Assessment of tree cover density of Sri Lanka using visual interpretation of open-source high-resolution imagery and geographic information system interface mapping

Abstract

Trees are found in various formations, ranging from individual trees to randomly occurring tree clusters and systematically established tree plantations, as well as natural forests. Accurate information about trees, their distribution and density is crucial for the development of national policies, strategies, and management decisions related to tree planting, and environmental management. While some organizations and individuals have mapped forests, home gardens, and trees outside forests at different scales, the lack of comprehensive and systematic spatial distribution data on trees and tree cover density in Sri Lanka has been a significant challenge for policymakers. To address this issue, this study utilized the tree cover mapping (TCM) tool developed by U.S. Geological Survey. High-resolution images were visually interpreted within a geographic information system interface to map tree cover. The TCM tool employed a systematic sample grid, with a sampling interval of 200 m. The study encompassed 1.64 million sampling units, and mapping and interpretation were conducted at a scale of 1:3000. The resulting tree density map had a resolution of 200 m. Results show that 78% of the country’s land area has a tree cover density exceeding 10%, resembling open and sparse forests, while 64% of the country exhibits a tree cover density exceeding 40%, comparable to dense forest areas. The study found that although forest cover was limited in districts such as Colombo, Gampaha, and Jaffna, these areas still displayed a significant level of tree cover density, offering services and functions similar to those provided by forests.