You are here:
WATERSHED BIOGEOCHEMISTRY IN THE OREGON COAST RANGE: THE ROLE OF RED ALDER AND SALMON
Citation:
Compton, J E. AND M R. Church. WATERSHED BIOGEOCHEMISTRY IN THE OREGON COAST RANGE: THE ROLE OF RED ALDER AND SALMON. Presented at North American Forest Ecology Workshop, Corvallis, OR, June 19, 2003.
Description:
Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species influence is the role of N2-fixing red alder (Alnus rubra) in the biogeochemistry of Pacific Northwest forests. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder), particularly when near-coastal sites were excluded (r2 = 0.65 and 0.68, for nitrate-N and DON). Nitrate and DON concentrations were much more strongly related to broadleaf cover within the entire watershed than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands play an important role in watershed N export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4 to 30.8 kg N ha-1 yr-1), and described by a multiple linear regression combining broadleaf and mixed broadleaf-conifer cover (r2 = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds.
Anadromous fish also can serve as a significant source of nutrients and energy to the streams where they return and die, although these inputs are not traditionally considered in watershed nutrient budgets. To understand the relative importance of terrestrial and marine nutrients sources, we compared the watershed export of N, C and P with the delivery of these elements in returning chinook and coho salmon. Dissolved element export was 9.7 kg N ha-1 yr-1, 16 kg C ha-1 yr-1, and 0.19 kg P ha-1 yr-1. These data are compared with the flux of nutrients returning from the ocean as spawning salmon (0.03, 0.13 and <0.01% for N, C and P, respectively). The ratio of terrestrial to marine derived nutrients is very high, indicating that under present conditions, most of the C and N passing through the stream is derived from the watershed. Present-day salmon runs are approximately 1-5% of historic runs, however, indicating that marine-derived N, C and P were relatively more important before the decline of salmon runs. The forms and timing of nutrient supplies to streams, and their mode of entry into stream food webs, are important to consider in light of developing efforts for augmenting nutrient inputs to aid in salmon recovery.