Populus trichocarpa associated microbiomes vary across a natural temperature, elevation and rainfall gradient

Abiotic stresses like heat and drought pose major threats to forest ecosystems and are anticipated to increase due to anthropogenic climate change. One mechanism by which plants cope with stress conditions is tailoring their microbiomes to aid in stress tolerance. However, experimentally imposing abiotic stresses to examine impacts on tree microbiomes over long timescales is impractical. Naturally occurring climate gradients provide an opportunity to observe the longer-term effects of abiotic stress on soil and tree-associated microbiomes, potentially informing projections on how these relationships will respond to climate change. Populus trichocarpa is an ecologically important tree species, a major source of wood products, a potential bioenergy feedstock, and has emerged as a model species for both biological and ecological processes. To understand how belowground plant-microbe interactions changed across climatic gradients, we collected bulk soil, rhizosphere soil, and root endosphere samples from P. trichocarpa trees found along a 200 km transect over the Cascade Mountains in Washington, USA within the Cowlitz River Basin on the wetter, western side and along the Tieton River basin on the drier, eastern side. Precipitation rates and temperatures varied considerably over the ~1300-meter elevation span. We performed 16S and ITS2 amplicon sequencing to examine the bacterial/archaeal and fungal communities, as well as soil chemical and physical characteristics, stand characteristics, and host tree physiology. We hypothesized that the soil and P. trichocarpa-associated rhizosphere and root endosphere microbiomes would vary in response to the naturally occurring climate gradient along the transect. We found that for each niche sampled, bacterial/archaeal and fungal communities clustered into three distinct micro-climate groups: a wetter community common across the Cowlitz River sites, a drier community of the lower altitude Tieton River sites, and a high elevation community at the crest of the pass and along the upper Tieton River. The community differences between these micro-climate groups were diminished within the P. trichocarpa root endosphere, though still evident, indicating that while P. trichocarpa may have a selective and homogenizing effect on the microbiome, abiotic factors play a major role in the overall shaping of these microbial communities.