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Two morphological variants of Terminalia alata (Combretaceae) differed in leaf flushing phenology and spatial distribution in a Cambodian deciduous forest. The hairy-type trees displayed leaf exchange behavior in the middle of the dry season. The glabrous type flushed new leaves 3 months after the wet season started. The leafless period of the hairy type was estimated to be

Background: Dry deciduous forest (DDF) is distributed throughout the monsoon area of South-east Asia. Leaf morphological and photosynthetic traits are essential for adaptation to the environmental variability and usually change with tree height and season.Aims: To clarify the differences in leaf traits as influenced by tree height, light conditions and season for two dominant dipterocarps of DDF: Dipterocarpus tuberculatus and Shorea obtusa.Methods: We measured changes in leaf traits with tree height, namely leaf mass per area (LMA), maximum photosynthetic rate (Amax), stomatal conductance (gs) and nitrogen concentration (N), in the rainy and early and mid-dry seasons. To identify the effects of height and light intensity on leaf traits, we conducted multiple regression analyses using leaf traits as the dependent variables and the height, light and season as independent variables.Results: Tree height was more important for gradients in most leaf traits, especially in LMA in both species, than light intens...

We developed an empirical model to estimate aboveground carbon density with variables derived from airborne Light Detection and Ranging (LiDAR) in tropical seasonal forests in Cambodia, and assessed the effects of LiDAR pulse density on the accurate estimation of aboveground carbon density. First, we tested the applicability of variables used for estimating aboveground carbon density with the original LiDAR pulse density data (26 pulse m−2). Aboveground carbon density was regressed against variables derived from airborne LiDAR. Three individual height variable models were developed along with a canopy density model, and three other models combined canopy height and canopy density variables. The influence of forest type on model accuracy was also assessed. Next, the relationship between pulse density and estimation accuracy was investigated using the best regression model. The accuracy of the models were compared based on seven LiDAR point densities consisting of 0.25, 1, 2, 3, 4, 5 and 10 pulse m−2. The best model was obtained using the single mean canopy height (MCH) model (R 2 = 0.92) with the original pulse density data. The relationship between MCH and aboveground carbon density was found to be consistent under different forest types. The differences between predicted and measured residual mean of squares of deviations were less than 1.5 Mg C ha−1 between each pulse density. We concluded that aboveground carbon density can be estimated using MCH derived from airborne LiDAR in tropical seasonal forests in Cambodia even with a low pulse density of 0.25 pulse m−2 without stratifying the study area based on forest type.

We measured the sap flux densities of 12 deciduous trees in a tropical dry deciduous forest with high seasonality of available water located in Cambodia and evaluated the seasonal trends in transpiration and leaf phenology. For all trees, the minimum transpiration was recorded in the middle of the dry season, and almost all trees restarted transpiration before the first monsoon rain. The occurrence of the ‘paradox’ of leaf phenology was confirmed. During the dry season, transpiration was controlled by leaf phenology and decreased with an increase in the duration of the leafless period. In contrast, during the wet season, daily changes in transpiration were determined by changes in evaporative demand. Transpiration during the dry season accounted for more than 30% of the annual total among trees, and at the stand scale, the dry season contribution was 38%. The dry season transpiration was not negligible for the water balance in this ecosystem. The soil water condition in the shallow layer, where the main root system is located, was not the source of transpiration during the dry season. This implied that the root probably extended to a deep layer and absorbed water. The relationships between the mean canopy stomatal conductance and vapour pressure deficit revealed that most trees were isohydric. Isohydric behaviour controlling stomatal openness to avoid xylem hydraulic failure was also confirmed at the stand scale and was advantageous for these trees, allowing them to continue transpiring under the high evaporative demand during the dry season. Copyright © 2015 John Wiley & Sons, Ltd.

