Modeling the Effects of Anadromous Fish Nitrogen on Riparian Forest Balance

Wild anadromous fish such as Pacific Chinook salmon (Oncorynchus tshawytscha) and steelhead (Oncorhyncus mykiss) were once abundant in Idaho, where they deposited their carcasses, rich in marine-derived nutrients (MDN).  This study investigates the long-term cumulative effect of presence versus absence of anadromous fish nitrogen on the carbon balance of riparian forests along historically salmon-bearing streams in the North Fork Boise River watershed, Idaho.  The ecosystem process model BIOME-BGC is used to develop a representative forest ecosystem and predict the impact of decades of addition and continuing absence of MDN on net ecosystem exchange (NEE).  The model is parameterized using local ecophysiology and site data and validated using field measurements of leaf area and soil moisture.  Multi-decadal model runs with and without nitrogen deposition at levels consistent with estimates of historical deposition are run to determine the effects of these nutrients on the forest's carbon balance.  Results indicate that MDN may increase NEE by up to eight times relative to no nutrient addition, whereas the continued loss of marine nitrogen may lead to decreasing NEE relative to historical conditions.  Understanding the long-term impacts of MDN to inland watersheds will help inform forest management and nutrient-loss mitigation efforts.