Escape from harmful soil biota at high elevations: Plant-soil feedbacks along stress gradients.

Plant-soil feedbacks (PSFs) have strong effects on plant community diversity and composition, but we know less about how the abiotic environment alters PSFs. In the context of the stress-gradient hypothesis (SGH), we predicted less negative PSFs in alpine versus prairie systems and tested this with three alpine and four prairie species and soil from three elevational gradients in western Montana, USA. All species were smaller when grown in prairie soil than in alpine soil, and sterilization of soil increased the growth only of alpine species in prairie soil. Training prairie soil with conspecifics decreased the biomass of both alpine and prairie species, but training alpine soil had no effect. PSFs were three times more negative when plants were grown in low-elevation prairie soils than in alpine soils. Consistent with the original ideas for the SGH, the frequency of positive PSFs was higher in alpine soil than in prairie soil, and the frequency of negative PSFs was higher in prairie soil than in alpine soil. Our results suggest that alpine species may escape more intense negative PSFs at lower elevations and that PSFs will not limit the upward movement of plant species as temperatures warm.