Factors affecting interannual variability in transpiration in a tropical seasonal forest in northern Thailand: Growing season length and soil drought

Tropical seasonal forests play an important role in global and regional carbon cycling and climates. Annual transpiration and primary productivity in tropical seasonal forests should be affected by the growing season length and physiological controls during the growing season. We investigated the year-to-year variations in the transpiration period as a measure of the growing season length in a teak (Tectona grandis Linn. Q plantation in northern Thailand using sap flux measurements obtained over a 4-year period and examined the effect of soil drought on transpiration during the mid-growing season. The beginning and end of the transpiration period differed appreciably between years, corresponding to differences in the timing of soil moisture changes. These differences resulted in approximately 60 days interannual variation in the length of the transpiration period during the observation period, indicating that soil moisture changes are a major cause of large interannual. variation in the transpiration period. Transpiration control caused by soil drought was sometimes observed during the transpiration period. The results suggest that soil moisture has two potential impacts on annual transpiration at this site; through modification of the length of the transpiration period, and through physiological control during the transpiration period. This regime contrasts with temperate deciduous forests and hill evergreen forests, another typical forest type in Thailand.