Modeling the effects of anadromous fish nitrogen on the carbon balance of riparian forests in central Idaho, USA

Background/Question/Methods

Wild anadromous fish such as Pacific Chinook salmon (Oncorynchus tshawytscha) and steelhead (Oncorhyncus mykiss) were once abundant in Idaho, where their carcasses, rich in marine-derived nutrients (MDN) were deposited onto nutrient poor riparian soils.  This study investigates the long-term cumulative effect of presence versus absence of MDN on the carbon balance of riparian forests along historically salmon-bearing streams in the North Fork Boise River watershed, Idaho, USA.  The ecosystem process model BIOME-BGC is used to simulate the impact of decades of addition and continuing absence of MDN on riparian forest net primary productivity (NPP) and net ecosystem exchange (NEE).  The model is parameterized using locally measured ecophysiological data for the tree species, 17 years of local climate data, and site information. Model evaluation is done using field measurements of leaf area index (LAI) and soil moisture.

Results/Conclusions

The parameterized model correctly predicted LAI and soil moisture across the six sites that we simulated.  Factors limiting the NPP and the NEE at our sites varied from year to year.  When soil water availability or temperature was limiting, the additional MDN did not result in increasing productivity.  However, in years with a favorable climate, NPP increased by as much as 20% relative to the non-salmon scenario. This increase in NPP resulted in a doubling of the NEE; indicating that riparian forests are a much stronger carbon sink when MDN are present relative to a continued lack of MDN deposition. When scaled up to the almost 20,000km of salmon bearing streams within the Columbia Basin, these results indicate that salmon had a large impact on the regional vegetative carbon balance.