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Carbon stocks in central African forests enhanced by elephant disturbance

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Large herbivores, such as elephants, can have important effects on ecosystems and biogeochemical cycles. Yet, the influence of elephants on the structure, productivity and carbon stocks in Africa’s rainforests remain largely unknown. Here, we quantify those effects by incorporating elephant disturbance in the Ecosystem Demography model, and verify the modelled effects by comparing them with forest inventory data from two lowland primary forests in Africa. We find that the reduction of forest stem density due to the presence of elephants leads to changes in the competition for light, water and space among trees. These changes favour the emergence of fewer and larger trees with higher wood density. Such a shift in African’s rainforest structure and species composition increases the long-term equilibrium of aboveground biomass. The shift also reduces the forest net primary productivity, given the trade-off between productivity and wood density. At a typical density of 0.5 to 1 animals per km², elephant disturbances increase aboveground biomass by 26–60 t ha⁻¹. Conversely, the extinction of forest elephants would result in a 7% decrease in the aboveground biomass in central African rainforests. These modelled results are confirmed by field inventory data. We speculate that the presence of forest elephants may have shaped the structure of Africa’s rainforests, which probably plays an important role in differentiating them from Amazonian rainforests.
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1Dipartimento per la Innovazione nei sistemi Biologici, Agroalimentari e Forestali, University of Tuscia, Viterbo, Italy. 2Laboratoire des Sciences du Climat
et de l’Environnement, IPSL-LSCE CEA/CNRS/UVSQ, Gif-sur-Yvette, France. 3Biogéosciences, UMR 6282 CNRS, Université Bourgogne Franche-Comté,
Dijon, France. 4Embrapa Agricultural Informatics, Campinas, Brazil. 5NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA,
USA. 6Department of Biology, Saint Louis University, St. Louis, MO, USA. 7Wildlife Conservation Society, New York, NY, USA. 8Núcleo de Estudos e
Pesquisas Ambientais, Universidade Estadual de Campinas, Campinas, Brazil. 9School of Informatics, Computing, and Cyber Systems, Northern Arizona
University, Flagstaff, AZ, USA. *e-mail:
Megaherbivores and large herbivores (terrestrial vertebrates
with body mass greater than 1,000 kg and 45–1,000 kg,
respectively) can have profound effects on ecosystems and
biogeochemical cycles by consuming biomass, transporting nutri-
ents and changing plant mortality15. The extinction of most mega-
herbivores at the end of the Pleistocene induced cascading effects on
plant communities and ecosystem functioning13,5. Megaherbivores
and most large herbivores are now endangered, and their disappear-
ance may have important ecological repercussions14. Elephants,
one of the last remaining megaherbivores, are classified as vul-
nerable (Loxodonta) or endangered (Elephas) by the International
Union for Conservation of Nature Red List6. The ecosystem-engi-
neering role of savannah elephants (L. africana Blumenbach, 1797)
has been studied extensively7 but much less is known about the role
of forest elephants (L. cyclotis Matschie, 1900) in African rainfor-
ests. Forest elephants are rapidly declining in numbers8 and have
mostly received attention for their role as seed dispersers911. Forest
elephants are found in west and central African forests; they are not
found in Amazonia, nor is any comparable species. The presence of
elephants in central African rainforests could partly explain some
of their distinctive features compared with Amazonian forests.
Despite similar climate and soil conditions, central African forests
have a lower average stem density (426 ± 11 stems ha1), larger tree
diameters (average 31 cm) and higher mean aboveground biomass
(AGB) (~360–430 Mg ha1 dry weight) compared to Amazonian
forests (~600 ± 11 stems ha1, ~25 cm and ~260–340 Mg ha1,
respectively)1214. Although Amazonia has some high-AGB areas,
elephants may contribute to biome-scale differences between
the two continents over long timescales. Forest elephants kill and
browse trees smaller than 30 cm in diameter that are located on and
near trails used for movement; a size class subject to strong light
competition15. We hypothesize that the chronic thinning of those
small trees by elephants alleviates competition for resources in the
low canopy strata, allowing surviving trees to attain large sizes—a
process that gives rise to higher total AGB stocks at the stand level.
