Search

To identify the soil carbon stock changes from croplands/grasslands to forestlands in Japan, we compared the soil carbon stock of a cropland/grassland with that of an adjacent forestland at 22 different sites. With respect to a 0–30-cm depth basis, the soil carbon stock in the cropland/grassland was the same as that in the forestland; however, it was less than that in the forestland when an equivalent mass approach was used. The ratio of the soil carbon stock in the forestland to that in the cropland/grassland was 1.11 on average, which is comparable with previous mass-corrected paired sampling studies. The ratio was affected by two factors: tree age and current vegetation. Mean annual temperature, mean annual precipitation, phosphate absorption coefficient, litter quantity, and former land use did not affect the ratio. According to a comparison with the results of a paired study for deforestation, the application of the reciprocal of the soil carbon stock ratio of deforestation to the ratio of afforestation was slightly overestimated. Further studies are necessary to establish a general conclusion. Some previous studies, including those involving non-mass-corrected data, are possibly biased, and more studies using the paired sampling method with equivalent mass basis must be conducted to provide the general factor for soil carbon stock change in the future.

Spatiotemporal prediction of the response of planted forests to a changing climate is increasingly important for the sustainable management of forest ecosystems. In this study, we present a methodology for estimating spatially varying productivity in a planted forest and changes in productivity with a changing climate in Japan, with a focus on Japanese cedar (Cryptomeria japonica D.Don) as a representative tree species of this region. The process-based model Biome-BGC was parameterized using a plant trait database for Japanese cedar and a Bayesian optimization scheme. To compare productivity under historical (1996–2000) and future (2096–2100) climatic conditions, the climate scenarios of two representative concentration pathways (i.e., RCP2.6 and RCP8.5) were used in five global climate models (GCMs) with approximately 1-km resolution. The seasonality of modeled fluxes, namely gross primary production, ecosystem respiration, net ecosystem exchange, and soil respiration, improved after two steps of parameterization. The estimated net primary production (NPP) of stands aged 36–40 years under the historical climatic conditions of the five GCMs was 0.77 ± 0.10 kgC m-2year-1(mean ± standard deviation), in accordance with the geographical distribution of forest NPP estimated in previous studies. Under the RCP2.6 and RCP8.5 scenarios, the mean NPP of the five GCMs increased by 0.04 ± 0.07 and 0.14 ± 0.11 kgC m-2year-1, respectively. The increases in annual NPP were small in the southwestern region because of the decreases in summer NPP and the small increases in winter NPP under the RCP2.6 and RCP8.5 scenarios, respectively. Under the RCP2.6 scenario, Japanese cedar was at risk in the southwestern region, in accordance with previous studies, and monitoring and silvicultural practices should be modified accordingly.

In the past two decades, conversion of forests to rubber (Hevea brasiliensis) plantations has spread widely in continental Southeast Asia. In addition to the effect of the establishment of new rubber plantations, the impact of the replanting of rubber trees, i.e., second-rotation plantation establishment, on the soil organic carbon (SOC) pool is unknown because of the short history of land management at such sites. We examined whether the first-rotation rubber plantation (RUB_1st) soils have more of a legacy effect from previous natural forests than the second-rotation plantation soils in Cambodia, so as to improve SOC stock estimates for a series of land use changes in a tropical moist climate zone, e.g., the lowlands of continental Southeast Asia. The total SOC stock, estimated for soil masses equivalent to 0–30 cm depth, was larger in natural forest (FOR) plots (52.7 MgC ha−1) than in RUB_1st (38.4 MgC ha−1), early-stage second-rotation plantation (RUB_2nd_A1–6; tree age ≤6 years; 37.2 MgC ha−1), and middle-stage second-rotation plantation (tree age >6 years; 33.9 MgC ha−1) plots. Values for factors related to SOC dynamics, such as the mass and carbon/nitrogen (C/N) ratio of leaf litter, soil C/N ratio, soil aluminum content, and bulk density of coarse plant residue (>2 mm), were also higher in FORs than in the rubber plantations. The SOC pools in both high- and low-density fractions of topsoil were lost largely within the first 10 years of forest–rubber plantation conversion. The loss of SOC in the low-density fraction in the RUB_1st might have been compensated to some extent by the remaining plant residue in these former forests. Although weeding resulted in the mixing of temporarily growing C4 grasses with soil in RUB_2nd_A1–6 plots, these grasses might have contributed minimally to the SOC pool in the long term, as reflected by C stable isotope ratios. Accordingly, we presented a conversion factor of SOC stock as 0.66 ± 0.18 for the 9-year transition period from forests to rubber plantations on a flat topography in a tropical moist climate zone of continental Southeast Asia, and concluded that the replanting of old rubber plantations should take priority over the forest-rubber plantation conversion as possible to reduce the emission of greenhouse gas under the land use change.

