Tree island area in oil palm agroforests directly and indirectly drives evaporative fraction

Abstract. Evapotranspiration (ET) – the combined water flux from soil and vegetation to the atmosphere – is a key component of water cycling and climate regulation, and strongly affected by land-use changes. The evaporative fraction (EF), representing the proportion of available energy allocated to ET, is often preferred over ET as a target variable in studies involving repeated measurements under varying weather conditions. In Sumatra’s lowlands in Indonesia, (evapo)transpiration of dominant land-use types including oil palm monocultures is well studied; however, there is a lack of studies assessing ET (or EF) across diverse mosaic landscapes and types of land-use such as oil palm agroforests. Across 52 experimental plots – forest restoration patches known as “tree islands” – in an oil palm landscape (EFForTS-BEE), we tested whether the experimental treatments ‘planted tree diversity’ and ‘tree island area’ influence ET and EF as derived from UAV (uncrewed aerial vehicle)-based thermography and subsequent energy balance modeling. A random partition linear model showed that planted tree diversity (1, 2, 3, or 6 species) did not affect plot-level ET or EF, whereas tree island area (25, 100, 400, or 1600 m²) had a positive effect, with EF increasing by 17 % from the smallest to the largest tree islands. A structural equation model revealed that the effect of tree island area on EF was mediated by both direct and indirect pathways. Specifically, a strong direct effect of island area on EF (Std.Beta = 0.44, p < 0.001) was complemented by an indirect pathway through increased observed woody plant diversity and stand structural complexity. Stand structural complexity had a positive effect on EF (Std.Beta = 0.20, p < 0.05), while neither the vegetation index GNDVI nor tree height variability had significant effects. The observed tree-island-area effect can be explained by a decrease of EF along an edge gradient detected inside the larger tree islands. Our findings suggest that larger tree islands enhance ET and EF through structural and biodiversity-related mechanisms. This underscores the importance of tree islands in human-modified landscapes, not only as biodiversity refugia but also as functional elements that support climate regulation.