The Potential Utility of Stable Isotopes for Food Web Analysis in Douglas-Fir and Red Alder Riparian Forests of Western Oregon

Abstract

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.