| Budget Amount *help |
¥3,600,000 (Direct Cost: ¥3,600,000)
Fiscal Year 2006: ¥1,100,000 (Direct Cost: ¥1,100,000)
Fiscal Year 2005: ¥2,500,000 (Direct Cost: ¥2,500,000)
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| Research Abstract |
A soil-plant-air continuum multilayer model was developed to simulate the seasonal changes of evapotranspiration and canopy net assimilation (A_n) in a tropical deciduous forest. The model was applied to observations in a teak plantation in a dry tropical climate, northern Thailand. We also investigated the timing of the leaf flush and the end of the growing season from the perspective of the availability of soil moisture for carbon gain, which is balanced by precipitation, canopy interception, soil evaporation, soil water uptake by transpiration, and discharge. Numerical simulations using both a constant and a temporally variable leaf area index (LAI) were conducted to determine whether soil water is temporally and continuously available for tree assimilation by examining the positive canopy net assimilation (An) and the duration of positive An (DPA), assuming unsaturated soil. The model with temporal changes in LAI captured seasonal changes in soil surface moisture, and the simulated
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transpiration agreed with seasonal changes in heat pulse velocity, corresponding to the water use of individual trees, assuming a soil texture with a relationship between volumetric soil water content and soil water potential relatively similar to that measured at 0.1 m deep. The model with constant LAI showed that An became negative in the dry season because the assimilation limitations caused by stomatal conductance were reduced by severe soil drought and the simultaneous increase in leaf temperature increased dark respiration. Thus, the experiment emphasized unfavorable conditions for carbon gain in the dry season. The start of the longest DPA (LDPA) in a year approached the timing of leaf flush in the teak plantation after the spring equinox, and the end appeared earlier than that of all canopy duration periods. Therefore, the estimation may predict the growing season length of plantations in dry tropical areas. However, when the leaf flush occurred around the time of the spring equinox, before the rainy season, other factors such as increasing day length may have been responsible, in addition to soil moisture favorable for carbon gain. The model sensitivity analysis suggested that a smaller LAI and slower maximum rate of carboxylation likely extend the LDPA because soil water from the surface to rooting depth is maintained longer at levels adequate for carbon gain by decreased canopy transpiration. The experiments also suggest that lower soil hydraulic conductivity and deeper rooting depth can postpone the end of the LDPA by increasing soil water retention and the soil water capacity, respectively. Less
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