Estimating survival rates in ecological studies with small unbalanced sample sizes: an alternative Bayesian point estimator

Increasingly, the survival rates in experimental ecology are presented using odds ratios or log response ratios, but the use of ratio metrics has a problem when all the individuals have either died or survived in only one replicate. In the empirical ecological literature, the problem often has been ignored or circumvented by different, more or less ad hoc approaches. Here, it is argued that the best summary statistic for communicating ecological results of frequency data in studies with small unbalanced samples may be the mean of the posterior distribution of the survival rate. The developed approach may be particularly useful when effect size indexes, such as odds ratios, are needed to compare frequency data between treatments, sites or studies

Spatial and temporal variability of plant-available soil water in Congo Basin and its relationship with tree species distributions

Regional-scale patterns of tropical rainforest tree composition can be due to climate (rainfall, dry season length), geology and/or soil properties (chemical fertility, available water). In Amazonia, soil fertility and dry season length appears to be the main factor to explain this pattern. However, in the Congo Basin, geology has been proposed to explain the pattern of some commercial timber species. Since the geological substrates of this area have similar chemical properties, we hypothesized that this pattern could be explained by the plant-available soil water (PAW). We used a soil water balance model similar to RisQue in the Congo Basin over the period from 2000 to 2010, with a decade time step, and with a spatial resolution of 8 km. The input parameters of this model were the maximum plant-available soil water (PAWmax), rainfall and evapotranspiration. The output parameter was the maximum number of successive decades when PAW was null, named extreme drought index (EDI). Finally we carried out a map of EDI at Congo Basin scale that we compared with maps of the spatial pattern of 31 commercial species. We showed that Arenosols, as expected, but also other soils like Ferralsols, have the lowest PAWmax of the Congo Basin. We evidenced no or low correlations between the map of EDI and maps of the spatial pattern of each of the 31 commercial species. Other factors, not taken into account in this study, might explain this result like the water table level and variable forest rooting depth in function of soil type. (Résumé d'auteur

Acacia auriculiformis production in the Mampu agroforestry zone on the Batéké plateau, Democratic Republic of Congo

peer reviewedThe Mampu agroforestry zone on the Batéké plateau in the Democratic Republic of Congo, which has been managed with Acacia auriculiformis shade trees for over twenty years by local communities, supplies subsistence products and fuel wood to Kinshasa. Thanks to international grant funding, this agroforestry system, which integrates traditional slashand- burn cultivation, has been replicated in many places across the RDC, but its performance has never been assessed. The aim of this study was to estimate Acacia auriculiformis production in terms of total biomass and usable biomass for charcoal (stems and branches more than 4 cm in diameter) as part of the agroforestry system. To do so, two local allometric equations for total and usable biomass were adjusted from destructive testing data. Using existing inventory data (n = 112 plots), we identified significant structural heterogeneity throughout the rotation period (8-10 years) but also among plots of the same age. Despite this heterogeneity, which may be accounted for by environmental conditions on site and/or by differences in the handling of plot management techniques, production is comparable to that observed at other sites, averaging 145 tonnes per hectare over 10 years. The Mampu agroforestry system has many advantages, including direct services creating rural employment and combined production of subsistence goods and charcoal, but also indirect services such as avoided deforestation and carbon sequestration. The system's sustainability and dissemination should nevertheless be discussed

The importance of tree allometry for local-scale variation in aboveground biomass

Aboveground biomass (AGB) plays a critical role in determining the long-term dynamics of carbon in tropical forests. Consequently, understanding what factors are important in controlling AGB in tropical forests has major implications for projecting the terrestrial carbon stocks, in the context of an increasingly uncertain future. In this study, we aimed to explore the local-scale AGB variation in two forest sites in northern Congo, representative of contrasted forest types under the same climate but growing on vastly different soils and parent material (quartzite substrate for CIB and sandstone substrate for Mokabi). Tree diameter was measured in 36 permanent forest plots of 1-ha in each site, and tree allometry (total tree height, height of the first branch and crown dimensions) was measured on a subsample of 18 plots of 1-ha in each site. Allometric data were available for a total of 2202 trees (1040 for CIB and 1162 for Mokabi) covering a large range of diameters (10-200 cm). We first developed site-specific allometric models that were used to estimate AGB at plot level. We then explore the determinants of AGB variation at plot level using multiple regressions and mixed linear models. For a given diameter, trees tended to be taller and to have deeper crown in the Celtis forest of the CIB (rich soils), while they tended to have larger crown in the Manilkara forest of the Mokabi (sandy soils). Similar trends were reported within species for the sixteen species shared by both sites, suggesting an environmental control of tree allometry. We found that AGB strongly varied between the two forest sites, with greater AGB per hectare in the Celtis forest of the CIB site. Within-site AGB variation was positively related to basal area, though between-site AGB variation was determined by tree allometry (height-diameter and crown allometries). These results have strong implications for forest biomass and carbon monitoring

