Region effects influence local tree species diversity

Global patterns of biodiversity reflect both regional and local processes, but the relative importance of local ecological limits to species coexistence, as influenced by the physical environment, in contrast to regional processes including species production, dispersal, and extinction, is poorly understood. Failure to distinguish regional influences from local effects has been due, in part, to sampling limitations at small scales, environmental heterogeneity within local or regional samples, and incomplete geographic sampling of species. Here, we use a global dataset comprising 47 forest plots to demonstrate significant region effects on diversity, beyond the influence of local climate, which together explain more than 92% of the global variation in local forest tree species richness. Significant region effects imply that large-scale processes shaping the regional diversity of forest trees exert influence down to the local scale, where they interact with local processes to determine the number of coexisting species.Ecologists generally agree that large-scale patterns of diversity reflect a balance between regional processes of species production, extinction, and dispersal, on one hand, and within-region sorting of species based on adaptations to physical conditions of the environment, as well as competition among species for limiting resources, on the other hand (1–4). Nonetheless, the spatial scale down to which region effects extend has not been well resolved (5–7), but has wide-ranging implications for understanding the origins of patterns in local species richness. If the species richness of a local assemblage were strictly limited by competition and other local interactions among populations, new species could not be added without others being forced out, and we would expect to find a common relationship between diversity and local environmental conditions across regions (6). However, if unique historical and biogeographic features of each region influenced within-region diversification and extinction (8–10), these region-specific effects might contribute to the global pattern in local species richness. Efforts to disentangle these effects have met with limited success and have led to a long-standing discussion of the relationship between local and regional diversity (6, 11–21).Analyses designed to distinguish local and regional influences on diversity have found strong region effects in some cases (8, 22) and weak or nonexistent region effects in others (15, 23–25). However, most studies that failed to find significant region effects either have addressed large, biologically heterogeneous samples, or they have used local samples (e.g., 0.1-ha “Gentry” plots) that are too small to characterize the diversity of local assemblages adequately (26, 27). Additionally, many large-scale samples have been compiled from maps generated from presence-only museum records or from coarse-scale atlases that document the extent of species occupancy, not actual local occurrence. Such data often undercount local species richness. Moreover, many tests of the diversity–environment relationship have analyzed data on local communities that extend over broad ranges of ecological conditions (e.g., tropical rainforests to arctic tundra and hot deserts; ref. 25) with a range of biomes and vegetation types unevenly represented among regions. These sampling issues have confounded the testing of region effects.In this study, we analyze a dataset of tree species richness from the Center for Tropical Forest Science—Forest Global Earth Observatories (CTFS—ForestGEO; www.forestgeo.si.edu/; ref. 28) to disentangle the influences of local climate and regional factors, i.e., differences between regions resulting from unique histories and geographic settings, on the global biodiversity pattern. The data represent 47 forest dynamics plots distributed worldwide (Fig. 1) with a median size of 25 ha, within which all individual trees equal to or greater than 1 cm diameter at breast height (DBH) were identified and counted (SI Appendix, Table S1). Plots of this size are large enough to include adequate samples of species richness, but small enough to avoid substantial heterogeneity in climate and vegetation structure within them. The CTFS data are complete censuses, and the species richness in each plot is accurate. Many previous studies have been based on data from herbarium records of coarse-scale species range maps, and species richness is generally underestimated, considerably so in some cases. Moreover, the forest plots represent a single vegetation type surveyed over a range of environmental conditions. We assembled for each plot a set of local plot characteristics and climate data, and used generalized linear models to characterize the relationship between number of tree species and local plot variables and to test the additional statistical effect of region on local species richness. Our analyses were repeated for species richness with tree DBH ≥ 1 cm and DBH ≥ 10 cm.Open in a separate windowFig. 1.Global distribution of the 47 CTFS plots. The number associated with each plot is its size in hectares. The base vegetation map is the 2012 MODIS global land cover map (www.landcover.org/data/lc/) with IGBP Land Cover Type Classification.