No growth stimulation by CO2 enrichment in alpine glacier forefield plants

Since 1850, glaciers in the European Alps have lost around 40% of their original area, releasing bare forefields, which are colonized by alpine pioneer species, setting the scene for later successional stages. These expanding pioneer communities are likely less restricted by resources and competition than late‐successional systems. We thus hypothesized that rising atmospheric CO₂ concentration will enhance plant growth in these high‐elevation communities. Nine characteristic, perennial glacier forefield species were assembled in microcosms and grown at a nearby experimental site in the Swiss Alps (2440 m a.s.l.). The communities were exposed to an elevated CO₂ concentration of 580 ppm by free‐air CO₂ enrichment for three seasons. Four study species were additionally grown in isolation in containers, half of which received a low dose of mineral fertilizer (25 kg N ha^‐1 a^‐1) in order to explore a potential nutrient limitation of the CO₂ response. Responses of growth dynamics and peak season biomass of the two graminoid species, four forbs and three cushion forming species were analysed by repeated nondestructive assessments and a final biomass harvest. After three seasons, none of the species were stimulated by elevated CO₂, irrespective of mineral nutrient addition, which by itself enhanced growth in the fertilized plants by +34% on average. Increased CO₂ concentration did not affect total (above‐ plus belowground) biomass but reduced aboveground biomass by ?35% across all species, even in the fast growing ones. This reduced aboveground biomass was associated with higher biomass partitioning to roots. Foliar nonstructural carbohydrate concentration increased and nitrogen concentration in leaves decreased under elevated CO₂. We observed downward adjustment of photosynthetic capacity by on average ?26% under long‐term exposure to 580 ppm CO₂ (assessed in graminoids only). Our results indicate that glacier forefield pioneers, growing under harsh climatic conditions are not carbon limited at current atmospheric CO₂ concentration.