Biomass and Carbon Content Allocation of Six-year-old Anisoptera Costata Korth . , nd Dalbergia Cochinchinensis Pierre , Plantations in Lao PDR

The objective of our study was to examine the biomass and carbon contents (including soil) of two six-year-old native species, Anisoptera costata and Dalbergia cochinchinensis. Forest plantation is an important strategy to restore degraded forest ecosystem not only for increasing timber supply, but also for socio-economics, environmental and biodiversity purposes. In addition, forest plantations helps to increase the biomass density and carbon content besides improving soil conditions. Increasing forest plantations can help reduce carbon dioxide emission in the atmosphere. In this study, the biomass and carbon contents of trees and soil in plantations were determined. The two species A. costata and D. cochinchinensis, both planted June 2002, were selected for this study. Stem, branch, leaf and root components were measured to obtain biomass and carbon contents. Results of the study showed that a total biomass of 5.63 ±1.23 t ha-1 and 5.42 ±0.94 t ha-1 were accumulated in A. costata and D. cochinchinensis, respectively. Meanwhile, the carbon contents recorded were 2.58 ±0.45 t ha-1 and 2.50 ±0.32 t ha-1 in A. costata and D. cochinchinensis, respectively. However, soil organic carbon content was found to be higher in D. cochinchinensis 0.68 ±0.05% than A. costata 0.57 ±0.02 %. These data will be useful in designing forest plantation for long term management using native tree species.


Introduction
According to FAO, the world's forest plantation accounted for less than 7% or 264 million ha of total forest areas, in which 78% are productive and 22% are protective.Most of this was established through afforestation, three-quarter of all planted forest consist of native species, while one-quarter comprises introduced species.It is estimated that the world's forests store 289 gigatonnes (Gt) of carbon (C) in their biomass alone and decreased 0.5 Gt annually during the period 2005-2010 (FAO, 2010).The total C storage in forest plantations nowadays is approximately 11.8 Gt with an increase of 0.178 Gt yr-¹, tropical forests that spread over 13.76 million km² worldwide account for 60% of the global forest (FAO, 2010; Dutca et al., 2009) and plays a key role in global C cycle both in terms of C flux and volume of C stored (Baishya et al., 2009).The estimations made by Houghton and Goodale (2004) regarding C storage in plantations during the 1990s indicated a global level rate of 0.19 Gt yr-¹.They also estimated the annual C storage rate in tropical plantations (0.11 Gt yr-¹), and in the temperate and boreal plantations (0.081 Gt yr-¹).Carbon management in forests is a global concern to mitigate the increased concentration of green-house gases in the atmosphere (Singh and Lodhiyal, 2009).At the global level, forest plantations represent less than 10% of deforested areas and tree planting currently compensates about 0.3% of the carbon released by deforestation (Montagnini and Porras, 1998;Dutca et al., 2009).The establishment of plantations has been proposed as a tool for forest restoration of degraded lands because of their effects on vegetation structure, microclimate and soil (Affendy et al., 2009).Plantations of multi-purpose tree species can play an important role in restoring productivity, ecosystem stability, and biological diversity of degraded tropical lands (Parrotta, 1992;Piotto et al., 2004).In addition, reforestation is being considered as a mitigation option to reduce the increase in atmospheric carbon dioxide and predicted climate change.Forestry-based C storage projects are thus being introduced in many tropical countries but assessment of C storage potentials is made difficult by a lack of species-level information (Kraenzel et al., 2003).The ability of these plantations to sequester C has received renewed interest since C sequestration projects in developing nations could receive investments from companies and governments wishing to offset their emission of green-house gases through the Kyoto Protocol's Clean Development Mechanism (CDM) (Losi et al., 2003).There are two common methods being used to estimate biomass and C content for both plantation and natural forest: volume equation and regression equation (Francis, 2000;Losi et al., 2003).The role of soil in the global C cycle has also been receiving greater scientific attention (Lugo and Brown, 1993).
In Lao PDRs, there were 306,000 ha of forest plantation up to 2009, in which more than 50% was rubber (DoF, 2009), but assessment of the C potentials has not been done.The government aims to increase the national forest cover of up to 70% of total land area by the year 2020 through natural regeneration in more than six million ha and another 0.5 million ha by plantations especially in degraded forests including fallow forests and logged-over forests as stipulated in the Forestry Strategy 2020 (MAF, 2005).The objective of this study was to examine the allocation of biomass and C contents (including soil) of two six-year-old native species, A. ostata and D. cochinchinensis.

