[A-HW30] 水循環・水環境

[AHW30-P17] Seasonal changes in the canopy water balance of Japanese plantation forest

*Linh Thuy Pham¹、Yuichi Onda¹、Xinchao Sun²、Sean Andrew Hudson¹、Janice Elaine Hudson¹、Yupan Zhang¹、Chiu Wei Chen³ (1.University of Tsukuba、2.Tianjin University、3.Tokyo University of Agriculture and Technology)

キーワード：seasonal changes, stemflow, throughfall, canopy interception loss, thinning

Stemflow, throughfall, and canopy interception are some of the most critical parameters for hydrologic behavior and watershed management. When rainfall occurs within a forest ecosystem, small amounts of rainfall will be trapped by the tree canopy (Canopy interception – Ic) until it becomes saturated and canopy storage is exceeded, throughfall (TF) – precipitation which reaches the forest floor by passing through or dripping from the canopy, will occur. When the trunk storage capacity is exceeded, water will flow down the tree trunk and reach the ground as stemflow (SF). Among the factors that can affect canopy water balance, seasonal change is one that requires further investigation (because of the differences in meteorological parameters). Previous research on canopy water balance and interception loss has mostly focused on the growing season - when plants have a greater demand for water (Ahmadi et al, 2009). However, it is also critically important to know the amount of water that will be in the water budget of the catchment delivered via stemflow and throughfall because it helps to better regulate streamflow between seasons, especially in the dry season. (Hewlett, 1982). In Japan, 60% of coverage is the forest with 44% being plantation forests. However, the plantation forests are not well managed with a very high density of trees, leading to undeveloped understory vegetation cover which causes low soil infiltration, low groundwater levels, high soil runoff, and soil erosion. To combat these negative processes silvicultural activities such as thinning, especially strip thinning - in which plantations are thinned parallel to the direction of the slope - have been widely adopted in Japan. The research site was in mountain Karasawa - covered by dense cypress plantation forest, located in the northeast of Sano city, Tochigi prefecture of Japan, where strip thinning was implemented on November of 2011. During thinning, 50% of trees were cut, stand density was lowered from 2198 to 1099 trees/ha, and the basal area was reduced from 50.4 to 26.2 m²/ha. Canopy water balance parameters were recorded from November of 2010 (1 year before thinning) until November of 2012 (1 year after thinning). Throughfall was measured by 20 tipping rain gauges distributed in 2x2 m grid under the canopy, stemflow was collected with a polyvinyl tube around the trunk, connected with a draining hole to a bucket equipped with an Odyssey water level probe. Meteorological factors such as rainfall, temperature, humidity, wind speed were recorded by the weather station. After analysis, the data showed some changes in the canopy water balance before and after thinning, and also some differences between seasons. Within the partitioning of Pg, TF has the largest proportion (64.2%), followed by Ic (25.2%) and SF (10.6%). After thinning, the TF rate increased to 12%, the SF rate decreased to 4%, and the Ic rate decreased to 8%. With the decrease in Ic, there will be more water in the soil matrix during the dry season. TF, SF, and Ic amounts tend to be higher in summer and autumn but lower in winter and spring. TF and SF also tended to be higher during summer and autumn, but Ic rate is higher during winter and spring.