We estimated the aboveground net primary production (ANPP) in five self-thinning jack pine (Pinus banksiana) stands in Wood Buffalo National Park, NWT, Canada. The stands (11 to ca.175 years old) w...

In forested areas of Fukushima, Japan, radiocesium emitted by the Fukushima Daiichi nuclear power plant accident has accumulated on the forest floor and in surface mineral soils. To estimate and model the mobility of radiocesium in forest soils, the concept of physical variations in soil organic matter is useful. We applied density fractionation to soil samples collected in two different forest stands in Fukushima; a deciduous broadleaf forest (DBF) and an evergreen needleleaf forest (ENF). To determine the relative contributions of the two density fractions to retention of radiocesium in surface mineral soils in a temperate forest region we investigated the concentrations and stocks of radiocesium in the low- and high-density fractions (LF and HF, respectively) of the soils, as well as in the litter, and temporal changes therein between 2012 and 2014. The 137Cs stock in the litter decreased from 2012 to 2014, whereas in the soil, as the sum of the LF and HF, it increased. On average, more than 40% of the 137Cs stock in surface mineral soil (depth of 0–5 cm) was retained in LF particles, which were composed mainly of particulate organic matter. Although the LF accounted for only 10.3% of the dry mass of the soil on average, it was also characterized by an average concentration of 137Cs that was 7.8 times higher than that of the HF, which was composed of organo-mineral particles. The mean increase in the 137Cs stock in the LF from 2012 to 2014 was equivalent to 70.1% and 52.5% of those in the soils in the DBF and ENF, respectively. The relationship between loss on ignition and 137Cs content indicated that the spatial heterogeneity of radiocesium in bulk soil was substantially affected by the heterogeneity in particulate organic matter. The coarse-size (>250 μm) of the LF suggested that it was less physically protected against microbial attack. Accordingly, the LF was considered to act as a large, but temporal reservoir of radiocesium that was originally retained in the litter layer. In contrast, the accumulation of 137Cs in the HF was low, especially in 2013–2014. The HF (or organo-mineral particles) might have a more important role than the LF in 137Cs fixation over a long period, and should be continuously studied as part of the current radiocesium monitoring network.

High-resolution data on spatial variations in the productivity of bryophytes and the mass of the organic layer are important components in geochemical models for northern forests. We investigated spatial variations in growth rates of the feather moss Hylocomium splendens and the thickness of the organic layer in relation to environmental factors on a north-facing slope in Interior Alaska. The forest floor on the 600 m × 600 m slope was classified into four types, i.e., Classes I–IV, characterized mainly by the dominance of leaf litter, H. splendens, Pleurozium schreberi, and Sphagnum spp., respectively. Spatial autocorrelation analyses showed clustering patterns for some tree and forest floor variables, which were associated with the distribution of the four classes on the slope. Mean growth (length) rates of H. splendens were high in Class II, while growth rates by weight were high in Class I due to a high weight-to-length ratio of shoots. The thickness of the organic layer in bryophyte patches ranged from 6.0 to 40.7 cm and was also controlled by slope orientation. The presence of large trees, particularly Betula neoalaskana, mediated the relationship between topography and forest floor characteristics.

