Spatial partitioning among tree species within an area of tropical montane gallery forest in south-eastern Brazil

The present contribution analyses the relationship between environmental variables and tree community spatial organisation within an area of tropical montane gallery forest in Itutinga, south-eastern Brazil (21 degrees 21'22"S and 44 degrees 36'50"W, 920 m of altitude). A main gradient was searched from the stream margins to the forest boundaries with the surrounding montane grasslands. To accomplish this, 28 10 x 30 m plots were laid out along this gradient. All trees with DBS (diameter at base of the stem) greater than or equal to 5 cm were identified and measured for their DBS and total height. Soil and topographic variables were obtained for all plots. Soil variables included pH, levels of mineral nutrients and organic matter, textural properties and soil moisture, this latter measured monthly during one year. The same variables were obtained for adjacent areas of montane grassland in order to investigate differences with the forest interior. The relationship between tree species distribution and environmental variables was assessed with traditional direct gradient analysis, along the SMB (Streamside-Middle-Border) gradient, and canonical correspondence analysis (CCA). The latter largely confirmed the former, since topographic variables related to the SMB gradient were the most strongly correlated to the species' distribution. Although some soil chemical variables showed a significant variation along the SMB gradient, even extending to the grassland, this was considered an effect more than a cause of a main soil water and vegetational gradient. The spatial partitioning among tree species within the gallery forest as well as the definition of the sharp forest-grassland boundary may be seen as emergent properties of complex species-environment interactions triggered by differences in soil water regime. Even the establishment of a predominantly light-demanding tree community on the sunbathed forest border could possibly stem from soil water variations.