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Soil moisture is a crucial variable mediating soil‐vegetation‐atmosphere water exchange. As climate and land use change, the increased frequency and intensity of extreme weather events and disturbances will likely alter feedbacks between ecosystem functions and soil moisture. In this study, we evaluated how extreme drought (2015/2016) and postfire vegetation regrowth affected the seasonality of soil water content (0–8 m depth) in a transitional forest in southeastern Amazonia. The experiment included three treatment plots: an unburned Control, an area burned every three years (B3yr), and an area burned annually (B1yr) between 2004 and 2010. We hypothesized that (a) soil moisture at B1yr and B3yr would be higher than the Control in the first years postfire due to lower transpiration rates, but differences between burned plots would decrease as postfire vegetation regrew; (b) during drought years, the soil water deficit in the dry season would be significantly greater in all plots as plants responded to greater evaporative demand; and (c) postfire recovery in the burned plots would cause an increase in evapotranspiration over time, especially in the topsoil. Contrary to the first expectation, the burned plots had lower volumetric water content than the Control plot. However, we found that droughts significantly reduced soil moisture in all plots compared to non‐drought years (15.6%), and this effect was amplified in the burned plots (19%). Our results indicate that, while compounding disturbances such as wildfires and extreme droughts alter forest dynamics, deep soil moisture is an essential water source for vegetation recovery. Plain Language Summary: As extreme droughts and wildfires have become more common in southeastern Amazonia, they could alter soil moisture availability, but the intensity of these compounding effects still needs to be understood. This study investigates the impact of the extreme drought of 2015/2016, postfire vegetation recovery, the return of water to the atmosphere, and their interacting effects on water distribution across the soil profile in three treatment plots: an unburned Control, an area burned every three years, and an area burned annually from 2004 to 2010. Contrary to expectations, we found that soil moisture in the burned plots decreased in the years following the last experimental fire (2010), even as the vegetation recovered. Drought events significantly reduced soil moisture across all plots, but this effect was amplified in the burned plots. As land use changes increase and climate change intensifies, such drought‐induced soil drying may become more common, exacerbating the impacts of wildfires on ecosystem resilience and forest health across much of the region. Key Points: During the driest months of the year, burned forests showed significant reductions in soil moisture compared with unburned forestsThe drought event of 2015/2016 caused greater reductions in soil moisture in the burned forest than in the unburned forestBoth burned and unburned forests increased water use throughout the 8‐m soil column from 2011 to 2018 [ABSTRACT FROM AUTHOR]

Understanding how savanna soil properties influence vegetation diversity and function is a major challenge in ecological studies. We investigated the effects of soil properties on woody species density, richness, composition, and vegetative phenology (inferred by the Normalized Difference Vegetation Index (NDVI)) in two alluvial and two interfluvial savanna sites (1 block of 10 plots of 20 × 50 m in each site), in the Brazilian Cerrado. We showed that plots in alluvial savannas present less fertile soils and have lower plant densities and species richness and higher seasonality of NDVI than plots in the interfluvial savannas. The species composition of the sites was associated with the P, Fe, K, and Mn content of the soil. Soil K, Fe, and Ca contents were the main variables associated with plant density in a linear mixed model (LMM) that explained 79% of data variability (r2c = 0.79%), and K, Fe, and Al were the main predictors to explain species richness (r2c = 0.81%). Soil K, pH, and Silt were the best predictors of the seasonality in NDVI (r2c = 0.14%). We highlight the all‐encompassing effect of K soil content on species density, richness, composition, and NDVI and argue that this macronutrient and a few other soil properties (e.g., P, Fe, Al, and Silt) are the main factors mediating plant responses to water and nutrient stress in woody savanna communities occurring in the Cerrado–Amazônia transition. [ABSTRACT FROM AUTHOR]

