The spatial distribution of species assemblages is often described using three components of species diversity: alpha or local diversity, beta diversity and gamma or regional diversity. Beta diversity plays a pivotal role in linking local and regional diversity and it captures a fundamental facet of the spatial pattern of species assemblages.
Aimed at understanding why some forests of the world exhibit higher tree beta-diversity values than others, ecologists from many countries conducted a study in 10 large-scale forest plots (20-ha tropical forest plot in Xishuangbanna included) worldwide. Prof. CAO Min of Xishuangbanna Tropical Botanical Garden (XTBG) participated in the study.
The researchers asked the following specific questions: (1)What is the contribution of environmentally related variation versus pure spatial and local stochastic variation to tree beta diversity assessed at the scale of the forest plot? (2)At what resolution (i.e. size of sampling unit) are those components of tree beta diversity more apparent? (3)How does tree species richness of the plot affect its beta diversity? (4)What determines the variation in tree beta diversity observed across regions/continents?
To address those questions, the researchers compared the tree beta diversity found within 10 permanent stem-mapped forest plots that comprise tropical, subtropical and temperate forests distributed world-wide. The non-directional variation in tree species composition among cells of the plot was their measure of beta diversity. They compared the beta diversity of each plot with the value expected under a null model. They also apportioned the beta diversity into four components: pure topographic, spatially structured topographic, pure spatial and unexplained. They used linear mixed models to interpret the variation of beta diversity values across the plots.
In the study the researchers demonstrated the usefulness of stem-mapped forest plot data to compare the variation of tree beta diversity from one region to another. The ecologists predicted that a significant portion of the observed differences in beta diversity will be explained by different amounts of environmental variation of the areas being compared, once the effects of gamma species richness and sampling unit size are taken into account.
The study entitled “The variation of tree beta diversity across a global network of forest plots” has been published in Global Ecology and Biogeography, 21 (12):1191-1202, DOI: 10.1111/j.1466-8238.2012.00770.x
