Transpiration of Acacia plantations in a managed tropical peatland Sumatra, Indonesia

Waterlogged and anoxic conditions facilitate the preservation of carbon-rich peat layers in tropical peatlands coexisting with peat swamp forests. Peatlands in Southeast Asia, which host one-third of the tropical peatland area, have high temperatures throughout the year and high soil moisture availability, which support high evapotranspiration rates. The majority of existing land cover in Southeast Asia peatland is a canopy-covered ecosystem. Therefore, these ecosystems are considered to support high transpiration rates. However, the understanding of transpiration rates and their governing factors for existing land cover in Southeast Asia peatlands remains poorly understood due to limited measurements.

Here, we quantified transpiration rates and explored governing factors in Acacia crassicarpa plantations (fast-growing species, harvested on a 4-5 year rotation) in the coastal peatland of Eastern Sumatra, Indonesia, between 2020 and 2022. Transpiration was quantified by measuring in situ sap flow rate using the HFD8-50 and SFM1 (ICT International, Australia) during the plantation age of 2 to 4 years. We measured the sapwood cross-sectional area using an increment borer (Haglöf, Sweden). In addition, we implemented a sampling strategy that considered tree size, azimuth, height, and radial factors, to account for the variability and upscaling from tree to stand level of transpiration.

Our results showed that the greatest source of variability to determine transpiration were the radial and tree size. On a diel and daily basis, tree transpiration was affected by vapour pressure deficit and solar radiation. Further, we did not observe a relationship between seasonal rainfall variations and transpiration. We found that stand-level transpiration in deeper groundwater level sites (around -80 cm below peat surface) was higher by 20% than those in shallower sites (around -40 cm below peat surface) due to the higher stand density and total sapwood area. Overall measured transpiration rates (0.8 – 1.0 mm d^-1) represent 20-24 % of evapotranspiration measured by eddy covariance. This study provides the first insights into the eco-hydrological characteristics of the Acacia crassicarpa plantation and improves the understanding of water balance from this globally important ecosystem.