Mapping Multi-decadal Mangrove Forest Change in the Philippines: Vegetation Extent and Impacts of Anthropogenic and Climate-Related Factors

Abstract

There is a continuous decline of mangrove forests in the Philippines due to anthropogenic activities and natural disturbances. Through the years, monitoring of mangrove extent was done as part of the many local efforts to manage mangrove forests. However, existing mangrove cover estimates were generated by varied sources with different methodologies and classification techniques. This study aims to utilize a standardized method to detect multi-decadal spatio-temporal mangrove extent with the use of Landsat-derived Mangrove Vegetation Index (MVI) calculated in Google Earth Engine (GEE). Mangroves were mapped per region and areal changes were calculated from the period 2000 to 2020. A decline of 29,000 hectares was recorded from 2000 to 2020, observed in 12 out of the 17 Philippine administrative regions. The variations in rainfall, typhoons, sea surface height, temperature, and occurrence of land use/cover conversion were seen to have impacted the structure and extent of mangrove communities. Results show that higher multi-decadal mangrove losses were observed in regions with higher precipitation change (r = 0.68 to 0.99), higher maximum sea surface temperature (r = 0.39), more frequent typhoons (r = 0.43), and those exposed to extreme heating (r = 0.47) and precipitation events (r = 0.51). Areas that were least devastated by typhoons (fewer than 10 typhoons) have recorded increased vegetation cover such as in Northern Mindanao (Region 10), Autonomous Region in Muslim Mindanao (ARMM), and Zamboanga Peninsula (Region 9). Reduction in precipitation can increase salinity which lowers seedling survival, growth rates, and productivity; while the impact of typhoons is mainly attributed to physical damages brought by strong waves and wind. Some of the previous mangrove areas were found to be converted to fishponds and built-up areas brought by the increasing demands for the economical use and development of land. Understanding the impacts of these natural and human-induced drivers will aid in formulating effective conservation and resource management measures. Further, the proposed standardized mapping workflow allowed detailed analysis of these impacts as reflected by the spatio-temporal changes in mangrove extent.