We conducted a study that evaluated the likelihood of capturing food web relationships with stable isotopes using two dominant producers, red alder and Douglas-fir, in the Oregon Coast Range. Foliage from 10 riparian forests dominated by either red alder or Douglas-fir was analyzed for δ¹⁵N and δ¹³C to determine the isotopic variability within a forest stand and across stands. We then conducted a sensitivity analysis using the IsoError model (Phillips and Gregg 2001) to determine whether δ¹⁵N and δ¹³C could successfully discriminate between red alder and Douglas-fir in a food web study. We found greater δ¹⁵N variation within and across sites for Douglas-fir (within stand: ≤2.58‰, average across stands: 1.72‰) compared to red alder (within stand: 0.59‰, average across stands: 0.27‰). δ¹³C variation was similar for both species within and among forest stands. On average, δ¹⁵N and δ¹³C of red alder and Douglas-fir differed by <2.0‰ in our forests. The sensitivity analysis found δ¹⁵N had limited use for predicting the dietary contribution of Douglas-fir and red alder to a consumer due to the high variability of δ¹⁵N in Douglas-fir and small differences in δ¹⁵N between the species. We had greater success with δ¹³C where overall variability was lower, and source differences were higher, than δ¹⁵N. Despite physiological differences that led us to believe red alder and Douglas-fir would have distinct δ¹⁵N and δ¹³C values, our results demonstrated that it would be difficult to discriminate between food webs based on these producers.