Responses Of Tree Architecture To The Neighborhood Shading And Crowding In Subtropical Forests In Tiantong,Zhejiang Province

The difference in tree architecture results from the plant's inherent genetic structure and the shaping of external biological competition and environment.The diversity of tree architecture is vital for the spatial arrangement of forest canopy and the full utilization of light and water resources of plants.It is the main driver for the coexistence of species and maintenance of ecosystem productivity.Previous studies on plant architecture and resource investment strategies were mostly based on species characteristics,whereas lacking the overall understanding of the tree neighboring effects on tree architecture.Vertical shading and horizontal crowding of neighbors are the main drivers affecting the changes of tree architecture and resource utilization,however their causal association is not yet clear.The research in this thesis was conducted in an evergreen broad-leaved forest(1hectare)located in Tiantong,Zhejiang province.Each individual was measured for tree architecture traits including crown size(crown area,relative crown area and crown depth),stem taper,stretch direction of branch,leaf dispersion and tree branch bifurcation ratio(step branch bifurcation ratio R_b0:1,step branch bifurcation ratio R_b1:2,step branch bifurcation ratio R_b2:3,branch bifurcation ratio),leaf economic traits and wood mechanical traits like mean leaf area,specific leaf area,leaf nitrogen content,leaf phosphorus content,twig wood density,and also for the topographical factors,soil nutrients and moisture content.The overlapping area of the crown that each neighbor shaded on the target tree was calculated using spatial analysis to represent the shading effect of the neighbors.At the same time,according to the diameter at breast height of the target tree,the crowding neighbors was selected,the basal area and number of crowding neighbors per area were calculated to represent the crowding effect.Finally,the linear mixed-effect model was used to analyze how tree architecture,branch bifurcation ratio,leaf economic traits and wood mechanical traits response to the shading effect alone and in combination with crowding effect.In the linear mixed-effect model,plant species were set as random variables,the shading and crowding effect,the height of target tree,the distance between the target and neighboring trees and the environmental factors were set as fixed variables.The following are the main results from the thesis:1.The shading of neighbors had important effects on tree architecture,branch bifurcation ratio,leaf economic traits.The shading of neighbors explained 57%,44%,1%,6%,and 38%variation of crown area,relative crown area,crown depth,stem taper and leaf dispersion,respectively;explained 2%variation of step branch bifurcation ratio Rb0:1;explained 1%variation of leaf phosphorus contents,respectively.In addition to the shading area,the height of target trees and their spatial distance from the neighbors also affected the architectural and some other traits.Specifically,tree height explained1%,23%,and 67%variation of the crown area,relative crown area and crown depth;explained 3%variation of the step branch bifurcation ratio Rb2:3;explained 0.2%and2%variation of mean leaf area and specific leaf area.Besides,change in the tree architecture and other traits were also related to environmental factors.Specifically,soil nutrients explained 2%,0.4%,and 36%variation of crown area,depth and stretch direction of branch;explained 1%variation of the step branch bifurcation ratio Rb1:2;explained 0.5%,3%,0.6%,and 6%variation of mean leaf area,specific leaf area,leaf nitrogen content and twig wood density;soil moisture explained 1%and 15%variation of the crown area and the stretch direction of branch.2.When considering the crowding effect of the neighboring crown together,the results of liner mixed-effect model showed that the shading and crowding effect of the neighbors affected the plant architecture together and explained more variation of the tree architecture than branch bifurcation ratio,leaf economic traits and wood mechanical traits.The crown area was mainly explained by the total shading area of the neighbors(1.7%),tree height(31.3%)and the soil nutrients(0.8%);the relative crown area was explained by the total shading area of the neighbors(1.6%),the sum of basal area of crowding neighbors per unit area(0.2%),the density of the crowding neighbors(1.1%)and soil nutrients(0.6%);crown depth was explained by the sum of basal area of crowding neighbors per unit area(0.2%),the density of the crowding neighbors(0.7%)and its own tree height(42.6%);the stem taper was explained by the sum of basal area of crowding neighbors per unit area(0.2%),the density of the crowding neighbors(2.4%),tree height(0.7%),soil nutrients(0.6%)and soil moisture(0.7%);the leaf dispersion was explained by the sum of basal area of crowding neighbors per unit area(1.3%)and tree height(40.5%).Branch bifurcation ratios and step branch bifurcation ratios were basically not affected by shading and crowding of neighbors,except for that the step branch bifurcation ratio R_b0:1 was explained by the total shading area of the neighbors(0.2%),tree height(15.1%)and the soil nutrients(0.2%);.Mean leaf area was explained by the soil nutrients(0.8%);specific leaf area was explained by tree height(1.0%)and soil nutrients(1.7%);leaf nitrogen content was explained by tree height(0.3%)and soil nutrients(0.2%);leaf phosphorus content was explained by the density of the crowding neighbors(0.5%)and soil moisture(0.2%);twig wood density was explained by tree height(0.2%),soil nutrients(1.2%)and moisture(0.3%).The above results indicated that the effect of shading and crowding of neighbors on tree architecture was far greater than they on branch bifurcation ratio and leaf economic traits.This thesis proved that the shading effect of neighbors led to a heavier horizontal investment of crown,and the crowding effect of neighbors led to a heavier vertical investment of the crown.In conclusion,this study demonstrated that the shading and crowding of neighbors have opposite effect on tree architecture and investment strategies,resulting in the diversity of tree architecture and functional strategies in the forest.This study provided new evidence for the differentiation of forest tree architecture from the perspective of neighboring effect,which is helpful to deepen the theory of species coexistence.