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Within the tropics, the species richness of tree communities is strongly and positively associated with precipitation. Previous research has suggested that this macroecological pattern is driven by the negative effect of water‐stress on the physiological processes of most tree species. This implies that the range limits of taxa are defined by their ability to occur under dry conditions, and thus in terms of species distributions predicts a nested pattern of taxa distribution from wet to dry areas. However, this ‘dry‐tolerance’ hypothesis has yet to be adequately tested at large spatial and taxonomic scales. Here, using a dataset of 531 inventory plots of closed canopy forest distributed across the western Neotropics we investigated how precipitation, evaluated both as mean annual precipitation and as the maximum climatological water deficit, influences the distribution of tropical tree species, genera and families. We find that the distributions of tree taxa are indeed nested along precipitation gradients in the western Neotropics. Taxa tolerant to seasonal drought are disproportionately widespread across the precipitation gradient, with most reaching even the wettest climates sampled; however, most taxa analysed are restricted to wet areas. Our results suggest that the ‘dry tolerance' hypothesis has broad applicability in the world's most species‐rich forests. In addition, the large number of species restricted to wetter conditions strongly indicates that an increased frequency of drought could severely threaten biodiversity in this region. Overall, this study establishes a baseline for exploring how tropical forest tree composition may change in response to current and future environmental changes in this region. Revisado por pares

Northwester Amazonian forest conservation a challenge for ecologists. The actual deforestation rates in Amazonian rain forests are extremely high. The worst case scenario could lead to an almost total disappearance of largest tropical forest mass that no wades exists on the earth, in a relatively short time (Laurance el al. 2001). Patterns of rain forest plant diversity in this area reaches exceptional high values per unit area (Gentry 1988a, Valencia et al. 1994, ter Steege et al.20039 To guarantee an effective conservation planning, basic knowledge on the distribution of individual species assemblages is necessary. In spite of the fact that information concerning to plan communities has much increased in the last decade; most studies have focused on the trees because they are the most conspicuous elements in the forests (Gentry 1988b, Duivenvoorden 1995, 1996 Pitman et al. 1999 , 2001,ter Steege et al. 2000, Condit ET al.2002). However, it is well known that vascular plant diversity in tropical rain forests in also well represent by other growth forms, such as climebers, shrubs, epiphytes and herb (Gentry and Dobson 1987, Divenvoorden 1994, Balslev et al . 1998, Galeano et al .1998). Doctorado

Resumen: El presente estudio se llevó a cabo en diez hectáreas de una parcela permanente establecida en el Parque Nacional Natural Amacayacu, Amazonia colombiana. Donde se evaluó el efecto de la variación ambiental y la configuración espacial sobre los patrones florísticos de las especies arbóreas a escala local en un bosque de tierra firme. Se estudió la variación florística y ambiental en cuadrantes de 20x20m. Adicionalmente, se consideraron diferentes categorías de abundancia de las especies arbóreas de dosel y de sotobosque (total, alta, media y baja). Se utilizó el Análisis de Correspondencia Linealizado DCA y el Análisis de Correspondencia Canónica CCA, seguido de una partición de la variación, para cuantificar la magnitud a la cual el ambiente y la limitación en dispersión determinan la variación florística. La fracción espacial, que representa procesos de autocorrelación como la limitación en dispersión, se analizó mediante el método de Coordenadas Principales de Matrices Vecinas PCNM. La diversidad beta de la parcela fue baja, aparentemente. El PCNM aparece como el método de análisis más apropiado para estudios a esta escala. Las diferencias florísticas explicadas a lo largo de la parcela de 10-has fueron principalmente asociadas con procesos biológicos como la limitación en dispersión. La mayor parte de la variación florística, no obstante, no fue explicada por las variables ambientales o espaciales consideradas. En conclusión, estos resultados sugieren que procesos aleatorios son determinantes esenciales de la variación espacial de las especies arbóreas de dosel y sotobosque, a escala local en bosques de tierra firme en el Parque Nacional Natural Amacayacu. Palabras clave: abundancia, análisis espacial, diversidad beta, distribución de especies, rareza, flora amazónica. Abstract: This study was carried out in a 10-Ha permanent plot established in the Amacayacu National Park, Colombian Amazonia. We assessed the extent at which floristic patterns of tree species were determined by either the environmental variation or the spatial configuration in tierra firme forests. Floristic and environmental variation was assessed on quadrants of 20x20 m. To run the analysis, four different categories of abundance of tree canopy and understory species were considered (all, high, medium and low). We used Detrended Correspondence Analysis and Canonical Correspondence Analysis, followed by a variation partitioning, to analyze the patterns of species distribution in order to quantify the role played by both environmental filtering and dispersal limitation. The spatial template, which represents dispersal limitation, was evaluated using the Principal Coordinates of Neighbour Matrix (PCNM) method, which appears as the most appropriate method to analyze the spatial template at this scale. Beta diversity was relatively low within the plot. The floristic differences explained along the 10-has plot were mainly associated to biological processes, such as dispersal limitation. The largest proportion of community variation in our dataset was unexplained by either environmental or spatial data. In conclusion, these results support random processes as the major drivers of the spatial variation of tree species at a local scale on tierra firme forests in the Amacayacu National Park. Maestría