Canopy conductance controls on water use by Korean forests
TERRECO WP 1-05
From 08/2010 to 06/2014Principal Investigator: Joon Kim, Hyojung Kwon, Dennis Ochuodho Otieno, John Tenhunen
Staff: Eun-Young Jung, Minseok Kang, Junghoon Lee
Grant: IRTG 1565 WP I TERRECO - Complex Terrain and Ecological Heterogeneity - Evaluating ecosystem services in production versus water yield and water quality in mountainous landscapes
The importance of securing water resources and their efficient management has attracted more attention recently in Korea due to the local development of water deficits. In water budget analysis, however, evapotranspiration (ET) has been approximated as the residual in the water balance equation or estimated from empirical equations and assumptions. To minimize the uncertainties in these estimates, it is necessary to directly measure ET. In this study (Kwon et al. 2009), using the eddy covariance technique, we have measured ET in a mixed forest in the Seolmacheon catchment in Korea from September 2007 to December 2008. During the growing season (May-July), ET in this mixed forest averaged about 2.2 mm d−1, whereas it was on average 0.5 mm d−1 during the non-growing season in winter. The annual total ETin 2008 was 581 mm y−1, which is about 1/3 of the annual precipitation of 1997 mm. Despite the differences in the amount and frequency of precipitation, the accumulated ET during the overlapping period (i.e., September to December) for 2007 and 2008 was both ~110 mm, showing virtually no difference. The omega factor, which is a measure of decoupling between forest and the atmosphere, was on average 0.5, indicating that the contributions of equilibrium ET and imposed ET to the total ET were about the same. The results suggest that ET in this mixed forest was controlled by various factors such as net radiation, vapor pressure deficit, and canopy conductance.
ET was also measured in the Gwangneung site, a deciduous forest in a complex terrain located northeast of Seoul (Kang et al. 2009). By using multi-year observations from 2005 to 2007, we quantified ET and analyzed its temporal variation and control mechanisms based on the radiatively coupled combination equation. During the study period, the averaged annual precipitation was about 1500 mm of which ¼ returned to the atmosphere as ET. The decoupling parameter omega which indicates the measure of interaction between vegetation and the atmosphere averaged 0.37. Thus, ET is to a greater extent influenced by imposed ET (hence, air saturation deficit and surface conductance).
The joint study of these sites and additional sites in Haean catchment within the TERRECO project have as their goal an improved understanding of the biological structures and physiological responses that determine canopy conductance. The role of understory vegetation in overall forest water use is a second focus. The studies are intended to aid in obtaining reliable estimates of water balances in small catchments in Korea.
Abstract 2013: Temperate deciduous forest is the major terrestrial ecosystem in South Korea. Forests supply several types of services to the local communities as well as non-local communities and recently its important role in regulations of water and carbon quantity has been received public attention with regard to climate change and water shortage problems. In this regard, the quantification of water and carbon in the forest ecosystem is critical starting point of evaluation of their services. Therefore, we examined water fluxes and budgets, which can be directly linked to regulating services of forest, for temperate deciduous forests in South Korea. As the first step of this concept of the research, four specific questions were raised: (1) what determines tree water use? (2) what controls the overstory and understory transpiration (Eo and Eu)? (3) how change the contributions of Eo and Eu to the total ecosystem evapotranspiration (Eeco) over time? (4) does forest water use and water use efficiency change along elevation gradients? To answer these questions, detailed plot studies were conducted in Gwangneung (GN) and Haean Catchments. A plot in GN was a natural climax deciduous forest, ca. 200 year-old and dominated by Quercus serrata and Carpinus laxiflora forming the overstory layer. Four plots in Haean were located at different elevations, one at 450 m, two at 650 m, and one at 950 m. All four plots were young deciduous forests, ca. 30 year-old and dominated by Quercus spp., i.e., Q. mongolica, Q. dentata, and Q. serrata. For all study sites, tree water use (TWU) and forest stand transpiration were estimated from sap flux density measured by two sap flow methods, thermal dissipation probes and stem heat balance, and sapwood area determined by the increment borer. Eeco was measured only for GN forest site by the eddy covariance measurements placed at the 40 m height tower. Leaf Δ13C isotope analysis was applied for the common species of Q. mongolica for Haean sites to examine water use efficiency, which is the ratio of carbon gains and water losses through stomata, at different elevations. First, we found a general relationship of TWU to stem diameters in a sigmoid function regardless of species and elevations. This empirical relationship can be used to estimate the maximum TWU of the trees with known stem diameters. Second, we observed that the canopy development controlled not only Eo but also Eu, via changing microclimate of the understory layer. Consequently, their contributions to Eeco changed over time with changing canopy development and microclimate; for example, the contributions of Eo and Eu were decreased to 42% - 58% during monsoon rainy season, i.e., July and August. Last, stand transpiration and water use efficiency were significantly decreased with increasing elevations, thus, with decreasing mean air temperature (Ta), since a gradient in Ta caused the gradients in vapor pressure deficit and the growing season length, which determined the quantity of the annual stand transpiration. The results of the plot level studies can be utilized to calibrate and validate the spatial simulation models such as PIXGRO and HYDRUS to upscale those processes to the catchment level. Moreover, quantified forest water fluxes and budgets and defined relationships with environmental factors can be linked to estimate carbon fluxes of the forests as combining phloem sap Δ13C isotope analysis. All this information of fluxes can be the inputs for the economical evaluations of forest ecosystem services. In this way, they can help improve sustainable management strategies for the forests in South Korea.
Key words: deciduous forest, transpiration, overstory, understory, elevation gradient
