Characteristics And Influencing Factors Of Canopy Tree Growth In Tiantong Evergreen Broad-leaved Forest

The vertical stratification of forests is a complex phenomenon shaped by the interaction between trees and their surrounding environment.The primary contributors to forest biomass,canopy trees encounter different biotic and abiotic environments and form different growth characteristics during their growth process from sapling to mature tree across the stratified layers.However,the characteristics and influencing factors of tree growth in different tree layers remain poorly understood.And this issue is critical in a subtropical evergreen broad-leaved forest with complex vertical structures and high species diversity.Based on a 10-year vegetation survey data from a20 ha forest dynamic monitoring plot in Tiantong,Zhejiang,this study divided trees into three layers(understory layer,subcanopy layer,and canopy layer)based on their relative canopy positions during field investigations.Non-disturbed communities with intact vertical structures(i.e.,three complete tree layers)were selected as the main study area.And combined with environmental investigation data,including topography and soil,the growth and distribution characteristics of trees in different tree layers were analyzed.Then we used linear mixed-effects models and hierarchical partitioning to quantify the relative contribution of initial size,neighborhood interactions,and the abiotic environment in determining tree growth across the stratified layers.The main results are as follows:(1)From the understory layer to the canopy layer,the absolute growth increment of the basal area of canopy trees increased while the relative growth rate significantly decreased.Moreover,the coefficients of variation of these two growth indices exhibited significant differences among different forest layers,with the highest coefficient of variation observed for the absolute growth increment of trees in the understory layer and the highest coefficient of variation observed for the relative growth rate of trees in the subcanopy layer.The proportion of trees in the canopy layer was the highest,while the subcanopy layer had the highest number of individuals;and the diameter distribution of trees in the understory,subcanopy,and canopy layers followed an inverse "J" shape,skewed normal,and normal distribution,respectively.The number and competition index of heterospecific neighbors were significantly higher than those of conspecific neighbors for trees in all three layers.(2)The absolute growth of trees in the understory layer was mainly positively influenced by their initial size,while trees in the subcanopy layer were positively influenced by the initial size and negatively affected by heterospecific competition.And trees in the canopy layer were primarily negatively impacted by heterospecific competition.The negative effect of heterospecific competition on tree growth significantly increased from the understory to the canopy layer,while the positive effect of initial size gradually diminished and showed a negative impact in the canopy layer.And the effect of the abiotic environment on the overall trend showed an increase,with the greatest impact in the canopy layer.In conclusion,there are differences in the canopy tree growth in different tree layers,which are closely related to the distribution characteristics of trees and biotic and abiotic environments in different tree layers.The diameter of trees in the understory layer shows an inverted "J" type distribution,and most of them are small-dimeter individuals,with relatively few conspecific neighbors and small intraspecific competition.Therefore,the relative growth rate of trees in the understory layer is higher,but the absolute growth is still relatively small due to the small initial size and growth space.Thus,individual trees in the understory layer tend to grow rapidly to the subcanopy layer to obtain more vertical growth space.The vertical height range of the subcanopy layer is wide,so trees have more vertical growth space.Many tree individuals of different diameter classes stay here,forming a growth pattern dominated by initial size and neighborhood competition.However,when trees are in the canopy,their diameter distribution is normally distributed.Some of the small-diameter trees that have just reached the canopy layer in the growing stage still need to compete with their larger neighbors for light resources and have apparent habitat preferences,while some of the large-diameter individuals are in the senescence stage and their growth declines,making the initial size a negative effect on growth.In summary,the vertical stratification of the forest significantly impacts canopy tree growth,especially the differences in light availability and growth space due to the vertical stratification playing an essential role in canopy tree growth.The research emphasizes the significance of integrating both the vertical structural characteristics of forest stands and the individual-level traits of trees in comprehending the dynamics of canopy tree growth.It also indicates the need to consider the varying responses of individuals in different tree layers to the changes in light resources and growing space resulting from silvicultural thinning,when formulating forest management strategies.