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

Plants using crassulacean acid metabolism (CAM) for photosynthesis are particularly adapted to dry conditions, as they can focus on night-time carbon uptake and still exhibit considerable productivity. However, gas exchange measurements of CAM plants at the ecosystem level are scarce. Only a few studies to date report on the carbon dioxide (CO₂) exchange of CAM plants using the eddy covariance (EC) method. We monitored the ecosystem CO₂ exchange of an Agave sisalana plantation using the EC method in semi-arid Kenya. Measurements lasted 65 days and began during a wet period that gradually transitioned to a dry period. High productivity periods of A. sisalana occurred during the initial wet period with a mean CO₂ uptake of −1.1 µmol m⁻² s⁻¹ (dry period: +0.3 µmol m⁻² s⁻¹). High productivity was related to significant day- and nighttime carbon uptake, indicating direct CO₂ fixation via the C3 pathway during daytime. With decreasing soil moisture, mean daytime net CO₂ exchange became a notable carbon source (from +1.0 to +4.0 µmol m⁻² s⁻¹), suggesting a shift of A. sisalana towards strict CAM photosynthesis in response to soil drying. Our results demonstrate A. sisalana's high photosynthetic plasticity in relation to soil moisture dynamics and its significance for ecosystem-scale CO₂ fluxes.