Fungal and bacterial communities across meadow-forest ecotones in the western Cascades of Oregon

Meadows are natural dynamic features of forested mountain landscapes of the Pacific Northwest. Proportions of meadows and forests change with environmental conditions and disturbance history. We investigated the belowground microbial communities associated with these two vegetation types and how they change across the meadow-forest transition at two sites in Oregon. Soils were sampled along replicate transects extending from meadow into forest. We quantified total bacterial and fungal biomass using direct microscopy and described the composition of bacterial and fungal communities using a DNA-based fingerprinting technique. Bacterial biomass was similar in meadow and forest soils, but fungal biomass was significantly higher in forest soil. Meadow and forest soils had distinct communities of bacteria and fungi. Bacterial communities near the meadow-forest boundary reflected current vegetation, but fungal communities under meadow vegetation near the forest edge were intermediate in composition between those found in meadow and forest soils. The more gradual transition observed with fungal communities may reflect the influence of tree roots and their associated ectomycorrhizal fungi or possibly colonization by saprotrophic fungi associated with tree litter accumulating near the forest edge. Invasion of forest-associated fungi into the meadow soils may presage subsequent expansion of forest vegetation into meadows.