Ectomycorrhizal (EM) fungi are a prominent and ubiquitous feature of forest soils, forming symbioses with most tree species, yet little is known about the magnitude of their impact on forest carbon cycles. A subset of EM fungi form dense, perennial aggregations of hyphae, which have elevated respiration rates compared with neighboring non-mat soils. These mats are a foci of EM activity and thereby a natural laboratory for examining how EM fungi impact forest soils. In order to constrain the contributions of EM fungi to forest soil respiration, we quantified the proportion of respiration derived from EM mat soils in an old-growth Douglas-fir stand in western Oregon. One dominant genus of mat-forming fungi, Piloderma, covered 46% of the soil surface area. Piloderma mats were monitored for respiration rates over 15 months and found to have on average 17% higher respiration than non-mat soil. At the stand level, this amounts to roughly 8% of soil respiration due to the presence of Piloderma mats. We calculate that these mats may constitute as much as a third of autotrophic respiration, based on respiration rates from trenched plots in a neighboring forest stand.