Parsing the roles of abiotic and biotic factors in Douglas-fir seedling growth

SummaryMany studies have confirmed the potential importance of mycorrhizas to plant growth and community structure. Needed now are studies that focus on the relative ecological importance of mycorrhizas, specifically how the effect of mycorrhizas compares with other environmental factors present in soil that may affect plant growth. In a greenhouse, we grew Douglas-fir seedlings in soil collected from 6 sites in a dry region in British Columbia, Canada, predicting that these soils would vary in the ectomycorrhizal fungi present, and in several physical characteristics. In addition, we imposed a gradient of soil moisture. Using model selection based on information theory and multimodel inference, we ranked the relative importance of several abiotic (watering level, pH, C:N) and biotic (ectomycorrhizal richness and % colonization by ectomycorrhizal fungi) variables in models predicting seedling biomass. Variation in abiotic factors, namely watering level and pH, tempered by the effects of ectomycorrhizal richness, contributed most to variation in seedling growth. For young Douglas-fir seedlings growing in disturbed environments, small shifts in the species richness of ectomycorrhizal fungi bear importance, yet overall remain subordinate to abiotic gradients. By utilizing natural variability in soil abiotic properties and mycorrhizal community composition along with experimental manipulations of additional factors, the experimental approach utilized here helps to move us beyond simply testing for the significance of mycorrhizas for plants to understanding the relative importance of mycorrhizas in comparison with other influential ecological factors.