Rekruttering og diversitet hos trearter i en tørkeutsatt skog i nordøstre Thailand

Forest communities are complex because many factors are associated with the spatial patterns of woody species. The main objective of this thesis is to provide a better understanding of how woody species composition and diversity, as well as its components, are related and respond to environmental variables within a forest landscape. The environmental variables are both natural environmental factors and human-induced changes to a local environment. To capture a whole forest community assemblage and to understand how woody species respond to the environment through their life span, seedling, sapling and adult stages are thus considered. The thesis consists of four papers. The studies were conducted in a bamboo-deciduous forest, in northeastern Thailand. Three of the papers are observational studies and one is an experimental study. The first observational study attempts to find the most important determinants of woody species richness and diversity at different life stages. The second study examines if productivity and human disturbance can function as determinants of evenness in the relative abundance of growth stages (seedlings, saplings and adults) of woody species. The third study is concerned with niches of woody species, examining if their niches shift across the life stages. The experimental study was set up to determine effects of bamboos, which have become dominant in the forest, on woody seedlings regeneration. Overall, the results show that environmental variables rather than human disturbance were important in explaining the variation in woody species richness and diversity (Paper I). Productivity and human disturbance significantly explained the variation in evenness of adults and saplings, respectively, in which evenness decreased with productivity and human disturbance (Paper II). The soil phosphorus content was found to be the most important variable for seedlings as it had a negative relationship to their richness, evenness and diversity (Paper I and II). The sapling stage is the most susceptible to human disturbance which influences sapling richness, evenness and diversity (Paper I and II). Adult richness and diversity can be predicted by forest structure (i.e. number of bamboo clumps and canopy cover) (Paper I). The basal area (i.e. productivity) was the most important determinant for adult evenness (Paper II). When examining the environmental or niche requirements among the growth stages within a species, the nine most abundant species in the forest showed changes in their niche requirements across the growth stages (Paper III). The results from the experiment clearly showed that bamboos suppress regeneration of woody seedlings through their deep shade and litter on the forest floor. Seedling abundance and species richness were reduced by bamboo canopies, and removal of bamboo litter increased seedling abundance and species diversity (Paper IV). Although current human activities do not appear to have severe negative effect on community properties, a long term study and experiments are needed to clarify the effects of human disturbance on this forest community. A good management strategy for woody species should consider the situation for each life stage, since the environmental requirements of a species change across the ontogenetic stages. Finally, bamboos should be incorporated in management strategies when the aim is to maintain woody species richness and diversity in these forest ecosystems