Latitudinal gradient in beta diversity of forest communities in America

Beta diversity is an important component of biological diversity,measuring compositional change in species assemblages across temporal and spatial scales.Understanding spatial variation in beta diversity and the underlying mechanism(s) has become the focus of biodiversity studies in recent years.Although many studies have proved latitudinal gradient in species richness,whether beta diversity shows latitudinal trends still remain controversial.In the present study,we assessed the latitudinal gradient in beta diversity at community level.We selected inventory data of 131 0.1 hm2 forest plots from South and North America,including various types of forest communities.These forest plots were a subset of 226 mature forest plots which were systematically gathered by late Alwyn H.Gentry during 22 years.We formulated two theoretical frameworks to facilitate the analyses.First,since species range size generally increase with latitude,there will be more obvious overlapping in species distribution toward the pole;this in turn will lead to latitudinal gradient in beta diversity.Second,since latitudinal decline in species richness at larger spatial grain is more abrupt than that at smaller spatial grain,the discrepancy in species richness of different spatial grains(beta diversity) will decrease with latitude.To catch more information of spatial variation in species composition within communities,we used both multiplicative and additive partitioning methods to calculate beta diversity.Simple correlation and regression were used to test the relationships between latitude and species diversity.The results showed that(1) beta diversity increased significantly along with decreasing latitude,irrespective which partitioning method was used.Generally,beta diversity in tropical and subtropical forests was comparable,and was much higher than that of temperate forests.This pattern can be easily derived by the latitudinal decrease in species range size measured by mean number of subplots occupied by all species within community,and the differences between regression slopes of species richness against latitude at larger and smaller grains;(2) gamma diversity of the studied communities was mainly contributed by beta diversity,with an average of 78.2%.Along with increasing latitude,the relative contributes of beta diversity to total gamma diversity decreased significantly;(3) beta diversity in south hemisphere was relatively higher than that of north hemisphere.This might reflect the regional differences in the histories of species evolution and environmental changes.In addition,we discussed the proper usage of multiplicative and additive partitioning methods.To our knowledge,this is the first attempt to test latitudinal gradient in beta diversity of forest ecosystems at community level.The results of our study have important implications for not only understanding the determinants of spatial variation in biodiversity but also for biodiversity conservation.