Effects Of Plant’s Light Utilization And Water Transport Capacity On Stratification Of Tree Architecture In Evergreen Broad-leaved Forests

Tree architecture as a basic unit for shaping community structure and vertical layer, refers to the overall shape of a tree and the spatial position of its components. The characteristics of tree architecture is a comprehensive result of plant adapting to variable environment conditions and biological competition. There is now mounting evidence that, tree architecture varies with changes of forest vertical layers due to tree species posited different vertical layers having different light utilization and water transport capacities within a given forest. Most emperical studies have focused only on the description of spatial pattern of tree architecture. Thus, it is poorly to understood that how does ecological process of light utilization and water transport affects the variation in tree architecture along forest vertical layers. Therefore, linking tree architecture and its light utilization and water transport capacity help us to deepen the theory of relationships between tree architecture and its function, and to understand deeply the processes of plants adaption strategies and community assembly. We conducted the experiments based on the ecological and physiological linkages between tree architecture and light utilization and water transport capacity in Tiantong National Forest Park, Zhejiang Province. Tree architecture and physiological characteristics of light utilization and water transport capacity were investigated in three10×10m2shrubs and three20×20m2Schima superba communities. Tree architecture includes tree height (H), crown depth and area (CD and CA), basal diameter (BD), number of leaves borne by twig (LN), individual mean leaf area (LMA), length and diameter of petiole (PD and PL), number of branch borne by trunk (BN), number of first level branch borne by second level branch (1BN), number of twig borne by first level branch (0BN), angle between twigs (0BA), and angle between first level branches (1BA). While light utilization and water transport characteristics include crown exposure index (CEI), net photosynthesis and transpiration rates (Pn and Tr), stomatal conductance (Cond), sap flux density and sap flow (Js and E), Huber value (HV) and leaf water potential (LWP). According to metabolic theory, the relationships between tree architecture and its light utilization and water transport capacities were determined at community and species levels. Thus, at each levels, One-way ANOVA or Independent-samples T Test was used to determine the differences in tree architecture with light utilization and water transport capacity among forest vertical layers. Further, Principal Component Analysis (PCA), Linear regression and Partial correlation analysis were used to show the relationships between tree architecture, light utilization and water transport capacity, while Zero-model was used to simulate niche differentiation along forest vertical structure. We try to reveal the physiology mechanism of species coexistence and the variation in tree architecture along forest vertical layers in the evergreen broad-leaved forest. We observed the following results:(1) Tree architecture were different between forest vertical layers. Specifically, H, CA, CD BD, LN, LMA, PL, PD,BN were significantly largest in canopy layer, intermediate in sub-canopy layer, while lowest in understory layer (p<0.05). However,1BN,0BN,0BA and1BA showed the opposite trends to the above specified tree architecture. Those above differences resulted in canopy trees having the bigger and compact crown, wheares understory plants displaying the smaller and open crown. Thus, it’s showed that the variation in tree arcitectures across vertical layers led to the difference in appearance of crown between canopy trees and understory plants.(2) CEI and Pn reflected light utilization capacity of plants. In this study, CEI and Pn increased significantly along vertical layers (p<0.05). In addition, CEI has a significant linear relationship with tree architecture (including H, CD, CA, BD, LN, MLA, BN,0BN,1BN,0BA,1BA, PL and PD)(p<0.05). Those results demonstrated that light utilization capability drived tree architecture changing along vertical layers height in EBLFs.(3) Js, E, Tr and Cond reflected water transport capacity of plants. In this study, Js, E, Tr, Cond were significantly increased along vertical layers height (p<0.05), while Huber value showed opposite trends. Tr, Cond, Js, E and Huber values were reduced into integrated traits i.e., PCI and PC2, based on PCA. Thus, PCI and PC2had the significant linear relationships aganist tree architecture (p<0.05). The overall results demonstrated that water transport capacity drived tree architecture changing along vertical layers in EBLFs.(4) The absolute value of partial correlation coefficient between CEI and tree architecture increased along vertical layers, while the coefficient between PC1or PC2and tree architecture decreased in shrubs and S.superba community, when CEI, PCI and PC2were selected as the control variables respectively. Those showed that light utilization capacity was mainly responsible for variation in tree architecture of canopy layer, while water transport capacity was the driver for the change in architecture of understory layer.(5) Tr, Pn, Cond and Js were used as resources utilization traits of plants.The varaition on those traits along vertical layers reflected niche divergence among species. In this study, Tr, Pn, Cond and Js increased significantly with vertical layers (p<0.05). Additionally, the random values of Tr, Pn Cond and Js simulated from Zero-model were significantly different with its actual values in forest. Hence those results proved that niche divergence of resources utilization between potential canopy species and understory species resulted in species coexistence along forest vertical structure.(6) Stoma density (SD) were significantly correlated with Tr, Pn, Cond and Js(p <0.05). In this study, stomatal density differed between canopy, sub-canopy and understory layers (p<0.05), and its simulated random values significant differed with its actual values across forest vertical layers in Zero-modeling. In addation, canopy trees had significant larger values than understory plants for Tr, Pn, Cond, JS and SD (p<0.05). That is to say that canopy trees have higher light utilization and water transport capacity to transport water from soil into canopy layer due to its higher SD, thus canopy trees can be grown in canopy layer. On the contrary, understory plants have lower SD, which caused understory plants can’t producing higher light utilization and water transport capacity to lift water from soil into canopy layer, and led to understory plants only surviveing in the understory layer. Therefore, the difference of SD between canopy and understory plants may be one of the reasons for species coexistence along forest vertical structure.In conclusion, light utilization and water transport capacity were different between canopy and understory plants, which led to tree architecture changing along vertical layers, and further caused different vertical layers having diversified morphological appearance in EBLFs. In addition, Tr, Pn, Cond and Js represented the resources utilization niche of plants, which diverged across forest vertical layers. The differentiations in Tr, Pn, Cond and Js between canopy and understory plants were mianly determined by SD. In this study, Tr, Pn, Cond and Js were significantly correlated with SD, and SD varied along vertical layers. Therefore, we concluded that the difference of SD between canopy and understory plants was probably one of the reasons for species coexistence along forest vertical structure in EBLFs.