A Geospatial and Socio-economic Assessment of Tree Density in Home Gardens as Trees Outside Forests Sources in Agro-ecological Regions: Upper Mahaweli Catchment, Sri Lanka

Abstract

Homegardens (HGs) serve as crucial 'Trees Outside Forest Sources' (TOFS) systems, offsetting tree cover loss resulting from extensive deforestation in the Upper Mahaweli Catchment (UMC) over the past two centuries. This study examined the temporal and spatial distribution of HGs in relation to agro-ecological regions (AERs) within the UMC boundary, assessed the tree density of HGs, and analyzed the biophysical and socio-economic factors influencing tree density. Geospatial assessment utilized Landsat Images and tree density estimation was based on a random sample of 500 HGs in the UMC with a minimum diameter at breast height (dbh) of >10cm. Regression analysis was employed to identify factors affecting tree density. The spatial assessment revealed that in 2017, the AER of IM3a covered the highest percentage area (66.5%) whereas IM1a recorded the greatest incremental percentage change (22.8%) from 1992-2017, while the lowest change was observed in IL2 (0.03%). The highest tree density was observed in IU2 (858 trees/ha), and the average density in WM2a (505 trees/ha), comparable to that of South-Asian tropical rainforests. However, there is a concerning trend of rapid spread of exotic species contributing to the high tree density observed in IU2, WM2a, and IU3d. The multiple linear regression model (p<0.01, R2 = 58.62%) indicated spatial, agronomic and institutional factors positively influenced tree density. Conversely, the HG area square, and demographic factors had a significant negative impact on tree diversity (p<0.01). To ensure the ecological sustainability of the UMC, the study proposes to introduce an appropriate incentive package to enhance tree density in HGs, with native species.