Forest disturbances associated with edge effects, wildfires, and windthrow events have impacted large swaths of the tropics. Defining the levels of forest disturbance that cause ecologically relevant reductions in fruit and seed (FS) production is key to understanding forest resilience to current and future global changes. Here, we tested the hypotheses that: (1) low‐intensity experimental fires alone would cause minor changes in FS production and diversity in a tropical forest, whereas synergistic disturbance effects resulting from edge effects, wildfires, droughts, and blowdowns would drive long‐term reductions in FS diversity and production; and (2) the functional composition of FS in disturbed forests would shift toward tree species with acquisitive strategies. To test these hypotheses, we quantified FS production between 2005 and 2018 in a large‐scale fire experiment in southeast Amazonia. The experimental treatments consisted of three 50‐ha plots: a Control plot, a plot burned annually (B1yr) and a plot burned every three years (B3yr) between 2004 and 2010. These plots were impacted by edge effects, two droughts (2007 and 2010), and a blowdown event in 2012. Our results show that FS production remained relatively high following low‐intensity fires, but declined where fires were most severe (i.e., forest edge of B3yr). The number of species‐producing FS declined sharply when fires co‐occurred with droughts and a windthrow event, and species composition shifted throughout the experiment. Along the edge of both burned plots, the forest community became dominated by species with faster relative growth, thinner leaves, thinner bark, and lower height. We conclude that compounding disturbances changed FS patterns, with a strong effect on species composition and potentially large effects on the next generation of trees. This is largely due to reductions in the diversity of species‐producing FS where fires are severe, causing a shift toward functional traits typically associated with pioneer and generalist species. [ABSTRACT FROM AUTHOR]

Trees balance temporal asynchrony in carbon source and sink activity by accumulating and using non‐structural carbon (NSC). Previous work has demonstrated differences in the amount and distribution of NSC stored in stemwood in tropical tree species and related these patterns in NSC distribution to tree growth and mortality rates. However, we still do not know how changes in the amount and location of starch, a major component of NSC in stemwood, influence the seasonal carbon dynamics of mature trees and how this may reflect storage‐growth trade‐offs.In this work, we hypothesized that combining two life history traits, here leaf habit (evergreen/semi‐deciduous) and the anatomical distribution of starch within the stemwood (parenchyma storage and fibre storage), would allow us to explain differences in the seasonal interplay between carbon sources and sinks and the use and accumulation of starch in the tree stem. We expected semi‐deciduous/fibre‐storing species to have greater seasonal amplitudes of carbon source and sink activity, and therefore greater variation in starch content and stronger storage‐growth trade‐offs than evergreen/parenchyma‐storing species.We measured monthly increments in stem radial growth, soluble sugars and starch every 3 months during 2019 in Dacryodes microcarpa (semi‐deciduous/fibre‐storing species), Ocotea leucoxylon (evergreen/parenchyma‐storing species) and Sacoglottis guianensis (semi‐deciduous/parenchyma‐storing species).We found seasonal changes in starch but not sugars in the semi‐deciduous species, with greater amplitude in the fibre‐storing species that also had greater storage capacity and stem respiration rates. The fibre‐storing species further showed a negative relationship between starch consumption/accumulation and growth during the rainy season, suggesting a trade‐off between growth and storage, with starch accumulating in some cases when growth was low.Synthesis. Our results show the influence of seasonal starch storage on carbon dynamics in three species of tropical trees that differ in leaf phenology and starch storage traits. Semi‐deciduous/fibre‐storing species have greater temporal variation in carbon sink activities and more seasonally dynamic starch content. Since the fibre‐storing species we studied are slower‐growing and longer‐lived trees with lower mortality rates compared to the parenchyma‐storing species, these results may provide clues about how storage traits could influence their survival and life span. [ABSTRACT FROM AUTHOR]

Tropical forest fragmentation from agricultural expansion alters the microclimatic conditions of the remaining forests, with effects on vegetation structure and function. However, little is known about how the functional trait variability within and among tree species in fragmented landscapes influence and facilitate species' persistence in these new environmental conditions. Here, we assessed potential changes in tree species' functional traits in riparian forests within six riparian forests in cropland catchments (Cropland) and four riparian forests in forested catchments (Forest) in southern Amazonia. We sampled 12 common functional traits of 123 species across all sites: 64 common to both croplands and forests, 33 restricted to croplands, and 26 restricted to forests. We found that forest-restricted species had leaves that were thinner, larger, and with higher phosphorus (P) content, compared to cropland-restricted ones. Tree species common to both environments showed higher intraspecific variability in functional traits, with leaf thickness and leaf P concentration varying the most. Species turnover contributed more to differences between forest and cropland environments only for the stem-specific density trait. We conclude that the intraspecific variability of functional traits (leaf thickness, leaf P, and specific leaf area) facilitates species persistence in riparian forests occurring within catchments cleared for agricultural expansion in Amazonia. [ABSTRACT FROM AUTHOR]

