Role of riparian red alder in the nutrient dynamics of coastal streams of the Olympic Peninsula, Washington, USA

One result of clear-cut logging in the Pacific Northwest is that many watersheds; are now dominated by riparian stands of red alder Alnus rubra (Bong). This species colonizes disturbed areas quickly and can limit the establishment of coniferous forest species. In the Northwest, inputs of nutrients from decaying salmon carcasses have been reduced with declining salmon runs, and nitrogen-rich red alder litter may provide a critical source of nutrients to streams. We hypothesized that high-nutrient inputs from red alder forests would translate into more productive and nutrient-rich stream ecosystems, compared with streams bordered by coniferous species. Leaf litter inputs and chemistry, surface water chemistry, and seston and periphyton nutrient dynamics were measured in six streams in the Hoh River Watershed on the western Olympic Peninsula, Washington, during 1999-2000; three streams were dominated by riparian red alder and three in old-growth coniferous forest. Litter inputs to a red alder-dominated stream were three times greater than litter into an old-growth stream. Although total carbon concentration was similar, nitrogen concentration of red alder litter was approximately three times greater than coniferous litter. Alder litter concentrations of other limiting elements, such as Ca, Cu, Mg, K, P, and Zn, were also significantly higher than conifer needles. Phosphorus and Mg concentrations of suspended particulate matter were significantly higher in streams dominated by red alder. Periphyton biomass was significantly higher in streams dominated by alder and had increased levels of magnesium. These data suggest that red alder forests may provide important subsidies of limiting elements that fuel food webs in Pacific Northwest streams. This might be especially important in stressed systems, such as those that have experienced drastic resource removal through forest harvesting or reduced salmon runs.