Aboveground Biomass Production of Rhizophora apiculata Blume in Sarawak Mangrove Forest

Problem statement: Mangrove forests are found in tropical and subtrop ical coastal tidal regions. Rhizophora apiculata Blume is one of the most important species in mang rove forest. It is also one of the commercial mangrove timber species in Asia-Pacific region which dominates large areas of mangrove in this region. In order to under stand forest ecosystem characteristics and to establish the proper management system, a precise e stimation of biomass is necessary. The objective of this study is to quantify the aboveground biomas s production and stem volume of R. apiculata in Awat-Awat mangrove forest, Sarawak. Approach: Seven representative trees were used in this study for sampling from February 2011 to March 2011 . Allometric relationships were examined using either independent variable Diameter (D) or c mbination of quadratic of D and Height (DH). Results: The best fit of allometric equations were developed from the combination of quadratic of D and H (y = 0.055 ×0.948, R = 0.98) which is more recommended to estimate biomass and stem volume of R. apiculata in Awat-Awat mangrove forest, Sarawak. Total aboveground biomass and stem volume of R. apiculata were 116.79 t h −1 and 65.55 m h, respectively. Conclusion: Aboveground biomass and stem volume is closely rel ated with tree diameter and height which indicates that abovegroun d biomass and stem volume will increase with increasing diameter and height of R. apiculata.


INTRODUCTION
Mangroves are coastal forests that affects by tides, soil texture and marine salinity. This forest is found in tropical and subtropical coastal tidal regions. As a component of wetlands, mangroves has also been recognized as one of the most productive ecosystem which grow on sheltered shores and estuaries in the tropics and sub-tropical area (Gandaseca et al., 2011). Mangroves are very well adapted to grow in sea and brackish water. They have roots that typically grow in anaerobic sediment and receive oxygen through aerating tissue which communicates to the air through lenticels on the aerial roots and trunks (Lai et al., 1993).
The species R. apiculata Blume is one of the most important species in mangrove forest. It is also one of the commercial mangrove timber species in the Asia-Pacific region which is dominate large areas of mangrove in the region and often grows as pure stand (Ong et al., 2004). Mangrove forests in Malaysia, especially in Sarawak is about 175,153 ha which are found along the coastline (Bennet and Reynold, 1993). The values of mangroves consist of both monetary and non-monetary such as forest industry, fisheries industry, wildlife conservation, tourism and environment protection (Bennet and Reynold, 1993;Lai et al., 1993).
Studies found that quantification of tree biomass can be very difficult and expensive since it involves tree felling, unearthing root systems, weighing and drying samples. Such activities are very expensive for any other purpose than research (Specht and West, 2003). Therefore, attention has been paid to develop techniques to estimate tree biomass from easily measured tree characteristics known as 'allometry'. Allometry is a powerful tool for estimating biomass production from easily measured tree characteristic such as stem diameter and height that are quantifiable in the field (Komiyama et al., 2005). Common allometric equations have been reported for aboveground biomass (Brown et al., 1989;Brown, 1997;Ketterings et al., 2001) and also for mangroves biomass (Komiyama et al., 2002;Ong et al., 2004;Nguyen and Ninomiya, 2007) elsewhere.
The estimation of aboveground biomass of mangrove is not only provides increasingly valuable means for making comparisons among ecosystems but could also use to evaluate the productivity pattern, nutrient cycle and energy flow (Kusmana et al., 1992). Furthermore, in order to understand the forest ecosystem characteristics, the measurement of tree biomass is important. The main objective of this study is to assess the biomass production of R. apiculata in Awat-Awat mangrove forest, Sarawak, Malaysia.

Study site:
This study site is situated in Awat-Awat mangrove forest, Lawas Sarawak, Malaysia Fig. 1 and all of R. apiculata are 15 years old. Forest inventory and sampling were done from February 2011 to March 2011. Within sample plot (100×50 m), all tree heights and diameters were measured. The Diameter (D) of trees was measured using diameter tape at 30 cm above the highest prop-roots while tree Height (H) was measured using hypsometer (Komiyama et al., 2005). To estimate the aboveground tree biomass in this forest, seven tree samples were selected for sampling (2 small, 3 medium and 2 big size trees). In order to choose the representative trees for sampling, D and H data for all trees inside tree plot was sorted from the lowest to the highest value then divided into three classes namely small (0-12 cm), medium (12-24 cm) and big (24-36 cm). The mean values of each class were selected for calculation.
All selected trees were harvested and each sample tree was divided into several components named (1) stem, (2) branch and twig and (3) leaf. After each tree sample was completely felled, its main stem were separated into each component as 0-2, 2-4, 4-6 m log to the top. All tree components were weighted to acquire the total fresh weight. About±10 cm disc stem sample was taken from each part together with plant sample such as branches and leaves and brought to the laboratory. The sample was oven dried for two days. Total dry weight of those plant parts (stem, branch and leaf) were estimated as dry/fresh weight ratios (Brown, 1997). Allometric relationships were examined using independent variable D or combination of quadratic of D and H. The relationship between independent variable and components biomass and stem volume was described by a power function, Yi = aD b or Yi = a (D 2 H) b , where a and b are regression constant, D is tree diameter (cm), H is total height (m) and Yi is the dry biomass (kg) of a tree component i.e., stem, branch and leaf (Heryati et al., 2011). To choose the most appropriate biomass prediction of R. apiculata, both methods for stand biomass prediction were compared. The aboveground biomass was determined summing of the biomass of stem, branch and leaf. The total aboveground biomass of R. apiculata was calculated from the summation of tree biomass found from sampling plot. All data for biomass and stem volume were converted into hectares.