Fire–vegetation feedbacks potentially maintain global savanna and forest distributions. Accordingly, vegetation in savanna and forest ecosystems should have differential responses to fire, but fire response data for herbaceous vegetation have yet to be synthesized across biomes. Here, we examined herbaceous vegetation responses to experimental fire at 30 sites spanning four continents. Across a variety of metrics, herbaceous vegetation increased in abundance where fire was applied, with larger responses to fire in wetter and in cooler and/or less seasonal systems. Compared to forests, savannas were associated with a 4.8 (±0.4) times larger difference in herbaceous vegetation abundance for burned versus unburned plots. In particular, grass cover decreased with fire exclusion in savannas, largely via decreases in C4 grass cover, whereas changes in fire frequency had a relatively weak effect on grass cover in forests. These differential responses underscore the importance of fire for maintaining the vegetation structure of savannas and forests. [ABSTRACT FROM AUTHOR]

Copyright of Global Change Biology is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

By favouring long-distance dispersal, anemochory is often associated with open and patchy habitats, whereas zoochory enables short-distance dispersal and prevails in closed and extensive habitats. Brazilian savanna (Cerrado) on shallow rocky soils (RS) have open vegetation and are patchily distributed, whereas savanna on deep soils (DS) have dense vegetation and occur in large stands. Thus, we predicted that zoochory would be favoured in DS and anemochory would characterise RS. We tested the hypothesis that vegetation structure and distribution patchiness are related to the characteristic dispersal mechanisms associated with closed and open habitats. We compared the abundance of dispersal mechanisms between DS and RS with generalised linear mixed models and used an indicator species analysis to identify species associated with each vegetation type. There was no difference between DS and RS in the number of species associated with one or the other dispersal mechanism. We found fewer-than-expected zoochorous individuals and more-than-expected anemochorous individuals in RS, and more zoochorous indicator species in DS. Habitat patchiness and openness are related to the composition of dispersal mechanisms. Open vegetation on shallow rocky soils favours long-distance dispersal relative to zoochory. This implies reduced plant establishment and increased leaflessness, being harsher on animal dispersers. Protected areas are often concentrated in RS, whereas DS are rapidly converted into croplands due to their excellent aptitude for mechanised agriculture. Conserving DS and RS requires a balanced complementary approach, that ensures that all landscape elements are adequately represented in protected areas. [ABSTRACT FROM AUTHOR]

Climatic seasonality provokes considerable variability in the physical conditions of streams. Although few studies have assessed systematically the temporal effects of climate on aquatic communities, the diversity and structure of odonate larval assemblages (suborders Anisoptera and Zygoptera) are predicted to be influenced by seasonality. To evaluate this hypothesis, we tested the predictions that (1) the genera and abundance richness of zygopteran and anisopteran larvae will decrease during periods of high precipitation, (2) the genera richness and abundance of the two suborders will be associated inversely with temperature, and (3) the composition of the zygopteran assemblage will be more affected by seasonality, in particular precipitation, than that of the Anisoptera, whose composition will be influenced primarily by temperature. We collected odonate larvae every three months over a 6-year period at a stream located in the Cerrado–Amazon forest transition zone and compared these data with data on the local climate from the same period. Our results indicate that the seasonal variation in precipitation had a significant effect on the abundance of both odonate suborders, but that, while the anisopteran genera richness was affected by precipitation, the zygopteran richness was not. We showed that temperature affected the abundance of both suborders, but had no effect on genera richness. As we predicted, the composition of the zygopteran larval assemblage was affected more by the climatic variables than the Anisoptera. In addition, while the Zygoptera were affected primarily by precipitation, the Anisoptera were influenced most by temperature. The results indicate that the timing of the emergence of odonate larvae is synchronized with the months preceding the rainy season, when a high density of larvae are recorded, which may reflect a strategy for both the avoidance of the effects of fluctuations in water levels and the exploitation of the relatively abundant food sources found in the terrestrial environment. Temperature also appears to contribute to this synchrony, by either accelerating or delaying the process of larval emergence. [ABSTRACT FROM AUTHOR]

