plant species level, browser impacts were variable. Although browsers negatively influenced recruitment of all species, they had little to no impact on the mortality of some dominant species, resulting in a long‐term, browser‐driven shift in woody species composition that was largely mediated via their differential effects on plant mortality rates rather than recruitment. Synthesis. Our results demonstrate unequivocally that, even in the absence of fire, native browsing ungulates can exert dramatic ‘top‐down’ controls in semi‐arid savannas, influencing all aspects of woody plant demography. Besides suppressing woody plant recruitment, browsers can also have substantial cumulative long‐term impacts on growth and mortality rates of woody plants, including adults, which can differ between species in ways that fundamentally alter the structure and function of woody vegetation. In semi‐arid rangelands, intact communities of native browsing ungulates thus provide a critical ecosystem service by regulating woody cover, and their removal (or extinction) from these systems can lead to rapid woody encroachment.

The Reforestation of Africa?

1poses a challenging question for African ecologists and environmentalists: do we, in the subcontinent, face not a contraction, but a vast and inevitable expansion of subtropical tree cover, driven by levels of CO 2 that have not been seen in the past several million years? Higgins and Scheiter’s paper warrants our attention because it projects, for the first time, the continent-wide implications of the decade-old hypothesis, originally formulated by South African ecologists, of CO 2 -driven woody expansion in fire-prone savannas. 2 The paper supports concerns that the expansion of woodlands and forest may be an imminent threat to ecosystem structure, function and biodiversity across extensive landscapes in the sub-continent. 3 If its projections are correct, then we stand on the brink of massive ecosystem change in the ‘savanna–complex’ vegetation (i.e. tropical grasslands, savanna and forests) of Africa. But how credible are these projections? Unlike more intensively researched temperate ecosystems, the vegetation structure and land cover of huge tracts of sub-Saharan Africa may be highly sensitive to increasing levels of atmospheric CO 2 . Vast areas of the subcontinent are currently dominated by C 4 grasses – a photosynthetic mode that owes much of its competitive advantage to the low CO 2 levels of pre-industrial and, even more so, glacial times. 4 Grasses do not require the large amounts of carbon that woody plants do to support their photosynthetic tissue. This low carbon demand for growth allows grasses to outcompete woody plants under low CO 2 conditions by building up a flammable layer of grass fuel – the savanna fire trap – that immolates slower growing woody plants and maintains the system in its grassy state. Under high levels of CO 2 , trees are thought to regain the advantage, escaping the fire trap and converting the system into forest. 2 This mutable balance of trees versus grasses, mediated by atmospheric CO 2 levels and fire, results in ‘bi-stable’ systems 5 in which

Caught in a fire trap: Recurring fire creates stable size equilibria in woody resprouters

Globally, fire maintains many mesic habitats in an open canopy state by killing woody plants while reducing the size of those able to resprout. Where fire is frequent, tree saplings are often suppressed by a "fire trap" of repeated topkill (death of aerial biomoass) and resprouting, preventing them from reaching adult size. The ability to tolerate repeated topkill is an essential life-history trait that allows a sapling to persist until it experiences a long fire-free interval, during which it can escape the fire trap. We hypothesized that persistence in the fire trap results from a curvilinear relationship between pre-burn size and resprout size, which causes a plant to approach an equilibrial size in which post-fire biomass recovery is equal to fire-induced biomass loss. We also predicted that the equilibrial stem size is positively related to resource availability. To test these hypotheses, we collected data on pre-burn and resprout size of five woody plant species at wetland ecotones in longleaf pine savanna subjected to frequent burning. As expected, all species exhibited similar curvilinear relationships between pre-burn size and resprout size. The calculated equilibrial sizes were strong predictors of mean plant size across species and growing conditions, supporting the persistence equilibrium model. An alternative approach using matrix models yielded similar results. Resprouting was less vigorous in dry sites than at wet sites, resulting in smaller equilibrial stem sizes in drier sites; extrapolating these results provides an explanation for the absence of these species in xeric uplands. This new framework offers a straightforward approach to guide data collection for experimental, comparative, and modeling studies of plant persistence and community dynamics in frequently burned habitats.

Growth of juvenile and sapling trees differs with both fire season and understorey type: Trade‐offs and transitions out of the fire trap in an Australian savanna

AbstractCanopy tree populations in mesic savannas are often bimodal with few saplings but many smaller individuals of indeterminate age that repeatedly suffer topkill and regenerate from underground tissues. Little is known about growth rates or mechanisms that allow subadult trees to reach the canopy. The wooded savannas of northern Australia have high frequencies of dry‐season fires. In a 32 400‐m2 field experiment, 2405 juveniles (&lt;150‐cm height) and saplings (150–499 cm) of the eucalypt canopy species were individually marked and measured the year prior to fires set in three different seasons and again at the end of the growing season (without fires) a year later. Trees in unburnt plots served as controls. All fire treatments were repeated in plots dominated by the most common understorey, a native annual grass (sorghum) and in plots dominated by perennial native species; these produce different fuels for fires and competitive regimes for young trees. After early dry‐season fires, height growth of larger juveniles and all saplings was significantly enhanced, especially in sorghum. After late dry‐ or wet‐season fires, juvenile trees grew well, but all of the small saplings (150‐ to 299‐cm height) were reduced to ‘juveniles’ and did not recover pre‐fire heights but, instead, produced many new basal (coppice) stems. Late, dry‐season fires reduced more than 80% of large saplings (300–499 cm) to juvenile size in sorghum, whereas in non‐sorghum, 60% of the trees grew to poles (500–999 cm). The results demonstrate that juvenile and sapling growth responses to fire and the probability of subadult trees reaching the canopy are related to fire–understorey interactions, and suggest that the mechanisms include morphological and carbohydrate storage dynamics which vary with tree size and life history stage. The key to successful management of a sustainable woody canopy lies in the understorey.

Fine-scale Patchiness of Burns in a Mesic Eucalypt Savanna Differs with Fire Season and Sorghum Abundance

Canopy tree populations in the eucalypt savannas of northern Australia consistently show bottlenecks in the transition of young trees to the canopy layer. This is most likely due to frequent fires, where young trees are caught in a 'fire trap', suffer topkill and regenerate from underground tissues year after year. Little is known of fire behaviour at the spatial scale of individual juvenile trees. This paper presents field measurements of small-scale, 10 cm (microsite), burn patterns and patch sizes following set fires in the early dry season, late dry season, and wet season. Fire treatments were repeated in plots dominated by a native annual grass (Sorghum) and in others with little or none of this grass. Both late dry season and wet season fires left few or no microsites unburnt, regardless of understorey type. With early dry season fire, however, understorey type made a difference: 47% of the microsites were left unburnt in plots with little or no Sorghum compared to only 5% in Sorghum-dominated plots. Further, the early dry season fire pattern was very patchy, resulting in both large and small areas of burnt vegetation. In Sorghum-dominated plots, more than half the patches were greater or equal 10 m in size and only a third were < 3 m in size, whereas in plots with little or no Sorghum, 99% of burnt patches were < 10 m in size, and 41% were < 0.5 m in size. Thus, only a regime of early dry season fire with little or no Sorghum in the understorey creates potential 'windows of escape' for juvenile trees. The study has implications for management of savanna trees by fire.