COS 284-4 - Soil microbial communities differentially influence Pseudotsuga menziesii seedling physiology across the elevation gradient of the Western Oregon Cascades.

Abstract: The Cascade Mountain Range provides terrain to support an extensive elevation gradient in the Pacific Northwest. These mountains facilitate ecosystem diversity - low elevation forests are mild and humid whereas higher elevation forests are defined by aridity and short growing seasons. Douglas-fir (Pseudotsuga menziesii) is the most abundant conifer tree in Pacific Northwestern forests and ranges from the coastal forests to 1500 meters up the Western Cascade elevation gradient. In a previous study we found that soil microbial community species pools associated with P. menziesii vary across the Cascade elevation gradient. However, the extent to which plant-microbial interactions may also vary in these different climates is not well understood. To address this knowledge gap we set up a reciprocal inoculation greenhouse experiment in which P. menziesii seedlings sourced from 6 sites across the elevation gradient were exposed to microbiota sourced from either the highest or the lowest site. We then recorded plant physiological measurements including growth, germination speed, stem diameter, bud burst timing, and mycorrhizal colonization rates. We found that the microbial communities sourced from lower elevations inhibit seedling fitness, whereas microbial communities sourced from higher elevations do not. These results support the broader ecological theory that biotic factors limit plant growth in moist, mild environments whereas abiotic factors limit plant growth in arid, harsh environments.