The transport of radiocesium from the canopy and quantification of the spatial distribution of radiocesium in the soil of konara oak forests are important to better understand the variability of 137Cs stocks in the soil between proximal and distal stem areas as well as fine-scale variations around the tree trunk. Moreover, a better understanding of fine-scale spatial variabilities of 137Cs concentrations and stocks will provide insights for optimizing soil sampling strategies to provide a more robust estimation of contamination at the stand scale. This study aims to elucidate the transport of 137Cs by stemflow in a radioactively contaminated konara oak forest in Tsukuba, Japan by describing and quantifying the fine-scale spatial distribution of 137Cs in the soil and preferential flowpaths of stemflow on the tree stem by a dye tracing experiment. 137Cs concentrations and stocks were higher in the soils of the proximal stem area than distal stem area when they corresponded with the preferential flowpaths of stemflow. There was a significant relationship between canopy projection area of individual trees and average soil 137Cs concentrations and stocks, even though canopies of the trees overlapped. Our results demonstrate that the spatiality of 137Cs concentrations and stocks in the soil of the proximal stem area are governed (at least partially) by the preferential flowpaths of stemflow along the tree trunk. In addition, higher 137Cs concentrations and stocks in the near-trunk soils of trees with larger crown areas might be caused by an enhanced ability to capture dry deposition.

Black spruce (Picea mariana (Mill.) B.S.P.) is a dominant species on north-facing slopes located on the permafrost in the Alaskan Interior, where tree growth rates vary significantly across slope gradients. To better understand the effects of nitrogen (N) and mycorrhizal associations on plant growth, we investigated the relationships between tree growth and isotope ratios of nitrogen (δ15N) in foliage and roots, because plant δ15N values reflect tree dependencies on N uptake by mycorrhizae. We established two transects (one along a slope gradient, and a second on a north-to-east axis at a fixed elevation) and four plots on the slope gradient transect. We measured current shoot growth of saplings on transects and aboveground biomass (AGB) in the plots. We collected foliage and fine root samples and measured their δ15N signatures and N concentrations. The AGB and shoot growth varied widely across the gradient: values were higher at high elevations and at the east–northeast aspect. Foliage δ15N values and N concentrations were positively correlated with tree growth, but growth rate was negatively correlated with the dependency on mycorrhizae for N uptake. Thus, black spruce had a reduced dependency on mycorrhizae for N uptake on the upper slope, where conditions were N-rich. On the lower slope, where the soil active layer was shallow, black spruce growth was limited and had an elevated dependency on mycorrhizae for N uptake.

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 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.

Density fractionation is now a potential approach for distinguishing among the functions of different soil organic carbon (SOC) pools in a forest ecosystem. The co-existence of evergreen and deciduous forest types in seasonally dry tropics is favorable for clarifying the effect of contrasting plant functional trait on the size and quality of SOC pools under the high temperature regime. We examined whether forest type (deciduous vs. evergreen), mineral factor (aluminum and iron oxide) and climatic condition (temperature and precipitation) affect the size and quality of SOC pools in bulk soil and density fractions in a seasonally dry tropical forest of continental Southeast Asia. The forest type and mineral factor affected the quality and size, respectively, of SOC pools. The forest type had significant effects on carbon-to-nitrogen (C:N) ratios of leaf litter and free form of light fraction. However, variations in the size of the SOC pools at a depth of 0–30 cm and in each topsoil density fraction were explained exclusively by the mineral factor, i.e., aluminum oxide extractable by acid ammonium oxalate, regardless of variations in forest type and precipitation pattern. It was suggested that the surface area of clay mineral was the dominant factor for the storage capacity of SOC in the study area. The aluminum and related oxides might also increase intra-aggregate space for occluded form of light fraction. The impact of the climatic factor on the SOC pools was not found in a significant level though its effect including humid tropics on the composition of SOC remains a subject of future investigation. The linear models with variables of SOC and soil organic nitrogen favorably estimated the size of the C and N pools in the light fraction independent of the forest type. These regression models help to estimate the wide-area distribution of labile SOC pools of plant detritus form and its digital mapping, combined with a dataset of bulk SOC content. And it will provide incentives for C sequestration practice under forest management in seasonally dry tropical forests through the spatially-defined assessment of reduction effect of C emissions.

