Philippine Journal of Science 2nd Draft: 14 pages 150 (S1): 361-374, Special Issue on Biodiversity Corrected: 17 Feb 2021 ISSN 0031 - 7683 04:12 PM Date Received: 02 Sep 2020 26_MS_20-207B Detection of Forest Cover Disturbances Using Synthetic Aperture Radar (SAR) Time Series Data in the Mount Pulag National Park: A Key Biodiversity Area in Luzon, Philippines Bernard Peter O. Daipan* Department of Forest Biological Sciences College of Forestry, Benguet State University La Trinidad, Benguet 2601 Philippines Forest cover disturbances continue to occur at a very alarming rate, which greatly contributes to the loss of biodiversity resources. Therefore, an approach to detect and measure historical deforestation would be of utmost importance – particularly in the near real-time forest monitoring and management, biodiversity conservation, and estimation of carbon emissions. This study introduced a simple approach using synthetic aperture radar (SAR) time-series data, primarily the C-band Sentinel-1A (S1A) VH-polarization with a 10-m resolution, in detecting forest cover disturbances from 2016–2019 in Mount Pulag National Park (MPNP), a protected area (PA) within the island of Luzon and a priority biodiversity conservation area. The process involved using codes in Google Earth Engine (GEE) to produce a forest cover disturbance map; a grid-based geospatial and statistical analysis using the geographic information system; and data validation, and determination of possible causes of disturbances using Google Earth Pro (GEP). The results derived from Sentinel-1 data were also compared to existing optical data, which is a Landsat-based generated disturbance map. The output of this paper revealed that a total of 577 forest disturbances, with an equivalent area of 154.58 ha, were detected from a three-year period. For Landsat-generated data, 402 disturbances were detected with an area of 176.07 ha. However, based on the accuracy assessment using positive predictive value (PPV), the radar data produced higher accuracy (80%) compared to the optical data (73%). The visual observation and interpretation of satellite imageries made it clear that the primary drivers of forest cover disturbances in Mount Pulag are agricultural expansion (48%), forest clearings (26%), landslides (12%), slash-and-burn or “kaingin” (12%), and forest fires (2%). Keywords: forest disturbances, geospatial techniques, Google Earth Engine, Mount Pulag National Park, Sentinel-1A, time-series analysis INTRODUCTION gas emissions such as carbon (Whittle et al. 2012) and it is estimated that approximately 20% of the global In this study, the term forest cover disturbance was used to CO2 emissions emanated from these disturbances (van denote all forest changes as a result of both deforestation der Werf 2009). According to the State of the World’s and forest degradation (Hirschmugl et al. 2020). Forest Forests of 2020 (FAO and UNEP 2020), deforestation cover disturbance is the major source of greenhouse and forest degradation continue to occur at alarming *Corresponding Author: [email protected] rates, which threaten biodiversity resources. Agricultural 363 Philippine Journal of Science Daipan: Detection of Forest Cover Disturbances Vol. 150 No. S1, Special Issue on Biodiversity Using SAR in Mount Pulag expansion remains to be one of the leading drivers of a simple approach using highly accessible open-source this problem. Thus, a mechanism was developed by software with SAR time-series data to detect forest cover the United Nations Framework Convention on Climate disturbances in one of the PAs in the Philippines that Change (UNFCCC) for reducing emissions from needs immediate intervention in conserving its dwindling deforestation and forest degradation, plus the sustainable forest cover due to unlawful activities. It further identified forest management, and the enrichment of carbon stocks the causes of deforestation through visual observation (REDD+) by compensating developing countries to and interpretation of historical satellite imageries using protect forest resources (Mitchell et al. 2017). To qualify GEP. Satellite images from GEP are imperative tools in for the compensation, the REDD+ strategy requires all producing precise and detailed maps of various forest developing countries to establish a national measurement, types and are very useful in the identification, mapping, reporting, and verification (MRV) system to quantify and monitoring of these forest ecosystems (Garcia 2019). changes in forest cover (FCPC 2016). According to the United Nations Environment Programme (UNEP), the GEP helps to see and evaluate the natural Likewise, detecting and measuring forest cover resource and environmental changes in a way that makes disturbances and developing