To test this hypothesis, elephant disturbance was incorporated
into a mechanistic forest-stand model (ED216; see Methods). The
model simulations were evaluated against measurements of tree
size and wood density at two Congo Basin sites (see Methods) with
contrasting elephant disturbance11,17. The ED2 model simulates
horizontal and vertical vegetation heterogeneity in long-term for-
est succession, plant competition for resources leading to mortal-
ity and stochastic disturbance events (for example, tree fall). Plant
functional diversity in ED2 is represented by three plant functional
types (PFTs): early-successional trees (shade-intolerant, fast-grow-
ing pioneers, low wood density), mid-successional trees (interme-
diate) and late-successional trees (shade-tolerant, slow-growing,
canopy-dominant and high wood density) (see the PFT parameters
in Supplementary Table 1). We represented elephant disturbance by
increasing the mortality of trees with diameters <30 cm based on
observations of plant survival rates from browsing or trampling18,19.
Mortality was inversely proportional to tree size and proportional
to animal population density4. We performed idealized site-level
simulations to analyse the sensitivity of forest properties to differ-
ent animal densities that are representative of central Africa. These
densities ranged from 0.25 to 5 individuals km2 (refs. 8,20).
Here we show that elephant disturbance changes forest structure,
increases AGB and average tree diameter, and reduces stem density,
Carbon stocks in central African forests enhanced
by elephant disturbance
Fabio Berzaghi 1,2,3*, Marcos Longo 4,5, Philippe Ciais 2, Stephen Blake6,7, François Bretagnolle3,
Simone Vieira 8, Marcos Scaranello4, Giuseppe Scarascia-Mugnozza1 and Christopher E. Doughty 9
Large herbivores, such as elephants, can have important effects on ecosystems and biogeochemical cycles. Yet, the influence
of elephants on the structure, productivity and carbon stocks in Africa’s rainforests remain largely unknown. Here, we quantify
those effects by incorporating elephant disturbance in the Ecosystem Demography model, and verify the modelled effects by
comparing them with forest inventory data from two lowland primary forests in Africa. We find that the reduction of forest stem
density due to the presence of elephants leads to changes in the competition for light, water and space among trees. These
changes favour the emergence of fewer and larger trees with higher wood density. Such a shift in African’s rainforest structure
and species composition increases the long-term equilibrium of aboveground biomass. The shift also reduces the forest net
primary productivity, given the trade-of f between productivity and wood density. At a typical density of 0.5 to 1 animals per km2,
elephant disturbances increase aboveground biomass by 26–60 t ha1. Conversely, the extinction of forest elephants would
result in a 7% decrease in the aboveground biomass in central African rainforests. These modelled results are confirmed by
field inventory data. We speculate that the presence of forest elephants may have shaped the structure of Africa’s rainforests,
which probably plays an important role in differentiating them from Amazonian rainforests.
Corrected: Author Correction
NATURE GEOSCIENCE | VOL 12 | SEPTEMBER 2019 | 725–729 | 725
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... Elephants are the largest and among the most long-lived, intelligent, and socially complex terrestrial animals (Filippi et al., 2017;Healy et al., 2014;McComb et al., 2000;Wittemyer et al., 2005;, and tropical rainforests are the most diverse ecosystems on the planet (Connell, 1978). As keystone species and ecosystem engineers (Berzaghi et al., 2019;Blake et al., 2009), patterns of forest elephant movements may have profound impacts on the structure and composition of their habitats, which feeds back into shaping future elephant movement trajectories in an oscillating cycle (Blake et al., 2009). This chapter summarizes movement data from the Republic of Congo, Gabon, and the Central African Republic to describe the evolutionary ecology ballet between forest elephants and their environment and the unfortunate role that humans now play as choreographers. ...
... These include reduced fitness of browsed species, potentially increased fitness of competitors of browsed species, reduced fitness for consumers that share diet items with elephants, increased fitness for their competitors, and so on through the web of interactions. This influences the balance of competitive interactions among trees in Central African forests toward slow growing, high wood density species, with globally relevant implications for carbon sequestration and climate change (Berzaghi et al., 2019). ...