Understanding the role of environmental conditions and geographic space on species distributions is a major goal of ecological studies. Here, we investigate the effects of soil properties, a native bamboo (Actinocladum verticillatum (Nees) McClure ex Soderstr) ground cover and geographic distance on species richness and turnover in the Brazilian Cerrado. We established three transects along a forest–savanna gradient (14°41′S and 52°20′W), from the lowland next to a stream to the top of a hill, where we sampled the woody plant community (DBH ≥ 3 cm), soil properties and ground cover of A. verticillatum. We evaluated changes in species composition with a principal coordinates analysis, and the effects of the environmental predictors on species richness using generalised linear models and on species turnover using generalised dissimilarity modelling. We found that species richness increases with K concentration in the soil and decreases with Zn concentration in the soil and bamboo cover. The predictors explained 62% of species turnover, with environmental parameters (soil properties, bamboo cover and altitude) having the most significant contribution (80%), followed by the spatially structured environmental variation (20%). The most important environmental predictors of woody species turnover were soil concentration of Zn, P and K, altitude, and bamboo cover. Changes in species richness and composition in the studied gradient were strongly associated with plot height, bamboo cover and soil content of K, Zn and P. Our findings indicate that nutrient availability and competition with bamboo explain vegetation structure and composition in this region. [ABSTRACT FROM AUTHOR]

Summary: Non‐structural carbon (NSC) storage (i.e. starch, soluble sugras and lipids) in tree stems play important roles in metabolism and growth. Their spatial distribution in wood may explain species‐specific differences in carbon storage dynamics, growth and survival. However, quantitative information on the spatial distribution of starch and lipids in wood is sparse due to methodological limitations.Here we assessed differences in wood NSC and lipid storage between tropical tree species with different growth and mortality rates and contrasting functional types. We measured starch and soluble sugars in wood cores up to 4 cm deep into the stem using standard chemical quantification methods and histological slices stained with Lugol's iodine. We also detected neutral lipids using histological slices stained with Oil‐Red‐O.The histological method allowed us to group individuals into two categories according to their starch storage strategy: fiber‐storing trees and parenchyma‐storing trees. The first group had a bigger starch pool, slower growth and lower mortality rates than the second group. Lipid storage was found in wood parenchyma in five species and was related to low mortality rates.The quantification of the spatial distribution of starch and lipids in wood improves our understanding of NSC dynamics in trees and reveals additional dimensions of tree growth and survival strategies. [ABSTRACT FROM AUTHOR]

High rates of biological nitrogen fixation (BNF) are commonly reported for tropical forests, but most studies have been conducted in regions that receive substantial inputs of molybdenum (Mo) from atmospheric dust and sea‐salt aerosols. Even in these regions, the low availability of Mo can constrain free‐living BNF catalyzed by heterotrophic bacteria and archaea. We hypothesized that in regions where atmospheric inputs of Mo are low and soils are highly weathered, such as the southeastern Amazon, Mo would constrain BNF. We also hypothesized that the high soil acidity, characteristic of the Amazon Basin, would further constrain Mo availability and therefore soil BNF. We conducted two field experiments across the wet and dry seasons, adding Mo, phosphorus (P), and lime alone and in combination to the forest floor in the southeastern Amazon. We sampled soils and litter immediately, and then weeks and months after the applications, and measured Mo and P availability through resin extractions and BNF with the acetylene reduction assay. The experimental additions of Mo and P increased their availability and the lime increased soil pH. While the combination of Mo and P increased BNF at some time points, BNF rates did not increase strongly or consistently across the study as a whole, suggesting that Mo, P, and soil pH are not the dominant controls over BNF. In a separate short‐term laboratory experiment, BNF did not respond strongly to Mo and P even when labile carbon was added. We postulate that high nitrogen (N) availability in this area of the Amazon, as indicated by the stoichiometry of soils and vegetation and the high nitrate soil stocks, likely suppresses BNF at this site. These patterns may also extend across highly weathered soils with high N availability in other topographically stable regions of the tropics. [ABSTRACT FROM AUTHOR]