The Atlantic Forest, Humid Chaco, and Dry Chaco are major eco-regions in Paraguay, but information on forest carbon stocks in these forest types remains limited. To establish a system to measure, report and verify forest carbon change under the REDD+ mechanism, we developed new allometric equations to estimate tree biomass in each eco-region. Three models of total and aboveground biomass were developed from destructive sampling data. The models performed well, explaining ≥96% of the variation in aboveground and total biomass, although the best model differed among the eco-regions. The inclusion of height and wood density improved fit in the Atlantic Forest model, but including wood density did not enhance the Humid and Dry Chaco models. Our models will improve the estimation of biomass in Paraguay because they provide better estimates of total and aboveground biomass in each eco-region than pan-tropical generic models. Discipline: Forestry and forest products Additional key words: allometric models, Atlantic forests, Chaco forests, living biomass including root, REDD-plus * Corresponding author: satoo@affrc.go.jp Received 24 September 2014; accepted 20 November 2014. Introduction The REDD+ mechanism (Reducing Emissions from Deforestation and forest Degradation and the role of conservation, sustainable management of forests and enhancement of forest carbon stocks in developing countries) provides financial incentives to help developing countries establish ways to reduce emissions from deforestation and forest degradation. Because the measurement, reporting, and verification (MRV) of forest carbon change are indispensable for REDD implementation, the use of a ground-based inventory with remote sensing is recommended to monitor carbon stocks on a national scale (UNFCCC 2009). For a ground-based inventory, permanent sample plots allow the monitoring of carbon stocks by area (e.g. Fox et al. 2010, Samreth et al. 2012). Estimates of carbon stocks in living biomass can be obtained using allometric equations with measured field variables such as diameter and tree height in the plots (Metzker et al. 2012). However, the main source of uncertainty in biomass estimates lies in the choice of allometric model (Molto et al. 2013). Although the proportion of forested area has continuously declined during the last 20 years in Paraguay (Huang et al. 2007), information on forest carbon stocks and allometric models remains limited. Generic allometric equations developed by Chave et al. (2005) can be used to estimate forest biomass and carbon stocks across diverse tropical forest types (Gibbs et al. 2007, Asner et al. 2009, Mitchard et al. 2013), including Atlantic Forest (Vieira et al. 2008) and Humid and Dry Chaco (Gasparri et al.

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

Evergreen forests in tropical monsoon Asia experience a severe dry season lasting several months. Deep soil moisture is responsible for evergreen forest transpiration during this season. This study provides preliminary information on soil physicochemical properties in a large soil profile (800 cm deep) and evaluated the influences of deep-soil moisture dynamics on the transpiration of evergreen forests in Cambodia using a water flow model. In general, changes in soil physicochemical properties with depth were small, excluding topsoil. The soil texture in the large soil profile was characterized by very little gravel and a large sand fraction. The soil carbon stock was 71.7 and 130.1 Mg C ha − 1 at depths of 0–150 and 0–800 cm, respectively. The coarse pore volume (− 100 ≤ ψ K s ) clearly decreased with depth from 100 to 600 cm and were extremely low at 600–700 cm. In the water flow simulation, a rooting depth deeper than 400 cm was necessary for the evergreen forest to maintain transpiration during the dry season. On the other hand, when the K s of deep soil (550–750 cm depth) was assumed to be higher than that estimated by the cylinder sample, a rooting depth of 800 cm was necessary, since most of the soil water that was replenished during the rainy season was rapidly lost during the dry season. Also, it was necessary for evergreen forests to effectively use soil water within the deep root zone (400–800 cm depth). The compacted deep soil horizons played an important role in maintaining the moist conditions in the root zone of an evergreen tropical monsoon forest in Asia that experienced a severe dry season.

