The distinct rhizomorphic mats formed by ectomycorrhizal Piloderma fungi are significant features of the organic soil horizons of coniferous forests throughout the Pacific Northwest. Through their activities, EM mats can create a unique soil environment and likely result in the establishment of distinct microbial communities and activities. As part of an NSF-funded Microbial Observatory at the H.J. Andrews Experimental Forest in western Oregon, we are determining the microbial community structure and function of EM mats in a Douglas-fir-dominated forest ecosystem. Previous surveys have shown Piloderma to be a dominant mat-forming EM genus with an average chitinase activity ~1.5 times greater than non-mat soils. These facts indicate that Piloderma mats likely plays an important ecological role in this nitrogen-limited forest ecosystem. This poster will present the results of a year-long temporal study examining the seasonal dynamics of microbial community composition and chitinase activity in Piloderma mat and corresponding non-mat soils. The results of this study aim to elucidate the response of the EM microbial communities to the phenological allocation of carbon.