Effects of Leaf Area Change on Transpiration and Productivity of 50-year-old Pinus koraiensis Stands in Mt. Taehwa

Abstract

Forests play an important role as a conduit connecting biosphere and atmosphere, providing important feedback controls on the water and carbon balance of the entire earth system. In other words, forest water and carbon uses, uptake and release of water and carbon by forest, are important to understand how lithosphere, atmosphere and biosphere interact for sustainability of earth system especially under the substantial changes by natural disturbances as well as human management. To enhance the understanding of ecosystem responses to such changes, I tried to quantify the effect of leaf area changes on water and carbon balance, more specifically, transpiration and productivity of 50-year-old Pinus koraiensis stand. For leaf area changes, I conducted two different approaches. First, I performed the defoliation of individual trees, which mimics the reduction of leaf area by insect, herbivore, environmental stress or pruning, by removing 30 and 60% of branches. Second, I conducted the leaf area reduction by 20 and 40% thinning, which is the most common forest management practice to improve the quality of timber production and sustainable forest management. To evaluate the effect of leaf area changes on water and carbon at tree and stand level, transpiration, which was measured with Granier-type heat dissipation sensor, and growth measured with dendroband were compared at different treatments. In addition, canopy carbon net assimilation was estimated using Canopy Conductance Constrained Carbon Assimilation model. I found 1) thirty percent artificial defoliation treatment decreased water stress of individual tree and increased leaf hydraulic conductance, resulted in the compensation of transpiration at the tree level. However, 60% defoliation treatment was not able to compensate the leaf area loss with the increase of hydraulic conductivity and resulted in the decrease of tree-level transpiration. Canopy carbon assimilation in 30% defoliation treatment showed no change due to the compensation of transpiration, however, those of 60% defoliation treatments decreased. This implied that the mild defoliation event can be overcome by the enhanced physiological processes of Pinus koraiensis, due to less water stress and better light environment. However, severe defoliation was not able to easily recover in transpiration and productivity. 2) In thinning experiment, there was the increase of precipitation throughfall at thinned plots, which resulted in the increase of soil water content and sapflux density, and leaf- and tree-level transpiration. However, stand transpiration decreased ~30 % and 50 % at 20 % (mild-thinned) and 40 % (heavy-thinned) plots, respectively. Therefore, unlike transpiration compensation with mild defoliation of individual tree, thinning decreased stand transpiration even at the mild density (e.g., 20 %). However, thinning treatment enhanced the DBH increment at the tree level in both intensities. The enhanced diameter growth at the tree level also increased stand net primary production at light-thinned plots without statistical significance. Therefore, water use efficiency increased with thinning treatments. My study quantified the transpiration and productivity and carbon assimilation of Pinus koraiensis plantation in Mt. Taehwa under various leaf area indices with different treatments. Leaf reduction at different levels, tree and stand, respond differently in water and carbon relations. At the tree level, transpiration was compensated at the mild defoliation, but transpiration decreased with thinning treatment. Photosynthesis increased in both treatments due to less water stress and better light environment. Consequently, water-use-efficiency enhanced with in both treatments. As a result, my study provided insight for water and carbon use of Pinus koraiensis plantations for forest ecosystem management.