Impacts of climate variability on tree demography in second growth tropical forests: the importance of regional context for predicting successional trajectories

Naturally regenerating and restored second growth forests account for over 70% of tropical forest cover and provide key ecosystem services. Understanding climate change impacts on successional trajectories of these ecosystems is critical for developing effective large‐scale forest landscape restoration (FLR) programs. Differences in environmental conditions, species composition, dynamics, and landscape context from old growth forests may exacerbate climate impacts on second growth stands. We compile data from 112 studies on the effects of natural climate variability, including warming, droughts, fires, and cyclonic storms, on demography and dynamics of second growth forest trees and identify variation in forest responses across biomes, regions, and landscapes. Across studies, drought decreases tree growth, survival, and recruitment, particularly during early succession, but the effects of temperature remain unexplored. Shifts in the frequency and severity of disturbance alter successional trajectories and increase the extent of second growth forests. Vulnerability to climate extremes is generally inversely related to long‐term exposure, which varies with historical climate and biogeography. The majority of studies, however, have been conducted in the Neotropics hindering generalization. Effects of fire and cyclonic storms often lead to positive feedbacks, increasing vulnerability to climate extremes and subsequent disturbance. Fragmentation increases forests’ vulnerability to fires, wind, and drought, while land use and other human activities influence the frequency and intensity of fire, potentially retarding succession. Comparative studies of climate effects on tropical forest succession across biogeographic regions are required to forecast the response of tropical forest landscapes to future climates and to implement effective FLR policies and programs in these landscapes.     

What controls the distribution of tropical forest and savanna?

Forest and savanna biomes dominate the tropics, yet factors controlling their distribution remain poorly understood. Climate is clearly important, but extensive savannas in some high rainfall areas suggest a decoupling of climate and vegetation. In some situations edaphic factors are important, with forest often associated with high nutrient availability. Fire also plays a key role in limiting forest, with fire exclusion often causing a switch from savanna to forest. These observations can be captured by a broad conceptual model with two components: (1) forest and savanna are alternative stable states, maintained by tree cover-fire feedbacks, (2) the interaction between tree growth rates and fire frequency limits forest development; any factor that increases growth (e.g. elevated availability of water, nutrients, CO(2)), or decreases fire frequency, will favour canopy closure. This model is consistent with the range of environmental variables correlated with forest distribution, and with the current trend of forest expansion, likely driven by increasing CO(2) concentrations. Resolving the drivers of forest and savanna distribution has moved beyond simple correlative studies that are unlikely to establish ultimate causation. Experiments using Dynamic Global Vegetation Models, parameterised with measurements from each continent, provide an important tool for understanding the controls of these systems.     

Evidence of non‐stationary relationships between climate and forest responses: Increased sensitivity to climate change in Iberian forests

Climate and forest structure are considered major drivers of forest demography and productivity. However, recent evidence suggests that the relationships between climate and tree growth are generally non‐stationary (i.e. non‐time stable), and it remains uncertain whether the relationships between climate, forest structure, demography and productivity are stationary or are being altered by recent climatic and structural changes. Here we analysed three surveys from the Spanish Forest Inventory covering c. 30 years of information and we applied mixed and structural equation models to assess temporal trends in forest structure (stand density, basal area, tree size and tree size inequality), forest demography (ingrowth, growth and mortality) and above‐ground forest productivity. We also quantified whether the interactive effects of climate and forest structure on forest demography and above‐ground forest productivity were stationary over two consecutive time periods. Since the 1980s, density, basal area and tree size increased in Iberian forests, and tree size inequality decreased. In addition, we observed reductions in ingrowth and growth, and increases in mortality. Initial forest structure and water availability mainly modulated the temporal trends in forest structure and demography. The magnitude and direction of the interactive effects of climate and forest structure on forest demography changed over the two time periods analysed indicating non‐stationary relationships between climate, forest structure and demography. Above‐ground forest productivity increased due to a positive balance between ingrowth, growth and mortality. Despite increasing productivity over time, we observed an aggravation of the negative effects of climate change and increased competition on forest demography, reducing ingrowth and growth, and increasing mortality. Interestingly, our results suggest that the negative effects of climate change on forest demography could be ameliorated through forest management, which has profound implications for forest adaptation to climate change.     