RESULTS
The percent of aboveground biomass of R. apiculata was found 62.55% for stem, 31.89% for branch and 5.57% for leaf in the Awat-Awat mangrove forest. Biomass production of stem was higher than the leaf and branch biomass of R. apiculata Table 1.    Regression coefficients using tree diameter (D) were found 0.97, 0.96, 0.95, 0.91 and 0.98 with stem volume, stem biomass, branch biomass, leaf biomass and aboveground biomass, respectively Fig. 2. The regression models using combination of quadratic of D and H with stem volume, stem biomass, branch biomass, leaf biomass and aboveground biomass were 0.99, 0.99, 0.93, 0.86 and 0.98, respectively. Comparatively, these r-squares values were higher than those were found for D and tree components and almost of the r-square values were closed to 1. Therefore, quadratic of D and H was used to estimate the aboveground biomass of R. apiculata.
The average diameter of R. apiculata was 14.45±0.375 cm with the average height of 12.16±0.217 m in the sample plot of Awat-Awat mangrove forest. Medium class (12-24 cm diameter) trees was the dominate species of R. apiculata in this forest plot with the density of 376 trees h −1 while the lower (100 trees h −1 ) was found for big class (24-36 diameters) of trees. Stem volume was found higher for big class (30.90 m 3 h −1 ) of mangrove followed by medium class (30.71 m 3 h −1 ) and small class (3.94 m 3 h −1 ). The estimated total aboveground biomass was found higher (55.45 ton h −1 ) for medium class trees followed by big class (53.80 t h −1 ) and small class (7.54 t h −1 ) of mangroves in this mangrove forest.

DISCUSSION
Biomass studies of mangroves have been done in many places of the world with many species i.e., Avicennia marina, Bruguiera gymnorrhiza, Ceriops tagal, Kandelia obovata, Rhizophora mucronata and R. apiculata. In present study, R. apiculata shows comparatively higher aboveground biomass than mixed mangroves (R. apiculata, R. mucronata, B. gymnorrhiza, B. parviflora, C. tagal, Xylocarpus granatum) in Langkawi Island, Malaysia (Norhayati and Latiff, 2001), C. tagal in Satun, Thailand (Komiyama et al., 2000) and K. obovata in Okinawa, Japan (Khan et al., 2009). Moreover, the biomass value of R. apiculata was comparable with the values recorded elsewhere Table 2. Studies revealed that the variation in above ground biomass depends on species but also on ecological circumstances and geographical location (Komiyama et al., 2008). Furthermore, Komiyama et al. (2008) noted that total aboveground biomass of Rhizopora sp. in the Pacific and Australia region ranges from 40.7-460 t h −1 . Norhayati and Latiff (2001) also reported 115.56 t ha −1 of biomass for mixed mangrove (R. apiculata, R. mucronata, B. gymnorrhiza, B. parviflora, C. tagal, X. granatum) forests in Langkawi Island, Malaysia. In this present study, the level of aboveground biomass R. apiculata was lower compared to those reported by Christensen (1978); Gong and Ong (1990); Hossain et al. (2008) and Norhayati and Latiff (2001) globally (Table 2). The proportions of biomass in this present study are unlikely among all components of R. apiculata. R. apiculata shows comparatively higher percentage of stem biomass (62.55%) compared to branch (31.89%) and leaf (5.57%). However, Heryati et al. (2011) mentioned that the amount of the stem biomass is closely related to the production of trees obtained through photosynthesis which is generally stored in the trunk. Similarly, Hossain et al. (2008) reported the same proportions of biomass stem>branch>leaf>reproductive components for B. parviflora in Kuala Selangor, Malaysia Table 3. All biomass proportion values found for R. apiculata in this present study are comparable with other studies elsewhere Table 3. The variation in biomass may be related to the ecology, species, plant density, growing season, plant age and global positioning of mangrove forests.

CONCLUSION
The combination of quadratic of D and H is recommended as variables for stem volume and biomass quantification of R. apiculata in Awat-Awat mangrove forest, Lawas, Sarawak. In addition the aboveground biomass and stem volume is related with tree diameter and height which indicates that aboveground biomass and stem volume will increase if diameter and height also increase. Total Aboveground Biomass in Awat-Awat mangrove forest Lawas is 116.79 t h −1 .