Litter plays key roles in forest ecosystems, and forest degradation is likely to spur a further decline in leaf litterfall inputs to forest soils. However, the effects on physicochemical surface-soil properties remain largely unknown, especially in seasonal tropical forest ecosystems. We initiated a litterremoval manipulation experiment in a Cambodian lowland evergreen forest undergoing intensive selective logging. Litter removal performed for 2 and 4.4 years respectively triggered an increase in bulk density and decrease in surface-soil carbon (C) and nitrogen (N) contents to 67 and 73% of the original levels, respectively. After only 2 years of treatment, bulk density rose to very high value (>1.40 Mg m) likely preventing further soil compaction, while the C and N reduction effects lasted over 2 years. Greater soil compaction occurs in stands with a smaller initial bulk density. However, C (N)-rich soils did not necessarily lose a greater relative proportion of C (N) than C (N)-poor soils. Although N remained above C following the litter removal, conservative trends in the C:N ratio suggested a limited capacity for N retention. Together, our data suggest that shifts in leaf litter inputs in response to localized human disturbances may have rapid and lasting consequences on physicochemical surface-soil properties; possibly accelerated by a tropical climate. Moreover, a speedy recovery to an adequate litter supply, at least before reaching the upper soil compaction limit, is essential to conserve forest ecosystems. Discipline: Soils, fertilizers and plant nutrition Additional key words: bulk density, carbon, forest degradation, nitrogen, soil compaction

Although information about the hydrologic and ecological features of lowland evergreen forests in central Cambodia has been collected since the beginning of the 21st century, measurements of the transpiration process remain very limited. This paper describes the differences detected in transpiration (q) and crown-level stomatal conductance (GS) between Calophyllum inophyllum (which undergoes successive leaf exchange) and Drypetes sp. (which performs irregular leaf exchange) (hereafter referred to as Calophyllum and Drypetes, respectively), and the analysis of these differences in consideration of their contrasting leaf phenologies. We evaluated q using sap f low measurements and obtained daily GS values. Calophyllum and Drypetes had high and low periods between which q and GS differed significantly. Within high/low periods, smaller scatter in the plot of GS versus vapour pressure deficit (D) was found in Calophyllum compared to Drypetes. For a given value of D, q in high periods was 1.3 and 1.9 times larger than in low periods for Calophyllum and Drypetes, respectively. The smaller scatter for Calophyllum was the result of relatively constant physiological activity that was maintained by successive leaf fall. For both species, high periods were recorded after remarkable leaf-fall events; thus, our current data implied that leaf phenology is one of the most important factors affecting transpiration. Discipline: Forestry and forest products Additional key words: leaf phenology, riparian zone, sap flux density, transpiration, vapour pressure deficit

Forest soils in the basalt region of Southeast Asia are important natural resources, due to their high agricultural potential and high capacity for carbon sequestration. To characterize the physicochemical properties and the components of the soil organic carbon of forest soils in the basalt region, five evergreen sites (E1–5) and one deciduous forest site (D1) were selected in Cambodia at elevations ranging from 132 to 908 m. The components of the soil organic carbon of each site were separated using a density fractionation approach, i.e. high(≥ 1.6 g cm−3) and low( 600 m), were strongly weathered and characterized by a lower pH, a lower level of exchangeable bases, and a more reddish color than the other evergreen forest soils. The soils of sites E3 and E4, located on a hillside and at the base of a hill, respectively, had a high soil effective cation exchange capacity in the B horizons compared to those at sites E1–2 and E5. The soil of site E5 in the isolated basalt region had characteristics resembling those in sites E1–2 except for its high exchangeable aluminum content. The site D1 soil on a hillside was relatively young and shallow, and black in color. The carbon stock in the six forest sites (0–30 cm in depth) was 40.8– 113.7 Mg C ha−1 for high-density fractions and 3.3–7.6 Mg C ha−1 for low-density fractions, respectively. The differences in vegetation types (deciduous vs. evergreen forests), mean annual temperature and free aluminum contents among forest sites were considered factors affecting the carbon content and carbon to nitrogen (C/N) ratio of high-density fractions. It was considered that the variously weathered parent materials, regulated by the relative position in the basalt plateau, were responsible for the gradient of soil morphology and soil nutrient conditions and characterized the soil carbon stock in the study area. Discipline: Forestry and forest products Additional key words: tropical monsoon forest * Corresponding author: e-mail jtori@affrc.go.jp Received 29 May 2012; accepted 19 September 2012.