... By selling credits for standing, healthy rainforests, the trees could be worth more alive than dead -and maintain the huge diversity of flora and fauna within them, including the forest elephants. Moreover, forests containing functional populations of forest elephants sequester more carbon that forests that do not, at globally relevant levels (Berzaghi et al., 2019). The forest elephant range states are rich in natural resources but are among the lowest ranking nations according to the human development index -indeed, the highest-ranking forest elephant range state ranks 119th (out of 189 countries) on that index (UNDP, 2020). ...
To survive, all organisms must maximize energy input and reproductive output and minimize risk. This applies to how they travel through their environment. Due to numerous mechanical and physical laws that scale allometrically, forest elephants (Loxodonta cyclotis), as the largest vertebrate inhabitants of Africa’s dense tropical forests, solve this optimization in rather different ways than the smallest, for example, shrews. In this chapter, we discuss how body size influences animal ranging and why elephants ought to have very large ranges. We then use GPS telemetry data we collected ourselves and additional data from published studies to characterize home range size and other movement metrics of forest elephants in Central Africa. We demonstrate how the availability of water, food, nutrients, social organization, sex, and personality combines to drive the movements of forest elephants. We conclude that these factors are largely trumped by a human-induced landscape of fear throughout the range of forest elephants. We explain how the combination of large body size and the extent of forest elephant movements lead to their profound ecosystem engineering impacts, which help maintain forest biodiversity and increase carbon sequestration. We then show how human activities, primarily poaching and infrastructure development, restrict elephant movements, with negative consequences for forest function that have globally relevant ramifications. We finally argue that if forest elephant movements in their present form are to be maintained, the planet’s rich nations must match and surpass the impressive legislation for protected areas made by forest elephant range states in their commitment to demand and create the economic conditions needed for the sustainable management of tropical forest resources, including elephants.KeywordsAnimal movementTropical forestCongo BasinConservationEcosystem engineerHome range
... Also, animal species were shown to have different net effects on ecosystems under different biophysical conditions, such as different rainfall regimes or soil textures 42,72 . As well, they can have different impacts in different ecosystem types, such as the positive effects of wolves in boreal forests but negative effects in grasslands 55 , or the positive effects of elephants in tropical forests but neutral or negative effects in savannah 42,[73][74][75] . Animal effects may vary with their population density. ...
... However, animal effects may also be non-linear. At a low population density, species might be functionally neutral, and may only become functionally effective at higher densities 75 . For instance, the effect of forest elephants on carbon storage is negligible at densities less than 0.25 km -2 but becomes increasingly positive at higher densities, and even becomes negative at densities beyond 4 km −2 (ref. ...
... For instance, the effect of forest elephants on carbon storage is negligible at densities less than 0.25 km -2 but becomes increasingly positive at higher densities, and even becomes negative at densities beyond 4 km −2 (ref. 75). ...
... If these slow-growing saplings reach maturity, however, many will provide fruits for elephants. Elephants then sow the seeds of these late-successional trees in their nutrient-rich dung (Berzaghi et al., 2019;Campos-Arceiz & Blake, 2011). In some cases, an animal species' ecological roles can have counteracting effects on F I G U R E 2 Examples of ecological functions of animals that influence vegetation structure and the approximate duration of the impact. ...
... Change-inducing feedback loops resulting from the functional extinction of animals can have important implications for carbon storage, nutrient cycling and biodiversity. Still, they may not be detected for tens to hundreds of years, especially within forested environments, due to the slow growth of trees (Berzaghi et al., 2019;Osuri et al., 2016;Peres et al., 2016;Poulsen et al., 2013). Determining the timescale over which a feedback loop operates may present unique challenges. ...
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Ecosystem functions in a series of feedback loops that can change or maintain vegetation structure. Vegetation structure influences the ecological niche space available to animals, shaping many aspects of behaviour and reproduction. In turn, animals perform ecological functions that shape vegetation structure. However, most studies concerning three‐dimensional vegetation structure and animal ecology consider only a single direction of this relationship. Here, we review these separate lines of research and integrate them into a unified concept that describes a feedback mechanism. We also show how remote sensing and animal tracking technologies are now available at the global scale to describe feedback loops and their consequences for ecosystem functioning. An improved understanding of how animals interact with vegetation structure in feedback loops is needed to conserve ecosystems that face major disruptions in response to climate and land‐use change.