As a feasibility study for applying a simplified method to estimate CO2 emissions from deforestation and forest degradation in tropical forests, we estimated the nationwide forest tree biomass carbon stock using data from 100 permanent sampling plots (PSPs) set by the Forestry Administration, Cambodia in seasonal forests in Cambodia. Averaged tree carbon stocks and SE-Standard error were at 158.8 ± 7.3 Mg C ha-1 for evergreen and semi-evergreen forests and 55.2 ± 6.9 Mg C ha-1 for deciduous forests in 1998 (the first census) and for the second census in 2000-2001, 163.8 ± 7.8 Mg C ha-1 and 56.2 ± 6.7 Mg C ha-1, respectively. The averaged tree biomass carbon stock differed significantly between the two forest types. Using the forest cover for 2006 and the averaged carbon stock for 2000-2001, the national-level forest tree carbon stock in the early to mid-2000s was estimated at 824.2 ± 39.2 Tg C for evergreen forests and 263.9 ± 31.3 Tg C for deciduous forests, and 1,088.1 ± 50.2 Tg C in total. By repeating this calculation for all forest areas by remote sensing and averaged tree carbon stock via ground-based measurement with PSPs, we could monitor the total tree carbon stock in nationwide forests in Cambodia. We also presented the possible reasons for uncertainty related to the present tree biomass carbon stock of forests and recommendations in order to improve the accuracy of the carbon stock using PSP systems in Cambodia.

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.

Soil carbon (C) stocks in forest ecosystems have been widely estimated to a fixed soil depth (i.e., 0–30 cm) to clarify temporal changes in the C pool. However, surface elevations change as a result of compaction or expansion of the soil under forest management and land use. On the other hand, the calculation of soil C stocks based on “equivalent soil mass” is not affected by compaction or expansion of forest soil. To contribute to the development of a forest C accounting methodology, we compared changes in soil C stocks over 4 years between depth- and mass-based approaches using original soil data collected at 0–30 cm depths in young plantations and secondary forests in West Java, Indonesia. Our methodology expanded on the mass-based approach; rather than using one representative value for the mass-based calculation of soil C stocks, we adjusted individual values, maintaining the coefficient of variance in soil mass. We also considered the effect of an increase or decrease in soil organic matter on equivalent soil mass. Both increasing and decreasing trends in soil C stocks became clearer when the mass-based approach was used rather than the depth-based approach. The trends in soil C stocks based on equivalent soil mass were particularly evident in the surface soil layers (0–5 cm) and in plantation sites, compared with those for soil profiles including subsurface soil layers (0–30 cm) and in secondary forests. These trends in soil C stocks corresponded with temporal trends in litter stocks. We suggest that equivalent mass-basis soil C stock for the upper 30 cm of soil be calculated based on multiple soil layers to reduce estimation errors. Changes in soil organic matter mass had little effect on the estimation of soil C stock on an equivalent mass basis. For the development of a forest C accounting system, the mass-based approach should be used to characterize temporal trends in soil C stocks and to improve C cycle models, rather than simpler methods of calculating soil C stocks. These improvements will help to increase the tier level of country-specific forest C accounting systems.

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.

We examined non-destructive methodologies for practicalities in monitoring anthropogenic greenhouse gas (GHG) emissions from tropical dry-land forest under the influence of various forms of human intervention. Spaceborne SAR withstood comparison with Landsat ETM+ in land cover classification of degraded tropical forest. For measurement of carbon stock and GHG flux per unit land area, the gain-loss method requires both growth rate and removal rate of forest carbon stock. However, the latter has rarely been obtained in tropical forest. For the stock-difference method, permanent sampling plot data can be used to estimate mean carbon stock per unit land area of each forest type. For cyclic land use that includes a clear-cutting stage such as slash-and-burn agriculture, chronosequential changes in carbon stock can be predicted by determining the time and spatial-distribution of cleared land. Changes in forest biomass by logging, storm-damage, etc., may be identified by monitoring the presence and diameter of the crowns of overstory trees. We developed five equations containing the parameter for crown diameter for estimating tree biomass. Overstory height can be a parameter for estimating ecosystem carbon stock of various plant communities, and forest height can be measured by airborne and spaceborne sensors, etc. Generic equations containing the parameter for overstory height are available for estimating community biomass of tropical and subtropical forests. PALSAR has an advantage over other remote systems by enabling frequent sensing and semi-direct biomass estimation using backscattering coefficients. However, no reasonable remote sensing methods exist for monitoring the amount of carbon loss by forest conversion and logging in forests with high biomass. To compensate for the faults of the present PALSAR methodologies and to enable practical and frequent monitoring of all types of forests by humans, it is vital to devise a new methodology to detect changes in high-biomass forests.

