Carbon storage in rubber plantations of various stand ages in Cambodia

I Introduction
Recently, a rapid increase has occurred in the rate of conversion of natural forests to rubber plantations on the Indochinese peninsula (4). Although this peninsula is a key region in the Reducing Emissions from Deforestation and Forest Degradation (REDD) project in Southeast Asia, information about the impact of land-use change in this area on the terrestrial carbon (C) cycle is currently lacking. Estimation of the major C pools (tree and understory biomass, deadwood, litter, and soil) in rubber plantations, using the same methodology applied to natural forests under the REDD scheme ( / ), may be the first step in evaluating the effect of deforestation in Indochina on the terrestrial C cycle. Accordingly, we investigated the major C pools in young rubber plantations of various ages in central Cambodia.
II    Material and methods
1. Study area
The study site is located in northern Kampong Thom province, Cambodia (12° 92-93' N, 105° 37-39' E). The mean annual temperature in the province is 27°C, and the annual rainfall from 1994 to 2004 ranged from 1,085 to 1,857 mm (3). The region has a pronounced dry season from November to February. The basaltic bedrock tis described as Triassic intermediate to mafic volcanic rock, and the soils are Ferralsols. The natural forest type of the study area is secondary dry evergreen forest, and the dominant tree species are Irvingia malayana and Peltophorum dasyrhachis (5).
2. Rubber plantations
Since 2001, secondary forests in the study area have been converted to rubber plantations, and at present, rubber plantations occupy about 4,700 ha in total. Prior to the establishment of rubber plantations, burning and cultivation were practiced by local people. Rubber trees are planted 3 m apart in rows spaced 6.5 m apart. Soil between the rows is plowed by heavy equipment to 20 cm depth to control weeds. We established one study plot each in 1-, 2-, 6-, and 7-year-old rubber plantations (R1, R2, R6, and R7, respectively) and two plots each in 8- and 9-year-old plantations (R8a, R8b, R9a, and R9b, respectively). The size of each plot was 30 m × 40 m.
3. Estimation of stand biomass
Tree (above- and belowground) biomass of each rubber tree was estimated from the basal area (ba, m2) and wood density (D, kg m 3 ) using the allometric equation proposed in Kiyono et al. ( 2 ) as follows,
Tree biomass (kg) = 4.08×ba 1.25×D 1.33 .
Understory biomass was estimated with an allometric equation using community height ( 2 ). The amount of deadwood (mainly dead tree stumps of former secondary forest) was calculated on the basis of stump girth and height and cavity proportion. Belowground deadwood was - not determined in this study.
4. Sampling and analyses of soil and litter
Soil and litter sampling was conducted in R1, R2, R6, and R9a. One sampling point was selected in each of these four study plots. Litter thickness (T, L, and F) was recorded and the litter was collected from a 50×50-cm-quadrat at each sampling point. Three replicate soil samples were collected from 0-5, 5-15, and 15-30 cm depths using 100-ml steel cylinders. For physical analysis, clay (<0.002 mm), silt (0.002-006 mm), and sand (0.06-2 mm) content was determined using the pipette method and wet sieving. Charcoal in coarse organic matter (>2 mm) was detected by wet sieving and visual check. The bulk density of fine soil was measured on an oven-dry (105°C, 24 h) basis. The total carbon (Total C) and total nitrogen (Total N) contents of the soil and litter samples were assessed using the dry combustion method (varioMAX, Elementar Corp.). Soil C stock (mg C ha 1 ) in each soil horizon was calculated by multiplying Total C by the bulk density and horizon thickness. Soil C density (mg C ha-' cm-') in each soil layer was calculated by dividing the soil C stock by the thickness of the soil layer.
Results and discussion
III
1. Field observations
Stands R1 and R2
The canopies of stands R1 and R2 were open. Grass species, both from the former secondary forest vegetation and newly invaded Poaceae, were observed on the forest floor (Photo 1). Tree stumps from the former forest were dry and hardly decayed. The mean litter thickness (T, L, and F) was 0.65 (R1) and 0.29 cm (R2). Charcoal (>2 mm) was observed on the soil surface. The clear plowing (Ap) horizons ranged from 7 to 22 cm in thickness.
Stand R6
The canopy of the stand was almost closed, and some trees were already bearing fruit. Poaceae and shade tolerant species (Chromolaena odorata, Cyrtococcum sp.) were distributed on the forest floor. The remaining tree stumps on the forest floor were relatively wet and in a state of advanced decomposition, accompanied by hard fungi and termites. The litter of rubber leaves had accumulated with a mean thickness of 0.70 cm. Plowing horizons were not clear and ranged from 5 to 15 cm in thickness.
