Future Of Tropical Forests Research Articles

Quantitative assessment of past and future tropical forest transition and its dynamic to streamflow of the catchment, Malaysia

Quantitative assessment of past and future tropical forest transition and its dynamic to streamflow of the catchment, Malaysia

The effect of repeated hurricanes on the age of organic carbon in humid tropical forest soil.

Increasing hurricane frequency and intensity with climate change is likely to affect soil organic carbon (C) stocks in tropical forests. We examined the cycling of C between soil pools and with depth at the Luquillo Experimental Forest in Puerto Rico in soils over a 30-year period that spanned repeated hurricanes. We used a nonlinear matrix model of soil C pools and fluxes ("soilR") and constrained the parameters with soil and litter survey data. Soil chemistry and stable and radiocarbon isotopes were measured from three soil depths across a topographic gradient in 1988 and 2018. Our results suggest that pulses and subsequent reduction of inputs caused by severe hurricanes in 1989, 1998, and two in 2017 led to faster mean transit times of soil C in 0-10 cm and 35-60 cm depths relative to a modeled control soil with constant inputs over the 30-year period. Between 1988 and 2018, the occluded C stock increased and δ13C in all pools decreased, while changes in particulate and mineral-associated C were undetectable. The differences between 1988 and 2018 suggest that hurricane disturbance results in a dilution of the occluded light C pool with an influx of young, debris-deposited C, and possible microbial scavenging of old and young C in the particulate and mineral-associated pools. These effects led to a younger total soil C pool with faster mean transit times. Our results suggest that the increasing frequency of intense hurricanes will speed up rates of C cycling in tropical forests, making soil C more sensitive to future tropical forest stressors.

Solar radiation and soil moisture drive tropical forest understory responses to experimental and natural hurricanes

AbstractTropical forest understory regeneration occurs rapidly after disturbance with compositional trajectories that depend on species availability and environmental conditions. To predict future tropical forest regeneration dynamics, we need a deeper understanding of how pulse disturbance events, like hurricanes, interact with environmental variability to affect understory demography and composition. We examined fern and sapling mortality, recruitment, and community composition in relation to solar radiation and soil moisture using 17 years of forest dynamics data (2003–2019) from the Canopy Trimming Experiment in the Luquillo Experimental Forest, Puerto Rico. Solar radiation increased 150% and soil moisture increased 40% following canopy trimming of experimental plots relative to control plots. All plots were disturbed in 2017 by Hurricanes Irma and Maria, so experimentally trimmed plots presented the opportunity to study the effects of multiple hurricanes, while control plots isolated the effects of a single natural hurricane. Recruitment rates maximized at 0.14 individuals/plot/month for ferns and 0.20 stems/plot/month for saplings. Recruitment and mortality were distributed more evenly over the 17 years of monitoring in experimentally trimmed plots than in control plots; however, following Hurricane Maria demographic rates substantially increased in control plots only. In experimentally trimmed plots, the largest community compositional shifts occurred as a result of the trimming events, and compositional changes were greatest for control plots after Hurricane Maria in 2017. Pioneer tree and fern species increased in abundance in response to both simulated and natural hurricanes. Following Hurricane Maria, two dominant pioneer species, Cyathea arborea and Cecropia schreberiana, recruited abundantly, but only in control plots. In trimmed plots, increased solar radiation and soil moisture shifted understory species composition steadily toward pioneer and secondary‐successional species, with soil moisture interacting strongly with canopy trimming. Thus, both solar radiation and soil moisture are environmental drivers affecting pioneer species recruitment following disturbance, which interact with canopy opening following hurricanes. Our results suggest that if hurricane disturbances increase in frequency and severity, as suggested by climate change predictions, the understory regeneration of late‐successional species, such as Manilkara bidentata and Sloanea berteroana, which prefer deeper shade and slightly drier soil microsites, may become imperiled.

Sasaoka Masatoshi and Fujiwara Takahiro, eds. Tropical Forest Conservation for Whom? Implications for Future Tropical Forest Governance from Field Research. Tokyo: Shinsensha, 2021, 251+xxviip.

