Patterns of tree species composition across tropical African forests and within central African moist forests: the need for adapted management and conservation strategies

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Le Corum, Montpellier, 19-23 June 2016

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BSTRACTS /

ORAL PRESENTATIONS

O64-05 – S64 Towards an unified vision of the central african forests

Thursday 23 June / 08:00-10:00 – Antigone3

Can we predict forest composition across space and time in Central Africa

MAXIME RÉJOU-MÉCHAIN

_{, FRÉDÉRIC MORTIER}

_{, NICOLAS BARBIER}

_{, JEAN-FRANÇOIS BASTIN}

_,

FABRICE BÉNÉDET

_{, XAVIER BRY}

_{, JÉRÔME CHAVE}

_{, GUILLAUME CORNU}

_{, GILLES DAUBY}

_{, JEAN-LOUIS DOUCET}

_,

ADELINE FAYOLLE

_{, SYLVIE GOURLET-FLEURY}

1_{French Institute of Pondicherry, LIAG, 605001, Pondicherry, India} 2_{CIRAD, UPR BSEF, 34398, Montpellier, France}

3_{IRD, UMR AMAP, 34000, Montpellier, France}

4_{Université Montpellier, UMR I3M, 34095, Montpellier, France} 5_{CNRS, UMR 5174 EDB, 31062, Toulouse, France}

6_{IRD, UMR DIADE, 34000, Montpellier, France}

7_{Gembloux Agro-Bio Tech, Unite de gestion des ressources forestieres et des milieux naturels (GRFMN), 5030, Gembloux, Belgium}

Background. Predicting the current and future natural distributions of species is challenging, especially in the tropics where large remote areas remain poorly known. Such challenge can only be met with an in-depth understanding of the drivers of species distribution, a well-designed and extensive survey and appropriate statistical models.

Method. In this study, we use a large dataset of forest inventories from logging companies, which provides information on the abundance of 123 tree genera, in 140,000 plots spread over four Central African countries. In order to predict the current and future distribution of these tree genera, we use a set of bioclimatic, geological and anthropogenic variables. We rely on a recently published methodology, called Supervised Component Generalized Linear Regression (SCGLR), which identifies the most predictive dimensions among a large set of predictors.

Result. Using a calibration and validation scheme, we show that the distribution of most tree genera can be well predicted over the whole study area at the present time. At the community level, the floristic and functional composition of tree genera is also inferred with a good accuracy. Finally, using spatially explicit null models, we show that species-climate association are in most cases not better than chance, thus challenging our ability to predict how forest composition will be affected by climatic changes.

Conclusion. Overall, our study shows that tropical tree distributions can be predicted with good accuracy at the present time, offering new perspectives to manage tropical forests at large spatial scales, but that predicting shifts in species distribution under climate change scenarios is challenging.

O64-06 – S64 Towards an unified vision of the central african forests

Thursday 23 June / 08:00-10:00 – Antigone3

Functional shifts within Central African rainforests

JEAN-FRANCOIS BASTIN

_{, FRÉDÉRIC MORTIER}

_{, MAXIME RÉJOU-MÉCHAIN}

_{, FABRICE BÉNÉDET}

_,

GUILLAUME CORNU

_{, JEAN-LOUIS DOUCET}

_{, ADELINE FAYOLLE}

_{, MAHLET TADESSE}

_{, SYLVIE GOURLET-FLEURY}

_,

RAPHAEL PÉLISSIER

1_{CIRAD, UPR BSEF, F-34000, Montpellier, France}

2_{Gembloux Agro-Bio Tech, Unité de gestion des ressources forestieres et des milieux naturels, 5030, Gembloux, Belgique} 3_{Georgetown University, Department of Mathematics and Statistics, DC, Washington, USA}

4_{IRD, UMR AMAP, F-34000, Montpellier, France}

Background: Understanding the reaction of ecosystems to climate change and anthropogenic pressure is a central question in ecology and environmental sciences. In the terrestrial tropics, theoretical and empirical works suggest that once external disturbances have reached a given threshold, forest-savanna systems can switch from one state to another. Considering the multiplicity of the tropical forest systems, we make the assumption that numerous shifts may actually occur within the forest itself, without changes in forest cover but with risks of critical modifications in forest functioning.

Methods: To test this hypothesis, we used a finite mixture of regression models aiming at simultaneously predicting and grouping forest functional profiles at the stand level with respect to anthropogenic pressure, climate and soil. The model is built on a dataset of more than 140 000 plots of 0.5-ha each gathered from Central African forest companies. Forest stand functions are analyzed through two key functional traits: the successional status - pioneer vs. non-pioneer trees- and the leaf phenology - evergreen vs. deciduous trees. Results: Our model captured a significant part of variation in the functional composition over the study area and revealed how anthropogenic pressure, climate change, soils or their combination lead to profound modifications within the forests. In particular, we showed that shifts from evergreen to deciduous stands can be mediated both by anthropogenic pressure or climate change. Discussion: This work shows for the first time how external forcing may jointly lead to multiple shifts in the functional composition of tropical forests. Our model allowed to predict directional changes in forest functioning according to anthropogenic pressure and climate thus opening new perspectives in theoretical ecology, global vegetation modelling and in the understanding of the vulnerability of tropical forests to global changes.