Fire-Mediated Forest Encroachment in Response to Climatic and Land-Use Change in Subtropical Andean Treelines

We used dendroecological techniques to analyze the effects of rainfall and grazing on fire regime and its implications for tree regeneration in subtropical mountains of northwestern Argentina during the 20th century, a period characterized by increasing rainfall and decreasing land-use intensity. We dated fire scars and establishment of Alnus acuminata (the dominant tree species) in six watersheds along a 600 km latitudinal range. We correlated fire frequency with rainfall records and performed Superposed Epoch Analyses to assess the relationship between rainfall and fire events during the century, and in two sub-periods: 1930–1965 (low rainfall, high grazing) and 1966–2001 (high rainfall, low grazing). We performed permutation analyses to assess the association between fire events and tree establishment, and to describe the spatial distribution of fires and forests in relation to hillslope aspect. Rainfall was associated with regional fires at interannual and decadal scales: fire probability increased after growing seasons with above-average rainfall and through the century, in concurrence with rainfall increase. The climatic control of fire was stronger under lower land-use intensity. Tree establishment was temporally associated with fire events, which occurred mainly in north facing slopes, where grassland cover is more extensive and forest colonization more likely. These results suggest that fire is limited by the availability of fine fuels, which is enhanced by high rainfall and reduced grazing; and tree establishment is limited by the competition with grasses. Consequently, increasing rainfall and decreasing grazing favored higher fire frequency, thus promoting forest encroachment during 20th century.     

Alternative stable equilibria and critical thresholds created by fire regimes and plant responses in a fire‐prone community

Wildfire is a dominant disturbance in many ecosystems, and fire frequency and intensity are being altered as climates change. Through effects on mortality and regeneration, fire affects plant community composition, species richness, and carbon cycling. In some regions, changes to fire regimes could result in critical, non‐reversible transitions from forest to non‐forested states. For example, the Klamath ecoregion (northwest United States) supports extensive conifer forests that are initially replaced by hardwood chaparral following high‐severity fire, but eventually return to conifer forest during the fire‐free periods. Climate change alters both the fire regime and post‐fire recovery dynamics, potentially causing shrubland to persist as a stable (i.e. self‐renewing) vegetation stage, rather than an ephemeral stage. Here, we present a theoretical investigation of how changes in plant traits and fire regimes can alter the stability of communities in forest‐shrub systems such as the Klamath. Our model captures the key characteristics of the system, including life‐stage‐specific responses to disturbance and asymmetrical competitive interactions. We assess vegetation stability via invasion analysis, and conclude that portions of the landscape that are currently forested also can be stable as shrubland. We identify parameter thresholds where community equilibria change from stable to unstable, and show how these thresholds may shift in response to changes in life‐history or environmental parameters. For instance, conifer maturation rates are expected to decrease as aridity increases under climate change, and our model shows that this reduction decreases the fire frequencies at which forests become unstable. Increases in fire activity sufficient to destabilize forest communities are likely to occur in more arid future climates. If widespread, this would result in reduced carbon stocks and a positive feedback to climate change. Changes in stability may be altered by management practices.     

