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Forest ecosystem parameters related to the amount of evapotranspiration and rain interception are key elements to successful hydrological modeling. Thus, we evaluated ASTER (Advanced Spaceborne Thermal Emission and Reflection radiometer) reflectance bands and optical indices for qualitative and quantitative estimation of various characteristics of tropical seasonal forests. Ground conditions were measured in 14 sites in Kampong Thom Province, Cambodia, representing six major tropical seasonal forest types: dry evergreen forest, mixed evergreen-deciduous forest, dry deciduous forest, regrowth of dry evergreen forest, moist evergreen forest, and swamp forest. We performed a discriminant analysis to classify forest types using ASTER reflectance bands and optical indices. We used Visible and near infrared Radiometer (VNIR) and Shortwave Length Infrared Radiometer (SWIR) surface reflectance, four vegetation indices: NDVI (Normalized Difference Vegetation Index); SR (Simple Ratio); DVI (Difference Vegetation Index), and MSAVI2 (Second Modified Soil Adjustment Vegetation Index), and three water content indices: SRWI (Simple Ratio Water Index); NDWI (Normalized Difference Water Index); and LWCI (Leaf Water Content Index), for the discriminant analysis. ASTER image products were acquired on January 12, 2002 in the dry season. We also performed regression analyses to identify an optical index closely correlated with forest qualitative characteristics such as tree density, tree height, basal area, and leaf area index (LAI). Each forest type showed a distinctive pattern in reflectance bands, demonstrating that satellite images can potentially be used for regional forest type classification.

We studied the morphology and physicochemical properties of soils under three different types of forest, i.e., dry evergreen forest (DEF), dry deciduous forest (DDF), and mixed forest with evergreen and deciduous trees (MF), in the dry evergreen forest zone of Kampong Thom Province, Cambodia. The morphological features of soils varied among the three different forest types. The physical characteristics of the soils in the study area were strongly correlated with soil texture. Clay content was clearly higher in DEF soils than in the DDF or MF soils. Bulk density was generally high (1.27–1.92), except in the surface horizons. It was especially high at depths of 100–200 cm and 160–200 cm in the DDF and MF soils, respectively. Total soil porosity was 0.32–0.44 (m m), except in the surface horizons, and was slightly higher in DEF soils. The DEF soils were characterized by a higher percentage of fine pores (less than −49 kPa) than the other pore classes. DDF soils were characterized by decreasing percentages of coarse pores (0 to −0.2 kPa; the point of capillary saturation), medium pores (−0.2 to −4.9 kPa), and small pores (−4.9 to −49 kPa), and by a concomitant increase in fine pores with depth. In MF soils, the proportion of small pores slightly decreased with depth. The soils were generally poor in cation-exchange capacity (CEC) and exchangeable cations (ECEC). ECEC and CEC were closely related to clay content. The stock of exchangeable Ca, Mg, and K was larger in DEF soils than in DDF soils. The pH (HO) of DDF soils was clearly higher than that of the other soil types at 0–50 cm in depth and showed different patterns in vertical changes. The stock of total carbon at 0–70 cm in depth was highest in MF soils.

Soil moisture conditions were observed in four types of forest in central Cambodia, where dry evergreen forests are distributed widely, to investigate differences of soil moisture in each forest and to clarify relationships between forest types and soil moisture conditions. Observations revealed that soil water contents were high during the rainy season in dry deciduous forest (DDF) and mixed forest (MF), which contained both evergreen and deciduous trees. Those areas have thinner tree crown density and less stand biomass than a dry evergreen forest (DEF). In contrast, during the dry season, water content was low in DDF and MF. That difference is attributable to the disparate evapotranspiration rates of forests caused by the tree crown density and stand biomass. Moreover, soil temperatures were affected by the type of forest. In areas with DDF forests, the temperatures were high in the months of April and May but were lower in MF, DEF, and DEFlog forests. Those differences were caused by inhibition of temperatures through shading effects of tree crowns and evapotranspiration by trees. Based on those observed data, this study clarified a relationship between forest stand type and soil moisture conditions in Kampong Thom forest area.

