Chatchai Tantasirin

Soil respiration and soil CO2 concentration in a tropical forest, Thailand

Soil respiration and soil carbon dioxide (CO2) concentration were investigated in a tropical monsoon forest in northern Thailand, from 1998 to 2000. Soil respiration was relatively high during the rainy season and low during the dry season, although interannual fluctuations were large. Soil moisture was widely different between the dry and wet seasons, while soil temperature changed little throughout the year. As a result, the rate of soil respiration is determined predominantly by soil moisture, not by soil temperature. The roughly estimated annual soil respiration rate was 2560 g C m−2 year−1. The soil CO2 concentration also increased in the rainy season and decreased in the dry season, and showed clearer seasonality than soil respiration did.

Throughfall under a teak plantation in Thailand: a multifactorial analysis on the effects of canopy phenology and meteorological conditions

Valuable teak (Tectona grandis Linn. f.) plantations cover vast areas throughout Southeast Asia. This study sought to increase our understanding of throughfall inputs under teak by analyzing the abiotic and biotic factors governing throughfall amounts and ratios in relation to three canopy phenophases (leafless, leafing, and leafed). There was no rain during the brief leaf senescence phenophase in our study. Leveraging detailed field observations, we employed boosted regression tree (BRT) analysis to identify the primary controls on throughfall amount and ratio during each canopy phenophase. Whereas throughfall amounts were always dominated by rainfall magnitude (as expected), throughfall ratios were governed by a suite of predictor variables during each phenophase. The BRT analysis demonstrated that throughfall ratio in the leafless phase was most influenced (in descending order of importance) by air temperature, rainfall amount, maximum wind speed, and rainfall intensity. Throughfall ratio in the leafed phenophase was dominated by rainfall amount. The leafing phenophase was an intermediate case where rainfall amount, air temperature, and vapor pressure deficit were most important. Our results highlight the fact that throughfall ratios are differentially influenced by a suite of meteorological variables during each canopy phenophase. Abiotic variables, such as rainfall amount and air temperature, trumped leaf area index and stand density in their effect on throughfall ratio. The leafing phenophase, while transitional in nature and short in duration, has a detectable and unique impact on water inputs to teak plantations. Further work is needed to better understand the biogeochemistry of leaf emergence in teak plantations.

Separating physical and biological controls on long‐term evapotranspiration fluctuations in a tropical deciduous forest subjected to monsoonal rainfall

Evapotranspiration (ET), especially in the mainland of the Indochina Peninsula, can impact and is impacted by the Asian monsoonal (AM) system, thereby prompting interest in its long-term variability. To separate the physical and biological factors controlling ET variability in a tropical deciduous forest under the AM influence, 7 year eddy covariance and ancillary measurements were collected and analyzed. The 7 year mean rainfall (Pr) and ET along with their standard deviations were 1335 ± 256 and 977 ± 108 mm (about 73% of Pr), respectively, suggesting close coupling between these two hydrologic fluxes. However, other physical and biological drivers decouple seasonal and annual variations of ET from Pr. To explore them, a big-leaf model complemented by perturbation analysis was employed. The big-leaf model agreed well with the measured ET at daily to multiyear time scales, lending confidence in its ability to separate biological and physical controls on ET. Using this formulation, both first-order and second-order Taylor series expansions of the total ET derivatives were applied to the big-leaf model and compared with measured changes in ET (dET). Higher-order and joint terms in the second-order expansion were necessary for matching measured and analyzed dET. Vapor pressure deficit (D) was the primary external physical controlling driver of ET. Leaf area index (LAI) and bulk stomatal conductance (gs) were shown to be the main significant biological drivers of the transpiration component of ET. It can be surmised that rainfall variability controls long-term ET through physical (mainly D) and biological (mainly LAI and gs) factors in this ecosystem.

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. f.) 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.

Interannual variation in leaf expansion and outbreak of a teak defoliator at a teak stand in northern Thailand

The leaf area index (LAI) is a key factor affecting tree growth in forests. Following the outbreak of a defoliator, the LAI declines, serving as a useful indicator in forest management. In this study, daily radiative transmittance from above the canopy, which decreases exponentially with increasing LAI, was measured in a teak plantation (Tectona grandis L. f.) in northern Thailand from March through July in 2001-2008. Volumetric soil moisture was also measured at depths of 0.1, 0.2, 0.4, and 0.6 m. The negative logarithmic value of the ratio of daily downward solar radiation on the forest floor to that above the canopy (NLR; -ln[Sb decrease/Sdecrease]), was calculated as an indicator of leaf flush and subsequent leaf expansion. The NLR data indicated that leaf expansion began in late March and continued to the beginning of May during all eight years (with the day the leaves began to expand defined as D(B)). In addition, the peak in NLR values (NLR(P)), corresponding to the lowest value of a 99% confidence interval, occurred in July. The day when NLR first reached NLR(P) was defined as D(P), which always occurred in June, 31-85 days after D(B). The NLR indicated an increase in the population of Hyblaea puera (a teak defoliator) that was associated with greatly decreased leaf areas during two growth periods (D(B)-D(P)): the earliest D(B)-D(P) in 2001 and the second-earliest D(B)-D(P) in 2008. In almost all cases, soil moisture data indicated that leaf expansion occurred after increases in soil moisture at depths of 0.1-0.4 m even without increases at 0.6 m; in contrast, increases in shallow soil moisture (0.1-0.2 m) were insufficient to trigger leaf expansion at the stand level. Periods of soil drought at 0.1-0.4-m soil depths inhibited leaf expansion, resulting in prolongation of the interval between D(B) and D(P) during those years in which the D(B)s occurred chronologically close to one another. Moreover, when drought did not limit leaf expansion, the D(B)-D(P) growth periods characterized by earlier D(B)s tended to be longer than those with later D(B)s.

Scale Dependency of Hydrological Characteristics in the Upper Ping River Basin, Northern Thailand

The scale dependence of the hydrological characteristics of a river basin was studied using three watersheds with different scales in northern Thailand. The discharge per unit area in the medium-scale watershed (Mae Chaem) had only small interannual changes even though large interannual changes occurred in the rainfall. The discharge per unit area in the small-scale watershed (Mae Tia) was about twice as large as in the medium-scale watershed and had larger interannual changes that were correlated with the interannual changes in rainfall. The long-term trend of discharge per unit area showed no distinct trend in either medium- or small-scale watersheds, whereas there was a distinct decreasing trend of low flow in the small-scale watershed. In the medium-scale watershed, however, this decreasing trend did not appear, suggesting that the land cover change in the uplands may have an influence on the discharge per unit area in the small-scale watershed, but only a minor influence on the discharge per unit area in the medium-scale watershed. The discharge per unit area in the microscale watershed (Huay Kog-Ma) was the largest and had the smallest seasonal change among the three watersheds. Even in the dry season, there was significant water flow in the microscale watershed.

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.