Scale matters: fire–vegetation feedbacks are needed to explain tropical tree cover at the local scale

At a broad (regional to global) spatial scale, tropical vegetation is controlled by climate; at the local scale, it is believed to be determined by interactions between disturbance, vegetation and local conditions (soil and topography) through feedback processes. It has recently been suggested that strong fire–vegetation feedback processes may not be needed to explain tree‐cover patterns in tropical ecosystems and that climate–fire determinism is an alternative possibility. This conclusion was based on the fact that it is possible to reproduce observed patterns in tropical regions (e.g. a trimodal frequency distribution of tree cover) using a simple model that does not explicitly incorporate fire–vegetation feedback processes. We argue that these two mechanisms (feedbacks versus fire–climate control) operate at different spatial and temporal scales; it is not possible to evaluate the role of a process acting at fine scales (e.g. fire–vegetation feedbacks) using a model designed to reproduce regional‐scale pattern (scale mismatch). While the distributions of forest and savannas are partially determined by climate, many studies are providing evidence that the most parsimonious explanation for their environmental overlaps is the existence of feedback processes. Climate is unlikely to be an alternative to feedback processes; rather, climate and fire–vegetation feedbacks are complementary processes at different spatial and temporal scales.     

Dispersal limitation drives successional pathways in Central Siberian forests under current and intensified fire regimes

Fire is a primary driver of boreal forest dynamics. Intensifying fire regimes due to climate change may cause a shift in boreal forest composition toward reduced dominance of conifers and greater abundance of deciduous hardwoods, with potential biogeochemical and biophysical feedbacks to regional and global climate. This shift has already been observed in some North American boreal forests and has been attributed to changes in site conditions. However, it is unknown if the mechanisms controlling fire‐induced changes in deciduous hardwood cover are similar among different boreal forests, which differ in the ecological traits of the dominant tree species. To better understand the consequences of intensifying fire regimes in boreal forests, we studied postfire regeneration in five burns in the Central Siberian dark taiga, a vast but poorly studied boreal region. We combined field measurements, dendrochronological analysis, and seed‐source maps derived from high‐resolution satellite images to quantify the importance of site conditions (e.g., organic layer depth) vs. seed availability in shaping postfire regeneration. We show that dispersal limitation of evergreen conifers was the main factor determining postfire regeneration composition and density. Site conditions had significant but weaker effects. We used information on postfire regeneration to develop a classification scheme for successional pathways, representing the dominance of deciduous hardwoods vs. evergreen conifers at different successional stages. We estimated the spatial distribution of different successional pathways under alternative fire regime scenarios. Under intensified fire regimes, dispersal limitation of evergreen conifers is predicted to become more severe, primarily due to reduced abundance of surviving seed sources within burned areas. Increased dispersal limitation of evergreen conifers, in turn, is predicted to increase the prevalence of successional pathways dominated by deciduous hardwoods. The likely fire‐induced shift toward greater deciduous hardwood cover may affect climate–vegetation feedbacks via surface albedo, Bowen ratio, and carbon cycling.     

Abrupt shifts in African savanna tree cover along a climatic gradient

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.     

Tree–Grass Coexistence in the Everglades Freshwater System

Mosaic freshwater landscapes exhibit tree-dominated patches —or tree islands—interspersed in a background of marshes and wet prairies. In the Florida Everglades, these patterned landscapes provide habitat for a variety of plant and animal species and are hotspots of biodiversity. Even though the emergence of patchy freshwater systems has been associated with climate histories, fluctuating hydrologic conditions, and internal feedbacks, a process-based quantitative understanding of the underlying dynamics is still missing. Here, we develop a mechanistic framework that relates the dynamics of vegetation, nutrients and soil accretion/loss through ecogeomorphic feedbacks and interactions with hydrologic drivers. We show that the stable coexistence of tree islands and marshes results as an effect of their both being (meta-) stable states of the system. However, tree islands are found to have only a limited resilience, in that changes in hydrologic conditions or vegetation cover may cause an abrupt shift to a stable marsh state. The inherent non-linear and discontinuous dynamics determining the stability and resilience of tree islands should be accounted for in efforts aiming at the management, conservation and restoration of these features.     

