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The water balance of tropical dry deciduous forests is less well understood than some other forest ecosystems. To help close this knowledge gap, we separately measured the evapotranspiration from the whole ecosystem (ETW), transpiration (TR) and interception loss (IL) from overstory trees, and evapotranspiration from the understory vegetation (ETU) in a tropical dry deciduous forest in Cambodia. It was found that ETW was equivalent to 73.7% of rainfall (P) at the annual scale. In the dry season, ETW corresponded to 120.1% of P, which indicates the utilization of soil water replenished during the wet season. The sum of transpiration estimated by the thermal dissipation (TD) method with the original coefficient (TRG), IL, and ETU was smaller than ETW, except for the middle of the dry season, due to an underestimation of TRG. Although recently reported calibration coefficients can reasonably correct TRG, future calibrations of the TD method are highly recommended for the precise evaluation of single-tree-scale transpiration in tropical dry forests. The annual contribution of the understory vegetation to ETW (ETU/ETW) was 34.6%, leading to the conclusion that the understory vegetation cannot be ignored when trying to gain a comprehensive understanding of the hydrologic cycle in tropical dry forests. The seasonal variations in ETU/ETW were mainly controlled by the leaf area index (LAI) of overstory trees, resulting from the overall stability of ETW and decreasing trend of ETU with increasing LAI in the wet season, with the opposite holding true in the dry season, i.e., decreasing ETW with the decline of LAI and less variations of ETU. Thus, LAI influenced both the seasonality and the annual contribution in ETU/ETW, exerting a notable influence on hydrological cycling in this forest.

We investigated the capabilities of a canopy height model (CHM) derived from aerial photographs using the Structure from Motion (SfM) approach to estimate aboveground biomass (AGB) in a tropical forest. Aerial photographs and airborne Light Detection and Ranging (LiDAR) data were simultaneously acquired under leaf-on canopy conditions. A 3D point cloud was generated from aerial photographs using the SfM approach and converted to a digital surface model (DSMP). We also created a DSM from airborne LiDAR data (DSML). From each of DSMP and DSML, we constructed digital terrain models (DTM), which are DTMP and DTML, respectively. We created four CHMs, which were calculated from (1) DSMP and DTMP (CHMPP), (2) DSMP and DTML (CHMPL), (3) DSML and DTMP (CHMLP), and (4) DSML and DTML (CHMLL). Then, we estimated AGB using these CHMs. The model using CHMLL yielded the highest accuracy in four CHMs (R2 = 0.94) and was comparable to the model using CHMPL (R2 = 0.93). The model using CHMPP yielded the lowest accuracy (R2 = 0.79). In conclusion, AGB can be estimated from CHM derived from aerial photographs using the SfM approach in the tropics. However, to accurately estimate AGB, we need a more accurate DTM than the DTM derived from aerial photographs using the SfM approach.

Influential stakeholders have highlighted many constraints inherent in conventional scientific forest management plans for community forestry (CF) and presented simpler alternatives. Nevertheless, some developing countries continue to use rigid, complex and high-cost plans and regulations. This article considers two issues: (1) why heavily-regulated or rigid CF systems were originally introduced and why they continue to be used in developing countries despite critiques and counterproposals, and (2) under what circumstances will such CF systems face an impasse, and what can be done to resolve the situation. Using Cambodia as a case study, we examine the development of a rigid CF system, review negative factors influencing the upland forested area, clarify the unfavorable situations arising from these factors and discuss likely problems associated with the CF management system. International organizations played a key role in the introduction and maintenance of rigid, complex and high-cost CF systems in Cambodia. Conflicts and crises arise when the administration prosecutes local farmers for illegal cultivation or deprives communities of CF management rights because of the expansion of commercial crop cultivation and the lack of adequate community management in response to unprecedented changes. A likely practical solution to the probable impasse is the development and funding of a functional network of CF management committees, rather than the adoption of an entirely new, alternative system.

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.

Development pressure has led to serious deforestation on the Indochina Peninsula. Particularly rapid defor-estation has occurred in easily accessible lowland areas, and it is thus important to accumulate knowledge about these forests immediately. We measured evapotranspiration rates for a lowland dry evergreen forest in Kampong Thom Province, central Cambodia, using the energy balance Bowen ratio (EBBR) method based on meteorological data collected from a 60-m-high observation tower. Daily evapotranspiration was higher during the dry season than during the rainy season of the Asian monsoon climate. The seasonal variation in evapotranspiration generally corresponded to the seasonal difference in the vapor pressure deficit. A multi-layer model was used to simulate the seasonal variation in evapotranspiration. The multilayer model also reproduced the larger evapotranspiration rate in the dry season than in the rainy season. However, observed values substantially exceeded model-calculated results during certain periods at the beginning of the dry season and in the late dry season. Moreover, during the rainy season, the model tended to overestimate evapotranspiration. The differences between these observed and simulated values may have been caused by seasonal characteristics of photosynthesis and transpiration in the lowland dry evergreen forest that were not considered in the model simulation.

A methodological characteristic of the REDD+ scheme is that it attempts to reduce deforestation by rewarding communities that change problematic land use practices. This has led to discussions on benefit sharing. This article focuses on incentives for alternative land use practices among village members living in frontier areas, especially in relation to support for sustainable land use and people’s livelihoods, and clarifies the issues that REDD+ projects are likely to face in this context. Although some documents regarding REDD+ projects have mentioned support to encourage such incentives, insufficient consideration has been given to the realities of the changes in frontier communities. REDD+ projects are unlikely to motivate members to embrace alternative land use practices if support or benefit sharing does not match members’ expectations. Here, we examine the changes in household (HH) income and structure, as well as in livelihood activities, experienced by Cambodian frontier villagers living at the site of a planned REDD+ project. During the nine years compared in this study, the frontier villages experienced broad and imbalanced changes in HH income owing to the rapid expansion of the cultivation of cash crops. Our results indicate that benefit sharing or support inevitably becomes more difficult and challenging in frontier areas than in areas where subsistence production systems still predominate, although such frontiers could, in theory, yield maximum returns with regard to forest carbon balance if the REDD+ projects addressed benefit sharing and support and came to fruition.