Ecological Mechanisms Of Competition Affecting The Growth Of Masson Pine(Pinus Massoniana)

Competition is the most important driver of tree growth decline and hence mortality at the local scale.However,the mechanisms underlying how competition affects tree growth(and further mortality)remain unclear.To ascertain the mechanisms underlying competition-induced growth decline(and further mortality),a field experiment was conducted in a homogeneous high-density Pinus massoniana forest in southern Jiangxi Province,China.This dissertation includes the following three sections.First,I quantified the effects of competition on tree growth(growth vigour quantified by aboveground biomass and radial growth rate)using the data of field survey data and harvest sampling.Second,I determined the architecture traits,biomass allocation,non-structural carbohydrates(NSC)content,C-N-P stoichiometry and water use efficiency(WUE)for representative trees with different growth vigour(flourishing,moderate,dying and dead),and then analysed how tree growth vigour related to these indices.Finally,I analysed the ecological mechanisms of competition-induced growth decline comprehensively using a structural equation model(SEM),and predicted the tree growth rate using functional traits.The main results were as follows.(1)Both tree size and growth rate were negatively correlated with competition intensity quantified by neighbour competition index(i.e.,Hegyi＿H).The R~2 of the linear regression models of diameter at breast height,tree height,aboveground biomass per year,and radial growth rate and competition intensity were 0.94,0.77,0.59,and0.18,respectively.Meanwhile,the ordinal logistic regression also showed that tree growth vigour was significantly correlated with competition intensity.The competition intensity differed among tree growth vigour levels and was 19.45,44.85,98.56,and173.71 for flourishing,moderate,dying,and dead trees,respectively.These results demonstrated that competition is the main driver of growth decline and hence mortality for this forest.(2)Tree growth vigour in competition was related to tree height-diameter(H-D)scaling exponent.The H-D scaling exponent of flourishing,moderate,dying,and dead trees were 0.58,0.51,0.49,and 0.43 respectively,showing a pattern of decrease as tree growth vigour deteriorated.The H-D scaling exponent of flourishing trees was significantly higher than that of other trees.Concurrently,tree growth vigour was related to branch length-diameter(L-d)scaling exponent but not to branching angle.The branch length-diameter(L-d)scaling exponent of flourishing,moderate,and dying trees were 1.34,1.33,and 1.50,respectively,showing a pattern of increase as tree growth vigour deteriorated.The L-d scaling exponent of dying trees was significantly higher than that of other trees.Furthermore,tree growth vigour was also related to biomass allocation.The lowest leaf mass fraction(LMF)was found in dying trees(7.48%),which was 34.21%and 27.02%lower than that of flourishing trees(11.37%)and moderate trees(10.25%),respectively.(3)Tree growth vigour in competition was related to NSC characteristics.Compared to flourishing trees,the NSC(including soluble sugar and starch)pool(biomass weighted)of dying trees decreased by 27.31%.Specifically,the dying trees had a significantly lower NSC concentration in leaves,twigs and branches,and decreased by 20.88%,21.56%,and 22.39%,respectively,compared with flourishing trees.Additionally,tree growth vigour was not a function of C,N and P concentration and their ratios but was related to N stoichiometric homeostasis(H_N).The H_N of flourishing trees was‘strictly homeostatic’,which was significantly stronger than the‘homeostatic’of moderate trees and the‘weakly homeostatic’of dying trees.This result implied that carbon starvation and lower H_N were the potential ecological mechanisms of competition inducing growth decline and even mortality.(4)Tree growth vigour in competition was a function of its WUE.The WUE of dying trees was significantly lower than that of flourishing and moderate trees,with21.66%and 21.20%lower than the flourishing and moderate trees,respectively.Meanwhile,tree radial growth rate was positively correlated with WUE.These results demonstrated that the difference in water use strategy was also an important mechanism in determining tree fate in competition.(5)The structural equation model(SEM)explained 74.6%of the variation of tree radial growth rate,showing that competition had no direct effects but indirect effects through WUE,functional traits and biomass allocation(here is LMF)on tree radial growth rate.LMF had the greatest effects(0.539),including both direct effects(0.493)and indirect effects(0.046),on tree radial growth rate.Functional traits are good predictors of the tree radial growth rate(R~2=0.485).Branch diameter(BD)and leaf non-structural carbohydrates content(LNSC)were the two most powerful predictors,contributing 33.67%and 33.46%to the prediction of growth rate,respectively.The radial growth rate was predicted better after integrating functional traits,WUE,and LMF as predictors(R~2=0.661).In the integrated prediction model,LMF was the most powerful predictor,followed by BD and LNSC,and their relative contributions to the model were 33.22%,23.87%,and 22.55%,respectively.Overall,competition induced growth decline(and hence mortality)of trees through the decrease of aboveground exposed area and leaf biomass,NSC deficiency,lower H_N and WUE,and their interactions.Additionally,tree radial growth rate was predicted effectively by functional traits,LMF,and WUE.The most powerful predictors were LMF,BD,and LNSC.This study improved our understanding of competition,enriched the relevant theory and provided theoretical references for forest management and prediction of forest dynamics.