Vulnerability to forest loss through altered postfire recovery dynamics in a warming climate in the Klamath Mountains

In the context of ongoing climatic warming, certain landscapes could be near a tipping point where relatively small changes to their fire regimes or their postfire forest recovery dynamics could bring about extensive forest loss, with associated effects on biodiversity and carbon‐cycle feedbacks to climate change. Such concerns are particularly valid in the Klamath Region of northern California and southwestern Oregon, where severe fire initially converts montane conifer forests to systems dominated by broadleaf trees and shrubs. Conifers eventually overtop the competing vegetation, but until they do, these systems could be perpetuated by a cycle of reburning. To assess the vulnerability of conifer forests to increased fire activity and altered forest recovery dynamics in a warmer, drier climate, we characterized vegetation dynamics following severe fire in nine fire years over the last three decades across the climatic aridity gradient of montane conifer forests. Postfire conifer recruitment was limited to a narrow window, with 89% of recruitment in the first 4 years, and height growth tended to decrease as the lag between the fire year and the recruitment year increased. Growth reductions at longer lags were more pronounced at drier sites, where conifers comprised a smaller portion of live woody biomass. An interaction between seed‐source availability and climatic aridity drove substantial variation in the density of regenerating conifers. With increasing climatic water deficit, higher propagule pressure (i.e., smaller patch sizes for high‐severity fire) was needed to support a given conifer seedling density, which implies that projected future increases in aridity could limit postfire regeneration across a growing portion of the landscape. Under a more severe prospective warming scenario, by the end of the century more than half of the area currently capable of supporting montane conifer forest could become subject to minimal conifer regeneration in even moderate‐sized (10s of ha) high‐severity patches.     

Ectomycorrhizal fungal diversity decreases in Mediterranean pine forests adapted to recurrent fires

Fire is a major disturbance linked to the evolutionary history and climate of Mediterranean ecosystems, where the vegetation has evolved fire‐adaptive traits (e.g., serotiny in pines). In Mediterranean forests, mutualistic feedbacks between trees and ectomycorrhizal (ECM) fungi, essential for ecosystem dynamics, might be shaped by recurrent fires. We tested how the structure and function of ECM fungal communities of Pinus pinaster and Pinus halepensis vary among populations subjected to high and low fire recurrence in Mediterranean ecosystems, and analysed the relative contribution of environmental (climate, soil properties) and tree‐mediated (serotiny) factors. For both pines, local and regional ECM fungal diversity were lower in areas of high than low fire recurrence, although certain fungal species were favoured in the former. A general decline of ECM root‐tip enzymatic activity for P. pinaster was associated with high fire recurrence, but not for P. halepensis. Fire recurrence and fire‐related factors such as climate, soil properties or tree phenotype explained these results. In addition to the main influence of climate, the tree fire‐adaptive trait serotiny recovered a great portion of the variation in structure and function of ECM fungal communities associated with fire recurrence. Edaphic conditions (especially pH, tightly linked to bedrock type) were an important driver shaping ECM fungal communities, but mainly at the local scale and probably independently of the fire recurrence. Our results show that ECM fungal community shifts are associated with fire recurrence in fire‐prone dry Mediterranean forests, and reveal complex feedbacks among trees, mutualistic fungi and the surrounding environment in these ecosystems.     

Long-Term Tree Cover Dynamics in a Pinyon-Juniper Woodland: Climate-Change-Type Drought Resets Successional Clock

Woody vegetation has expanded in coverage over the past century in many places globally, exemplified by pinyon-juniper changes in the Southwestern United States. Extreme drought is one of the few non-management drivers besides fire that might reverse such cover changes, but this has not been well documented. Here, we assess 68 years of tree cover dynamics across an elevation gradient of a pinyon-juniper woodland using aerial photographs (1936 and 1959) and QuickBird imagery (2004). Canopy cover increased 32% from 1936 to the onset of a major drought (2002). The largest relative increase in canopy cover occurred from 1936 to 1959 at the higher elevations, but these gains were eliminated by fires occurring from 1959 to 2002, during which time lower elevations with low canopy cover exhibited the greatest relative increases. The 2002–2004 drought reduced canopy cover by 55%, which eliminated gains in cover that occurred since 1936. Relative tree cover loss was highest at low elevations with low tree cover, but absolute tree cover loss was greater in areas of high tree cover, which increased with elevation. The loss of more than half of the canopy cover during a 2-year drought period was much greater than losses due to fire or possible increases due to historic land use (for example, grazing). These results suggest that regional-scale climatic influences may be more important than land use legacies in controlling tree cover of these and perhaps other semiarid woodlands over longer time scales—notable given that similar episodes of tree mortality are projected in coming decades with climate change.     

