Responses Of Plant Diversity-productivity Relationship To Canopy Gap Disturbance In Tiantong Evergreen Broad-leaved Forest

Forest productivity plays an important role in regulating global climate change and carbon budget.Previous studies have shown that species diversity,functional diversity,and structural diversity(structural heterogeneity of forest ecosystem in horizontal and vertical directions)are beneficial to productivity.However,species,functional diversity and structural diversity have potential interrelationships,and canopy gap disturbance also has an impact on the above factors.The superposition of these factors makes it more complex to analyze their impact on productivity,and provides an opportunity to explore their potential ecological mechanisms.In eastern subtropical forests,species diversity,functional diversity,and structural diversity are often affected by extreme weather such as typhoons.Species,functional diversity and structural diversity are often affected by canopy gap disturbance,and the impact of canopy gap disturbance on them may vary with spatial scale.Partially because of the confounding effects of environmental factors,the underlying relationships between species diversity,functional diversity,structural diversity,canopy gap disturbance and productivity at different spatial scales remain poorly understood.In this study,we use data from a 20-ha evergreen broad-leaved forest dynamic plot in Tiantong,Zhejiang Province to explore the mechanisms on how species diversity,functional diversity and structural diversity may impact productivity.On this basis,the impact of canopy gap disturbance on the relationships between species diversity,functional diversity,structural diversity and productivity,as well as whether the impact change with spatial scale,are analyzed.The main results and conclusions are shown below:(1)Linear regression analysis showed that species richness,species composition,stand density,diameter layers,tree size diversity(DBH Shannon diversity)and initial biomass had significant positive effects on productivity.Through multiple regression analysis,it was found that species richness,species composition,tree size diversity,stand density and initial biomass had significant positive effects on productivity.Structural equation path analysis and impact effect results showed that the total effect was tree size diversity,stand density,initial biomass,species richness and species composition.The results show that the total effect of structural diversity on productivity is greater than that of species diversity,and the resource complementarity caused by structural diversity is the main driving factor of productivity.(2)The results of one-way ANOVA showed that canopy gap disturbance increased species richness,stand density and productivity,and changed species composition,but decreased tree size diversity and community-weighted mean specific leaf area.The results of multiple regression showed that species composition,community-weighted means of specific leaf area,stand density,tree size diversity,initial biomass and canopy gap disturbance had significant correlation with productivity.The results of structural equation modeling showed that canopy gap disturbance had an indirect effect on productivity through stand density and tree size diversity,and tree size diversity had the greatest effect on productivity.When the overall plot divided into two parts: disturbed quadrats and undisturbed quadrats,the results of disturbed quadrats show that only stand density and tree size diversity have significant correlation with productivity,and the total effect of tree size diversity is the largest,which indicates that niche complementary effect plays a leading role in productivity in disturbed quadrats.But not in undisturbed quadrats,the results showed that stand density,species composition,community-weighted mean specific leaf area and initial biomass had significant correlation with productivity,and the total effect of initial biomass was the largest,indicating that the hypothesis of vegetation quantity dominated by initial biomass played a leading role in productivity in undisturbed quadrats.These results indicate that canopy gap disturbance promotes niche complementary effect of structural diversity and reduces the effect of initial biomass on productivity.(3)The results of the overall plot showed that with the increase of spatial scale,the canopy gap disturbance changed from the indirect effect of stand density and tree size diversity(20 m scale)on productivity to the community-weighted means of maximum DBH(50 m,100 m scale).In disturbed quadrats,with the increase of spatial scale,the factor with the largest total effect on productivity changed from tree size diversity(20 m scale)to the community-weighted means of maximum DBH(50m scale),and finally became the topography factor(100 m scale).In undisturbed quadrats,with the increase of spatial scale,the factor with the greatest impact on productivity changed from initial biomass(20 m scale)to species composition(50 m scale),and finally to topography factor(100 m scale).However,with the increase of spatial scale,the impact of ecological processes on productivity decreases,the impact of environmental filtration on productivity increases,and the impact of canopy gap disturbance on the relationship between species,functional diversity,structural diversity and productivity gradually weakens.In conclusion,considering the environmental factors,this study systematically clarified the effects of canopy gap disturbance on the relationship between species,functional diversity,structural diversity and productivity.The impact of structural diversity on forest productivity is more important than species and functional diversity,which is mainly related to canopy gap disturbance promoting niche complementarity effect through structural diversity.However,with the increase of spatial scale,the explanation degree of ecological processes such as niche complementarity,selection effect and mass ratio to productivity decreases,and the explanation degree of environmental factors becomes stronger,leading to gap interference the influence on their relationship decreases with the increase of scale.The results deepen our understanding of the relationship between biodiversity and productivity,and provide a theoretical and scientific basis for the development of adaptive strategies to cope with climate change by biodiversity conservation in forest management.