Sasaoka Masatoshi and Fujiwara Takahiro, eds. <i>Tropical Forest Conservation for Whom? Implications for Future Tropical Forest Governance from Field Research</i>. Tokyo: Shinsensha, 2021, 251+xxviip.

Small understorey trees have greater capacity than canopy trees to adjust hydraulic traits following prolonged experimental drought in a tropical forest.

Future climate change predictions for tropical forests highlight increased frequency and intensity of extreme drought events. However, it remains unclear whether large and small trees have differential strategies to tolerate drought due to the different niches they occupy. The future of tropical forests is ultimately dependent on the capacity of small trees (<10cm in diameter) to adjust their hydraulic system to tolerate drought. To address this question, we evaluated whether the drought tolerance of neotropical small trees can adjust to experimental water stress and was different from tall trees. We measured multiple drought resistance-related hydraulic traits across nine common neotropical genera at the world's longest-running tropical forest throughfall-exclusion experiment and compared their responses with surviving large canopy trees. Small understorey trees in both the control and the throughfall-exclusion treatment had lower minimum stomatal conductance and maximum hydraulic leaf-specific conductivity relative to large trees of the same genera, as well as a greater hydraulic safety margin (HSM), percentage loss of conductivity and embolism resistance, demonstrating that they occupy a distinct hydraulic niche. Surprisingly, in response to the drought treatment, small trees increased specific hydraulic conductivity by 56.3% and leaf:sapwood area ratio by 45.6%. The greater HSM of small understorey trees relative to large canopy trees likely enabled them to adjust other aspects of their hydraulic systems to increase hydraulic conductivity and take advantage of increases in light availability in the understorey resulting from the drought-induced mortality of canopy trees. Our results demonstrate that differences in hydraulic strategies between small understorey and large canopy trees drive hydraulic niche segregation. Small understorey trees can adjust their hydraulic systems in response to changes in water and light availability, indicating that natural regeneration of tropical forests following long-term drought may be possible.

Complete or overcompensatory thermal acclimation of leaf dark respiration in African tropical trees.

Summary Tropical climates are getting warmer, with pronounced dry periods in large areas. The productivity and climate feedbacks of future tropical forests depend on the ability of trees to acclimate their physiological processes, such as leaf dark respiration (R d), to these new conditions. However, knowledge on this is currently limited due to data scarcity.We studied the impact of growth temperature on R d and its dependency on net photosynthesis (A n), leaf nitrogen (N) and phosphorus (P) contents, and leaf mass per unit area (LMA) in 16 early‐successional (ES) and late‐successional (LS) tropical tree species in multispecies plantations along an elevation gradient (Rwanda TREE project). Moreover, we explored the effect of drought on R d in one ES and one LS species.Leaf R d at 20°C decreased at warmer sites, regardless if it was expressed per unit leaf area, mass, N or P. This acclimation resulted in an 8% and a 28% decrease in R d at prevailing nighttime temperatures in trees at the intermediate and warmest sites, respectively. Moreover, drought reduced R d, particularly in the ES species and at the coolest site.Thermal acclimation of R d is complete or overcompensatory and independent of changes in leaf nutrients or LMA in African tropical trees.

Testing for changes in biomass dynamics in large-scale forest datasets.

Tropical forest responses to climate and atmospheric change are critical to the future of the global carbon budget. Recent studies have reported increases in estimated above-ground biomass (EAGB) stocks, productivity, and mortality in old-growth tropical forests. These increases could reflect a shift in forest functioning due to global change and/or long-lasting recovery from past disturbance. We introduce a novel approach to disentangle the relative contributions of these mechanisms by decomposing changes in whole-plot biomass fluxes into contributions from changes in the distribution of gap-successional stages and changes in fluxes for a given stage. Using 30years of forest dynamic data at Barro Colorado Island, Panama, we investigated temporal variation in EAGB fluxes as a function of initial EAGB (EAGBi ) in 10×10m quadrats. Productivity and mortality fluxes both increased strongly with initial quadrat EAGB. The distribution of EAGB (and thus EAGBi ) across quadrats hardly varied over 30years (and seven censuses). EAGB fluxes as a function of EAGBi varied largely and significantly among census intervals, with notably higher productivity in 1985-1990 associated with recovery from the 1982-1983 El Niño event. Variation in whole-plot fluxes among census intervals was explained overwhelmingly by variation in fluxes as a function of EAGBi , with essentially no contribution from changes in EAGBi distributions. The high observed temporal variation in productivity and mortality suggests that this forest is very sensitive to climate variability. There was no consistent long-term trend in productivity, mortality, or biomass in this forest over 30years, although the temporal variability in productivity and mortality was so strong that it could well mask a substantial trend. Accurate prediction of future tropical forest carbon budgets will require accounting for disturbance-recovery dynamics and understanding temporal variability in productivity and mortality.