150 Years of Tree Establishment, Land Use and Climate Change in Montane Grasslands, Northwest Argentina

Degraded grasslands resulting from intensive land use appear to be highly resistant to tree invasion due to interactions between land use, climate, grazing and fire. We describe long-term patterns of tropical montane forest regeneration into degraded grasslands and analyze their relationships with historical changes in rainfall, grazing and fire in Los Toldos valley (Northwest Argentina), cloud forest life zone (1600 m asl). We used dendrochronological techniques to reconstruct spatial and temporal patterns of Podocarpus parlatorei establishment (the dominant tree species in secondary forests) and grassland fires for the last 150 yr. We assessed current livestock spatial distribution along the valley through feces sampling. Inferred tree establishment patterns ( i.e ., considering age structure and mortality) were analyzed in relation to temporal and spatial patterns of grazing and fire derived from our own analyses and from government statistics, and to rainfall patterns derived from previous dendrochronological reconstructions. Current grazing intensity was higher close to the local township. Fire occurrence increased with periods of above-average rainfall (higher fuel productivity), and tended to increase with distance to township (less grazing). Tree establishment in grasslands was spatially associated with high grazing intensity and low fire frequency, and temporally associated with periods of high grazing intensity and below-average rainfall. Despite climatic and land-use changes leading to conditions potentially favorable for trees ( i.e ., more rainfall, less grazing), grasslands persist in this study area, likely due to the direct (saplings burning) and indirect (soil degradation and desiccation) effects of recurrent fires, enhanced by decreasing grazing and increasing rainfall.
Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp     

The impacts and implications of an intensifying fire regime on Alaskan boreal forest composition and albedo

Climate warming and drying are modifying the fire dynamics of many boreal forests, moving them towards a regime with a higher frequency of extreme fire years characterized by large burns of high severity. Plot‐scale studies indicate that increased burn severity favors the recruitment of deciduous trees in the initial years following fire. Consequently, a set of biophysical effects of burn severity on postfire boreal successional trajectories at decadal timescales have been hypothesized. Prominent among these are a greater cover of deciduous tree species in intermediately aged stands after more severe burning, with associated implications for carbon and energy balances. Here we investigate whether the current vegetation composition of interior Alaska supports this hypothesis. A chronosequence of six decades of vegetation regrowth following fire was created using a database of burn scars, an existing forest biomass map, and maps of albedo and the deciduous fraction of vegetation that we derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery. The deciduous fraction map depicted the proportion of aboveground biomass in deciduous vegetation, derived using a RandomForest algorithm trained with field data sets (n=69, 71% variance explained). Analysis of the difference Normalized Burn Ratio, a remotely sensed index commonly used as an indicator of burn severity, indicated that burn size and ignition date can provide a proxy of burn severity for historical fires. LIDAR remote sensing and a bioclimatic model of evergreen forest distribution were used to further refine the stratification of the current landscape by burn severity. Our results show that since the 1950s, more severely burned areas in interior Alaska have produced a vegetation cohort that is characterized by greater deciduous biomass. We discuss the importance of this shift in vegetation composition due to climate‐induced changes in fire severity for carbon sequestration in forest biomass and surface reflectance (albedo), among other feedbacks to climate.     

