Daniel Sabatier

Continental-scale patterns of canopy tree composition and function across Amazonia

The worlds greatest terrestrial stores of biodiversity and carbon are found in the forests of northern South America, where large-scale biogeographic patterns and processes have recently begun to be described. Seven of the nine countries with territory in the Amazon basin and the Guiana shield have carried out large-scale forest inventories, but such massive data sets have been little exploited by tropical plant ecologists. Although forest inventories often lack the species-level identifications favoured by tropical plant ecologists, their consistency of measurement and vast spatial coverage make them ideally suited for numerical analyses at large scales, and a valuable resource to describe the still poorly understood spatial variation of biomass, diversity, community composition and forest functioning across the South American tropics. Here we show, by using the seven forest inventories complemented with trait and inventory data collected elsewhere, two dominant gradients in tree composition and function across the Amazon, one paralleling a major gradient in soil fertility and the other paralleling a gradient in dry season length. The data set also indicates that the dominance of Fabaceae in the Guiana shield is not necessarily the result of root adaptations to poor soils (nodulation or ectomycorrhizal associations) but perhaps also the result of their remarkably high seed mass there as a potential adaptation to low rates of disturbance.

A spatial model of tree α-diversity and tree density for the Amazon

Large-scale patterns of Amazonian biodiversity have until now been obscured by a sparse and scattered inventory record. Here we present the first comprehensive spatial model of tree α-diversity and tree density in Amazonian rainforests, based on the largest-yet compilation of forest inventories and bolstered by a spatial interpolation technique that allows us to estimate diversity and density in areas that have never been inventoried. These data were then compared to continent-wide patterns of rainfall seasonality. We find that dry season length, while only weakly correlated with average tree α-diversity, is a strong predictor of tree density and of maximum tree α-diversity. The most diverse forests for any given DSL are concentrated in a narrow latitudinal band just south of the equator, while the least diverse forests for any given DSL are found in the Guayana Shield and Amazonian Bolivia. Denser forests are more diverse than sparser forests, even when we used a measure of diversity that corrects for sample size. We propose that rainfall seasonality regulates tree α-diversity and tree density by affecting shade tolerance and subsequently the number of different functional types of trees that can persist in an area.

An analysis of the floristic composition and diversity of Amazonian forests including those of the Guiana Shield.

A large number of newly published and unpublished hectare plots in Amazonia and the Guiana Shield area allow an analysis of family composition and testing of hypotheses concerning alpha-diversity in the south American rain forest. Using data from 94 plots the family-level floristic patterns in wet tropical South America are described. To test diversity patterns, 268 plots are used in this large area. Contrary to a widely held belief, western Amazonian plots are not necessarily the most diverse. Several central Amazonian plots have equal or even higher tree diversity. Annual rainfall is not a good estimator for tree diversity in the Amazonia area and Guiana shield. Plots in the Guiana Shield area (and eastern Amazonia) usually have lower diversity than those in central or western Amazonia. It is argued that this is not because of low rainfall or low nutrient status of the soil but because of the small area of the relatively isolated rain forest area in eastern Amazonia and the Guiana Shield. The low diversity on nutrient-poor white sand soils in the

The influence of soil cover organization on the floristic and structural heterogeneity of a Guianan rain forest

The impact of soil cover organization on the forest community has been studied in a 19-ha tract at Piste de St Elie station in French Guiana. 195 species each represented by at least 10 individuals were chosen from records of the position, diameter at breast height (dbh) and precise identification by botanical sampling of 12104 ligneous plants (dbh ≥ 10 cm).Spatial variations in the soil were mapped using the method proposed by Boulet et al. (1982). The soil mapping units correspond to the successive stages of evolution of a currently unbalanced ferralitic cover. These stages describe firstly the thinning by erosion of the microaggregated upper horizon and secondly the mineralogical changes under more or less extended hydromorphic conditions. The degree of evolution of ferralitic cover is also related to the hydrodynamic functioning and chemical properties of the soil. Geological substrate, topography and slope have also been taken into account.Analysis of the influence of environmental variables on plant cover has been performed using the Ecological Profiles method and Correspondence Analysis (CA) of the table of ecological profiles.The forest community seems to be dependent on the soil and the topographical features that govern it. There are significant, exclusive soil-species links for each soil functioning mapping unit. However, the highest proportion of significant positive links is connected with a thick microaggregated horizon (25%). Several species are of real value as indicators and more particularly enable differentiation between the forest stands of typical ferralitic soil and the ones of thinned out, transformed and hydromorphic soils. The CA of the species by environmental variables matrices reveals two significant factorial axes. The first can be associated with the drainage mainly related to the thinning of the soil and the second with the hydromorphic conditions related to the topography. The vegetation ordination of the stands (≅ 0.25 ha) delimited in the various soil domains clearly shows that changes in ferralitic cover and in particular the transition from soil with deep vertical drainage to soil with superficial lateral drainage is accompanied by substantial changes in the forest community.