... Biodiversity monitoring has largely relied on field observations taken by highly trained survey guides, but new technologies, including camera traps (Steenweg et al., 2017), audio recordings (Aide et al., 2013;Pekin et al., 2012;Rappaport et al., 2020), and remote sensing (Davies & Asner, 2014;Moudrý et al., 2022;Simonson et al., 2014) have the potential to revolutionize methods for measuring species counts and biodiversity metrics. These exciting new developments could enable researchers, companies, and governments to track both loss and uplift of biodiversity at large scales, offering potential new pathways for valuing biodiversity and the ecosystem services it provides (Berzaghi et al., 2019;Schmitz et al., 2023). With governments beginning to recognize biodiversity as a global resource (Kunming-Montreal Global Biodiversity Framework, 2022), these monitoring technologies have great potential for use in conservation and nature-based solutions (NBS) projects worldwide. ...
Vegetation structural complexity and the diversity of animal communities are closely linked in vegetated ecosystems. These structure-diversity relationships have the potential to be used to predict biodiversity at large spatial scales using remote sensing data. However, structure-diversity relationships may not be generalizable across different ecosystems or even across ecotypes within a single ecosystem. To understand how structure-diversity relationships vary within the tree-grass mosaic of a savanna environment, we evaluated how bird diversity relates to vegetation structure at multiple scales and across environmental gradients in an East African savanna- the Selenkay Conservancy in southern Kenya. We obtained detailed characterizations of vegetation structure using Light Detection and Ranging (lidar) from Unoccupied Aerial Vehicle (UAV) surveys, and related vegetation structure metrics to bird diversity metrics (Shannon diversity and species richness) collected at 50 sites spread inside and outside of the Selenkay Conservancy. We compared structure-diversity relationships across environmental gradients, including soil type (red and black soils) and protected status (inside and outside the conservancy). We also compared structure-diversity models at multiple scales, testing how relationships changed with scale. We found significant structure-diversity relationships with improved performed at larger spatial scales (≥ 50 m radius or 0.79 ha circular plots). Models of Shannon diversity performed better than those of species richness. While most structure-diversity relationships only applied to specific soil types, certain models showed the potential to be generalized across soil types, explaining ~55-59% of the variance. We found that strong relationships exist between vegetation structure and bird diversity in savannas. While most structure-diversity relationships were only applicable to specific soil types, several vegetation metrics were able to track bird diversity across the entire landscape, performing well in both red and black soil sites. These results demonstrate the potential to use airborne remote sensing to monitor biodiversity across savanna environments.
... Furthermore, most studies on megaherbivore impacts and carbon stocks focus on elephant impacts on above ground carbon stocks (Marshall et al. 2012, Berzaghi et al. 2019, Davies and Asner 2019. A recent perspective highlights the importance of quantifying a diversity of carbon pools to better understand how herbivory impacts long-term carbon storage (Kristensen et al. 2022). ...
Recent studies suggest that wild animals can promote ecosystem carbon sinks through their impacts on vegetation and soils. However, livestock studies show that intense levels of grazing reduce soil organic carbon (SOC), leading to concerns that rewilding with large grazers may compromise ecosystem carbon storage. Furthermore, wild grazers can both limit and promote woody plant recruitment and survival on savanna grasslands, with both positive and negative impacts on SOC, depending on the rainfall and soil texture contexts. We used grazing lawns in one of the few African protected savannas that are still dominated by megagrazers (> 1000 kg), namely white rhinoceros Ceratotherium simum , as a model to study the impact of prolonged and intense wild grazing on SOC stocks. We contrasted SOC stocks between patches of varying grazing intensity and woody plant encroachment in sites across different rhino habitat types. We found no differences in SOC stocks between the most‐ and least grazed plots in any of the habitats. Intermediately grazed plots, however, had higher SOC stocks in the top 5 cm compared to most and least grazed plots, but only in the closed‐canopy woodland habitat and not in the open habitats. Importantly, we found no evidence to support the hypothesis that wild grazing reduces SOC, even at high grazing intensities by the world's largest megagrazer. Compared to the non‐encroached reference plots, woody encroached plots had higher SOC stocks in soils with low clay content and lower SOC stocks in soils with high clay content, although only in the top 5 cm. Accordingly, our study highlights that wild grazers may influence SOC indirectly through their impact on tree‐grass ratios in grassy ecosystems. Our study thus provides important insights for future natural climate solutions that focus on wild grazer conservation and restoration. Keywords: fire, grazing impact, rewilding, soil carbon, white rhinoceros, woody encroachment
