Temperature and soil microbial community interact to alter performance across populations of a native plant

Abiotic and biotic factors interact to shape the expression of traits and fitness. In contemporary environments, species must respond not only to novel climates but also to altered biotic interactions, and these pressures likely differ across the landscape, which affects population persistence and range limits. Here, we tested the interactive effects of soil microbial communities and temperature variation on plant traits and fitness. We predict that plants are locally adapted to their home soil communities and temperatures, and that local soil microbes could mitigate stresses associated with exposure to elevated temperatures. We conducted a growth chamber experiment using a perennial plant native to the southeastern U.S.A., Elephantopus tomentosus (Asteraceae, L.), in which we exposed accessions from populations in the center of the range and the northern range edge to a fully factorial combination of three soil communities and two temperature levels, reflecting the current average temperature in the central portion of the range vs. potential future climates (+ 4°C). To test whether plants are adapted to their local soil microbial community, we inoculated pots with field soils collected from the center of the range, the range-edge, or with a sterilized control. Plants experienced a greater reduction in reproductive fitness under elevated temperatures when exposed to sterile soil than when grown in live soil. Additionally, plants in central soils had greater fecundity at elevated temperatures than those in northern range-edge soils, which is congruent with expectations and with the finding that seeds sourced from northern populations exhibited a greater reduction in fecundity under elevated temperatures than those sourced from the warmer central populations. Central soil communities increased the growth rate of both accessions, likely due to central-range specific microbes. Our study revealed that plant fitness varied with both temperature and soil microbes, highlighting the importance of accounting for biotic interactions in studies of the biological evolution to climate change.