Habitat heterogeneity and dispersal limitation are widely considered to be the two major mechanisms in determining tree species distributions. However, few studies have quantified the relative importance of these two mechanisms at different life stages of trees. Moreover, rigorous quantification of the effects of dominant tree species in determining species distributions has seldom been explored.
Dr. HU Yuehua and his teachers of Xishuangbanna Tropical Botanical Garden (XTBG) tested the hypothesis that the distribution of tree species is regulated by different mechanisms at different life history stages. They particularly hypothesised that dispersal limitation regulates the distribution of trees at early life stages and that environmental factors control the distribution of trees as they grow, because of niche differentiation resulting from environmental filtering.
The study was conducted in a 20-ha tropical seasonal rain forest plot in Xishuangbanna, southwest China (21°37′08″N, 101°35′07″E). Trees in 400-m2 quadrats in a 20-ha plot were grouped into four classes on the basis of the diameter at breast height (DBH) that roughly represent different stages in the life history of trees. A neighbourhood index was computed to represent a neutral spatial autocorrelation effect. They used both biotic (dominant species) and abiotic (topography and soil) predictor variables to model the distribution of each target species while controlling for spatial autocorrelation within each of the DBH classes.
The results showed that the relative importance of neutral and niche processes in regulating species distribution varied across life stages.
Their study revealed that dispersal limitation, represented as a neighbourhood index, has the largest effect on the distribution of trees across life stages whereas environmental factors mainly affect the distribution of large trees. This implies that if species are not analysed at multiple life stages, a biased conclusion may be reached regarding the mechanisms maintaining species coexistence.
The study entitled “Dominant species and dispersal limitation regulate tree species distributions in a 20-ha plot in Xishuangbanna, southwest China” has been published online in Oikos, doi: 10.1111/j.1600-0706.2011.19831.x