COS 245-3 - Mapping the Spread of Coconut Palms (Cocos nucifera) across 270 Pacific Atolls

Abstract: Remote Pacific atolls support unique forests, productive coral reefs, and rapidly growing populations exceeding half a million people. But despite the popular perception of atolls as pristine, untouched islands, many have been extensively modified by the establishment of coconut (Cocos nucifera) plantations. Coconut palms are economically essential to many island communities, but use large amounts of groundwater and lack nesting habitat for the seabirds that deliver nutrients to atoll soils. As many atolls’ native ecosystems and freshwater resources are already facing immense pressure from invasive species and climate change, a regional assessment of the impacts of coconut proliferation across small islands is needed to prioritize areas for conservation and management. However, such an assessment is impossible without up-to-date vegetation maps, and Pacific atolls comprise one of the last major terrestrial ecosystems that has not been mapped with remote sensing. To fill this knowledge gap, we produced vegetation maps of 250 Pacific atolls and coral islands using a first-of-its-kind high spatial resolution satellite imagery collection (2,418 sq. km) and a random forest algorithm with geometric lighting corrections.

We find that coconut palms now cover about 30% of the land area on Pacific atolls and roughly half of the forested area, and that coconut coverage correlates with an island's average rainfall and the presence of coconut cultivation. Historical accounts indicate the current extent of coconut palm monocrops vastly exceeds prehuman distributions, suggesting the present domination of atoll vegetation by coconut palms is a human-driven land cover change affecting an immense proportion of an ecosystem already imperiled by invasive species and climate change. Our classification algorithm achieved up to 97% accuracy for broad vegetation classes (palms, broadleaf trees, shrubs, non-vegetated land) by leveraging textural information from imagery; the overall accuracy of the up-scaled algorithm applied to all islands was ~85%. These maps will be made publicly available to help island managers and communities estimate the potential for vegetation-based freshwater and ecosystem conservation in the remote Pacific. Ongoing historical and economic analysis will help identify and prioritize areas where coconut plantations are no longer harvested and could be managed to restore hydrological and ecological functions.