Both evergreen and deciduous forests (Efs and Dfs) are widely distributed under similar climatic conditions in tropical monsoon regions. To clarify the hydraulic properties of the soil matrix in different forest types and their effects on soil water storage capacity, the soil pore characteristics (SPC) were investigated in Ef and Df stands in three provinces in Cambodia. Soils in the Ef group were characterized in common by large amounts of coarse pores with moderate pore size distribution and the absence of an extremely low Ks at shallow depths, compared to Df group soils. The mean available water capacity of the soil matrix (AWCsm) for all horizons of the Ef and Df group soils was 0·107 and 0·146 m3 m−3, respectively. The mean coarse pore volume of the soil matrix (CPVsm) in the Ef and Df groups was 0·231 and 0·115 m3 m−3, respectively. A water flow simulation using a lognormal distribution model for rain events in the early dry season indicated that variation in SPC resulted in a larger increase in available soil water in Ef soils than in Df soils. Further study on deeper soil layers in Ef and each soil type in Df is necessary for the deeper understanding of the environmental conditions and the hydrological modelling of each forest ecosystem. Copyright © 2010 John Wiley & Sons, Ltd.

In dry evergreen forests in Kampong Thom Province, Cambodia, regoliths over 10 m thick are widely distributed. At one study site in an evergreen forest, a 9.4-m-deep soil profile was excavated and certain physical properties, including soil hardness, soil water content, and extent of root systems, were measured in rainy and dry seasons. A new finding was that the evergreen forests used soil water from regolith in the dry season. The root system penetrated more than 9 m into the soil and withdrew soil water. Soil hardness from the surface to a depth of 4 m changed each dry and rainy season from extremely hard to soft. The reason for the change was related to soil texture; the silty soils readily changed their binding power in response to water content. A seasonal change in soil hardness was also observed at other points by penetration tests indicating very hard surface soils to a depth of 4.3 m at the end of the dry season, very soft surface soils to a depth of 1.0 m at the beginning of the rainy season (apparent soil thickness was 4.2 m), and 3.5 m of soft surface soil at the end of the rainy season. Thick regoliths and silty surface soil layers were found to store water and maintain dry evergreen forests in central Cambodia.

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.

This study of a water cycle was conducted in an evergreen forest located in the Mekong River Basin in central Cambodia. At the observation site, we measured the dynamics of the spatial distribution of groundwater levels. The groundwater movement was analyzed two-dimensionally using boundary conditions and parameters that had been observed in the field. The climate in the research area is dominated by two seasons, which occur annually: a rainy and a dry season. The groundwater levels are generally high during the rainy season and low during the dry season. Groundwater levels were measured along a stream, which flowed through the study site. The streambed was visible at the head of the stream in January. At the next downriver well point, the streambed appeared in March. Finally, it became visible at all well points in April, meaning that surface runoff had disappeared temporarily and instead flowed underground during the ensuing dry period. Groundwater levels of the studied lateral flow perpendicular to the stream that seeped and infiltrated into the stream were 1.2–2.5 m deep (in April), which was the lowest level recorded for the year. During that period, the depth of the groundwater of the studied lateral flow fell by as much as 56 mm per month. In addition, the lateral flow groundwater infiltrated into groundwater of the stream during that period. The groundwater level fluctuation was estimated based on a two-dimensional analysis of lateral flow perpendicular to the stream using a numerical simulation model with soil physical parameters and observed boundary conditions. The observations of ground water fluctuations were well reproduced. Deep seepage of groundwater was estimated using a uniform boundary condition that allowed efflux through the bottom, estimated as being approximately 30 mm per year. The simulated deep seepage rate was considered plausible considering other hydrological components such as soil water storage fluctuation.