The status of kelp forests and their vulnerability to climate change are of global significance. As the foundation for productive and extensive ecosystems, understanding the long-term trends in kelp is critical to coastal ecosystem management, climate resiliency, and restoration programs. In this study, we curate historical US government kelp inventories, develop methods to compare them with contemporary surveys, and use a machine learning framework to evaluate and rank the drivers of change for California kelp forests over the last century. Historical surveys documented Macrocystis and Nereocystis kelp forests covered approximately 120.4 km2 in 1910-1912, which is only slightly above surveys in 2014-2016 (112.0 km2). These statewide comparisons, however, mask dramatic regional changes with increases in Central California (+57.6%, +19.7 km2) and losses along the Northern (-63.0%, -8.1 km2), and Southern (-52.1%, -18.3 km2) mainland coastlines. Random Forest models rank sea otter (Enhydra lutris nereis) population density as the primary driver of kelp changes, with benthic substrate, extreme heat, and high annual variation in primary productivity also significant. This century-scale perspective identifies dramatically different outcomes for California’s kelp forests, providing a blueprint for nature-based solutions that enhance coastal resilience to climate change.
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Whales have been titled climate savers in the media with their recovery welcomed as a potential carbon solution. However, only a few studies were performed to date providing data or model outputs to support the hypothesis. Following an outline of the primary mechanisms by which baleen whales remove carbon from the atmosphere for eventual sequestration at regional and global scales, we conclude that the amount of carbon whales are potentially sequestering might be too little to meaningfully alter the course of climate change. This is in contrast to media perpetuating whales as climate engineers. Creating false hope in the ability of charismatic species to be climate engineers may act to further delay the urgent behavioral change needed to avert catastrophic climate change impacts, which can in turn have indirect consequences for the recovery of whale populations. Nevertheless, whales are important components of marine ecosystems, and any further investigation on existing gaps in their ecology will contribute to clarifying their contribution to the ocean carbon cycle, a major driver of the world’s climate. While whales are vital to the healthy functioning of marine ecosystems, overstating their ability to prevent or counterbalance anthropogenically induced changes in global carbon budget may unintentionally redirect attention from known, well-established methods of reducing greenhouse gases. Large scale protection of marine environments including the habitats of whales will build resilience and assist with natural carbon capture.
Large herbivores play unique ecological roles and are disproportionately imperiled by human activity. As many wild populations dwindle towards extinction, and as interest grows in restoring lost biodiversity, research on large herbivores and their ecological impacts has intensified. Yet, results are often conflicting or contingent on local conditions, and new findings have challenged conventional wisdom, making it hard to discern general principles. Here, we review what is known about the ecosystem impacts of large her-bivores globally, identify key uncertainties, and suggest priorities to guide research. Many findings are generalizable across ecosystems: large herbivores consistently exert top-down control of plant demography , species composition, and biomass, thereby suppressing fires and the abundance of smaller animals. Other general patterns do not have clearly defined impacts: large herbivores respond to predation risk but the strength of trophic cascades is variable; large herbivores move vast quantities of seeds and nutrients but with poorly understood effects on vegetation and biogeochemistry. Questions of the greatest relevance for conservation and management are among the least certain, including effects on carbon storage and other ecosystem functions and the ability to predict outcomes of extinctions and reintroduc-tions. A unifying theme is the role of body size in regulating ecological impact. Small herbivores cannot fully substitute for large ones, and large-herbivore species are not functionally redundant-losing any, especially the largest, will alter net impact, helping to explain why livestock are poor surrogates for wild species. We advocate leveraging a broad spectrum of techniques to mechanistically explain how large-herbivore traits and environmental context interactively govern the ecological impacts of these animals.