In the evergreen forests in Kampong Thom Province, Cambodia, soil depth and soil hardness apparently change from the rainy season to the dry season. In contrast, in deciduous and mixed forests, these parameters hardly change in either season. The apparent changes in soil depth and hardness would be strongly affected in the dry season by physical properties, including clay content and transpiration rates. At the DEFlog plot, hard soils were confirmed in the dry season from just below the surface to 4 m in depth. This fact suggests that the roots siphon off soil water from very deep under the surface of the ground. Assuming that the effective porosity of the soils was 0.09 m m, the soil water consumption rate in the dry season from just below the surface to 4.1 m in depth, occurring mainly by transpiration, was approximately 369 mm. In addition to this, a deeper soil layer above the groundwater level loses considerable soil water in the dry season.

Water storage capacity (WSC), which is based on effective porosity in a soil profile or watershed scale, is one of the indicators for evaluating the water conservation function in a forested area. The effect of soil water content (SWC) on WSC was compared in this study between Cambodia and Japan. We studied four experimental plots. The DEF-plot is located in dry evergreen forest, the DEFlog-plot, in selectively logged dry evergreen forest, and the MF-plot, in mixed (evergreen and deciduous trees) forest in Kampong Thom Province, Cambodia. The JPN-plot is located in a natural forest consisting of fir and Japanese hemlock in Kochi Prefecture, Japan. The effect was evaluated using the index for the effect of SWC on WSC (ESW = WSC/WSC); WSC is the typical WSC calculated from the effective porosity (estimated by the difference in SWC at saturation and -49 kPa), examined using the pressure plate method and soil thickness based on a soil survey. WSC is a modified WSC that considers soil water in a WSC evaluation, which is computed by removing the effective porosity filled with water from WSC. SWC was measured using a soil moisture gauge and was observed at depths of 30 and 100 cm from the surface in the three plots in Cambodia and at depths of 10, 30, and 50 cm in the JPN-plot. The ESW in the three plots in Cambodia was in the range of 0.6 to 0.8 from January through April and below 0.5 from June through October. In contrast, the ESW for the JPN-plot remained almost constant at 0.5 to 0.7 throughout the year. Seasonal variations in the ESW were considerable in the three plots in Cambodia and small at the JPN-plot. These results suggest that although the capacity for temporal rainwater storage was almost the same throughout the year in Japan, it decreased greatly in the rainy season in Cambodia.

Lake Tonle Sap, the largest inland water body in Southeast Asia, encompasses unique ecosystems and wildlife adapted to large seasonal fluctuations in water level. The permanent waterlogged area of the lake is encircled by a vast floodplain, the inundated woodland being dominated by (Lecythidaceae), probably a major vegetation type of this ecotone, although human impact has degraded the floodplain vegetation and developed forest is restricted to a narrow band along the shore in the lowest water season. The aspects of seasonally inundated vegetation (i.e., variations in physiognomy, species composition, stratification, and distribution) on the coastal side of the floodplain (approximately 4 km in depth), located adjacent to the southern part of Siem Reap, was analyzed. Quantitative data for phytosociological evaluation were collected at 67 quadrats (10 m × 10 m each) during the low water seasons in 2005 and 2006, the sampling plots being classified by Two- Way Indicator Species Analysis (TWINSPAN) and ordinated by Detrended Correspondence Analysis (DCA). Two vegetation zones (i.e., extensive cropland and disturbed woodland), seven vegetation types (i.e., “Cultivated field,” “Fallow field,” “Shrub,” and “Tall-shrub” in the extensive cropland zone and “Scrub,” “Open forest,” and “Closed forest” in the disturbed woodland zone), and vigorous invasion and/or regeneration of over the study area were identified. Human impact (e.g., plowing, burning, and cutting for firewood) seemed to be inversely related to both duration of flooding and maximum water depth and to be the main cause of degradation of seasonally inundated vegetation.