Tropical forest fires have become more common due to interactions between deforestation, land clearing, and drought. Forest recovery following fires may be limited by nitrogen. Biological nitrogen fixation (BNF) is the main pathway for new nitrogen (N) to enter most ecosystems, but BNF may be constrained by other nutrients, such as molybdenum and phosphorus, which are required for the process. We studied the role of BNF 7 years into the recovery of southeastern Amazon tropical forests that were burned experimentally either annually or every 3 years between 2004 and 2010. We hypothesized that, compared with unburned primary forests, BNF in burned forests would increase due to the depletion of N in the aboveground biomass pool and that soil concentrations of molybdenum and phosphorus from ash inputs would increase, reducing their potential constraints on BNF. Despite the fires depleting about half the aboveground N pool and rapid rates of recovery in leaf biomass and litterfall production, we found low rates of both free-living and symbiotic BNF. Higher concentrations of molybdenum and phosphorus in the mineral soils of the burned forests indicated that these elements were likely not limiting BNF. Our results suggest that despite the N demand for regrowth post-fire, substantial N stored in soils likely downregulates BNF. Overall, we found surprisingly low BNF rates (< 1.2 kg N ha−1 y−1) in this region of the Amazon, which contrasts with the traditional paradigm that (1) tropical forests fix large quantities of N and (2) that BNF increases after forest disturbance. [ABSTRACT FROM AUTHOR]

Aims Different plant functional groups display diverging responses to the same environmental gradients. Here, we assess the effects of environmental and spatial predictors on species turnover of three functional groups of Brazilian savannas (Cerrado) plants—trees, palms and lianas—across the transition zone between the Cerrado and Amazon biomes in central Brazil. Methods We used edaphic, climatic and plant composition data from nine one-hectare plots to assess the effects of the environment and space on species turnover using a Redundancy Analysis and Generalized Dissimilarity Modeling (GDM), associated with variance partitioning. Important Findings We recorded 167 tree species, 5 palms and 4 liana species. Environmental variation was most important in explaining species turnover, relative to geographic distance, but the best predictors differed between functional groups: geographic distance and silt for lianas; silt for palms; geographic distance, temperature and elevation for trees. Geographic distances alone exerted little influence over species turnover for the three functional groups. The pure environmental variation explained most of the liana and palm turnover, while tree turnover was largely explained by the shared spatial and environmental contribution. The effects of geographic distance upon species turnover leveled off at about 300 km for trees, and 200 km for lianas, whereas they were unimportant for palm species turnover. Our results indicate that environmental factors that determine floristic composition and species turnover differ substantially between plant functional groups in savannas. Therefore, we recommend that studies that aim to investigate the role of environmental conditions in determining plant species turnover should examine plant functional groups separately. [ABSTRACT FROM AUTHOR]

Drought, fire, and windstorms can interact to degrade tropical forests and the ecosystem services they provide, but how these forests recover after catastrophic disturbance events remains relatively unknown. Here, we analyze multi‐year measurements of vegetation dynamics and function (fluxes of CO2 and H2O) in forests recovering from 7 years of controlled burns, followed by wind disturbance. Located in southeast Amazonia, the experimental forest consists of three 50‐ha plots burned annually, triennially, or not at all from 2004 to 2010. During the subsequent 6‐year recovery period, postfire tree survivorship and biomass sharply declined, with aboveground C stocks decreasing by 70%–94% along forest edges (0–200 m into the forest) and 36%–40% in the forest interior. Vegetation regrowth in the forest understory triggered partial canopy closure (70%–80%) from 2010 to 2015. The composition and spatial distribution of grasses invading degraded forest evolved rapidly, likely because of the delayed mortality. Four years after the experimental fires ended (2014), the burned plots assimilated 36% less carbon than the Control, but net CO2 exchange and evapotranspiration (ET) had fully recovered 7 years after the experimental fires ended (2017). Carbon uptake recovery occurred largely in response to increased light‐use efficiency and reduced postfire respiration, whereas increased water use associated with postfire growth of new recruits and remaining trees explained the recovery in ET. Although the effects of interacting disturbances (e.g., fires, forest fragmentation, and blowdown events) on mortality and biomass persist over many years, the rapid recovery of carbon and water fluxes can help stabilize local climate. [ABSTRACT FROM AUTHOR]