early warning systems are them coherent and meaningful, therefore allowing imperative (Kellndorfer 2019) to understanding the carbon development planners and policymakers to decide on cycle, near real-time forest monitoring, biodiversity taking constructive intervention on the causative factors. conservation and protection, and sustainable forest management because it provides reliable spatiotemporal data and information of deforestation activities (Finer et al. 2018). Therefore, an approach to detect and measure MATERIALS AND METHODS historical deforestation and track forest cover disturbances would be of utmost importance. Description of the Study Site Different approaches have been proposed and implemented The MPNP is the highest peak in Luzon, with 2,922 to detect changes in forest cover and the most common is masl, and is known as the third highest mountain in the through optical Landsat time-series data (Shimizu et al. Philippines next to Mount Apo and Mount Dulang-dulang 2019). Other optical data such as Moderate Resolution (Misachi 2019). It is one of the PAs in the region that Imaging Spectroradiometer (MODIS) has also been used encompasses portions of the provinces of Benguet and to detect forest disturbances due to forest fires (Olpenda Ifugao in the Cordillera Administrative Region (CAR), 2019); however, the use of optical remote sensing for and the Province of Nueva Vizcaya in Region 2 (Figure detecting forest cover disturbances can be challenging 1). The park has a total land area of 11,550 ha and it is in tropical regions because of the continuous cloud home to diverse endemic flora and fauna. Coniferous cover (Kellndorfer 2019). Conversely, radar remote forests are found in the lower elevations, mossy forests sensing such as the SAR has the ability to penetrate can be found in the higher elevations, while the summit is through clouds regardless of time and weather conditions normally covered with grassland and dwarf bamboos. It (Arellano et al. 2019). Moreover, the SAR signal can also is also recognized as a key biodiversity area, biodiversity penetrate through the forest canopy and gather important conservation priority area, center of plant diversity, and a information like forest structure and density, which are priority important bird area (Fernando and Cereno 2010). very useful in forest biomass estimation (Monzon et al. The study area was declared as a national park (NP) in 2015). The L-band SAR data are more suitable for forest 1987 through Presidential Proclamation number 75. change assessment because of its longer wavelength compared to the C-band data (Bouvet et al. 2018), but the availability of L-band data is very limited in Data many countries (Shimizu et al. 2019). Nonetheless, the The data used in the study are remotely-sensed imagery launched of Copernicus S1A and 1B satellites in 2014 collections of C-band SAR-S1A ground range detected – developed by the European Space Agency (ESA) – (GRD) scenes from GEE with an acquisition or instrument provides an opportunity in many tropical regions for free mode of the interferometric wide swath (IWS) or and accessible C-Band SAR data that can systematically routine collections for land and processed to backscatter monitor forest and detect disturbances for a cycle of 6–12 coefficient, or sigma naught, expressed in decibels (dB). d globally (Hirschmugl et al. 2020). The GRD scenes used for the time-series analysis in the study site were cross-polarization transmitter-receiver In the Philippines, the use of SAR data for forest VH (vertical-horizontal) dual bands with a 10-meter disturbance detection and mapping is relatively new resolution. The VH polarization bands were selected compared to optical data, perhaps due to difficulties in because of their applicability to change detection and data processing and analyzing. Thus, this study introduced high sensitivity to volume scattering caused by branches 364 Philippine Journal of Science Daipan: Detection of Forest Cover Disturbances Vol. 150 No. S1, Special Issue on Biodiversity Using SAR in Mount Pulag Figure 1. Location of the study area. and leaves in a forest canopy (NASA 2020). Compared 10 m x 10 m resolution was applied; the orbit properties to raw S1A data from the ESA website, the S1A image was descending; for the polarization, the VH band was collections from GEE were already pre-processed using selected; finally, the filtered instrument mode is IWS. To Sentinel-1 Toolbox for each scene (Arellano et al. 2019). remove the speckle noise, a smoothing radius of 50 was Furthermore, the data used were already terrain corrected performed using a speckle filtering code. The filtered using a digital elevation model, radiometric calibrated, VH bands for the multi-temporal datasets (2016 and and