Ecosystem-scale crassulacean acid metabolism (CAM) gas exchange of a sisal (Agave sisalana) plantation

Plants using crassulacean acid metabolism (CAM) for photosynthesis are adapted to dry conditions by taking up carbon at night. Gas exchange measurements of CAM plants at the ecosystem level are rare, with only a few studies to date reporting CO₂ exchange using the eddy covariance (EC) method. We monitored the ecosystem CO₂ exchange of Agave sisalana using the EC method in an agricultural field in semi-arid Kenya over 65 days, starting in a wet period that gradually transitioned to a dry period. High productivity occurred during the wet period, with a mean net CO₂ uptake of −1.1 µmol m⁻² s⁻¹ (dry period: +0.3 µmol m⁻² s⁻¹). This was linked to significant day- and nighttime CO₂ uptake, indicating direct CO₂ fixation via the C3 photosynthetic pathway during daytime. As soil moisture decreased, mean daytime net CO₂ exchange increased considerably, from +1.0 to +4.0 µmol m⁻² s⁻¹, suggesting a shift towards strict CAM photosynthesis in response to soil drying. Our results demonstrate the high photosynthetic plasticity of A. sisalana with changing soil moisture and its significance for ecosystem-scale CO₂ fluxes.