| Leaf structural traits are closely related to leaf photosynthesis,which reflect the ability of trees to acquire external resources during growth.The discussion of intraspecific and interspecific factors affecting the photosynthetic capacity of leaves is conducive to exploring the variation mechanism of leaf photosynthetic capacity,and provides a theoretical basis for the establishment of photosynthetic model.In this study,six common broad-leaved tree species of Acer pictum subsp.mono,Acer tegmentosum,Betula platyphylla,Fraxinus mandschurica,Juglans mandshurica and Ulmus laciniata were studied in mixed broad-leaved Korean pine(Pinus koraiensis)forest in Liangshui National Nature Reserve of Heilongjiang Province.According to the shade tolerance of the tree species,the six tree species were divided into shade tolerant trees and shade intolerant trees,and each tree species was divided into large trees and small trees according to the tree height and diameter at breast height.The structural traits and photosynthetic traits of each sample tree were measured and 14 leaf trait indexes were calculated.The results showed that there were significant differences in leaf traits among trees with different shade tolerance.In terms of leaf chemical traits,the nitrogen(N)and phosphorus(P)contents,photosynthetic N and P utilization efficiency of shade intolerant trees were significantly higher than those of shade tolerant trees.In terms of leaf anatomical traits,the palisade tissue(PT)thickness of shade intolerant species was higher than that of shade tolerant species,and epidermis(ET)thickness of shade intolerant species was significantly lower than that of shade tolerant species.The maximum net photosynthetic rate(P_n)and stomatal conductance(G_s)of shade intolerant trees were significantly higher than those of shade tolerant trees.These results suggest that in the economic spectrum of leaves(LES),shade intolerant tree species are at the’quick investment-return’end,and devote more resources to the organization of carbon capture.Shade tolerant trees are at the’slow investment-return’end of the spectrum,devoting more resources to organizations that are resistant to environmental stress.The results of structural equation model(SEM)showed that the shade tolerance of trees had a significant direct effect on the photosynthetic capacity of plants.The stronger the shade tolerance of broad-leaved trees,the lower the leaf P_n.The leaf traits of large trees and small trees were significantly different.In terms of leaf chemical traits,the N and P contents and photosynthetic N and P utilization rates of small trees were significantly higher than those of large trees,which indicated that small trees invested more nutrients into photosynthetic organs for rapid growth.Among leaf anatomical traits,PT,ET and ST thickness of small trees were significantly lower than that of large trees,but the ratio of palisade tissue-leaf thickness ratio(PT/LT)of small trees was not significantly different from that of large trees,indicating that the thickness of each tissue composition of leaf changes with the growth of trees,but their proportions in leaves are stable.In addition,P_n,SLA and G_sof small trees were significantly higher than those of large trees,indicating that small trees were more stressed by environmental light resources,while large trees were more stressed by environmental water resources.These results indicate that the resource acquisition strategies of large trees are more conservative than small trees.The results of SEM showed that tree size had no significant direct effect on the photosynthetic capacity of plants,but it could indirectly affect P_n through the influence of leaf structural traits.Our study explains how shade tolerance and tree size affect tree photosynthesis directly or indirectly by affecting leaf structure traits on a local scale,providing new evidence for understanding the variation of leaf traits of different functional groups and trees of different sizes and their correlation. |
