Abstract: Modelling the local density of tropical saplings can provide insights into the ecological processes that drive species regeneration and thereby help predict population recovery after disturbance. Yet, few studies have addressed the challenging issues in autocorrelation and zero-inflation of local density. This paper presents Hierarchical Bayesian Modelling (HBM) of sapling density that includes these two features. Special attention is devoted to variable selection, model estimation and comparison. We developed a Zero-Inflated Poisson (ZIP) model with a latent correlated spatial structure and compared it with non-spatial ZIP and Poisson models that were either autocorrelated (Spatial Generalized Linear Mixed, SGLM) or not (generalized linear models, GLM). In our spatial models, local density autocorrelation was modeled by a Conditional Auto-Regressive (CAR) process. 13 explicative variables described ecological conditions with respect to topography, disturbance, stand structure and intraspecific processes. Models were applied to six tropical tree species with differing biological attributes: Oxandra asbeckii, Eperua falcata, Eperua grandiflora, Dicorynia guianensis, Qualea rosea, and Tachigali melinonii. We built species-specific models using a simple method of variable selection based on a latent binary indicator. Our spatial models showed a close correlation between observed and estimated densities with site spatial structure being correctly reproduced. By contrast, the non-spatial models showed poor fits. Variable selection highlighted species-specific requirements and susceptibility to local conditions. Model comparison overall showed that the SGLM was the most accurate explanatory and predictive model. Surprisingly, zero-inflated models performed less well. Although the SZIP model was relevant with respect to data distribution, and more flexible with respect to response curves, its model complexity caused marked variability in parameter estimates. In the SUM, the spatial process alone accounted for zero-inflation in the data. A refinement of the hypotheses employed at the process level could compensate for distribution flaws at the data level. This study emphasized the importance of the HBM framework in improving the modelling of density-environment relationships. (C) 2008 Elsevier B.V. All rights reserved.

Abstract: Tropical vegetation is a major source of global land surface evapotranspiration, and can thus play a major role in global hydrological cycles and global atmospheric circulation. Accurate prediction of tropical evapotranspiration is critical to our understanding of these processes under changing climate. We examined the controls on evapotranspiration in tropical vegetation at 21 pan-tropical eddy covariance sites, conducted a comprehensive and systematic evaluation of 13 evapotranspiration models at these sites, and assessed the ability to scale up model estimates of evapotranspiration for the test region of Amazonia. Net radiation was the strongest determinant of evapotranspiration (mean evaporative fraction was 0.72) and explained 87% of the variance in monthly evapotranspiration across the sites. Vapor pressure deficit was the strongest residual predictor (14%), followed by normalized difference vegetation index (9%), precipitation (6%) and wind speed (4%). The radiation-based evapotranspiration models performed best overall for three reasons: (1) the vegetation was largely decoupled from atmospheric turbulent transfer (calculated from X decoupling factor), especially at the wetter sites; (2) the resistance-based models were hindered by difficulty in consistently characterizing canopy (and stomatal) resistance in the highly diverse vegetation; (3) the temperature-based models inadequately captured the variability in tropical evapotranspiration. We evaluated the potential to predict regional evapotranspiration for one test region: Amazonia. We estimated an Amazonia-wide evapotranspiration of 1370 mm yr(-1), but this value is dependent on assumptions about energy balance closure for the tropical eddy covariance sites; a lower value (1096 mm yr(-1)) is considered in discussion on the use of flux data to validate and interpolate models.

Abstract: White-sand forests represent natural laboratories of evolution over their long history throughout Amazonia and the Guiana Shield and pose significant physiological challenges to the plants and animals they host. The study of diversification in plant lineages comprising species endemic to white-sand forest can therefore give insights into processes of evolution and community assembly in tropical forests. In this article, we synthesize recent studies of white-sand forests to integrate patterns of plant species distribution with processes of lineage diversification and community assembly in the white-sand flora. We contrast lineages that have radiated uniquely in these habitats (e.g., Pagamea, Rubiaceae), with cosmopolitan lineages comprising specialists to white-sand forests and other habitats that may have arisen via ecological speciation across habitat gradients (e.g., Protium, Burseraceae). In both cases, similar suites of functional traits have evolved, including investment in dense, long-lived tissues that are well-defended structurally and chemically. White-sand endemics, therefore, play an important role in biodiversity conservation because they represent unique combinations of functional and phylogenetic diversity. Furthermore, white-sand endemics may respond differently than other tropical forest plant species to contemporary global changes because they comprise resilient functional types that may better withstand increased drought, temperature, and invasions of exotic pests in these regions. © 2016 The Association for Tropical Biology and Conservation.

Abstract: P>1. Relationships between tropical rain forest biomass and environmental factors have been determined at regional scales, e.g. the Amazon Basin, but the reasons for the high variability in forest biomass at local scales are poorly understood. Interactions between topography, soil properties, tree growth and mortality rates, and treefalls are a likely reason for this variability. 2. We used repeated measurements of permanent plots in lowland rain forest in French Guiana to evaluate these relationships. The plots sampled topographic gradients from hilltops to slopes to bottomlands, with accompanying variation in soil waterlogging along these gradients. Biomass was calculated for > 175 tree species in the plots, along with biomass productivity and recruitment rates. Mortality was determined as standing dead and treefalls. 3. Treefall rates were twice as high in bottomlands as on hilltops, and tree recruitment rates, radial growth rates and the abundance of light-demanding tree species were also higher. 4. In the bottomlands, the mean wood density was 10% lower than on hilltops, the basal area 29% lower and the height:diameter ratio of trees was lower, collectively resulting in a total woody biomass that was 43% lower in bottomlands than on hilltops. 5. Biomass productivity was 9% lower in bottomlands than on hilltops, even though soil Olsen P concentrations were higher in bottomlands. 6. Synthesis. Along a topographic gradient from hilltops to bottomlands there were higher rates of treefall, which decreased the stand basal area and favoured lower allocation to height growth and recruitment of light-demanding species with low wood density. The resultant large variation in tree biomass along the gradient shows the importance of determining site characteristics and including these characteristics when scaling up biomass estimates from stand to local or regional scales.

Abstract: 2. This hypothesis was tested by comparing a generalist and a specialist ladybird species belonging to the same genus. 3. A resorption index was calculated for females of both species subjected to several starvation regimes. This index indicated that over a period of fasting of 3 days, the intensity of resorption was greater in the generalist than the specialist. When food was again supplied, oogenesis resumed and within 1 day was faster in the generalist than in the specialist. 4. As predicted, the resorption of oocytes and replenishment occurred faster in the generalist than in the specialist species. This is the first time, to our knowledge, that the speed and intensity of the ovarian dynamics of a predatory insect have been linked to its way of life.