O64-03 – S64 Towards an unified vision of the central african forests

Thursday 23 June / 08:00-10:00 – Antigone3

Patterns of tree species composition across tropical African forests and within central African

moist forests: the need for adapted management and conservation strategies

SYLVIE GOURLET-FLEURY1, ADELINE FAYOLLE2, JEAN-LOUIS DOUCET2, MICHAEL D. SWAINE3,

1_{CIRAD - Department : BSEF, F 34398 Montpellier - France}

2_{Gembloux Agro-Bio Tech, Université de Liège Foresterie tropicale, Gestion des Ressources Forestières BIOSE Department 5030}

Gembloux - Belgique

3_{University of Aberdeen Department : Institute of Biological and Environmental Sciences Aberdeen - United Kingdom}

Background: Differences in the distribution of biota across Africa have been described for well over 100 years. There is however little information on the forest types at a regional scale. In this study we aimed to identify large-scale variation in tree species composition across tropical Africa, and within central Africa, to detail the structure and functioning of moist forests.

Methods: Distribution data were gathered for 1175 tree species in 455 samples from the literature scattered across tropical Africa, from Senegal to Mozambique, and including all types of tropical forests. The value of elevation and 19 climatic variables (BIOCLIM) were assigned to each sample. Management forest inventory data were assembled for 49,711 0.5-ha plots across central Africa, covering an area of more than six million hectares. Using ordinations, we determined the variations in species composition across tropical African forests and for central African moist forests we used both genus composition and forest structure. We defined floristic clusters and identified the characteristic species/genera at both levels of resolution. Results: We found floristic evidence for three main biogeographic regions across the tropical African forests, and described six floristic clusters with particular environmental conditions within these regions: Coastal and Upland for East Africa, Dry and Wet-Moist for West Africa, and Moist and Wet for Central Africa. Within the central African moist forests, we identified 7 forest types based on genus composition and forest structure. Most of these forests were composed of a mosaic of the structural derivatives of the Celtis (Ulmaceae) forest. Secondary Musanga (Moraceae) forest was located along roads and around main cities; mixed Manilkara (Sapotaceae) forest covers a huge area in the Sangha River Interval; and monodominant Gilbertiodendron (Fabaceae) forest was sparsely distributed along rivers.

Conclusions: The forest types identified across tropical African forests and within central African moist forests call for adapted management and conservation strategies. Specifically, the old-growth secondary Celtis forests that cover huge areas in central Africa should be managed for future timber productions, possibly complemented by artificial regeneration while the very specific and low productive Manilkara forests should be carefully managed with lower intensity practices.

O64-04 – S64 Towards an unified vision of the central african forests

Thursday 23 June / 08:00-10:00 – Antigone3

Complementarity of environmental factors explain spatial floristic variations in mixed lowland

rainforest of Cameroon

MOSES LIBALAH

_{, VINCENT DROISSART}

_{, NICOLAS BARBIER}

_{, BONAVENTURE SONKÉ}

_,

DONATIEN ZEBAZE

_{, GILLES DAUBY}

_{, GISLAIN MOFACK}

_{, NARCISSE KAMDEM}

_{, NICOLAS TEXIER}

_,

PIERRE PLOTON

_{, RAPHAEL PÉLISSIER}

_{, STÉPHANE MOMO}

_{, PIERRE COUTERON}

1_{Laboratory of Plant Systematics and Ecology, Biology, Higher Teachers’ Training College, 00237, Yaounde, Cameroon} 2_{Institut de Recherche pour le Développement (IRD), Unité Mixte de Recherche AMAP, 34398, Montpellier cedex 5, France} 3_{Herbarium et Bibliothèque de Botanique Africaine, Université Libre de Bruxelles, 1050, Bruxelles, Belgium}

Background. Analyzing floristic composition is important to understand species diversity and its response to varying environmental factors. As such, focus of previous papers has been on selected species or local scales. We assessed the explanatory power of environmental factors on species distribution in central African rainforest. We aimed to determine floristic gradients and question the extent to which soil and/or climate could explain these gradients.

Method. We sampled 61 1-ha plots (407 species, 28,944 individuals with diameter at breast height above 10 cm) distributed over six sites in evergreen to semi-deciduous forests on about 100,000 km2 _{area. Bioclimatic variables (temperature, precipitation) were}

extracted from newly improved remote sensing datasets and assigned to each plot. On a subset of 28 plots, 14 soil chemical and physical properties were also analyzed. We determined variation in species composition by correspondence analysis (CA) that emphasizes scarce species and non-symmetric correspondence analysis (NSCA) that emphasizes abundant species. In order to link floristic gradients to soil/or climate, we apportioned the total floristic variances with respect to the environmental factors.

Result. Floristic gradient was depicted by scarce species (Funtumia africana, Baphia leptobotrys) and observed a north-south shift concomitant with a prominent precipitation gradient on axis 1 (66.8% variance). The response of abundant species (Greenwayodendron suaveolens, Tabernaemotana crassa) was evident only at the third axis (16.6% variance) but poorly correlated with soil and climate gradients. Independently, soil explained more total variance (CA; 55.5% & NSCA; 56.2%) than climate (CA; 29.9% & NSCA; 34.3%). A residual analysis showed that soil and climate were non-redundant in predicting floristic variations. Both factors explained as much as 75.9% of floristic variation while this proportion sharply dropped when either the effect of soil or climate was factored out. While climate (precipitation intensity) discriminated forest sites (i.e. large spatial scale), soil properties (texture) differentiate plots (i.e. local spatial scale).

Conclusion. Our study highlights the influence of soil and climate in regulating floristic variation in central African rainforests. It emphasizes on the importance of accounting for both scarce and abundant species and recommends that non-environmental factors be considered to capture variation in abundant species.