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We investigated soil physical properties in three forest types in tropical lowland monsoon forests in central Cambodia under the same climatic conditions, i.e., Kanhaplic Haplustults in dry evergreen forest (KH-E), Arenic Haplustults in dry deciduous forest (AH-D), and Arenic Ultic Alorthods in mixed evergreen–deciduous forest (AA-M), to clarify the relationship between forest types and soil physical properties. The clay content was correlated with water content at ψ = −9.8 and −1500 kPa (WC10 and WC1500), available water capacity (AWC), and the van Genuchten (vG) parameter N (P AA-M > KH-E whereas vG parameter α had a high value in KH-E soil at 0–100 cm in depth. The cumulative AWC (AWCcl, mm) at a soil depth of 0–200 cm was higher in the AH-D than in the KH-E, and was not considered a major factor affecting the distribution of different forest types under the same climatic conditions. The unsaturated hydraulic conductivity (K) at 0–100 cm in depth, estimated by use of models, was higher in AH-D than in KH-E mostly at matric potential ψ > −10 kPa. The low K in KH-E at ψ > −10 kPa was considered favorable for evergreen trees to retain the soil water for the transpiration in the dry season, and the matric potential in KH-E showed more gentle decreases in the early dry seasons than AH-D. Thus the differences in K among generally sandy soil types could possibly affect the establishment of different forest types in the study area with the same climate.

We enumerated all trees 10 cm or more in DBH with respect to DBH, height, and species identity in 29 circular plots of 20-m radius from Kampong Thom, Kratie, and Mondolkiri Provinces, Cambodia. The composition data were analyzed using cluster analysis with group-averaging protocol, and Sorensen’s similarity index based on basal area data and the resulting clusters were also described with respect to height structure and indicator species. We found four main clusters corresponding to traditional qualitative forest types known as evergreen forest, deciduous forest, hill evergreen forest, and swamp forest. The evergreen cluster was further divided into two stand types of dry evergreen forest and two stand types of secondary evergreen forest. The deciduous forest cluster was divided into three stand types of deciduous dipterocarp forest and a mixed deciduous forest. We describe the correspondence between the forest stand types of our study and the many regional names previously used for the different forest types in varying classification systems. Some of the stand types, for example, an evergreen forest overtopped by deciduous dipterocarp (Dipterocarpus intricatus) or by a pine (Pinus merksii), and a D. obtusifolius stand on seasonally waterlogged habitat, seemed to be unique in Cambodia. The application of this method and the needs of regional forest mapping are discussed.

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 (m3 m−3), 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 Ca2+, Mg2+, and K+ was larger in DEF soils than in DDF soils. The pH (H2O) 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.

Changes of soil water conditions in a soil profile were observed and estimated using a one-dimensional vertical soil water movement model for a dry evergreen forest area of Kampong Thom Province, Cambodia. The research site was in a dry evergreen forest where a meteorological observation tower had been established. Soil water matric potentials were measured at 20-, 50-, 100-, 150-, 200-, and 250-cm depths in an observation plot. Groundwater levels were observed at the site. Soil water matric potentials at each observation depth in a soil profile were simulated using a one-dimensional water movement model that was based on Richards’ equation. Results of observations and simulation revealed the following. (1) The site’s water-saturated zone was close to the ground surface during the rainy season. Water conditions in the unsaturated zone, which was above the groundwater level, were influenced strongly by groundwater. The groundwater level was 400 cm deep even in the dry season. The entire soil profile, from the surface to the bottom, never dried completely. (2) At the beginning of the rainy season, at the surface and subsurface depths of 20 cm and 50 cm, respectively, soil matric potentials were increased by rainfall events, which often supply water to the ground surface. Meanwhile, matric potentials at 200 and 250 cm depths were influenced directly by groundwater; they retained high potential values even during the dry season. In the middle zone, at depths of 100 and 150 cm, the soil was quite dried; the minimum matric potentials in the rainy season resembled those of the surface zone. (3) At the beginning of the dry season, matric potentials at 20- and 50-cm depths decreased because of soil water loss by transpiration of trees and evaporation from the ground surface.

Leaf area index (LAI) is a key biophysical variable in most process-based models of forest ecosystems and water cycles. We compared the LAI of two types of tropical seasonal forest in Kampong Thom Province, Cambodia. The two forest types are extremes of crown-cover density, i.e., closed dry evergreen forest (DEF) and open dry deciduous forest (DDF), suggesting marked spatial variation in forest site conditions such as soil moisture. Monthly changes in LAI were estimated indirectly using a plant canopy analyzer and hemispherical photographs. Both methods of LAI estimation showed instrument errors, i.e., low reproducibility in the plant canopy analyzer data and LAI-saturation in hemispherical photograph data; nevertheless, LAI values differed between DEF and DDF. The average LAI from three years of measurements was about 4.6 times higher in DEF than in DDF. DDF exhibited much greater seasonality than DEF. The annual minimum LAI averaged 76% and 84% of the annual maximum LAI for DDF and DEF, respectively. LAI showed high peaks in the rainy season and decreased in the dry season. However, in DEF, LAI decreased twice annually, at the beginning of the dry season and the beginning of the rainy season. Seasonal changes in LAI could be approximated using a third-degree Fourier-series equation.