The Roles Of Forest Vertical Structure And Large-diameter Trees On Plant Diversity Along Elevational Gradients

As key attributes of forest structures and ecosystem functions,forest vertical structure and large-diameter trees participate in multiple key processes of plant community assembly such as resource allocation and competition,affecting the understory environment and plant growth significantly.Compared with the studies on environmental factors such as climate,soil,and topography,there is still a lack of systematic work on how forest vertical structure and large-diameter trees play a role in maintaining plant diversity across different forests layers.Across geographic regions with different climatic conditions,vegetation types,geological history,and human disturbance history,it is unclear how the regional effect affects vertical structures and large-diameter trees and act on the plant community assembly.Mountain ecosystems harbor abundant forests and large-diameter trees,and are characterized by rapidly changing environments along elevational gradients,serving as an ideal platform for evaluating the impacts of forest vertical structure on plant diversity.In this study,based on the detailed woody plant survey data covering 10 elevation gradients,153 permanent sampling plots with a total of 1192species and 57341 individuals across subtropical and tropical forest regions in China,I analyzed the determinants of the distributions of large-diameter trees along the elevational gradients.By integrating biotic and abiotic factors as well as phylogenetic diversity,I quantitatively assessed the extent to which vertical structure and large-diameter trees affected the taxonomic and phylogenetic diversity of plant communities and their relative importance on the diversity in different forest layers.The main results and conclusions are shown below:(1)The subtropical and tropical mountain ecosystems in China harbored abundant large-diameter trees.The most abundant families were Fagaceae,Pinaceae,Lauraceae,Ericaceae,and Cupressaceae,which accounted for 66%of the total large trees.The elevational patterns for biomass of large trees varied greatly across regions and peaked at mid-elevations in most mountains.Mean annual temperature and annual precipitation were the critical determinants of the biomass of large trees,while the effects of soil and topography were relatively weak,supporting the water-energy hypothesis.The lower biomass of large trees appearing at low elevations in the Mt.Tianmu and Mt.Guanshan,might be related to intense human disturbance in the eastern subtropical regions,indicating indirect effects of disturbance on biodiversity maintenances.(2)Forest vertical structure and large trees had significant impacts on the plant taxonomic diversity in several mountains,but the general relations were weak.Specifically,the vertical structure was positively related to species evenness in Mt.Yulong(also known as Jade Dragon Snow Mountain)(R~2=0.41,P<0.01)and Mt.Guanshan(R~2=0.54,P<0.05),and the biomass of large trees showed significantly positive relations to the richness in Mt.Dabieshan(R~2=0.57,P<0.05).Compared to the whole community,tree saplings and shrubs were more affected by vertical structure and large trees,suggesting the different effects on species diversity in different forests layers.The relationships between vertical structure and plant diversity were significantly shaped by regional effect.The effect of large trees on species evenness decreased significantly as elevation ranges increased(R~2=0.44,P<0.05).Soil and slope significantly affected species diversity along elevational gradients in Mt.Yulong,Mt.Gaoligong,and Mt.Dabieshan,supporting the habitat heterogeneity hypothesis.(3)Forest vertical structure and large trees showed inconsistent effects on plant phylogenetic and taxonomic diversity in most regions,suggesting the different effects of vertical structure on phylogenetic diversity.In Mt.Yulong,phylogenetic community structure tended to be more clustered with the increase of the complexity of the vertical structure(R~2=0.52,P<0.01),indicating the more closely related species within the community.There was a clear unimodal relationship along the elevational gradient between the biomass of large trees and phylogenetic diversity in Mt.Jinfoshan(R~2=0.37,P<0.05).The vertical structure had stronger effects on phylogenetic diversity of tree saplings and shrubs when compared with the whole community in multiple mountains.And the effects of vertical structure on tree saplings were stronger than that on shrubs,indicating the importance of vertical structure on forest regeneration.The impacts of vertical structure and other environmental factors on phylogenetic diversity were affected by regional effect.Climatic factors were the main determinants of phylogenetic diversity in the mountains with large elevational ranges,such as Mt.Gaoligong,Mt.Tianmu,and Mt.Damingshan.Habitat heterogeneity played important roles in shaping phylogenetic diversity in Mt.Gaoligong and Mt.Yulong.The effects of vertical structure on phylogenetic structure decreased significantly along with increased latitude and longitude(R~2=0.33,P<0.05;R~2=0.34,P<0.05),suggesting the importance of east-west disparity in human disturbance history and north-south disparity of vegetation types in maintaining plant phylogenetic diversity.In conclusion,based on the plant diversity data of 10 elevational gradients spanning 12latitudes and 20 longitudes,I analyzed the distribution patterns of large-diameter trees along elevational gradients and identified their driving mechanisms.And I evaluated the influences of vertical structure and large tree effects on plant taxonomic and phylogenetic diversity along elevations,and clarified the role of regional effect on vertical structure effects of plant diversity.These findings provide the evidences that the complexity of vertical structure plays a critical role in maintaining plant diversity and its spatial variations,highlighting the importance of protecting primary forests with complex structures and large trees in mountains.This study provides scientific guidance for deeply understanding the maintenance mechanisms of plant diversity across multiply spatial scales and biodiversity facets.