Effects Of Stand Structure And Diversity On Forest Biomass And Productivity From Local To Global Scales

Forest ecosystems play an important role in the global carbon cycle.Forest carbon cycling is influenced by both biotic（stand structure and diversity）and abiotic factors（climate and soil condition）.However,there is still no consensus on the relative importance of stand structure and diversity in maintaining forest biomass and productivity.Metabolic Theory of Ecology（MTE）has been proven to be reliable in the quantitative forest structure and dynamic,but it is also concerned about the less attention of the time and space of forest biomass and productivity.Therefore,the relative importance of stand structure and diversity on forest biomass productivity and stability first to be examined based on local investigate data.On this basis,the influence of stand structure and diversity of forest biomass productivity further be studied,and verified the deviation of observations from MTE predictions using regional and global forest dataset.Finally,the dynamic changes of the relationship between forest biomass and stand density over time were discussed.The main results are as follows:（1）Based on long-term monitoring data of tropical forests,the results shown that:（1）forest biomass and productivity are strongly affected by larget-sized tree(D_max),while their stability are dominated by biodiversity;（2）functional diversity and richness more important than other diversity indexes;（3）the diversity effect increased with the increase of plot size.（2）Based on the global forest productivity dataset,the results showed that:（1）Forest productivity increased significantly with climate factors and stand biomass,but decreased significantly with stand age;（2）Stand biomass and stand age was the most important factor in determining the forest productivity;（3）The relationship between forest productivity and biomass followed the MTE prediction,but there are some deviations from MTE among different forest types.（3）Based on the subtropical forest monitoring plots across altitude gradients,the effects of diversity,stand structure,and largest-tree sizes on forest biomass were explored.Results shown that:（1）stand structure and largest-tree sizes(D_max and H_max)were the main factors for predicting forest biomass,while diversity had weak effect across elevation gradients;（2）the slopes between diversity,stand density,and largest-tree sizes and forest biomass were basically consistent with the MTE（Metabolic Theory of Ecology）prediction（0,-1/3,5/3 and 5/2,respectively）,but there are different degrees of deviation in different elevation gradients.（4）Based on the global forest biomass data across broad climatic gradients,the spatial pattern and driving mechanism of forest biomass were futher explored:（1）The explanatory power of largest-tree sizes on forest biomass is higher than climate factors,diversity and stand density,which strongly supports the larger-sized trees hypothesis;（2）The relationship between stand density,diversity,largest-tree sizes(D_max,H_max and V_max)and forest biomass are basically consistent with the prediction of MTE;（3）the largest-tree effect did not change significantly with climate gradient,while density effect and diversity effect were affected by climate conditions and succession stages.In order to resolve some of the debates in previous ecological studies（such as positive,negative,and irrelevant BEF relationships）,it is necessary to explore how these relationships change over time.（5）Based on a global forest datasets,the temporal dynamic of the relationship between stand density and forest biomass,∝¹⁺,were explored.Therefore,the relationship between stand density and forest biomass depends on the self-thinning exponentγ.Results shown that:（1）the self-thinning exponent showed unified dynamic rule,that is,first decreasing and then increasing with forest age,while-3/2 or-4/3 was just a transient value in the dynamic self-thinning process;（2）the temporal dynamic curves of self-thinning exponents were inconsistent among different forest types,which may be mainly driven by stand biomass;（3）the dynamic self-thinning model,which was obtained from the type-Ⅲsurvival curve and the logistic growth model of community biomass,could fit the dynamic self-thinning rule from boreal to tropical forest well.Our study is the first work that described the complete process of the dynamic self-thinning rule,using global observational data and a mathematical model,which well integrates previous contradictory views on dynamic self-thinning and helps to resolve the classic debate in ecology——the“-3/2 or-4/3”debate.In this study,we investigated the driving mechanisms of forest biomass and forest productivity from regional to global scales.Our results showed that largest-tree sizes were the best variables to predict forest biomass and productivity,while the effects of climate factors and diversity were weak.The MTE also proved largest-tree sizes are good predictors of forest biomass and productivity.Although MTE can explain the forest structure and dynamic characteristics well,it will obviously deviate from the theoretical predictions at different forest stages and environmental conditions.Therefore,MTE should be improved in the future.Our study also confirmed the dynamic self-thinning rule during forest development from observations data and mathematical model,providing a new perspective for further study on the relationship between stand density and biomass in the future.