Megaherbivores perform vital ecosystem engineering roles, and have their last remaining stronghold in Africa. Of Africa's remaining megaherbivores, the common hippopotamus (Hippopotamus amphibius) has received the least scientific and conservation attention, despite how influential their ecosystem engineering activities appear to be. Given the potentially crucial ecosystem engineering influence of hippos, as well as mounting conservation concerns threatening their long-term persistence, a review of the evidence for hippos being ecosystem engineers, and the effects of their engineering, is both timely and necessary. In this review, we assess, (i) aspects of hippo biology that underlie their unique ecosystem engineering potential; (ii) evaluate hippo ecological impacts in terrestrial and aquatic environments; (iii) compare the ecosystem engineering influence of hippos to other extant African megaherbivores; (iv) evaluate factors most critical to hippo conservation and ecosystem engineering; and (v) highlight future research directions and challenges that may yield new insights into the ecological role of hippos, and of megaherbivores more broadly. We find that a variety of key life-history traits determine the hippo's unique influence, including their semi-aquatic lifestyle, large body size, specialised gut anatomy, muzzle structure, small and partially webbed feet, and highly gregarious nature. On land, hippos create grazing lawns that contain distinct plant communities and alter fire spatial extent, which shapes woody plant demographics and might assist in maintaining fire-sensitive riverine vegetation. In water, hippos deposit nutrient-rich dung, stimulating aquatic food chains and altering water chemistry and quality, impacting a host of different organisms. Hippo trampling and wallowing alters geomorphological processes, widening riverbanks, creating new river channels, and forming gullies along well-utilised hippo paths. Taken together, we propose that these myriad impacts combine to make hippos Africa's most influential megaherbivore, specifically because of the high diversity and intensity of their ecological impacts compared with other megaherbivores, and because of their unique capacity to transfer nutrients across ecosystem boundaries, enriching both terrestrial and aquatic ecosystems. Nonetheless, water pollution and extraction for agriculture and industry, erratic rainfall patterns and human-hippo conflict, threaten hippo ecosystem engineering and persistence. Therefore, we encourage greater consideration of the unique role of hippos as ecosystem engineers when considering the functional importance of megafauna in African ecosystems, and increased attention to declining hippo habitat and populations, which if unchecked could change the way in which many African ecosystems function.
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Megafauna (terrestrial vertebrate herbivores > 5kg) can have disproportionate direct and indirect effects on forest structure, function, and biogeochemical cycles. We reviewed the literature investigating these effects on tropical forest dynamics and biogeochemical cycles in relation to ecology, paleoecology, and vegetation modelling. We highlight the limitations of field‐based studies in evaluating the long‐term consequences of loss of megafauna. These limitations are due to inherent space‐time restrictions of field‐studies and a research focus on seed dispersal services provided by large animals. We further present evidence of a research gap concerning the role of megafauna in carbon cycling in tropical ecosystems. Specifically, changes in aboveground biomass might not be noticeable in short‐term studies because of slow vegetation dynamics requiring decades to respond to disturbance (i.e., defaunation). Nutrient cycling has received even less attention in relation to the role of megafauna in tropical forests. We present an approach to investigate the effects of megafauna from new perspectives and with various tools (notably, vegetation models) which can simulate long‐term dynamics in different environmental and megafauna density scenarios. Vegetation models could facilitate interaction between plant‐animal ecology and biogeochemistry research. We present practical examples on how to integrate plant‐animal interactions in vegetation models to further our understanding of the role of large herbivores in tropical forests. This article is protected by copyright. All rights reserved.
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During the warmer Holocene Period, two major climatic crises affected the Central African rainforests. The first crisis, around 4000 cal yr BP, caused the contraction of the forest in favor of savanna expansion at its northern and southern periphery. The second crisis, around 2500 cal yr BP, resulted in major perturbation at the forest core, leading to forest disturbance and fragmentation with a rapid expansion of pioneer-type vegetation, and a marked erosional phase. The major driver of these two climatic crises appears to be rapid sea-surface temperature variations in the equatorial eastern Atlantic, which modified the regional atmospheric circulation. The change between ca. 2500 to 2000 cal yr BP led to a large increase in thunderstorm activity, which explains the phase of forest fragmentation. Ultimately, climatic data obtained recently show that the present-day major rise in thunderstorms and lightning activity in Central Africa could result from some kind of solar influence, and hence the phase of forest fragmentation between ca. 2500 to 2000 cal yr BP may provide a model for the present-day global warming-related environmental changes in this region.
