The mechanisms underlying the community assembly of tree species remain controversial: niche-based mechanisms, neutral mechanisms, and joint mechanisms incorporating niche and neutral processes remain under discussion. Although a few studies have examined the relative importance of niche and neutral processes at specific size-classes, there has been no study to date that, models, directly, the impact of size-class on species assembly. The long-term monitoring of the dynamics of tree species is a direct way of examining the role of size-class in species assembly yet the extended life span of trees makes this challenging.
Prof. CAO Min and his team of Xishuangbanna Tropical Botanical Garden (XTBG) have investigated the processes of community assembly using size classes of trees. They used the inhomogeneous Poisson process (to model environmental impacts), the homogeneous Thomas process (modeling dispersal) and the inhomogeneous Thomas process (modeling the joint-effects of environment and dispersal) to simulate tree species’ distributions.
To quantify the effects of environmental variables on the species-area relationships (SARs) and pair correlation function (i.e. the g-function), they used topographic and soil variables as environmental variables.
Data from two seasonal tropical forests were used to investigate the mechanisms underlying the species-area relationships and the pair correlation function. The first of those datasets was from the 20-ha forest dynamics plot in Bubeng village, Xishuangbanna National Nature Reserve, Southwest China The second dataset originated from the 50-ha forest dynamics plot on Barra Colorado Island (BCI), Panama.
To model size-class effect on species’ distributions, they classified trees into different DBH size-classes as an effective option for handling the tree size issue. They modeled species’ distributions using 3 DBH (diameter at breast height) size-classes: 1 to <5 cm DBH, 5 to <10 cm DBH, and DBH>10cm.
To evaluate the effect of incorporating size-classes on the point patterns of species’ distributions, the pair correlation function (i.e., the g-function) was computed for all the simulated and real species’ distributions.
After synthesizing the results of the SARs and g(r), reflecting community and species levels patterns, respectively, the researchers have demonstrated that dispersal was the more dominant process in the determination of the spatial patterns of species across size-classes at both species and community levels for both the rainforests examined. Moreover, the role of dispersal in driving species’ distributions increased with size-class increasing, whereas that of environment decreased.
The researchers concluded that it is critical to use summary characteristics at both community and species level to identify the ubiquitous mechanisms that determine species’ distributions. Size-class effect contributed to species’ assembly through differential dispersal which is the most important process in both forests studied.
The study entitled “Size-Class Effect Contributes to Tree Species Assembly through Influencing Dispersal in Tropical Forests” has been published in PLoS ONE.