Stands R7, R8, and R9
The canopy was closed, and a large portion of the rubber trees had fruit (Photo 2). Sap harvesting had already begun for some rubber breeds. Shade-tolerant grasses were distributed locally on the forest floor. Tree stumps were almost completely decomposed, and termite activity had decreased. A mixture of rubber twigs and leaves had accumulated to a mean thickness of 2.13 cm. Plowing horizons were not clear and ranged from 5 to 8 cm in thickness.
2. Stand biomass
Despite fluctuations, our measured results suggested that the rubber tree biomass increased and the deadwood biomass decreased with plantation age (Table 1). Understory biomass increased early on (R1, R2 to R6), and then decreased in R9a and R9b, possibly as a result of canopy closure. These trends indicated that the C storage function of rubber plantations can be qualified in terms of age-related parameters in the C cycle model. When the C content of tree biomass was 0.5 g C g-', tree biomass C was 30.3 and 40.2 Mg C ha 1 in R9a and R9b, respectively (Table 1). These values were lower than the estimated tree biomass C in secondary forest plots of this region (133.5 Mg C ha-', Toriyama, data not published).
3. Soil and litter C stock
The soil C stock in the study area may be more related to soil texture than to stand age. The clay content of the upper soil layers (0-15 cm depth) was high in R1 and R2, and low in R6 (Table 2). Bulk density was slightly lower in R1 (Table 2), and Total C was low in R6 that showed low clay content. Accordingly, it might be difficult to relate soil C stock to stand age in this study. The mean soil C stock at 0-30 cm depth ranged from 34.3 to 48.4 Mg C ha-' in the four sampled plots (Table 2), lower than levels in secondary forest soil (62.7 Mg C ha " ¹ , Toriyama, data not published). Plowing at the time of rubber plantation establishment may have influenced the distribution of soil C stock within the soil profile. The mean soil C density at 5-15 cm depth was comparable to that at 0-5 cm in R1 and R2. In contrast, soil C density in R6 and R9a decreased at 0-15cm in depth. Rubber plantation stand age may be related to the quality of soil C through the decomposition of charcoal. The ratio of charcoal to coarse organic matter was high in R1 (>0.3) and relatively low in R6 and R9a (<0.2). The plots in order from highest to lowest C/N value were R1 > R2 > R6 > R9a, suggesting that the mixing of charcoal may have influenced these C/N values. Dry matter and C stock in litter (sum of T, L, and F) rapidly decreased from R1 to R2, and increased from R2 to R9 (Table 3). Although twigs (T) contributed greatly to litter C stock in both R1 and R9, our field observations suggested that twigs from the former secondary forest decomposed within a few years of rubber plantation management and that twigs from rubber trees became increasingly important to these levels.
IV Conclusions
We evaluated the major C pools in rubber plantations in central Cambodia and proposed a method of comparing C storage function between rubber plantations and natural forests that involves relating rubber plantation stand biomass to stand age. Future studies should incorporate field data sampled from widely ranged soil textures and older stand age than this study.
Acknowlegements
This study was funded by the Japanese Ministry of the Environment (Global Environment Research Fund A-0802). All soil samples in this study were imported through Narita airport after quarantine inspection with the import permit No. 21Y927 issued on December 1st, 2009 by Yokohama Plant Protection Station. The authors thank the Dr. Makoto Araki, Dr. Yasuhiro Ohnuki, Dr. Eriko Ito, Mr. Nang Keth, Mr. Pero for their assistance in field surveys in Cambodia. The authors are also deeply indebted to Prof. Seiichi Ohta, Dr. Yojiro Matsuura. and the staff members of Forest Site Environment, FFPRI, for the helpful advice on soil analysis. (1) GOFC-GOLD (2009) Reducing greenhouse gas emissions from deforestation and degradation in developing countries: a sourcebook of methods and procedures for monitoring, measuring and reporting, GOFC-GOLD Report version COP14-2. Y. (2006) Development of simplified methodologies for measuring and predicting biomass carbon stock. In: A fiscal report of forestation basic data collection aiming at small-scale environmental planting CDM projects 2005, 8-41, FFPRI, Tsukuba. (In Japanese) (3) National Institute of Statistics (2006) Year Book 2006. http://www.nis.gov.kh/nis/yearbooks/Yearbook06.htm (4) ZIEGLER, A., D., FOX, J., M., XU, J., C. (2009) The Rubber Juggernaut. Science 324: 1024-1025. (5) TANI, A., ITO, E., KANZAKI, M., OHTA, S., KHORN, S., PITH, P., TITH, B., POL, S., LIM, S. (2007) Principal forest types of three regions of Cambodia: Kampong Thom, Kratie, and Mondolkiri. In: Forest environments in the Mekong River basin, 201-213, Springer, Tokyo.
DW = Deadwood
COM = Coarse organic matter (>2 mm)
NA = Not available