To understand changes in land surface energy balance partitioning due to tropical deforestation, we use a physically based analytical formulation of the surface energy balance. Turbulent heat fluxes are constrained by the thermodynamic maximum power limit and a formulation for diurnal heat redistribution within the land‐atmosphere system. The derived turbulent fluxes of sensible and latent heat compare very well to in situ observations for sites with intact rainforest and soybean land cover in southeastern Amazonia. The equilibrium partitioning into sensible and latent heat flux compares well with observations for both sites, except for the soybean site during the dry season where water limitation needs to be explicitly accounted for. Our results show that tropical deforestation primarily affects the absorption of solar radiation and the water limitation of evapotranspiration, but not the overall magnitude of turbulent heat fluxes that is set by the thermodynamic maximum power limit. Plain Language Summary: Tropical deforestation impacts the local energy and water exchange between land surface and atmosphere, typically resulting in regionally warmer and drier climates. General circulation models still disagree in reproducing these changes and little has been done to derive them from first principles. Here, we present an alternative approach to describe the effects of tropical land conversion from forest to soy agriculture, based on a physical theory of land‐atmosphere interactions. We view land‐atmosphere exchange as the result of a heat engine strongly shaped by turbulent heat exchange. This provides a framework to derive analytical expressions of the turbulent fluxes from the limit by how much work this engine can maximally perform. By comparing these with observations from a tropical rainforest and a soybean field in Amazonia, we find that the diurnal variations of turbulent fluxes are very well estimated. This means that turbulent land‐atmosphere exchange is primarily constrained by the thermodynamic limit, irrespective of surface roughness and evapotranspiration, and suggests that one can estimate the primary impacts of tropical land use change from physical principles. Thus, using thermodynamic limits represents an alternative approach to investigate the highly complex nature of land‐atmosphere interactions and global change from first principles. Key Points: Diurnal variations of turbulent heat fluxes are well estimated by the maximum power limit for sites with contrasting land cover in AmazoniaLand cover affected primarily the absorption of solar radiation and the partitioning of turbulent fluxes into sensible and latent heatThermodynamic theory is well suited as a parsimonious approach to describe first‐order controls of land‐atmosphere interactions [ABSTRACT FROM AUTHOR]

Water quality in streams is determined by several factors, including geology, topography, climate, and anthropogenic changes. This study aimed to assess the effects of watershed physical, morphology, and precipitation seasonality on the water quality of two streams that supply drinking water to rural settlements and urban areas in the Cerrado-Amazonia transition region. We monitored 16 physico-chemical attributes of water at six different sample locations over three years (2013–2016). Our results indicate that eight of these physico-chemical attributes did not meet the standards for safe drinking water established by Brazilian legislation. Precipitation seasonality, degradation of riparian zones, stream length, and watershed slope were the most important predictors of impaired water quality. Our results highlight the importance of restoring and conserving riparian forests in order to maintain drinking water quality. [ABSTRACT FROM AUTHOR]