Data collection
In order to estimate the biomass and carbon content, four square inventory plots (20 m x 20 m) were laid out in the A. costata and D. cochinchinensis plantations.Diameter at breast height (DBH) and total height were measured in each plot.The plantations were assessed in terms of survival, DBH and total height.The relationship between DBH and height was examined using linear regression analysis.For estimation of individual tree biomass, fives trees were randomly sampled from each A. costata and D. cochinchinensis plantations.Trees were felled as close as possible to the ground and the height of remaining stumps were about 0.10 m.Above-ground biomass was separated into different components (stem, branch, and leaf).Then the stem was cut into meter-long logs in order to measure the fresh weight of each stem.Fresh weights of the different tree components were determined using a weighting scale in the field.A 5 cm thick disc was cut from each of the meter long log and was weighed.About 500 g of bough and small branches, and 200 g of leaves were sampled.Stem samples were oven dried at 90�C for 72 hours, bough and small branches for 48 hours, and leaves for 24 to 36 hours (until constant weight or less than 15% moisture contents).

Moisture content (%) = Sample fresh weight -Sample dry weight/Sample dry weight x 100
The dry biomass values of all components were estimated from moisture contents of samples which were used to calculate carbon content of a tree.The carbon content default value of 0.46 (for DBH <10 cm) was used to estimate the carbon content of tree biomass while the root:shoot ratio default value of 20% of aboveground carbon was used as proposed by IPCC (2006).The relationship between DBH and tree biomass was constructed where the biomass was subsequently used to estimate the carbon storage in the six-year-old plantations.

Dry biomass of tree component (kg) = sample dry weight/sample fresh weight x total of fresh weight of tree component
Soil samples were collected at the centre of each sub-plot from different soil layers (0-20 cm, 21-40 cm and 41-60 cm) to analyze the soil organic carbon (SOC) and soil organic matter (SOM).SOC was obtained using the Walkey-Black (1934) method while the SOM was determined by multiplying % SOC by 1.724.

Data analysis
The mean values and standard errors of DBH, height, fresh weight, dry weight, %SOM and %SOC were analyzed using MS Excel 2007 and SAS 9.1.3for Windows (SAS Institute Inc., USA).One-way analysis of variance (ANOVA) was used to test the differences between variances.Duncan's Multiple Range Test (DMRT) was used to examine the differences of among values.Significant differences for all values were determined at p ≤0.05.

Growth performance of planted species
The average DBH and heght of A. costata and D. cochinchinensis are shown in Table 1.The survival rate after six years of planting was higher in A. costata (73.1%) with 813 trees ha-¹ compared to D. cochinchinensis (67.5%) with 750 tree/ha).The results also showed that linear regression equation for DBH and height was higher in D. cochinchinensis (R²=0.77)than A. costata (R²=0.51).While, the total basal area was shown higher in A. costata (29.26 m² ha-¹) than D. cochinchinensis (22.21m² ha-¹).
This may be explained by a slower growth rate in D. cochinchinensis (mean annual increment for DBH 0.63 cm year-¹ and height 0.53 m year-¹) compared to A. costata (mean annual increment for DBH 0.77 cm year-¹ and height 0.61 m year-1) at the same age and site condition.Details of growth performance of some native plantation trials in Central part of Lao PDR were compiled in Table 2..

