| 18 Jun 2025
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.
General comments
The manuscript is of very high quality. I have some minor comments and technical correction that, in my view, would benefit the text.
Specific comments
First paragraph of section 4.2: This entire paragraph discusses the (lack of) effect of planted tree diversity on EF. I think this discussion is somewhat not relevant and maybe misleading, since the observed tree diversity (the actual tree diversity of the environments in the time of measuring) had a positive effect on EF. The lack of effect of planted tree diversity is just and artifact of the lack of correlation between planted diversity and current diversity (due to mortality and other ecological mechanism not addressed in the manuscript). For example the second sentence: “This contrasts with widespread observations of increasing ecosystem functions with greater tree diversity, as reported for aboveground productivity (Zheng et al., 2024).”, is problematic because the authors are talking about planted tree diversity and not actual observed diversity and, additionally, the authors did found indirect effect of tree diversity on EF via structural complexity. My suggestion is to re-write this paragraph focusing on why planted tree diversity did not affected observed tree diversity and consequently EF.
The discussion presented in lines 434-437 contradicts the discussion presented in the first paragraph of section 4.2 (commentary above).
Technical corrections
Standardize the supplementary material naming. You call it supplementary material in the main text, and Appendix at the end. Same for supplementary figure naming (ie.: S1 vs. A1)
Figure S1/A1 is not a “conceptual structural equation model, relationships and assumed mechanisms”, ratter a scatter plot ET/EF vs. Shortwave radiation. The authors should include the graphical representation of the conceptual structural equation model, and scatter plots of individual relationships (ie.: diversity vs. ET).
Table S1 is not shown in the appendix.
Figure 3: the path from “Island Area” to “Structural Complexity” can be confused with a path from “Observed woody plant diversity” to “Structural Complexity”. Maybe change the figure a bit to make this path more clear.
Line 395. Supplementary material S3 not shown.