77 Journal of Environmental Science and Management 22-1: 77-86 (June 2019) ISSN 0119-1144 GIS-assisted Carbon Stock Assessment of Loboc- Bilar Mahogany Plantation, Bohol, Philippines JESAM ABSTRACT The study determined the carbon budget of the Loboc-Bilar Mahogany (Swietenia Tomas D. Reyes, Jr.1 macrophylla King.) Plantation in the province of Bohol, Philippines within the months of June to October 2018. The plantation straddles two municipalities, Loboc and Bilar. It is a popular destination for local and international tourists due to its compelling tunnel-like vegetative scenery. Delineation of the plantation boundary was fine-tuned using both image digitization and ground survey. A random sampling method was applied in conjunction with Geographic Information System (GIS) software to spatially distribute sampling plots in the research area. Several carbon pools were assessed, namely: aboveground biomass, necromass or ground biomass, and belowground biomass. Allometric and other mathematical equations were used in the calculation of biomass density, stored carbon and carbon dioxide equivalents. The plantation 1 Bohol Island State University (BISU) had 29,428.03 Mg of stored carbon in the biomass distributed over a total land area Bilar Campus, Zamora, Bilar 6317, of 115.21 ha, yielding an estimated stored carbon density of 255.43 Mg ha-1. The Bohol, Philippines monetized value of stored carbon in the whole plantation amounted to US$486,003.96. Corresponding author: Key words: allometric equation, carbon budget, carbon stock, mahogany plantation [email protected] INTRODUCTION The recent weather abnormalities experienced in tropical storms Sendong (Washi), Yolanda (Haian) and the Philippines and around the world are indications Seniang, which together claimed thousands of lives, of a changing climate (IPCC 2001; Lasco and Pulhin destroyed billions worth of infrastructure and agricultural 2003; Labata et al. 2012). This has been brought by crops, and devastated cities and provinces. The situation the ever increasing carbon dioxide (CO2) concentration is likely to worsen because the country is the third most in the atmosphere causing global warming. Worldwide, vulnerable in the world to climate change (Labata et al. the concentration of atmospheric CO2 has risen in the 2012; Cruz et al. 2017). beginning of the industrial era in 1970 from approximately 277 parts per million (ppm) to 399.4 + 0.1 ppm in 2015 (Le Even as the country receives some of the most Quere et al. 2016). Initially, the atmospheric CO2 increase extreme effects of climate change, because of its vast above pre-industrial levels was primarily caused by the forest reserves, it also has the seventh greatest potential release of carbon to the atmosphere from deforestation among tropical countries to mitigate climate change and other land-use-change activities (Ciais et al. 2013). through sequestration of the greenhouse gas - carbon Then emissions from fossil fuels followed and became dioxide (Cruz et al. 2017). Unfortunately, this potential the dominant source of anthropogenic emissions to the is not being realized, because much forest land in atmosphere since around 1920 (Archer et al. 2009). Its the Philippines is continuously being converted into relative share has continued to increase until present croplands for food production, without regard to the and may reach an alarming rate in the very near future. effects on local carbon pools. As forests are converted into agricultural land, they become a net source of Both the frequency and intensity of storms and greenhouse gas. The agriculture and forestry sectors in typhoons are believed to be caused by climate change. the Philippines contribute more than 33% of the country’s In the Philippines, an average of 20 typhoons enter its total greenhouse gas emissions (Forestry Statistics 2017). area of responsibility and the last 10-year records reveal an increase in number and intensity leading to increased However, productivity does not necessitate a destruction and disaster. Recent examples include reduction in the carbon storage capacity of the land. For 78 Carbon Stock Assessment of Mahogany Plantation in Bohol, Philippines For example, silviculture has the potential to preserve and lies at the southern part of Bohol province. The the livelihood and sustenance value of the land while plantation has a total land area of 115.21 ha with elevation increasing net carbon stocks. In fact, plantations can ranging from 250 to 350 meters above sea level (masl) sometimes sequester more carbon than natural forests. and its topography is characterized by moderately rolling Lasco and Pulhin (1998) estimated that tree plantations terrain with a sandy clay loam type of soil. The area has in the Philippines sequester carbon at the annual rate of tropical monsoon climate which belongs to the Climatic 2.6 m t, with some plantations approaching 15 t ha-1 yr-1. Type IV under the Corona Classification. This means The sequestration potential of a certain tree plantation, that rainfall is more or