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Tropical forest mortality is controlled by both biotic and abiotic processes, but how these processes interact to determine forest structure is not well understood. Using long‐term demography data from permanent forest plots at the Paracou Tropical Forest Research Station in French Guiana, we analysed the relative influence of competition and climate on tree mortality. We found that self‐thinning is evident at the stand level, and is associated with clumped mortality at smaller scales (<2 m) and regular spacing of living trees at intermediate (2.5–7.5 m) scales. A competition index ( CI ) based on spatial clustering of dead trees was used to build predictive mortality models, which also accounted for climate interactions. The model that most closely fitted observations included both the CI and climatic variables, with climate‐only and competition‐only models less informative than the full model. There was strong evidence for U‐shaped size‐specific mortality, with highest mortality for small and very large trees, as well as sensitivity of trees to drought, especially when temperatures were high, and when soils were water saturated. The effect of the CI was more complex than expected a priori: a higher CI was associated with lower mortality odds, which we hypothesize is caused by gap‐phase dynamics, but there was also evidence for competition‐induced mortality at very high CI values. The strong signature of competition as a control over mortality at the stand and individual scales confirms its important role in determining tropical forest structure. The complexity of the competition‐mortality relationship and its interaction with climate indicates that a thorough consideration of the scale of analysis is needed when inferring the role of competition in tropical forests, but demonstrates that climate‐only mortality models can be significantly improved by including competition effects, even when ignoring species‐specific effects. Synthesis . Empirical models such as the one developed here can help constrain and improve process‐based vegetation models, serving both as a benchmark and as a means to disentangle mortality processes. Tropical vegetation dynamic models would benefit greatly from explicitly considering the role of competition in stand development and self‐thinning while modelling demography, as well as its interaction with climate.
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Megaherbivores are known to influence the structure, composition, and diversity of vegetation. In Central Africa, forest elephants act as ecological filters by breaking tree saplings and stripping them of foliage. Much less is known about impacts of megafauna on Southeast Asian rain forests. Here, we ask whether herbivory by Asian megafauna has impacts analogous to those of African forest elephants. To answer this, we studied forest (1) structure, (2) composition, (3) diversity, and (4) tree scars in Belum and Krau, two protected areas of Peninsular Malaysia, and compared the results with those obtained in African forests. Elephants are abundant in Belum but have been absent in Krau since 1993. We found that stem density and diversity, especially of tree saplings, were higher in Krau than in Belum. Palms and other monocots were also more abundant in Krau. In Belum, however, small monocots (<1 m tall) were very abundant but larger ones (>1 m tall) were virtually absent, suggesting size-selective removal. The frequency of stem-break scars was equal at Belum and Krau but less than in Central Africa and greater than in the Peruvian Amazon where tapirs are the only megafauna. Pigs and tapirs could also contribute to the high frequency of tree scars recorded in Malaysian forests. Forest-dwelling elephants in Asia seem to have a reduced impact on tree saplings compared to African forest elephants, but a very strong impact on monocots.
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Defaunation is causing declines of large-seeded animal-dispersed trees in tropical forests worldwide, but whether and how these declines will affect carbon storage across this biome is unclear. Here we show, using a pan-tropical data set, that simulated declines of large-seeded animal-dispersed trees have contrasting effects on aboveground carbon stocks across Earth’s tropical forests. In our simulations, African, American and South Asian forests, which have high proportions of animal-dispersed species, consistently show carbon losses (2–12%), but Southeast Asian and Australian forests, where there are more abiotically dispersed species, show little to no carbon losses or marginal gains (±1%). These patterns result primarily from changes in wood volume, and are underlain by consistent relationships in our empirical data (~2,100 species), wherein, large-seeded animal-dispersed species are larger as adults than small-seeded animal-dispersed species, but are smaller than abiotically dispersed species. Thus, floristic differences and distinct dispersal mode–seed size–adult size combinations can drive contrasting regional responses to defaunation.