Defaunation impacts on seed survival and its effect on the biomass of future tropical forests

Large animal species, which provide important ecological functions such as dispersal of seeds or top–down control of seed predators, are very vulnerable in fragmented forests, being unable to survive in small fragments, and facing increasing hunting pressure. The loss of large animals affects two main ecological processes crucial for the tree reproductive cycle: seed dispersal of large seeds (e.g. provided by tapirs) and control of seed predator population (e.g. provided by large cats). The changes in both processes are expected to increase seed mortality since seeds are not dispersed away from conspecifics (causing increased pre‐dispersal mortality due to negative density dependent effects) and/or face increased predation after a dispersal event (post‐dispersal mortality). Although an extensive body of empirical knowledge exists on seed predation, the link between seed loss and adult tree community composition and structure is not well established, as well as the temporal scale seed changes affect adults. Using an individual‐based forest model (FORMIND), we evaluate the long‐term consequences of increased pre and post‐dispersal seed mortality on the future forest biomass retention of a Brazilian northeastern Atlantic forest. Our results show that forest biomass is significantly affected after 80–93% pre‐dispersal loss of large seeds, or post‐dispersal predation densities of 20–25 predators per parent tree. Large‐seeded tree species are at increased risk of local extinction causing up to 26.2% loss of forest biomass when both pre and post‐dispersal processes are combined. However, these changes can last up to 100 years after the occurrence of defaunation. In summary we conclude that large animal loss has the potential to reduce future forest biomass and tree species‐richness by impacting seed survival, and should be considered in the planning of biodiversity friendly landscapes as well as in calculations of the global carbon budget.

Climate change impacts on tropical forests: identifying risks for tropical Asia

There is growing evidence that global climate change is significantly altering forest ecosystems, and will continue to do so in the future. Changes in mean climate and climate extremes such as drought, storms, cyclones and wildfires can fundamentally alter species distribution, composition, phenology, and forest structure. This study reviewed the available evidence of climate change impacts on tropical forests. We selected 85 studies based on two selection criterias and recorded the impacts of climate change on different areas of tropical forests. The majority of the studies examined climate change impacts on forest structure and composition (72%), with few considering phenology (8%). This study focused on tropical Asian forests because of their high biodiversity values and their vulnerability to the interacting threats of forest fragmentation and climate change. The difference map (2080–2100 compared with 1980–2000) indicates a significant acceleration of mean warming (5–9 °C) and increase in mean precipitation (0.5–1 mm day-1) in the Himalayan Highlands, Tibetan Plateau and arid regions of South Asia. Based on this review, two issues were posed to direct future tropical forest research: (1) effect of climate change on the extinction risk of tropical trees and (2) integration of climate change risks into forest policy and management.

The future of tropical forests under the United Nations Sustainable Development Goals

ABSTRACTIn September 2015, member states of the United Nations unanimously adopted the Sustainable Development Goals (SDGs)—a set of 17 ambitions for the post-2015 global development agenda. The goals do not offer a prescriptive plan but establish levers of policy action that seek to improve the three pillars of sustainable development: society, environment, and the economy. To facilitate achieving the SDGs, it will be critical to identify context-specific opportunities and challenges for implementation. Tropical regions of the world currently host not only the highest levels of biodiversity but also some of the highest rates of urbanization and development globally. Moreover, tropical forest deforestation is a globally significant issue; it has adverse impacts on biodiversity, climate systems, and socioeconomic equality. Here, we provide a rapid overview and qualitative assessment of the academic and policy literature on development and tropical forests, using the framework of the SDGs to examine issues broadly relevant to both tropical forests and sustainable development. Our assessment gathers existing knowledge and reveals critical knowledge gaps. In doing so, we identify key synergies between SDGs and tropical forests. We also suggest potential pathways of influence to improve social, environmental, and economic conditions in these rapidly developing regions of the world.