Predicting the combined impacts of climate change and selective logging in timber production forests of Central Africa. [P-2215-03]

In the design and the implementation of current rules of Sustainable Forest Management (SFM), still too little account is taken of the sensitivity of tropical forests to climate change. In the Congo Basin, forests cover 220 million hectares and represent an economic sector of utmost importance for the rural development as well as for national and regional climate strategies. Hence, these forests constitute a major challenge for both adaptation and mitigation. A prerequisite to ensure the relevance and the effectiveness of SFM recommendations in this region is to elucidate the influence on forest dynamics of both climate change and harvesting pressure. This influence will likely consist of major shifts in structure and floristic composition. By opening the stands and increasing light availability, selective logging fosters the development of light demanding species. Some of these species, particularly the pioneers, are thought to be particularly drought sensitive so that global warming could strongly impact logged forests. The study of forest-climate-logging relationships needs therefore species-level predictions. However, the high diversity of tropical forests, in pair with the scarcity of data, hinders the correct fitting of species-specific models. To investigate the combined effects of climate and harvesting influence on Central African forests, we conducted long-term simulations of forest dynamics under several scenarios of climate change and timber harvesting. Climate scenarios were based on outputs from simulations of the atmospheric model ARPEGE-Climate of the French National Centre for Meteorological Research (CNRM), performed within the Coupled Model Intercomparison Project Phase 5 (CMIP5) and under several Representative Concentration Pathway (RCP) scenarios of the International Panel on Climate Change (IPCC). We also used outputs fields such as soil water and potential evapotranspiration from the model CARbon Assimilation In the Biosphere (CARAIB) of University of Liège obtained under the same climatic scenarios. Logging scenarios were implemented by considering a wide range of felling intensities. To carry out this work, we developed an innovative method based on a Mixture of inhomogeneous matrix models (MIMM) that permits to test and simulate the influence of timber harvesting and climate change on forest dynamics. While insuring a satisfactory fitting of vital parameters, such a methodology allowed us to reflect the diversity of tree ecological patterns, notably in response to climate variables. To do this, we simultaneously clustered species into groups according to species-specific ecological responses and identified group-specific explicative environmental and climate variables. To infer and validate model outputs, we used the M'Baïki site, in the Central African Republic (CAR), a unique experimental site that has been monitored for 30 years through a collaborative partnership with various French and CAR institutional and research organizations. (Texte intégral

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

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. (Texte intégral

Refining species traits in a dynamic vegetation model to project the impacts of climate change on tropical trees in Central Africa

African tropical ecosystems and the services they provide to human society suffer from an increasing combined pressure of land use and climate change. How individual tropical tree species respond to climate change remains relatively unknown. In this study, we refined the species characterization in the CARAIB (CARbon Assimilation In the Biosphere) dynamic vegetation model by replacing plant functional type morpho-physiological traits by species-specific traits. We focus on 12 tropical tree species selected for their importance in both the plant community and human society. We used CARAIB to simulate the current species net primary productivity (NPP), biomass and potential distribution and their changes in the future. Our results indicate that the use of species-specific traits does not necessarily result in an increase of predicted current NPPs. The model projections for the end of the century highlight the large uncertainties in the future of African tropical species. Projected changes in species distribution vary greatly with the general circulation model (GCM) and, to a lesser extent, with the concentration pathway. The question about long-term plant response to increasing CO2 concentrations also leads to contrasting results. In absence of fertilization effect, species are exposed to climate change and might lose 25% of their current distribution under RCP8.5 (12.5% under RCP4.5), considering all the species and climatic scenarios. The vegetation model projects a mean biomass loss of -21.2% under RCP4.5 and -34.5% under RCP8.5. Potential range expansions, unpredictable due to migration limitations, are too limited for offsetting range contraction. By contrast, if the long-term species response to increasing [CO2] is positive, the range reduction is limited to 5%. However, despite a mean biomass increase of 12.2%, a positive CO2 feedback might not prevent tree dieback. Our analysis confirms that species will respond differently to new climatic and atmospheric conditions, which may induce new competition dynamics in the ecosystem and affect ecosystem services