less evenly distributed throughout however, varies in terms of the type of species, age, and the year (Reyes and Ludevese 2015). The mahogany silvicultural practices applied which can enhance the plantation, including the rest of the Loboc Watershed, growth of trees and increase their biomass (Lasco and has an annual mean rainfall and temperature of 2,000 Pulhin 1998), and rotation period (Schroeder et al. 1992) mm and 24°C, respectively (Reyes and Bethune 2017). as cited by Racelis et al. (2008). Preliminary Activities Mahogany (Swietenia macrophylla King) is a fast growing tree and has been used to reforest the majority On-Screen Digitization. The canopy color of mahogany of the marginal areas in the Philippines, including the plantation is distinct from the surrounding forest Loboc-Bilar Mahogany Plantation in Bohol. Apart from vegetation. Google Earth Pro reveals the plantation as a its beauty and recreational significance, the plantation vegetation with distinct dark green color (Figures 1 and is known for having enormous amount of medium and 3) signifying a unique spectral signature. This makes it large-sized trees, typical of a forest plantation. However, easier to perform on-screen digitization of the plantation the carbon stock has not yet been quantified for this span. The output of digitization was saved as gpx and kml plantation, which is a critical first step in determining files for ground truthing using GPS receivers and further the ecological value of plantation on climate change GIS processing in desktop computer, respectively. mitigation. Ground Truthing and Plantation Boundary Thus, this study was conducted primarily to assess Delineation. Ground survey was conducted to determine the current carbon budget of the Loboc-Bilar Mahogany the real extent of the plantation boundaries. The gpx Plantation. Specifically, the study intended to apply files stored in GPS receivers were used as guide in the quantitative and systematic methodology for biomass and delineation of plantation extent. GPS tracks were saved carbon estimation with the application of GIS; quantify and imported in the GIS platform. Harmonization of the biomass and its stored carbon in all carbon pools the outputs of on-screen digitization and survey tracks using allometric and mathematical equations; and finally was carried out until fine-tuned plantation boundary was monetize the stored carbon in the whole plantation. determined. The Loboc-Bilar mahogany plantation was found to have a total land area of 115.21 ha. MATERIALS AND METHODS Determination of Plot Locations. Selection of survey Study Site. The carbon accounting study was locations of 100 sampling plots was performed using conducted in a three km stretch of mahogany stand several functionalities in ArcGIS. Following the along the boundaries of the municipalities of Loboc principle of random sampling, “create random points” and Bilar in the province of Bohol, Philippines (Figure in ArcGIS was used to distribute the plots without bias. 1). Initiated more than half a century ago, Loboc- This ensured that survey plots well represent the stand Bilar mahogany plantation was established through structure in the study area taking into consideration the a series of reforestation projects of the then Bureau of terrain particularly the relative position along the hills and Forestry, now Department of Environment and Natural flats including the distance in between them. (Figure 3). Resources (DENR), and the barangay and municipal local government units, to replace native trees lost from Sampling Plot Design. The nested plot design used in widespread slash-and-burn farming. This reforested area this study was modified from the sampling plot designs has now become a tourist destination and is home to the employed by Hairiah et al. (2001a and 2001b), Chave et al. Green Tunnel of Bohol which traverses the forest via the (2005), Racelis et al. (2008), Hairiah et al. (2011), Reyes national road. The forest stand is located at 9°40’ North et al. (2011) and Walker et al. (2012) with the intention of and 124°06’ East, about 45 km away from Tagbilaran City spreading as many plots as possible within the plantation. Journal of Environmental Science and Management Vol. 22 No. 1 (June 2019) 79 Figure 1. Google Earth map showing the location of the Loboc-Bilar mahogany plantation, Bohol, Philippines (in yellow color). Field Survey Snags and fallen dead branches were regularly collected for firewood by nearby community residents, thus, were Field Data Collection. Trees, regardless of species, with not included as part of the necromass or ground biomass diameter at breast height (DBH) of >30 cm were recorded (Figure 2). in a 20 x 20 m plot. Trees and other woody plants with DBH of 5-30 cm were sampled from within a smaller 5x5 m plot. Within the 5x5 plot, understorey vegetation, litter, and soil were assessed in four smaller plots of 1x1 m each. Non-destructive sampling was applied in the 20 x 20 m and 5 x 5 m plots, while destructive sampling was employed in the smallest plots.