Development of Carbon Management Strategies in Mangrove Forests of Banacon Island, Philippines

Abstract

Undeniably, climate change has grown as one of the most pressing issues of world today. Deforestation and forest degradation had contributed much to this problem since forests are important carbon sinks. Reflections from Rio +20 Earth Summit revealed that not much has been done to conserve natural resources since forest cover continuously disappears at alarming rates and carbon dioxide remains soaring and unabated. The need for sustainable forest management (SFM) reverberates until today as impoverish forest communities sob when they grapple with the harsh impacts of climate change, realizing that the forest which provides them subsistence is also direly shedding off. One such example is the mangrove forests where their essential services have degraded through time. This has affected local fisheries and even made coastal communities more vulnerable to threats of tidal surges and tsunami. In the Philippines, about seventy percent of its original mangroves have disappeared over the past century. Albeit this trend, deforestation rate has decreased today due to growing awareness of their ecological values. Further, the adoption of community-based forest management programs has started spurring efforts to restore coastal vegetation and protect existing growth mangroves. Indeed, local communities have critical roles to play in curving down deforestation as well as climate change. Among the successful cases of community-managed mangrove sites in the Philippines is the Banacon Island in Bohol Province. With an aim to develop stand management strategies for better carbon sequestration capacity of their plantation, this study was conducted. Recognizing that there is not much studies on geo-spatial analysis, allometric modelling, carbon stock capacity and stand density management of mangroves in the country, this study sought to integrate these facets into this report in hope that such information can help local community and government to improve and calibrate their current programs on mangroves. On landuse characterization aspect, the comparison between two available satellite images of different years (1993 and 2004) revealed three distinct forest landuses namely, dense mature stand, dense intermediate stand, and sparse mangrove. Using maximum likelihood image classification approach, dense mangrove cover was found to have increase from 146.5 to 285 ha after a decade. This was largely attributed to continuous planting effort of the local community. Dense mature (30-55 yr. old) and intermediate (≤ 29 yr. old) stands have expanded by forty one percent and sixty one percent, respectively. This progress also showed that the deforestation rate (due to thinning for pole and fuelwood) was compensated by planting activities with as much as +1.4 ha yr-1 and +7.5 ha yr-1, respectively. Developing appropriate allometric models for calculating tree biomass is of paramount importance to effective quantification of tree carbon stocks. Thus, sampling of 35 Rhizophora sylosa trees was conducted to develop allometric model specifically intended for the study site. Key results showed that larger portion of tree biomass is vested in belowground biomass (roots) than in aboveground biomass (stem, branch, and leaves). Diameter and height values were identified as significant co-predictor variables of tree biomass. Allometric models were then formulated in two formats: full and reduced. Full model uses both diameter and height as independent variables for tree biomass estimation while reduced model solely utilizes diameter values. Both allometric model forms yielded high coefficient of determination (R2) index thus suggesting their reliability of use. Full model was however preferred since diameter and height variables were found strongly correlated parameters with one another. Using full model can therefore reflect the contribution of both diameter and height on tree biomass calculation. Responding to the study goal of developing carbon management strategies for mangrove forests, the last chapter dealt mainly on the assessment of effects of forest management on mangrove carbon stocks. Using standard plot sampling technique and appropriate allometric models, biomass and carbon stock of mature (30-55 years) R. stylosa plantations were assessed based on three current stand management types being implemented in the site. These include: thinning, thinning with supplementary planting, and do nothing (non-thinning). Do nothing approach was identified as the most ideal since it produced the largest carbon stock. However, since the local community occasionally thins their plantations for poles and fuelwood, thinning and supplementary planting was therefore recommended by the study because of its ability to replenish density losses and augment carbon stocks. Another forest management aspect examined was plant spacing design. Based on biomass and carbon stock values of young R. stylosa stands (20 years old) that were established using of two different spacing: 0.5m x 0.5m

and 1.0m x 1.0m, values were found significantly larger in the former. Density, tree height and survival was also seen favorable in stands that were raised using 0.5m x 0.5m spacing. Therefore, closer spacing method was recommended as better plantation establishment strategy to achieve large carbon stocks. Species composition in natural stands also showed some likely effects on the carbon stocks of natural stands. Tree carbon stock was larger in Sonneratia-dominated stands where Sonneratia alba predominate. On the other hand, sediment carbon stock was bigger in Avicennia¬-dominated stands owing to its thicker OM layer and larger sediment mass. Thus, to further improve the carbon stocks of Avicennia-dominated stands, enrichment planting using Sonneratia alba was recommended to complement their already rich sediment carbon. Putting all together the relevant information about the effects of stand management, plant spacing and species composition on mangrove carbon stock, a schematic stand density management framework was proposed. This framework summarized the necessary conditions to achieve the most ideal stand density and carbon stock.