Trade-Offs Between Carbon and Water Fluxes Along a Land Use Intensity Gradient in Southeast Asian Forests and Plantations.

Land use intensity (LUI) significantly influences the biophysical and biogeochemical properties of the global landscape. The impact of LUI is exceptionally strong in Southeast Asia (SEA), where forests are increasingly being replaced by intensively managed plantations. Despite these transformations, comprehensive studies on how different LUI regulate carbon, energy, and water fluxes in this region remain scarce. In this study, we examine data from 16 eddy covariance (EC) flux tower sites in SEA, representing a total of 112 years of measurements. We aim to assess trade-offs in carbon fluxes, light use efficiency (LUE), and water use efficiency (WUE) across a gradient of LUI: low (primary forests), medium (secondary forests), and high (plantations). Our findings reveal that mature high LUI sites on mineral soil act as carbon sinks; however, their high evapotranspiration rates often exceed site-specific precipitation, making them susceptible to water stress. For example, mean annual carbon uptake at high LUI sites (ranged from -1.19 to -0.74 kg C m-2 year-1) outperformed low LUI sites (-0.85 to -0.02 kg C m-2 year-1). The strong carbon uptake on high LUI had an exception when the ecosystem was in the establishment phase and managed on peat soil (0.98 kg C m-2 year-1). However, WUE was greater at low LUI (mean annual: 2.63 to 6.50 g C kg-1 H2O-1) compared to high LUI sites (2.08 to 3.53 g C kg-1 H2O-1), illustrating a trade-off between carbon uptake and water use. Additionally, while high LUI sites required less radiation to achieve maximum gross primary productivity, their mean daily LUE was not higher compared to low LUI sites. These findings underscore the importance of carefully balancing carbon sequestration goals with water resource considerations and drought resilience when promoting plantation systems. Conversely, forest conservation offers advantages for water security and ecosystem resilience to face climate change.