Post-thinning Effects on Water Use Efficiency and Transpiration in a Korean Pine Plantation

Abstract

Understanding the underlying mechanism of forest management effects on forest ecosystem function is essential in forest management planning. One of main forest management practices, thinning alters microenvironmental conditions in forest stand. This causes changes of forest function like productivity and water use. The forest management effects have been intensively investigated, and understanding on the underlying mechanisms is increasing. However, they show variations among sites and species. Korean pine (Pinus koraiensis) is one of main plantation species in Korea because it provides various ecosystem services like timber and corn production. Most of Korean pine forests are planted and intensively managed, but understandings on the effects of management practices are limited. Thus, this study aimed (1) to quantify the effects of thinning on tree water use, productivity and water use efficiency, and (2) to develop stand transpiration model relating canopy conductance with stand structural attributes. For the first objective, thinning was conducted on Mt. Taehwa with two intensities. The tree water use, productivity and resultant water use efficiency were continuously monitored for four years in a 50-year-old Korean pine plantation. The heavy thinning (30% reduction in stand basal area) significantly improved tree water use and productivity, but light thinning (17% reduction in stand basal area) had minor effects. In addition, the tree size-growth relationship was different by thinning intensity. For the second objective, stand level transpiration estimated by sap flux density measurement from various stand density and sapwood area was used to develop a canopy conductance model. The canopy conductance model included a set of limitation function by environmental conditions. The reference canopy conductance and stomatal sensitivity to vapor pressure deficit showed a significant relationship with stand sapwood area. With inclusion of this relation, the developed model successfully reproduced the changes of stand transpiration with changes of stand sapwood area and climatic conditions. The findings of this study on the thinning intensity effects and dominant control of stand transpiration by stand sapwood area help to estimate changes of forest ecosystem function by forest management practices in Korean pine plantations, and can be used as guideline for forest management planning.