Patterns of tree species richness in forested wetlands

The number of tree species, or alpha diversity, in terrestrial forests decreases with increasing latitude. However, it is not known whether forests in flooded areas follow the same diversity patterns as upland forests. A good reason for doubt is the evidence that herbaceous wetland plants are most diverse at temperate latitudes. Wetlands are subject to strong environmental constraints, such as flooding, peat (acidity), cold, and salinity, which may be particularly limiting to tree growth and richness. We sampled 12 plots and collected published data from 245 plots of forested wetland around the world. The data were sorted into five categories based upon environmental constraints: tropical freshwater, temperate freshwater, temperate peat, tropical saline, and temperate saline. There was a significant difference in tree richness among the five categories (One-way ANOVA p<0.0001), from a mean of 31 species in tropical freshwater floodplains to 2 in temperate saline wetlands. Generally, tree richness increased with decreasing latitude, but the regression only accounted for 1.6% of the variation. It seems that species richness within forested wetlands is controlled by the cumulative number of environmental constraints.