Interspecific variability of δ13C among trees in rainforests of French Guiana: functional groups and canopy integration

Abstract The interspecific variability of sunlit leaf carbon isotope composition (δ13C), an indicator of leaf intrinsic water-use efficiency (WUE, CO2 assimilation rate/leaf conductance for water vapour), was investigated in canopy trees of three lowland rainforest stands in French Guiana, differing in floristic composition and in soil drainage characteristics, but subjected to similar climatic conditions. We sampled leaves with a rifle from 406 trees in total, representing 102 species. Eighteen species were common to the three stands. Mean species δ13C varied over a 6.0‰ range within each stand, corresponding to WUE varying over about a threefold range. Species occurring in at least two stands displayed remarkably stable δ13C values, suggesting a close genetic control of species δ13C. Marked differences in species δ13C values were found with respect to: (1) the leaf phenology pattern (average δ13C=–29.7‰ and –31.0‰ in deciduous-leaved and evergreen-leaved species, respectively), and (2) different types of shade tolerance defined by features reflecting the plasticity of growth dynamics with respect to contrasting light conditions. Heliophilic species exhibited more negative δ13C values (average δ13C=–30.5‰) (i.e. lower WUE) than hemitolerant species (–29.3‰). However, tolerant species (–31.4‰) displayed even more negative δ13C values than heliophilic ones. We could not provide a straightforward ecophysiological interpretation of this result. The negative relationship found between species δ13C and midday leaf water potential (Ψwm) suggests that low δ13C is associated with high whole tree leaf specific hydraulic conductance. Canopy carbon isotope discrimination (ΔA) calculated from the basal area-weighed integral of the species δ13C values was similar in the three stands (average ΔA=23.1‰), despite differences in stand species composition and soil drainage type, reflecting the similar proportions of the three different shade-tolerance types among stands.

CONSISTENCY BETWEEN ORDINATION TECHNIQUES AND DIVERSITY MEASUREMENTS: TWO STRATEGIES FOR SPECIES OCCURRENCE DATA

Both the ordination of taxonomic tables and the measurements of species diversity aim to capture the prominent features of the species composition of a community. However, interrelations between ordination techniques and diversity measurements are sel- dom explicated and are mainly ignored by many field ecologists. This paper starts from the notion of the species occurrence table, which provides a unifying formulation for different kinds of taxonomic data. Here it is demonstrated that alternative species weightings can be used to equate the total inertia of a centered-by-species occurrence table with common diversity indices, such as species richness, Simpson diversity, or Shannon information. Such an equation defines two main ordination strategies related to two different but con- sistent measures of species diversity. The first places emphasis on scarce species and is based on Correspondence Analysis and species richness (CA-richness strategy). The second, in which abundant species are prominent, relies on Non-Symmetric Correspondence Anal- ysis and Simpson diversity (NSCA-Simpson strategy). Both strategies are suitable for mea- suring a. and f3 diversity by analyzing the centered-by-species occurrence table with respect to external environmental or instrumental variables. In this paper, these two strategies are applied to ecological data obtained in a Neotropical rainforest plot. The results are then discussed with respect to the intrinsic characteristics of the community under analysis, and also to the broad classes of floro-faunistic data used in ecology (i.e., data gathered from museum or herbarium collections, exhaustive inventories in a reference plot, or enumeration through species-by-releves tables). The approach en- compasses several well-known techniques such as Correspondence Analysis, Non-Sym- metric Correspondence Analysis, Canonical Correspondence Analysis, and Redundancy Analysis, and provides greater insight into interrelations between ordination methods and diversity studies.

Within-plot relationships between tree species occurrences and hydrological soil constraints: an example in French Guiana investigated through canonical correlation analysis

Spatial relationships between tree species and hydrological soilconstraints are analysed within a 10-ha rainforest plot at PistedeSt Elie in French Guiana. We used canonical correlation analysis to crossdirectly the occurrence-by-species table of 4 992 individuals (d.b.h. ≥10 cm) belonging to 120 species with qualitative soil variablesandquantitative spatial data. Firstly, the list of species occurrences wasconfronted to nine soildescriptors characterising a weathering sequence from the initial well-drainedferralitic cover to transformed hydromorphic soil conditions. This analysisrevealed that, apart from some specialised species restricted to the swampsthatexperience prolonged water saturation, the most abundant species can be orderedalong two intermingled gradients of tolerance limiting their niche amplitude: amain gradient of tolerance to prolonged water saturation that appears downslopeduring the weathering sequence; a second gradient of less importance,displayingthe species intolerant of prolonged water saturation according to theirtolerance to temporary confinement of the uphill transformed soil systems duetothe late appearance of a perched water-table. The results support thehypothesisthat at Piste de St Elie, the constraining soil conditions imposed by surfacewater saturation are more important determinants for tree zonation of many treespecies than water shortage. Secondly, the list of species occurrences wasconfronted to a spatialdata table built from a trend surface regression of the tree coordinates. Thisanalysis indicated that soil drainage is the main structuring factor of thelocal multispecies spatial pattern. After partialling out the soil effect, themultispecies pattern revealed a broader scale of heterogeneity that we supposedto be linked to endogenous factors resulting from population dynamics.Implications of the results are then discussed in the perspective offuture research on tree zonation, local diversity pattern and communitystructuring in tropical rainforests.

