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Carbon stocks in central African forests enhanced by elephant disturbance

DOI:10.1038/s41561-019-0395-6
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Fabio Berzaghi
Fabio Berzaghi
Fabio Berzaghi
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Marcos Longo at Lawrence Berkeley National Laboratory
Marcos Longo
Philippe Ciais at Atomic Energy and Alternative Energies Commission
Philippe Ciais
Stephen Blake
Stephen Blake
Stephen Blake
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Abstract and Figures

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.
Simulated stand-scale AGB and NPP in response to different elephant population densities in the Congo Basin The percentage changes in ABG (left) and NPP (right) were calculated relative to a control simulation (without elephants) with equilibrium values of 388 Mg ha⁻¹ (AGB) and 33 Mg ha⁻¹ yr⁻¹ (NPP). Elephants were introduced at t = 0, after the control simulation. Elephant densities (individuals km⁻²) have been aggregated into bins: low (0.25), medium (0.5, 0.75, 1 and 2), high (3 and 4) and extreme (5).
Simulated stand-scale AGB and NPP in response to different elephant population densities in the Congo Basin The percentage changes in ABG (left) and NPP (right) were calculated relative to a control simulation (without elephants) with equilibrium values of 388 Mg ha⁻¹ (AGB) and 33 Mg ha⁻¹ yr⁻¹ (NPP). Elephants were introduced at t = 0, after the control simulation. Elephant densities (individuals km⁻²) have been aggregated into bins: low (0.25), medium (0.5, 0.75, 1 and 2), high (3 and 4) and extreme (5).
… 
Stand-scale effect of elephant disturbance a, Changes in stand properties and resource availability by size class. Gains (green) and losses (red) are relative to the control forest. DBH, diameter at breast height. b, Long-term effect of the prescribed density scenarios on equilibrium basal area and the composition of PFTs. c, Relative percentage of trees in each size class across density scenarios. Prescribed elephant densities have been aggregated as described in Fig. 1. The dashed lines and shaded areas represent the control (no elephants) at equilibrium and its standard deviation. The error bars indicate the standard deviation at equilibrium of the elephant-disturbed forests.
Stand-scale effect of elephant disturbance a, Changes in stand properties and resource availability by size class. Gains (green) and losses (red) are relative to the control forest. DBH, diameter at breast height. b, Long-term effect of the prescribed density scenarios on equilibrium basal area and the composition of PFTs. c, Relative percentage of trees in each size class across density scenarios. Prescribed elephant densities have been aggregated as described in Fig. 1. The dashed lines and shaded areas represent the control (no elephants) at equilibrium and its standard deviation. The error bars indicate the standard deviation at equilibrium of the elephant-disturbed forests.
… 
of the regional effect of elephant disturbance on AGB a, Change in AGB carbon (upscaled from stand to ha, see main text and Methods) compared with the control forest as a function of the prescribed elephant densities. The blue line is the fitted polynomial regression. Error bars represent confidence intervals. b, Elephant-induced increase in AGB across the central African rainforest zone (total area = 2.2 million km²) as a function of the total population and range occupied by elephants (see the main text and Methods). Historical population estimates from previous studies were used to calculate the (committed) loss of biomass carbon caused by progressive elephant extirpation8,24,25.
of the regional effect of elephant disturbance on AGB a, Change in AGB carbon (upscaled from stand to ha, see main text and Methods) compared with the control forest as a function of the prescribed elephant densities. The blue line is the fitted polynomial regression. Error bars represent confidence intervals. b, Elephant-induced increase in AGB across the central African rainforest zone (total area = 2.2 million km²) as a function of the total population and range occupied by elephants (see the main text and Methods). Historical population estimates from previous studies were used to calculate the (committed) loss of biomass carbon caused by progressive elephant extirpation8,24,25.
… 
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Articles
https://doi.org/10.1038/s41561-019-0395-6
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: fabio.berzaghi@lsce.ipsl.fr
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 mortality1–5. The extinction of most mega-
herbivores at the end of the Pleistocene induced cascading effects on
plant communities and ecosystem functioning1–3,5. Megaherbivores
and most large herbivores are now endangered, and their disappear-
ance may have important ecological repercussions1–4. 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 dispersers9–11. 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 ha−1), larger tree
diameters (average ≈ 31 cm) and higher mean aboveground biomass
(AGB) (~360–430 Mg ha−1 dry weight) compared to Amazonian
forests (~600 ± 11 stems ha−1, ~25 cm and ~260–340 Mg ha−1,
respectively)12–14. 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 km−2 (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 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.
Corrected: Author Correction
NATURE GEOSCIENCE | VOL 12 | SEPTEMBER 2019 | 725–729 | www.nature.com/naturegeoscience 725
Content courtesy of Springer Nature, terms of use apply. Rights reserved
... 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). ...
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... 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. ...
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... 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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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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Foraging impacts of Asian megafauna on tropical rain forest structure and biodiversity
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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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Contrasting effects of defaunation on aboveground carbon storage across the global tropics
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  • Apr 2016
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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How do size distributions relate to concurrently measured demographic rates? Evidence from over 150 tree species in Panama
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  • May 2016
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.
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Forest biomass, productivity and carbon cycling along a rainfall gradient in West Africa
Article
  • Sep 2017
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.
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Poaching empties critical Central African wilderness of forest elephants
Article
  • Feb 2017
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.
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Above-ground biomass and structure of 260 African tropical forests
Article
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.
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