Phenology and plant functional type dominance drive CO₂ exchange in seminatural grasslands in the Pyrenees

Abstract

Abstract Understanding the mechanisms underlying net ecosystem CO 2 exchange (NEE) in mountain grasslands is important to quantify their relevance in the global carbon budget. However, complex interactions between environmental variables and vegetation on NEE remain unclear; and there is a lack of empirical data, especially from the high elevations and the Mediterranean region. A chamber-based survey of CO 2 exchange measurements was carried out in two climatically contrasted grasslands (montane v . subalpine) of the Pyrenees; assessing the relative contribution of phenology and environmental variables on CO 2 exchange at the seasonal scale, and the influence of plant functional type dominance (grasses, forbs and legumes) on the NEE light response. Results show that phenology plays a crucial role as a CO 2 exchange driver, suggesting a differential behaviour of the vegetation community depending on the environment. The subalpine grassland had a more delayed phenology compared to the montane, being more temperature than water constrained. However, temperature increased net CO 2 uptake at a higher rate in the subalpine than in the montane grassland. During the peak biomass, productivity (+74%) and net CO 2 uptake (NEE +48%) were higher in the subalpine grassland than in the montane grassland. The delayed phenology at the subalpine grassland reduced vegetation's sensitivity to summer dryness, and CO 2 exchange fluxes were less constrained by low soil water content. The NEE light response suggested that legume dominated plots had higher net CO 2 uptake per unit of biomass than grasses. Detailed information on phenology and vegetation composition is essential to understand elevation and climatic differences in CO 2 exchange.