Broad-scale spatial pattern of forest landscape types in the Guiana Shield

Abstract Detecting broad scale spatial patterns across the South American rainforest biome is still a major challenge. Although several countries do possess their own, more or less detailed land-cover map, these are based on classifications that appear largely discordant from a country to another. Up to now, continental scale remote sensing studies failed to fill this gap. They mostly result in crude representations of the rainforest biome as a single, uniform vegetation class, in contrast with open vegetations. A few studies identified broad scale spatial patterns, but only when they managed to map a particular forest characteristic such as biomass. The main objective of this study is to identify, characterize and map distinct forest landscape types within the evergreen lowland rainforest at the sub-continental scale of the Guiana Shield (north-east tropical South-America 10° North-2° South; 66° West-50° West). This study is based on the analysis of a 1-year daily data set (from January 1st to December 31st, 2000) from the VEGETATION sensor onboard the SPOT-4 satellite (1-km spatial resolution). We interpreted remotely sensed landscape classes (RSLC) from field and high resolution remote sensing data of 21 sites in French Guiana. We cross-analyzed remote sensing data, field observations and environmental data using multivariate analysis. We obtained 33 remotely sensed landscape classes (RSLC) among which five forest-RSLC representing 78% of the forested area. The latter were classified as different broad forest landscape types according to a gradient of canopy openness. Their mapping revealed a new and meaningful broad-scale spatial pattern of forest landscape types. At the scale of the Guiana Shield, we observed a spatial patterns similarity between climatic and forest landscape types. The two most open forest-RSLCs were observed mainly within the north-west to south-east dry belt. The three other forest-RSLCs were observed in wetter and less anthropized areas, particularly in the newly recognized “Guianan dense forest arch”. Better management and conservation policies, as well as improvement of biological and ecological knowledge, require accurate and stable representations of the geographical components of ecosystems. Our results represent a decisive step in this way for the Guiana Shield area and contribute to fill one of the major shortfall in the knowledge of tropical forests.

The lowland high rainforest : structure and tree species diversity

Diversity studies provide data useful to the basic knowledge of biodiversity as well as a baseline for many other research fields. Forest dynamics, descriptive and functional ecology, conservation and sustainable development projects, need such data that are still few for neotropical forests, and especially so for French Guiana. Data from previous studies in this region come from ‘Piste de Saint-Elie’ (Sabatier & Prevost 1990b, Prevost & Sabatier 1996) and Saul (Oldeman 1974, Mori & Boom 1987). The research program at Nouragues aims to study forest dynamics and natural regeneration processes in the forest, including seed dispersal by vertebrates. The establishment of the station as a permanent research site has provided opportunity as well as appropriate conditions to undertake such studies, and to add one more piece to the jigsaw (Milliken 1998). The first surveys of floristic and tree diversity at Nouragues were based on preliminary results from 1989 (Sabatier & Prevost 1990a) and 1995 (Poncy et al. 1998).

Unveiling the diet of elusive rainforest herbivores in next generation sequencing era? The tapir as a case study.

Characterizing the trophic relationships between large herbivores and the outstanding plant diversity in rainforest is a major challenge because of their elusiveness. This is crucial to understand the role of these herbivores in the functioning of the rainforest ecosystems. We tested a non-invasive approach based on the high-throughput sequencing of environmental samples using small plant plastid sequences (the trnL P6 loop) and ribosomal ITS1 primers, referred to as DNA metabarcoding, to investigate the diet of the largest neotropical herbivore, the lowland tapir. Sequencing was performed on plant DNA extracted from tapir faeces collected at the Nouragues station, a protected area of French Guiana. In spite of a limited sampling, our approach reliably provided information about the lowland tapirs diet at this site. Indeed, 95.1% and 74.4% of the plant families and genera identified thanks to the trnL P6 loop, respectively, matched with taxa already known to be consumed by tapirs. With this approach we were able to show that two families and eight new genera are also consumed by the lowland tapir. The taxonomic resolution of this method is limited to the plant family and genera. Complementary barcodes, such as a small portion of ITS1, can be used to efficiently narrow identifications down to the species in some problematic families. We will discuss the remaining limitations of this approach and how useful it is at this stage to unravel the diet of elusive rainforest herbivores and better understand their role as engineers of the ecosystem.