Urgent need for warming experiments in tropical forests.

Although tropical forests account for only a fraction of the planet's terrestrial surface, they exchange more carbon dioxide with the atmosphere than any other biome on Earth, and thus play a disproportionate role in the global climate. In the next 20 years, the tropics will experience unprecedented warming, yet there is exceedingly high uncertainty about their potential responses to this imminent climatic change. Here, we prioritize research approaches given both funding and logistical constraints in order to resolve major uncertainties about how tropical forests function and also to improve predictive capacity of earth system models. We investigate overall model uncertainty of tropical latitudes and explore the scientific benefits and inevitable trade-offs inherent in large-scale manipulative field experiments. With a Coupled Model Intercomparison Project Phase 5 analysis, we found that model variability in projected net ecosystem production was nearly 3 times greater in the tropics than for any other latitude. Through a review of the most current literature, we concluded that manipulative warming experiments are vital to accurately predict future tropical forest carbon balance, and we further recommend the establishment of a network of comparable studies spanning gradients of precipitation, edaphic qualities, plant types, and/or land use change. We provide arguments for long-term, single-factor warming experiments that incorporate warming of the most biogeochemically active ecosystem components (i.e. leaves, roots, soil microbes). Hypothesis testing of underlying mechanisms should be a priority, along with improving model parameterization and constraints. No single tropical forest is representative of all tropical forests; therefore logistical feasibility should be the most important consideration for locating large-scale manipulative experiments. Above all, we advocate for multi-faceted research programs, and we offer arguments for what we consider the most powerful and urgent way forward in order to improve our understanding of tropical forest responses to climate change.

André Aubréville (1897-1982), a pioneering forester and a visionary mind

André Aubréville fut un forestier d’exception. Formé à l’École forestière française de Nancy, il rejoignit dans les années 1920 les services forestiers coloniaux. Ses livres quifontencore référence témoignent d’un esprit encyclopé- dique, d’une grande lucidité et d’un talent de visionnaire. Comprenant combien l’avenir de la forêt tropicale était menacé par les activi- tés humaines, il mit en place des structures forestières susceptibles d’assurer ce qu’on nommerait aujourd’hui une production sou- tenue. Il devint en 1939 Inspecteur général des Eaux et Forêts de l’Afrique occidentale française mais était aussi déjà réputé comme botaniste spécialiste des Sapotacées et bio- géographe. À l’âge de 60 ans, il fut nommé Professeur au Muséum national d’histoire naturelle de Paris, et entra à l’Académie des Sciences de l’Institut de France. Travailleur in- fatigable, il considérait la botanique comme un outil permettant de mieux comprendre la complexité des forêts tropicales. S’insur- geant contre les généralisations abusives, il montra pourquoi la climatologie actuelle ou passée déterminait localement la compo- sition floristique et les contours de la forêt africaine, forêt malmenée par les feux dont il mesurait l’effet dévastateur et durable. Il fut le premier à entreprendre une véritable tropicalisation de la foresterie, à démontrer la faiblesse du concept de forêt primaire et à suggérer la valorisation des forêts secon- daires. Il ne doutait pas que le matériau bois tiendrait, cinquante ans plus tard, face aux matériaux concurrents, et que les espèces les plus demandées le demeureraient encore pour leurs qualités technologiques supé- rieures. L’ensemble de son œuvre appelait à une forme d’interventionnisme susceptible de préserver les ressources forestières tropi- cales de toute forme d’exploitation abusive. D’aucuns ont pu y voir l’expression d’un tem- pérament jugé trop colonial. Mais on peut tout autant aujourd’hui y déceler, par l’am- pleur des propositions formulées en faveur de la protection de la végétation forestière, l’œuvre d’un forestier résolument précurseur et visionnaire.

