Photosynthetic Gas Exchange and Carbon Assimilation of Oil Palm (Elaeis guineensis Jacq.) and Understory Vegetation on Drained Tropical Peatlands

This study evaluated the photosynthetic gas exchange of mature oil palm and understory vegetation in a drained tropical peatland plantation located in Riau, Indonesia. Measurements were taken six times a day on the 17th frond of 16-year-old palms and two dominant understory species (Asystasia gangetica and Nephrolepis biserrata) over six periods from February 2020 to March 2022. The photosynthetic rate of oil palm ranged from 18.44 to 43.47 µmol CO2/m2/s, and its stomatal conductance ranged from 0.050 to 0.067 mol H2O/m2/s. The transpiration rate ranged from 577.83 to 1,189.31 µmol H2O/m2/s. Stomatal conductance was strongly linked to transpiration (r = 0.775, p < 0.001). Understory species also showed high photosynthetic rates, with N. biserrata at 25.31–31.49 µmol CO2/m2/s and A. gangetica at 23.53–36.37 µmol CO2/m2/s. Diurnal patterns showed that physiological activity generally increased from morning to noon. These results demonstrate variation in leaf physiology and photosynthetic carbon assimilation in both canopy and understory plants on drained peatlands.