Copyright of Biotropica is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Widespread degradation of tropical forests is caused by a variety of disturbances that interact in ways that are not well understood.To explore potential synergies between edge effects, fire and windstorm damage as causes of Amazonian forest degradation, we quantified vegetation responses to a 30‐min, high‐intensity windstorm that in 2012, swept through a large‐scale fire experiment that borders an agricultural field. Our pre‐ and postwindstorm measurements include tree mortality rates and modes of death, above‐ground biomass, and airborne LiDAR‐based estimates of tree heights and canopy disturbance (i.e., number and size of gaps). The experimental area in the southeastern Amazonia includes three 50‐ha plots established in 2004 that were unburned (Control), burned annually (B1yr), or burned at 3‐year intervals (B3yr).The windstorm caused greater damage to trees (>10 cm DBH) in the burned plots (B1yr: 13 ± 9% of 785 trees; B3yr: 17 ± 13% of 433) than in the Control plot (8 ± 4% of 2,300; ± CI). It substantially reduced vegetation height by 14% in B1yr, 20% in B3yr and 12% in the Control plots, while it reduced above‐ground biomass by 18% of 77.7 Mg/ha (B1yr), 31% of 56.6 (B3yr), and 15% of 120 (Control). Tree damage was greatest near the agricultural field edge in all three plots, especially among large trees and in B3yr. Trunk snapping (70%) and uprooting (20%) were the most common modes of tree damage and mortality, with the height of trunk failure on the burned plots often corresponding with the height of historical fire scars. Of the windstorm‐damaged trees, 80% (B1yr), 90% (B3yr), and 57% (Control) were dead 4 years later. Trees that had crown damage experienced the least mortality (22%–60%), followed by those that were snapped (55%–94%) and uprooted (88%–94%).Synthesis. We demonstrate the synergistic effects of three kinds of disturbances on a tropical forest. Our results show that the effects of windstorms are exacerbated by prior degradation by fire and fragmentation. We highlight that understorey fires can produce long‐lasting effects on tropical forests not only by directly killing trees but also by increasing tree vulnerability to wind damage due to fire scars and a more open canopy. Our study shows that the effects of windstorms are exacerbated by prior degradation by fire and fragmentation. Understorey fires can produce long‐lasting effects on tropical forests not only by directly killing trees but also by increasing tree vulnerability to wind damage due to fire scars and a more open canopy. [ABSTRACT FROM AUTHOR]

The interaction between droughts and land-use fires threaten the carbon stocks, climate regulatory functions, and biodiversity of Amazon forests, particularly in the southeast, where deforestation and land-use ignitions are high. Repeated, severe, or combined fires and droughts result in tropical forest degradation via nonlinear dynamics and may lead to an alternate vegetation state. Here, we discuss the major insights from the longest (more than 10 years) and largest (150-hectare) experimental burn in Amazon forests. Despite initial forest resistance to low-intensity fires, repeated fire during drought killed the majority of trees, reduced canopy cover by half, and favored invasive grasses--but the persistence of this novel vegetation state is unknown. Forest edges, where drying, fire intensity and grass invasion are greatest, were most vulnerable. Crucial to advancing fire ecology in tropical forests, we need to scale these results to understand how flammability and resilience postfire varies across Amazon forest types. [ABSTRACT FROM AUTHOR]

Understanding the relationship between photosynthesis, net primary productivity and growth in forest ecosystems is key to understanding how these ecosystems will respond to global anthropogenic change, yet the linkages among these components are rarely explored in detail. We provide the first comprehensive description of the productivity, respiration and carbon allocation of contrasting lowland Amazonian forests spanning gradients in seasonal water deficit and soil fertility. Using the largest data set assembled to date, ten sites in three countries all studied with a standardized methodology, we find that (i) gross primary productivity ( GPP) has a simple relationship with seasonal water deficit, but that (ii) site-to-site variations in GPP have little power in explaining site-to-site spatial variations in net primary productivity ( NPP) or growth because of concomitant changes in carbon use efficiency ( CUE), and conversely, the woody growth rate of a tropical forest is a very poor proxy for its productivity. Moreover, (iii) spatial patterns of biomass are much more driven by patterns of residence times (i.e. tree mortality rates) than by spatial variation in productivity or tree growth. Current theory and models of tropical forest carbon cycling under projected scenarios of global atmospheric change can benefit from advancing beyond a focus on GPP. By improving our understanding of poorly understood processes such as CUE, NPP allocation and biomass turnover times, we can provide more complete and mechanistic approaches to linking climate and tropical forest carbon cycling. [ABSTRACT FROM AUTHOR]

Interactions between climate and land-use change may drive widespread degradation of Amazonian forests. High-intensity fires associated with extreme weather events could accelerate this degradation by abruptly increasing tree mortality, but this process remains poorly understood. Here we present, to our knowledge, the first field-based evidence of a tipping point in Amazon forests due to altered fire regimes. Based on results of a large-scale, long-term experiment with annual and triennial burn regimes (B1yr and B3yr, respectively) in the Amazon, we found abrupt increases in fire-induced tree mortality (226 and 462%) during a severe drought event, when fuel loads and air temperatures were substantially higher and relative humidity was lower than long-term averages. This threshold mortality response had a cascading effect, causing sharp declines in canopy cover (23 and 31%) and aboveground live biomass (12 and 30%) and favoring widespread invasion by flammable grasses across the forest edge area (80 and 63%), where fires were most intense (e.g., 220 and 820 kW·m-1). During the droughts of 2007 and 2010, regional forest fires burned 12 and 5% of southeastern Amazon forests, respectively, compared with <1% in nondrought years. These results show that a few extreme drought events, coupled with forest fragmentation and anthropogenic ignition sources, are already causing widespread fire-induced tree mortality and forest degradation across southeastern Amazon forests. Future projections of vegetation responses to climate change across drier portions of the Amazon require more than simulation of global climate forcing alone and must also include interactions of extreme weather events, fire, and land-use change. [ABSTRACT FROM AUTHOR]