Carbon dynamics in the future forest: the importance of long‐term successional legacy and climate–fire interactions

Understanding how climate change may influence forest carbon (C) budgets requires knowledge of forest growth relationships with regional climate, long‐term forest succession, and past and future disturbances, such as wildfires and timber harvesting events. We used a landscape‐scale model of forest succession, wildfire, and C dynamics (LANDIS‐II) to evaluate the effects of a changing climate (A2 and B1 IPCC emissions; Geophysical Fluid Dynamics Laboratory General Circulation Models) on total forest C, tree species composition, and wildfire dynamics in the Lake Tahoe Basin, California, and Nevada. The independent effects of temperature and precipitation were assessed within and among climate models. Results highlight the importance of modeling forest succession and stand development processes at the landscape scale for understanding the C cycle. Due primarily to landscape legacy effects of historic logging of the Comstock Era in the late 1880s, C sequestration may continue throughout the current century, and the forest will remain a C sink (Net Ecosystem Carbon Balance > 0), regardless of climate regime. Climate change caused increases in temperatures limited simulated C sequestration potential because of augmented fire activity and reduced establishment ability of subalpine and upper montane trees. Higher temperatures influenced forest response more than reduced precipitation. As the forest reached its potential steady state, the forest could become C neutral or a C source, and climate change could accelerate this transition. The future of forest ecosystem C cycling in many forested systems worldwide may depend more on major disturbances and landscape legacies related to land use than on projected climate change alone.     

Future climate change effects on US forest composition may offset benefits of reduced atmospheric deposition of N and S

Climate change and atmospheric deposition of nitrogen (N) and sulfur (S) are important drivers of forest demography. Here we apply previously derived growth and survival responses for 94 tree species, representing >90% of the contiguous US forest basal area, to project how changes in mean annual temperature, precipitation, and N and S deposition from 20 different future scenarios may affect forest composition to 2100. We find that under the low climate change scenario (RCP 4.5), reductions in aboveground tree biomass from higher temperatures are roughly offset by increases in aboveground tree biomass from reductions in N and S deposition. However, under the higher climate change scenario (RCP 8.5) the decreases from climate change overwhelm increases from reductions in N and S deposition. These broad trends underlie wide variation among species. We found averaged across temperature scenarios the relative abundance of 60 species were projected to decrease more than 5% and 20 species were projected to increase more than 5%; and reductions of N and S deposition led to a decrease for 13 species and an increase for 40 species. This suggests large shifts in the composition of US forests in the future. Negative climate effects were mostly from elevated temperature and were not offset by scenarios with wetter conditions. We found that by 2100 an estimated 1 billion trees under the RCP 4.5 scenario and 20 billion trees under the RCP 8.5 scenario may be pushed outside the temperature record upon which these relationships were derived. These results may not fully capture future changes in forest composition as several other factors were not included. Overall efforts to reduce atmospheric deposition of N and S will likely be insufficient to overcome climate change impacts on forest demography across much of the United States unless we adhere to the low climate change scenario.     

Coincident scales of forest feedback on climate and conservation in a diversity hot spot

The dynamic relationship between vegetation and climate is now widely acknowledged. Climate influences the distribution of vegetation; and through a number of feedback mechanisms vegetation affects climate. This implies that land-use changes such as deforestation will have climatic consequences. However, the spatial scales at which such feedbacks occur remain largely unknown. Here, we use a large database of precipitation and tree cover records for an area of the biodiversity-rich Atlantic forest region in south eastern Brazil to investigate the forest-rainfall feedback at a range of spatial scales from ca 10(1)-10(4) km2. We show that the strength of the feedback increases up to scales of at least 10(3) km2, with the climate at a particular locality influenced by the pattern of landcover extending over a large area. Thus, smaller forest fragments, even if well protected, may suffer degradation due to the climate responding to land-use change in the surrounding area. Atlantic forest vertebrate taxa also require large areas of forest to support viable populations. Areas of forest of ca 10(3) km2 would be large enough to support such populations at the same time as minimizing the risk of climatic feedbacks resulting from deforestation.     

