CO₂- or water-limited? Plant trait and physiological responses to reduced atmospheric pressure

Climate induced upslope migration of alpine plants is accompanied by a decline in atmospheric pressure, raising the question of whether plant performance at higher elevations is constrained by carbon availability and/or by water-related stress. Decreasing air pressure alters gas diffusivity, evaporative demand, and the partial pressure of CO₂, potentially shifting limitation from carbon acquisition towards plant water relations. However, in the field the impact of reduced air pressure is hardly to detect because it covaries with temperature, humidity, and radiation along natural elevational gradients.We thus addressed this question using ecotron experiments that isolate air-pressure effects from co-varying climatic factors. Mountain grassland species and model plants with contrasting functional strategies (Arabidopsis thaliana, Trifolium pratense, Hieracium pilosella, Brachypodium rupestre) were grown under atmospheric pressures corresponding to ~1,500–4,000 m a.s.l. (85–62 kPa), while temperature, radiation and air humidity were kept equal among the treatments. We quantified traits related to gas exchange (stomatal conductance, carbon isotope discrimination), carbon acquisition and allocation (photosynthetic efficiency, carbohydrate storage, biomass production), and nutrient status (leaf nitrogen and chlorophyll).Across species, reduced air pressure consistently lead towards higher photosynthetic energy-use efficiency and increased leaf carbohydrate pools (up to +40%), while aboveground biomass decreased. Gas-exchange revealed species-specific strategies: stomatal conductance increased or remained stable under low pressure in forb species, whereas grass responses depended on interactions with water availability. After 4 weeks results indicated a decreased carbon assimilation efficiency under low air pressure and a higher vulnerability to drought because of the higher Vapour-pressure deficit (VPD)These short- term results suggest that reduced air pressure is a relevant parameter for upwards-migrating mountain plants and may play an underestimated role in shaping composition and performance of alpine ecosystems.