Environmental determinants of tropical forest and savanna distribution: A quantitative model evaluation and its implication

AbstractThe distributions of tropical ecosystems are rapidly being altered by climate change and anthropogenic activities. One possible trend—the loss of tropical forests and replacement by savannas—could result in significant shifts in ecosystem services and biodiversity loss. However, the influence and the relative importance of environmental factors in regulating the distribution of tropical forest and savanna biomes are still poorly understood, which makes it difficult to predict future tropical forest and savanna distributions in the context of climate change. Here we use boosted regression trees to quantitatively evaluate the importance of environmental predictors—mainly climatic, edaphic, and fire factors—for the tropical forest‐savanna distribution at a mesoscale across the tropics (between 15°N and 35°S). Our results demonstrate that climate alone can explain most of the distribution of tropical forest and savanna at the scale considered; dry season average precipitation is the single most important determinant across tropical Asia‐Australia, Africa, and South America. Given the strong tendency of increased seasonality and decreased dry season precipitation predicted by global climate models, we estimate that about 28% of what is now tropical forest would likely be lost to savanna by the late 21st century under the future scenario considered. This study highlights the importance of climate seasonality and interannual variability in predicting the distribution of tropical forest and savanna, supporting the climate as the primary driver in the savanna biogeography.

Atlantic rain forest recovery: successional drivers of floristic and structural patterns of secondary forest in Southern Brazil

AbstractQuestionsThe growing importance of secondary forests has stimulated research on the patterns of structure, biomass, species diversity and species composition of successional tropical forests. Despite current knowledge, the future of tropical forests and their ability to recover from human impacts are still a challenge for researchers. The Atlantic Forest is one of the most endangered ecosystems on Earth, yet has been reduced in size and distribution due to human disturbance such that most of the remaining fragments are secondary forest at different successional stages. How do these forests recover structure and species composition during succession? What are the main drivers of community assembly during succession?LocationSubtropical Atlantic rain forest, South Brazil (29°42′S, 50°11′W).MethodUsing four replicate chronosequences, we described floristic and structural successional patterns of subtropical Atlantic Forest in Brazil, and investigated causal relationships with environmental, spatial and temporal variables in structuring tree species composition. The chronosequences were described by four successional stages, according to floristic (species composition and richness) and structural (density, basal cover and height of trees) characteristics, tested using ANOVA with randomization tests. Time since abandonment and environmental conditions of topsoil, relief and spatial components were considered as possible successional drivers. Hypotheses about causal relationships in the successional process were tested using path analysis, considering the influence of those variables on the successional forest communities.ResultsWe found that forest stands of 26–45 yr old displayed structural characteristics similar to those of old‐growth stands, but that species composition of successional forest was somewhat different from reference areas (old‐growth forests). In our study, time since abandonment overcame the influence of environmental and spatial variables on forest assembly patterns in the chronosequences.ConclusionsOur study is the first one describing floristic and structural successional patterns of the Atlantic rain forest in a subtropical context and proposing causal relationships between recovery age, environment and community composition of secondary successional forests. We conclude that the dynamic of forest succession is autogenic and not entirely predictable from local environmental conditions of soil, space and relief.

Changes of community composition at multiple trophic levels due to hunting in Nigerian tropical forests

Hunting in tropical forests decimates large mammals, and this may have direct and indirect effects on other trophic levels and lead to trophic cascades. We compared replicated sites of hunted and protected forests in southeastern Nigeria, with respect to community composition of primates, other mammals, birds, plant seedlings, and mature trees. We make predictions regarding the community composition at the different trophic levels. In forests where large primates are rare, we hypothesize that their ecological role will not be fully compensated for by small frugivores. We apply multivariate methods to assess changes in community composition of mammals, birds, and seedlings, controlling for any differences between sites in the other groups, including mature trees. Medium and large (4–180 kg) primates were much rarer in hunted sites, while porcupine and rock hyrax increased in abundance with hunting. In contrast, the community composition of birds was similar in both types of forests. Seedling communities were significantly related to the community composition of mammals, and thus strongly affected by hunting. In protected forests primate dispersed plant seedling species dominated, whereas in hunted forests the seedling community was shifted towards one dominated by abiotically dispersed species. This was probably both a consequence of reduced seed dispersal by primates, and increased seed predation by rodents and hyrax. Hence we found no evidence for buffering effects on tree regeneration through functional compensation by non‐hunted animals (such as birds). Our results highlight how seedling communities are changed by the complex plant–animal intera ctions, triggered by the loss of seed dispersers. The results predict a rarity of primate‐dispersed trees in future tropical forest canopies; a forest less diverse in timber and non‐timber resources.

