Red alder enriches riparian food webs of the Pacific Northwest

Although the role of mature conifers and large wood in riparian ecosystems is well documented, few studies have measured the ecological effects and interactions of conifers and hardwoods on food resources for riparian consumers.  In this study we synthesize the results of a multi-phased study to compare trophic interactions between conifer- and alder-dominated riparian food webs.  Strong differences between canopy types at lower trophic levels were observed with higher levels of nitrogen and litter entering streams at alder-dominated riparian habitats as well as higher invertebrate prey biomass.  Alder foliage in spring and summer enhanced insect production consumed by terrestrial and aquatic vertebrates while pulses of red alder leaves in fall subsidized the production of stream invertebrates into winter when food resources are scarce.  These patterns of increased availability roughly coincide with the arrival of songbirds and bats, and the growth of young salmonids.  However, at higher trophic levels, strong density responses to invertebrate prey availability did not consistently translate into differences in diet or growth.  Our results demonstrate enhanced linkages between aquatic and terrestrial food webs in deciduous canopies that should be taken into account in management decisions, particularly at broad spatial scales relevant to resource managers.