Does red alder (Alnus rubra) in upland riparian forests elevate macroinvertebrate and detritus export from headwater streams to downstream habitats in southeastern Alaska?

Jack J. Piccolo and Mark S. Wipfli
Can. J. Fish. Aquat. Sci./J. Can. Sci. Halieut. Aquat. 59(3): 503-513 (2002)

Abstract: We assessed the influence of riparian forest canopy type on macroinvertebrate and detritus export from headwater streams to downstream habitats in the Tongass National Forest, southeastern Alaska. Twenty-four fishless headwater streams were sampled monthly, from April to August 1998, across four riparian canopy types: old growth, clearcut, young-growth alder, and young-growth conifer. Young-growth alder sites exported significantly greater count (mean=9.4 individuals·m^–3 water, standard error (SE)=3.7) and biomass (mean=3.1 mg dry mass·m^–3 water, SE=1.2) densities of macroinvertebrates than did young-growth conifer sites (mean=2.7 individuals·m^–3 water, SE=0.4, and mean=1.0 mg dry mass·m^–3 water, SE=0.2), enough prey to support up to four times more fish biomass if downstream habitat is suitable. We detected no significant differences in macroinvertebrate export between other canopy types or in detritus export among different canopy types. Roughly 70% of the invertebrates were aquatic; the rest were terrestrial or could not be identified. Although we do not recommend clearcutting as a means of generating red alder, maintaining an alder component in previously harvested stands may offset other potentially negative effects of timber harvest (such as sedimentation and loss of coarse woody debris) on downstream, salmonid-bearing food webs.