Size-dependency of leaf physiological characteristics and nitrogen allocation in Robinia pseudoacacia and Cornus controversa

Abstract

Leaf traits such as leaf mass per area (LMA), leaf nitrogen (N) concentration, and its allocations are good indicators of physiological and structural characteristics of tree. Understanding these leaf-level traits of different size trees is important for analyzing their responses and predicting their effects on forest ecosystem structure and function. This study investigated the patterns of size (diameter at breast height, DBH) dependent changes in leaf traits of two species with different life-history strategies. In particular, the partitioning of leaf N into the different pools of the photosynthetic and structural apparatus of the species was analyzed. Crown top leaves from thirty-seven Robinia pseudoacacia L. (1.3 - 41.3 cm DBH), a light-demanding pioneer and N-fixing species, and fifty-two Cornus controversa Hemsl. (0.8 - 48.8 cm DBH), an intermediate shade-tolerant and mid-successional species were collected for the analysis. LMA of two species increased linearly with DBH, while leaf N concentration decreased nonlinearly in a concave and convex shape for R. pseudoacacia and C. controversa, respectively. Photosynthetic capacities (maximum carboxylation and maximum electron transport rate) of R. pseudoacacia stayed relatively constant over the entire DBH range, while those of C. controversa had peak values at intermediate DBH. For the leaf N allocation, N concentration in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) (NR), bioenergetics (NB), and cell wall (NW) were higher in R. pseudoacacia than C. controversa. In contrast, N concentration of light-harvesting (NH) was higher for C. controversa than that of R. pseudoacacia. As DBH increased, structural N (NW) showed concave patterns for both species. On the other hand, photosynthetic N (NR, NB, NH) in R. pseudoacacia and NB in C. controversa linearly decreased, but NH of C. controversa decreased nonlinearly and NR of C. controversa showed convex pattern reaching maximum value of 0.303% about at 6 cm. Our research revealed pronounced changes of leaf traits with tree size reflects the differences in life-history strategies, implying that trees alter their N allocation within the photosynthetic and structural apparatus in leaf according to the life history strategies as they grow.