Successional convergence, stochastic assembly and the future of tropical forests

AbstractIn this issue of the Journal of Vegetation Science, Dent et al. take advantage of a century‐long chronosequence of forest succession in central Panama, and highlight a lack of convergence to the tree composition of old‐growth stands. Does this suggest tropical forests assemble randomly, or is this an expected consequence of high diversity and patchy recruitment?

Residence times of woody biomass in tropical forests

Background: The woody biomass residence time (τw) of an ecosystem is an important variable for accurately simulating its biomass stocks. Methods and results: We reviewed published data from 177 forest plots across the tropics and found a six-fold variation (23–129 years) in τw across our dataset, with a median τw of ca. 50 years. This value is similar to the median default value across 21 vegetation models for tropical forests, although the range of values used in models is large (20 to 200 years). Conclusions: The notion of a constant τw across all tropical forests may be of limited utility, given the large observed variation in τw. We found that while there was little relationship between climate variables and τw, there was evidence that edaphic factors exerted a strong influence on τw. In both the Neotropics and the Paleotropics, τw was highest in heavily weathered soils, suggesting that low soil fertility and/or non-limiting soil physical conditions exert a critical influence on τw. There is considerable uncertainty in how τw will be affected by global environmental change, especially by increased atmospheric CO2. Even small changes in τw could significantly reduce the future tropical forest carbon sink predicted by many vegetation models.

Drought‐induced shifts in the floristic and functional composition of tropical forests in Ghana

The future of tropical forests under global environmental change is uncertain, with biodiversity and carbon stocks at risk if precipitation regimes alter. Here, we assess changes in plant functional composition and biomass in 19 plots from a variety of forest types during two decades of long-term drought in Ghana. We find a consistent increase in dry forest, deciduous, canopy species with intermediate light demand and a concomitant decrease in wet forest, evergreen, sub-canopy and shade-tolerant species. These changes in composition are accompanied by an increase in above-ground biomass. Our results indicate that by altering composition in favour of drought-tolerant species, the biomass stocks of these forests may be more resilient to longer term drought than short-term studies of severe individual droughts suggest.

The Future of Tropical Forests in the Tide of Global Climate Change

Tropical forests are disappearing at an alarming rate. It is estimated that approximately 13 million ha of forests are lost in the world annually (Food and Agriculture Organization of the United Na...

The productivity, metabolism and carbon cycle of tropical forest vegetation

Summary1. Tropical forests account for one‐third of the total metabolic activity of the Earth’s land surface. Hence, understanding the controls on tropical forest photosynthesis and respiration, and the allocation of the products of photosynthesis to canopy, woody tissue and rhizosphere, is important to understand global ecosystem functioning.2. I review how studies in tropical ecosystem ecology have progressed since their inception in the 1960s towards developing a quantitative, mechanistic and global description of the carbon cycle of tropical vegetation.3. I present a synthesis of studies in tropical forest sites in the Americas and Asia for which gross primary productivity (GPP) has been reported, and a subset of these sites for which net primary productivity (NPP) and ecosystem carbon use efficiency (CUE) have been estimated. GPP ranges between 30 and 40 Mg C ha−1 year−1 in lowland moist tropical forests and declines with elevation. CUE in tropical forests is at the low end of the global range reported for forests.4. A pathway and framework are presented to explain the link between photosynthesis and tropical forest biomass, and to explain differences in carbon cycling and biomass between forests. Variation in CUE and allocation of NPP can be as important as variation in GPP in explaining differences in tropical forest growth rates between sites.5. Finally, I explore some of the key questions surrounding the functioning and future of tropical forests in the rapidly changing conditions of the early Anthropocene.6. Synthesis.There have been significant recent advances in quantifying the carbon cycle of tropical forests, but our understanding of causes of variation amongst forests is still poor. Moreover, we should expect all tropical forests in the 21st century, whether intact or disturbed, to be undergoing rapid change in function and composition; the key challenge for tropical ecosystem ecologists is to determine and understand the major and most fundamental aspects of this change.