In this study, we demonstrate changes in leaf morphological and physiological traits with tree height from dark understory to bright canopy conditions in various tree species in the Cambodian tropical dry evergreen forest. The vegetation mainly consisted of Dipterocarpaceae and Myristicaceae and the canopy trees usually reached 30-40 m in height. We investigated 25 individuals of 18 tree species ranging from 0.8 to 33 m in height. We measured the leaf photosynthetic rate, stomatal conductance and respiration rate for 3 to 5 leaves per sampling position in the early dry season. All leaves were then divided into two parts: one for measuring dry weight, nitrogen content and δ13C; the other for observation of leaf morphology. The leaf morphological traits, such as leaf mass per area (LMA), cuticle thickness, palisade layer thickness, leaf hardness and stomatal density increased linearly with tree height. The leaf nitrogen content per unit leaf area (Narea) peaked at 10 m from the ground, though the nitrogen content per unit dry leaf mass (Nmass) decreased linearly with tree height. Higher LMA, cuticle thickness and hard leaves in canopy condition may contribute to high drought tolerance and physical strength. The leaf-area-based photosynthetic rate (Amax-area) peaked at an intermediate tree height of approximately 10 m, and then decreased toward the upper canopy. In contrast, the leaf-mass-based photosynthetic rate (Amax-mass) decreased linearly with tree height. Reduction of leaf nitrogen content and stomatal conductance mainly limit photosynthetic capacities with tree height. Overall, many leaf morphological traits could be summarized in a simple and significant relation with tree height, though increasing tree height, which is related to the micro-climatic gradient, leads to both nitrogen and stomatal constraints of leaf photosynthetic capacities, even when considering many different tree species.

We studied the relationships among the soil C stock in tropical monsoon forests, the type of forest, and the environmental factors in the lower Mekong basin in Cambodia. We analyzed nine soil profiles in evergreen and deciduous forests growing over sedimentary rock and basalt. Evergreen forest soils tended to have a larger C stock than deciduous forest soils within geological formations. In evergreen and deciduous forest soils, carbon stocks were 56.9 ± 30.0 (mean ± SD) and 34.9 ± 23.5 Mg C ha-1, respectively, in the 0- to 30-cm depth range, and 108.7 ± 53.0 and 53.2 ± 30.4 Mg C ha-1, respectively, in the 0- to 100-cm depth range. Soil C stock was highly positively correlated with soil water content in the dry season, which is likely affected by the openness of the forest canopy and by soil clay content.

The Reducing Emissions from Deforestation and Forest Degradation in Developing Countries (REDD) initiative requires accurate estimates of carbon stock changes in forested areas. However, estimating carbon emissions from stumps of various heights left by illegal loggers is difficult. To remedy this problem, we examined two methods of estimating diameter at breast height (DBH) from a reference diameter observation measured at any stump height. The one-reference diameter (OD) observation model estimates DBH from a single diameter observation using empirical coefficients derived mainly from emergent dipterocarp trees. The two-reference diameter (TD) observation model estimates DBH from two diameter observations and assumes a logarithmic relationship between diameter and height. Prediction data to establish the models were collected in Cambodian lowland evergreen forests that are undergoing intensive illegal logging of emergent dipterocarp trees for timber. The OD model performed better than the TD model in predicting DBH and is extremely practical, as it requires only a single diameter observation. Validation data previously collected in the Southeast Asian tropical forests established the general validity of the OD model. This study may improve the reliability of the REDD scheme by providing a reliable method to assess carbon emissions from Southeast Asian tropical forests.

In this paper, we have measured many meteorological elements associated with evapotranspiration using a 60-m-high meteorological observation tower, runoff and precipitation of broad-leaf forest watersheds in the Kampong Thom Province of central Cambodia. Moreover, an interception plot was established for calculating the rainfall interception ratio by forest coverage close to the tower. Analyzing many kinds of observed data, we evaluated Stung Chinit watershed, which had the largest drainage area and an annual loss of ~1200 mm. This value was in agreement with annual evapotranspiration estimated in 2004 by the Bowen ratio energy balance method. The annual canopy interception rate, which is one of the most important hydrological factors in the forest, was approximately 15%. Also, the rainfall interception process was reproduced well according to the multilayer model. The available renewable freshwater resources in forested areas of the Mekong River basin were estimated by modeling. Annual available renewable freshwater resources in forested areas of the Mekong ranged from 300 to 2200 mm/year. The mountainous area of west Cambodia had vast renewable freshwater resources, and almost all regions had 500 mm/year. This result mostly corresponded to the amount of water resources in the largest evergreen forest experimental watershed including several small watersheds in central Cambodia.