Decadal dynamics of tree cover in an Australian tropical savanna

Spatio‐temporal variation in tropical savanna tree cover remains poorly understood. We aimed to quantify the drivers of tree cover in tropical mesic savannas in Kakadu National Park by relating changes in tree cover over 40 years to: mean annual rainfall, fire activity, initial tree cover and prior changes in tree cover. Aerial photography, acquired in 1964, 1984 and 2004, was obtained for fifty sites in Kakadu that spanned a rainfall gradient from approximately 1200 to 1600 mm. The remotely sensed estimates of tree cover were validated via field survey. Linear mixed effects modelling and multi‐model inference were used to assess the strength and form of the relationships between tree cover and predictor variables. Over the 40 years, tree cover across these savannas increased on average by 4.94 ± 0.88%, but was spatio‐temporally variable. Tree cover showed a positive albeit weak trend across the rainfall gradient. The strength of this positive relationship varied over the three measurement times, and this suggests that other factors are important in controlling tree cover. Tree cover was positively related to prior tree cover, and negatively correlated with fire activity. Over 20 years tree cover was more likely to increase if (i) tree cover was initially low or (ii) had decreased in the previous 20‐year interval or (iii) there had been fewer fires. Across the examined rainfall gradient, the greater variability in fire activity and inherently higher average tree cover at the wetter latitudes resulted in greater dynamism of tree cover compared with the drier latitudes. This is consistent with savanna tree cover being determined by interactions between mean annual rainfall, tree competition and frequent fire in these tropical mesic savannas.     

Not only trees: Grasses determine African tropical biome distributions via water limitation and fire

Aim

Although much tropical ecology generally focuses on trees, grasses are fundamental for characterizing the extensive tropical grassy biomes (TGBs) and, together with the tree functional types, for determining the contrasting functional patterns of TGBs and tropical forests (TFs). To study the factors that determine African biome distribution and the transitions between them, we performed the first continental analysis to include grass and tree functional types.

Location

Sub‐Saharan Africa.

Time period

2000–2010.

Major taxa studied

Savanna and forest trees and C₄ grasses.

Methods

We combined remote‐sensing data with a land cover map, using tree functional types to identify TGBs and TFs. We analysed the relationships of grass and tree cover with fire interval, rainfall annual average and seasonality.

Results

In TGBs experiencing < 630 mm annual rainfall, grass growth was water limited. Grass cover and fire recurrence were strongly and directly related over the entire subcontinent. Some TGBs and TFs with annual rainfall > 1,200 mm had the same rainfall seasonality but displayed strongly different fire regimes.

Main conclusions

Water limitation to grass growth was fundamental in the driest TGBs, acting alongside the well‐known limitation to tree growth. Marked differences in fire regimes across all biomes indicated that fire was especially relevant for maintaining mesic and humid TGBs. At high rainfall, our results support the hypothesis of TGBs and TFs being alternative stable states maintained by a vegetation–fire feedback for similar climatic conditions.     

Altered climate leads to positive density‐dependent feedbacks in a tropical wet forest

Climate change is predicted to result in warmer and drier Neotropical forests relative to current conditions. Negative density‐dependent feedbacks, mediated by natural enemies, are key to maintaining the high diversity of tree species found in the tropics, yet we have little understanding of how projected changes in climate are likely to affect these critical controls. Over 3 years, we evaluated the effects of a natural drought and in situ experimental warming on density‐dependent feedbacks on seedling demography in a wet tropical forest in Puerto Rico. In the +4°C warming treatment, we found that seedling survival increased with increasing density of the same species (conspecific). These positive density‐dependent feedbacks were not associated with a decrease in aboveground natural enemy pressure. If positive density‐dependent feedbacks are not transient, the diversity of tropical wet forests, which may rely on negative density dependence to drive diversity, could decline in a future warmer, drier world.