Biomass and carbon content
The total fresh biomass of A. costata and D. cochinchinensis were estimated at 10.63 ±2.37 t ha-¹ and 8.26 ±1.61 t ha-¹, respectively.The allocation of fresh biomass were significantly different in both species was found to be in the order of stem (F=7.48,P=0.0522) > branch (F=0.00,P=1.000) > root (F=5.11,P=0.866) > leaf (F=1.63,P=0.2711) Figure 2 A. The total dry biomass of A. costata and D.cochinchinensis were then calculated from the moisture contents of sample component at 5.63 ±1.23 t ha-¹ and 5.42 ±0.94 t ha-¹, respectively.The allocation of dry biomass were also significantly different in both species were found to be in order of stem (F=0.02,P=0.8912) > branch (F=1.39,P=0.3038) > root (F=0.26,P=0.6348) > leaf (F=2.09,P=0.2222) Figure 2 B. While, shoot:root weight ratio in both species were 4.99 and 5.00 (fresh weight), 2.68 and 4.67 (biomass), and while 4.95 and 5.46 (carbon content) in D. cochinchinensis and A. costata.
Therefore, the estimated total C contents in A. costata and D. cochinchinensis were 2.59 ±0.56 t ha-¹ and 2.50 ±0.43 t ha-¹, respectively.The C contents in branches and leaves of D. cochinchinensis were slightly higher than (Table 3).Carbon contents in roots were insignificantly (p>0.05)different between the two species.On the other hand, the C content in stem was slightly higher in A. costata 1.31 ±0.31 t ha-¹ than D. cochinchinensis 1.19 ±0.18 t ha-¹(F=0.02,P=0.8854), branch (F=1.30,P=0.3172), root (F=0.29,P=0.6205) and leaf (F=1.19,P=0.2392).In this study, sample linear regression equation was used to determine the relationship among fresh weight, dry weight and carbon content of the two species (Figure 3).The best R² values of 0.75 (AGB, BGB and carbon) and R²=0.76 value (total fresh weight) were obtained by A. costata and D. cochinchinensis, respectively (Table 4).However, the survival rate can affect the biomass and carbon contents (Thaung et al., 2007).According to Priaasukmana (1991) and Weidelt (1976) found that if weeding is carriout late, dipterocarps or deciduous species are easily damaged and also the importance of maintenance an insufficient tending is the most common reason for failure.There are only a few studies reported in the Asian region related to A. costata and D. cochinchinensis.Yamada et al. (2004) reported that 15-year-old D. cochinchinensis in Thailand had a total biomass of 104 t ha-¹ and root biomass of 28.8 t ha-¹.Most of the studies examined on aboveground biomass (AGB) (without detailed information on stem, bark, bough, branches, and leaves) or belowground biomass (BGB).In this study, data from some studies on native and exotic species in neighboring countries were summarized (Table 5).The productivity on biomass and C content more or less depends on species capacity, environment (temperature, soil nutrient, tree densities, rainfall, and wood density), human activities, and silvicultural practices in both plantation and natural forests (Baishya et al., 2009).
Moreover, biomass and C content are likely to increase according to the age of species in the same site (Yamada et al., 2004;Petsri et al., 2007).Other studies found that exotic species have higher ability than indigenous species to adapt in environments such as dry zone, poor soil, low rainfall (Min et al., 2006;Yamada et al., 2004).Phongoudome et al. (2011) reported that in two -year-old plantation with spacing 2 m x 2 m, two indigenous species such as Cassia siamea and Peltophorum dasyrachis were (2.92 ±0.08 and 1.99 ±0.07 kg tree-¹) more than total biomass and C content than Acacia mangium, A. auriculiformis and Eucalyptus camaldulensis (1.96 ±0.12; 1.60 ±0.08 and 1.58 ±0.16 kg tree-¹), respectively.
The fresh biomass, dry biomass and C content of root, stem, branch and leaf increased with increasing of tree sizes (diameter) as well as sizes of the trees indicating that both species still produced and accumulated biomass for the next.

Soil organic matter and soil organic carbon content
Overall, total soil organic matter (SOM) and soil organic carbon (SOC) were low in both plantations according to soil standard (<2.0 low, 2.0-4.0 medium, and >4.0 high.However, SOM and SOC were higher in D. cochinchinensis (1.164% and 0.675%, respectively) than A. costata (0.974% and 0.565%, respectively).Both SOM and SOC were higher at 0-20 cm soil depth (Table 2) which was mostly attributed to the contribution of litter fall.The correlation between SOM and soil depth was high in A. costata and D. cochinchinensis plantations with R² values of 0.99 and 0.94, respectively.Similar relationship was also found in both plantations for SOC.Montagnini and Porras (1998) and Krazenzel et al. (2003) reported similar condition where SOM and total nitrogen showed higher value at 0-15 cm of soil depth than other soil depths.According to Piotto et al. (2004), mixed plantations with native species would contribute much to sustainable management because they provide a greater range of goods and services than pure species plantations.Nevertheless, the amount of total C in soil natural forest tends to be higher than plantation (Takahashi et al., 2007).

Conclusion
This study examined the biomass of native plantation and their capacity to store carbon in trees and soils.The results showed that the six-year-old A. costata and D. cochinchinensis plantations are both comparable in terms of biomass accumulation and C sequestration.Based on the results of this study, the data obtained may be useful for forest management in Lao PDRs and could be shared among neighboring countries to restore degraded forest ecosystem in the future.Therefore, better silvicultural practices in natural forest and plantation as well as agroforestry are needed to increase biomass and carbon content in trees and soils.The results of this study suggest that through careful species selection and management practices based on understanding of natural succession and nutrient cycling processes, forest plantation could be a promising tool for tropical forest rehabilitation and carbon sequestration.

Figure 2 .
Figure 2. Fresh biomass, dry biomass and carbon content allocation in six-year-old plantation

Table 3 .
Total carbon contents in trees components and soil of six-year-old plantations of Anisoptera costata and Dalbergia cochinchinensis.