Background:The impact of fire on carbon cycling in tropical forests is potentially large, but remains poorly quantified, particularly in the locality of the transition forests that mark the boundaries between humid forests and savannas. Aims:To present the first comprehensive description of the impact of repeated low intensity, understorey fire on carbon cycling in a semi-deciduous, seasonally dry tropical forest on infertile soil in south-eastern Amazonia. Methods:We compared an annually burnt forest plot with a control plot over a three-year period (2009–2011). For each plot we quantified the components of net primary productivity (NPP), autotrophic (Ra) and heterotrophic respiration (Rh), and estimated total plant carbon expenditure (PCE, the sum of NPP andRa) and carbon-use efficiency (CUE, the quotient of NPP/PCE). Results:Total NPP andRawere 15 and 4% lower on the burnt plot than on the control, respectively. Both plots were characterised by a slightly higher CUE of 0.36–0.39, compared to evergreen lowland Amazon forests. Conclusions:These measurements provide the first evidence of a distinctive pattern of carbon cycling within this transitional forest. Overall, regular understorey fire is shown to have little impact on ecosystem-level carbon fluxes. [ABSTRACT FROM PUBLISHER]

éLetter to the Editor [ABSTRACT FROM AUTHOR]

Changes in climate and land use that interact synergistically to increase fire frequencies and intensities in tropical regions are predicted to drive forests to new grass-dominated stable states. To reveal the mechanisms for such a transition, we established 50 ha plots in a transitional forest in the southwestern Brazilian Amazon to different fire treatments (unburned, burned annually (B1yr) or at 3-year intervals (B3yr)). Over an 8-year period since the commencement of these treatments, we documented: (i) the annual rate of pasture and native grass invasion in response to increasing fire frequency; (ii) the establishment of Brachiaria decumbens (an African C4 grass) as a function of decreasing canopy cover and (iii) the effects of grass fine fuel on fire intensity. Grasses invaded approximately 200 m from the edge into the interiors of burned plots (B1yr: 4.31 ha; B3yr: 4.96 ha) but invaded less than 10 m into the unburned plot (0.33 ha). The probability of B. decumbens establishment increased with seed availability and decreased with leaf area index. Fine fuel loads along the forest edge were more than three times higher in grass-dominated areas, which resulted in especially intense fires. Our results indicate that synergies between fires and invasive C4 grasses jeopardize the future of tropical forests. [ABSTRACT FROM AUTHOR]

Droughts can exert a strong influence on the regional energy balance of the Amazon and Cerrado, as can the replacement of native vegetation by croplands. What remains unclear is how these two forcing factors interact and whether land cover changes fundamentally alter the sensitivity of the energy balance components to drought events. To fill this gap, we used remote sensing data to evaluate the impacts of drought on evapotranspiration (ET), land surface temperature (LST), and albedo on cultivated areas, savannas, and forests. Our results (for seasonal drought) indicate that increases in monthly dryness across Mato Grosso state (southern Amazonia and northern Cerrado) drive greater increases in LST and albedo in croplands than in forests. Furthermore, during the 2007 and 2010 droughts, croplands became hotter (0.1–0.8 °C) than savannas (0.3–0.6 °C) and forests (0.2–0.3 °C). However, forest ET was consistently higher than ET in all other land uses. This finding likely indicates that forests can access deeper soil water during droughts. Overall, our findings suggest that forest remnants can play a fundamental role in the mitigation of the negative impacts of extreme drought events, contributing to a higher ET and lower LST. [ABSTRACT FROM AUTHOR]