Eco-physiological responses of <i>Hieracium pilosella</i> and <i>Trifolium pratense</i> to reduced air pressure

Climate change is a major factor shaping the distribution of plant species. A well-documented response consequence is the upward shift of plant species to higher elevations as they track their thermal niches. However, plants migrating upward face complex environmental changes shaped by multiple interacting factors. Among these, reduced air pressure remains relatively understudied, its effects are often confounded with other covarying parameters. This study investigated the direct impact of reduced air pressure on the eco-physiological responses of two plant species (Hieracium pilosella L. and Trifolium pratensis L.). The plants were grown for 4 weeks in controlled climatic chambers under different air pressures (85, 75, and 62 kPa), while all other environmental parameters were kept constant. At the end of the experiment, photosynthesis, chlorophyll fluorescence, growth, carbohydrate content, carbon stable isotopes, and plant nitrogen concentrations were determined. Reduced air pressure decreased growth, carbon isotopic discrimination and chlorophyll content, but increased CO2 fixation efficiency and carbohydrate accumulation in the leaves. These results suggest that reduced air pressure impacts plant performance during upslope migration and may, in turn, contribute to shaping future distribution patterns in alpine ecosystems.