Tomonori Kume

Modeling Seasonal Changes in the Temperature Lapse Rate in a Northern Thailand Mountainous Area

Temperature data in the mountain forest regions are often extrapolated from temperature data recorded at base stations at lower elevation. Such extrapolation is often based on elevation differences between target regions and base stations at low elevation assuming a constant temperature lapse rate throughout the year. However, this assumption might be problematic where slope circulation is active and decoupled from the regional circulation. To model the seasonal change in the lapse rate, the authors compared daily maximum (Tmax) and minimum temperatures (Tmin) observed at a mountain forest site (Kog–Ma; 1300-m altitude) with those observed at the bottom of the basin (Chiang–Mai; 314-m altitude) in northern Thailand, where slope circulation is active and decoupled from the regional circulation. The difference in Tmax between Kog–Ma and Chiang–Mai (DTmax; Kog–Ma minus Chiang–Mai) was relatively unchanged throughout the year. However, the difference in Tmin between Kog–Ma and Chiang–Mai (DTmin) changed seasonally. Thus, assuming a constant lapse rate throughout the year could cause large errors in extrapolating Tmin data in mountainous areas in northern Thailand. The difference DTmin was related to nighttime net radiation (Rn), suggesting that nocturnal drainage flow affects the determination of DTmin. This relationship would be useful in formulating seasonal changes in the lapse rate for Tmin. As Rn data are generally unavailable for meteorological stations, an index that relates to the lapse rate for Tmin and is calculated from Tmax and Tmin data is proposed. This index might be useful for accurately estimating Tmin values in mountainous regions in northern Thailand.

Wind Speed Response of Sap Flow in Five Subtropical Trees Based on Wind Tunnel Experiments

Aims: We evaluated the responses of tree sap flow to wind speeds in coniferous and broad-leaved plants under steady and unsteady wind conditions. Study Design:We performed sap flow andmicro-meteorological measurementson two conifers,Chamaecyparis obtusa var.formosana and Araucaria cunninghamii, and three broadleaved species,Swietenia mahagoni,Michelia formosanaand Plumeria acutifoliain a wind tunnel.

Sap-flow velocity reduction by soil water deficit observed in a Lithocarpus edulis forest on Kyushu Island, Japan *

This paper examines transpiration reduction due to soil water deficits observed in a Lithocarpus edulis forest on Kyushu Island, south-western Japan. Continuous sap flow measurements were performed on six trees to monitor changes in tree transpiration rates. A reduction in sap flow velocity was observed in the period between late-September and late-October 2003. Precipitation amounts for this period were limited resulting in a corresponding low soil matric potential. A clear reduction in sap flow velocity occurred when soil matric potential at a 10 cm depth was ＜－ 70 (kPa). Sap flow velocity scaled for1.0 kPa vapor pressure deficit declined by 42 ％ when soil matric potential at a 10 cm depth was ＜－ 70 (kPa) compared to  － 70 ( kPa) . In addition, intensive leaf- scale physiological measurements were performed on two days that had contrasting soil matric potentials. The intensive measurements showed lower leaf transpiration rates and stomatal conductance on the day with the lower soil matric potential. This finding suggests soil water deficits caused stomatal closure and therefore reduction in tree transpiration. Sap flow velocity and leaf water potential data showed that stem hydraulic conductance did not decline with low soil matric potential when compared to the day with higher soil matric potential.

Severe Drought Resulting from Seasonal and Interannual Variability in Rainfall and Its Impact on Transpiration in a Hill Evergreen Forest in Northern Thailand

Our previous study revealed that a hill evergreen forest in the Kog-Ma experimental watershed in northern Thailand, which is influenced by Asian monsoon cycles, transpired actively even in the late dry season. In this study, the impact of severe drought on the transpiration of this forest was investigated using data measured at the site over 8 years that showed seasonal and interannual variation in rainfall. To this aim, the impacts of soil drought on sap flow and water potential were examined during severe drought conditions. This site showed large interannual variation in the total amount of annual rainfall and in the length of the dry period. An unusually severe drought occurred in the late dry seasons of 1998 and 2004 as a result of the small amount of annual rainfall and a prolonged dry period coinciding with El Nino. Under the detected severe drought conditions in the late dry season of 2004, noticeable symptoms of water stress were apparent only in the smallest study tree. Decreases in sap flow velocity and water potential caused by soil drought were not apparent in larger trees. Deeper root systems of larger trees may explain the lower impact of severe drought on transpiration in larger trees. Transpiration in this forest could be maintained actively even under unusually severe drought conditions.