Species-area relationships of different plant functional groups in tropical monsoon rainforests (Hainan Island, China)

The positive relationship between the number of species in an area and the size of the habitat studied (species-area relationship, SAR) is the foundation of ecological theory and the most studied issues in ecology. However, very few studies have addressed SARs and their relationships to functional group richness and the extent of habitat area studied. We analyzed SARs in different functional groups focusing on two types of tropical monsoon rainforests on Hainan Island, China. The number of trees, shrubs, lianas, all species, deciduous species, evergreen species, thorn species, non-thorn species, simple-leaved species, and compound-leaved species were strongly related to the size of the area studied in the two tropical monsoon rainforest communities. And the size of the area studied explained over 94% of the total variations in the number of families and other groups studied. No significant differences were found for the slope of the species-area curve (Z) of different functional groups in two tropical monsoon rainforest communities except for shrubs that were taller in the Terminalia hainanensis community (TC) than in Liquidambar formosana community (LC). In the comparison of different functional groups divided by the same principle in the same tropical monsoon rainforest, no significant difference was found for the value of Z for different functional groups except that Z for lianas were higher than for shrubs in LC. However, the Z of lianas was higher than that of trees and shrubs, and evergreen species and species without thorns had higher Z values than deciduous species and species with thorns. No significant differences were found between simple-leaved species and compound-leaved species in TC. The results indicated the SARs of different plant functional groups can be well-described by the power function, although their slopes differed significantly.