Stronger Sensitivity of Plant Photosynthesis to Rising CO2 in High Elevation Ecosystems.

The CO2-fertilisation effect (CFE) on vegetation productivity is the major driver of the enhanced land carbon sink in recent decades. CFE theoretically increases with elevation due to the higher sensitivity of carboxylation to an increase of CO2 under lower CO2 partial pressure, but the elevation-dependent CFE pattern has been largely overlooked. By conducting a 6-year CO2 enrichment experiment (+100 ppm) in an alpine grassland, we show that elevated CO2 increased gross primary production (GPP) by 25.5% ± 4.6%. Water availability and plant biomass allocation modulates CFE during different seasons. A global synthesis of 10 CO2 enrichment experiments reveals that CFE increased with elevation. The satellite-based EC-LUE model also demonstrates a positive global elevation-dependent CFE pattern, albeit substantially weaker than that from experimental observations. Current terrestrial biosphere models, however, could not represent the elevation-dependent pattern, highlighting the need to improve the representations of plants' elevational physiological adaptation to rising CO2 in models.