The Relative Contribution Of Regional And Local Processes In Shaping Tree Diversity In Subtropical And Tropical Forests In China

Spatial patterns of plant diversity and its underlying mechanisms are the fundamental questions in ecology and biogeography.Many studies have been focusing on disentangling how ecological and evolutionary processes at different scales shape plant diversity patterns.However,the effects of regional processes on the patterns of local plant diversity are often ignored.Evaluating the joint effects of regional and local processes provides critical insights into assembly rules of biological communities.In this study,using the recently compiled vegetation data in subtropical and tropical forests in China,I filtered 3307 woody plant species in forest canopy layers from 2960 forest plots.These data were used to explore the relative contribution of regional and local processes in shaping tree diversity in these forests from the view of two scales(regional and local scales),three dimensions(taxonomic,phylogenetic and functional diversity),and two biodiversity measures(species presence-absence data and abundance data).The main results included:(1)The effects of regional and local processes on species diversity patterns.The geographical patterns varied greatly among three biodiversity measures.Species richness and Simpson’s index decreased as latitude increased,while species rarity had a clear decreased trend along longitudes.Using boosted regression trees to model species diversity,I found that regional species pool was among the best predictors for species richness and Simpson’s index(32.9% and 32.3% of total explained variance,respectively),but human activity was the most important predictor for species rarity(30.3% of total explained variance).Structural equation models revealed that regional and local processes co-determined the large-scale patterns of species diversity,and regional processes had direct or indirect impacts on local species diversity by regional species pool.For species richness and Simpson’s index,the effects of regional processes were larger than those of local processes.However,local environmental predictors,particularly in climatic seasonality,had larger impacts on species rarity.The results empirically demonstrate regional species pool,historical environmental factors,and human activities play an important role in understanding the drivers in shaping the patterns and variabilities of local species diversity.(2)The effects of regional and local processes on phylogenetic and functional diversity.All eight functional traits showed significant phylogenetic signals(P < 0.05),suggesting that all functional traits are significantly affected by their evolutionary history.There was a clear spatial mismatch between spatial patterns of phylogenetic and functional diversity.Phylogenetic diversity decreased as latitude and longitude increased,supporting the tropical niche conservatism hypothesis.However,functional diversity showed a clear decreased latitude trend,showing the northern parts of this region occupied more functional space and higher niche partitioning.Regional species pool and current climate were the main predictors of phylogenetic diversity pattern,whereas phylogenetic structures were mainly affected by regional historical climate and topographic heterogeneity.The main variables determining geographical patterns between functional richness and dispersion,with historical climate being important for functional richness,and current climate being important for functional dispersion.Structural equation models revealed regional mean annual temperature(path coefficient= 0.298***)and local temperature seasonality(path coefficient = 0.217***)were the most important predictors for functional dispersion based on species presence-absence data.The effects of local predictors on functional dispersion based on species abundance were larger than those of regional predictors.The results suggest that nichebased processes and historical processes jointly shape the patterns of local phylogenetic and functional diversity.(3)The effects of contemporary and historical factors on functional community composition.At community level,maximum tree height,wood density,specific leaf area,leaf nitrogen content,and leaf phosphorus content increased as latitude increased,whereas seed mass and leaf dry matter content showed a decreasing trend.Multivariate spatial autoregressive error model revealed that historical and current environmental predictors jointly shaped the patterns of community weighted mean of these traits,with larger impacts from contemporary predictors.Regional annual mean temperature and elevational range significantly affected seed mass and leaf-related traits.Regional historical climatic stability was positively correlated with maximum tree height,wood density,seed mass,and leaf area.For local predictors,plant functional traits were significantly negative with soil pH and temperature seasonality,yet positive with slope and topographical position index.The results suggest that regional and local environmental filtering simultaneously affect spatial patterns of plant functional composition.In conclusion,regional and local processes co-determined spatial patterns of tree diversity in subtropical and tropical forests in China.Except for contemporary climate and habitat heterogeneity,plant diversity pattern was affected by historical climatic stability and local climatic seasonality.These results indicate that the paleoclimatic stability has a legacy in shaping the plant diversity pattern in this region,even though the Quaternary glaciation has relatively weak effect in this region.This study demonstrates that considering simultaneously taxonomic,phylogenetic and functional diversity by integrating species presence-absence and abundance data provide new insights into better understanding the mechanisms of biodiversity patterns in local plant communities.