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In stable populations with constant demographic rates, size distributions reflect size-dependent patterns of growth and mortality. However, population growth can also affect size distributions, which may not be aligned with current growth and mortality. Using 25 y of demographic data from the 50-ha Barro Colorado Island plot, we examined how interspecific variation in diameter distributions of over 150 tropical trees relates to growth–diameter and mortality–diameter curves and to population growth rates. Diameter distributions were more skewed in species with faster increases/slower decreases in absolute growth and mortality with diameter and higher population growth rates. The strongest predictor of the diameter distribution shape was the exponent governing the scaling of growth with diameter (partial R 2 = 0.20–0.34), which differed among growth forms, indicating a role of life history variation. However, interspecific variation in diameter distributions was also significantly related to population growth rates (partial R 2 = 0.03–0.23), reinforcing that many populations are not at equilibrium. Consequently, although fitted size distribution parameters were positively related to theoretical predictions based on current size-dependent growth and mortality, there was considerable deviation. These analyses show that temporally variable demographic rates, probably related to cyclic climate variation, are important influences on forest structure.
Net primary productivity (NPP) is one of the most important parameters in describing the functioning of any ecosystem and yet it arguably remains a poorly quantified and understood component of carbon cycling in tropical forests, especially outside of the Americas. We provide the first comprehensive analysis of NPP and its carbon allocation to woody, canopy and root growth components at contrasting lowland West African forests spanning a rainfall gradient. Using a standardised methodology to study evergreen (EF), semi-deciduous (SDF), dry forests (DF) and woody savanna (WS), we find that (i) climate is more closely related with above and belowground C stocks than with NPP (ii) total NPP is highest in the SDF site, then the EF followed by the DF and WS and that (iii) different forest types have distinct carbon allocation patterns whereby SDF allocate in excess of 50% to canopy production and the DF and WS sites allocate 40-50% to woody production. Furthermore, we find that (iv) compared with canopy and root growth rates the woody growth rate of these forests is a poor proxy for their overall productivity and that (v) residence time is the primary driver in the productivity-allocation-turnover chain for the observed spatial differences in woody, leaf and root biomass across the rainfall gradient. Through a systematic assessment of forest productivity we demonstrate the importance of directly measuring the main components of above and belowground NPP and encourage the establishment of more permanent carbon intensive monitoring plots across the tropics.
Elephant populations are in peril everywhere, but forest elephants in Central Africa have sustained alarming losses in the last decade [1]. Large, remote protected areas are thought to best safeguard forest elephants by supporting large populations buffered from habitat fragmentation, edge effects and human pressures. One such area, the Minkébé National Park (MNP), Gabon, was created chiefly for its reputation of harboring a large elephant population. MNP held the highest densities of elephants in Central Africa at the turn of the century, and was considered a critical sanctuary for forest elephants because of its relatively large size and isolation. We assessed population change in the park and its surroundings between 2004 and 2014. Using two independent modeling approaches, we estimated a 78–81% decline in elephant numbers over ten years — a loss of more than 25,000 elephants. While poaching occurs from within Gabon, cross-border poaching largely drove the precipitous drop in elephant numbers. With nearly 50% of forest elephants in Central Africa thought to reside in Gabon [1], their loss from the park is a considerable setback for the preservation of the species.
We report above-ground biomass (AGB), basal area, stem density and wood mass density estimates from 260 sample plots (mean size: 1.2 ha) in intact closed-canopy tropical forests across 12 African countries. Mean AGB is 395.7 Mg dry mass ha?1 (95% CI: 14.3), substantially higher than Amazonian values, with the Congo Basin and contiguous forest region attaining AGB values (429 Mg ha?1) similar to those of Bornean forests, and significantly greater than East or West African forests. AGB therefore appears generally higher in palaeo- compared with neotropical forests. However, mean stem density is low (426 ± 11 stems ha?1 greater than or equal to 100 mm diameter) compared with both Amazonian and Bornean forests (cf. approx. 600) and is the signature structural feature of African tropical forests. While spatial autocorrelation complicates analyses, AGB shows a positive relationship with rainfall in the driest nine months of the year, and an opposite association with the wettest three months of the year; a negative relationship with temperature; positive relationship with clay-rich soils; and negative relationships with C : N ratio (suggesting a positive soil phosphorus–AGB relationship), and soil fertility computed as the sum of base cations. The results indicate that AGB is mediated by both climate and soils, and suggest that the AGB of African closed-canopy tropical forests may be particularly sensitive to future precipitation and temperature changes.