PALM SPECIES DISTRIBUTION AND SOIL MOISTURE IN A SWAMPY AREA OF THE ATLANTIC FOREST, SOUTH-EASTERN BRAZIL

Environmental patchiness, competition, and distribution of parent plants may result in fine-scale clusters of conspecific individuals and even of individuals of different species. If several species in a community exhibit an aggregated pattern, indicating control by some influencing factors, then a study on patterns of association between them will provide evidence of any grouping of the species into assemblages of similar response to the influencing factors (Greig-Smith 1983). Positive spatial associations may be the result of similar responses to local environmental factors. On the smallest scales, these patterns reflect the interactions between individual plants (Svenning 2001a), whereas on larger scales the spatial distribution of individuals is influenced by variations in edaphic conditions (Hutchings 1997). Recently much attention has been devoted to the effects of spatial heterogeneity on ecological processes occurring at various spatial scales (Clark et al. 1995, 1998; Svenning 2001a, b). This environmental heterogeneity of resource distribution may greatly affect the performance and spatial patterns of plant populations and, ultimately, the community structure (Svenning 2001a, b). Variation in soil, topography, and microclimate generates an environmental heterogeneity of resource distribution for plants, which will also create suitable patches for growth and establishment. If recruitment is higher in these patches there will be a spatial association between plants and patches (Barot et al. 1999). Some studies have shown that edaphic variation in palm density is frequent in tropical forests of Central America and Amazonia (Svenning 1999, Pacheco 2001, Vormisto 2002). As pointed out by Clark et al. (1998), when the distributions of more species are studied, it will be possible to determine the generality of edaphic effects and to understand the consequences for tropical forest structure and function. Here, our goals were to assess the relationship between heterogeneity in soil water availability and the spatial patterns of three tropical palm species in an evergreen swampy forest imbedded in a semideciduous forest fragment (Gomes et al. 2006). According to Scarano (2006), although South America has the world’s largest area of wetlands, including swampy forests and floodplains, Brazilian freshwater wetlands are scarcely studied. Since the study species occur from swampy areas at stream edges to rain and seasonal forests (Henderson et al. 1995), we asked whether their abundance was a response to small-scale variation in soil water availability. To develop some hypotheses on the factors that cause spatial population aggregation we analyzed (1) the distribution of soil moisture and its effects on palm density, and (2) the spatial association between each pair of palm species. Our predictions are that (1) population density would be higher in moisture patches if palms are moisturelimited, and that (2) different palm species would be associated in suitable patches if they shared the same requirements in relation to water availability for growth and recruitment. SHORT COMMUNICATIONS