Soil layers play an important role in hydrological processes, particularly in forested basins. For example, thick soil layers can store rainwater temporarily and drain it gradually. In the Mekong River Basin, especially in Cambodia, evergreen and deciduous forests are widely distributed. However, we have no data on soil conditions such as thickness and hardness that affect water storage capacity in forested areas. To clarify the water storage role of forest soils in the lower reaches of the Mekong River Basin, we investigated the distribution of soil thickness using a handy dynamic cone penetrometer in three Cambodian provinces: Kampong Thom, Kratie, and Mondul Kiri. Soil thickness was greater than 8 m in a deciduous forest in Kampong Thom. In some evergreen forests, apparent soil thickness varied from the rainy to dry seasons because of specific grain size distributions; for example, the soils were rock hard in the dry season and soft in the rainy season. In Kratie, deciduous forests prevailed and soil thickness was shallow at less than 3 m. In Mondul Kiri, forests were mainly formed on a basaltic plateau, and soil thickness was greater than 7 m. Soil thickness and hardness differed among the soil types in these three provinces, as did water storage capacity. The maximum water storage capacity was 1350 mm in an evergreen forest distributed on Acrisols in Kampong Thom. In contrast, the minimum water storage capacity was 225 mm in a deciduous forest distributed on Leptosols in Mondul Kiri. We constructed a soil thickness map for the study area in Kampong Thom that was mainly covered with evergreen forest; the distribution of soil thickness was related to elevation, with soil thickness increasing 0·13 m per 1 m increase in elevation, except in a deciduous mixed swamp forest area. Copyright © 2008 John Wiley & Sons, Ltd.

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 m3 m−3, 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.

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.

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 = WSCb/WSCa); WSCa 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. WSCb is a modified WSC that considers soil water in a WSC evaluation, which is computed by removing the effective porosity filled with water from WSCa. 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.

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.

The global forest carbon (C) stock is estimated at 662 Gt of which 45% is in soil organic matter. Thus, comprehensive understanding of the effects of forest management practices on forest soil C stock and greenhouse gas (GHG) fluxes is needed for the development of effective forest-based climate change mitigation strategies. To improve this understanding, we synthesized peer-reviewed literature on forest management practices that can mitigate climate change by increasing soil C stocks and reducing GHG emissions. We further identified soil processes that affect soil GHG balance and discussed how models represent forest management effects on soil in GHG inventories and scenario analyses to address forest climate change mitigation potential. Forest management effects depend strongly on the specific practice and land type. Intensive timber harvesting with removal of harvest residues/stumps results in a reduction in soil C stock, while high stocking density and enhanced productivity by fertilization or dominance of coniferous species increase soil C stock. Nitrogen fertilization increases the soil C stock and N2O emissions while decreasing the CH4 sink. Peatland hydrology management is a major driver of the GHG emissions of the peatland forests, with lower water level corresponding to higher CO2 emissions. Furthermore, the global warming potential of all GHG emissions (CO2, CH4 and N2O) together can be ten-fold higher after clear-cutting than in peatlands with standing trees. The climate change mitigation potential of forest soils, as estimated by modelling approaches, accounts for stand biomass driven effects and climate factors that affect the decomposition rate. A future challenge is to account for the effects of soil preparation and other management that affects soil processes by changing soil temperature, soil moisture, soil nutrient balance, microbial community structure and processes, hydrology and soil oxygen concentration in the models. We recommend that soil monitoring and modelling focus on linking processes of soil C stabilization with the functioning of soil microbiota. This review has been supported by the grant Holistic management practices, modelling and monitoring for European forest soils – HoliSoils (EU Horizon 2020 Grant Agreement No 101000289) and the Academy of Finland Fellow project (330136, B. Adamczyk). In addition to the HoliSoils consortium partners, Dr. Abramoff contributed on this study and her work was supported by the United States Department of Energy, Office of Science, Office of Biological and Environmental Research. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the United States Department of Energy under contract DE-AC05- 00OR22725.