Comparing the Resiliency of Organic Carbon Storage in Two Mountain River Basins to Extreme Disturbance

Abstract:

Mountains potentially store disproportionately large amounts of organic carbon (OC) along their river networks. Mountain river networks act as transient storage sites for OC in sediment and downed wood, resulting in significant biogeochemical processing of that OC, as well as potential storage on long (10²-10⁴ year) timescales. Here, we seek to understand the potential resiliency of OC storage in mountain river networks to an increase in extreme climatic events that cause significant disturbance of sediment and wood within a river basin. Disturbance events mobilize stored OC and commonly result in the net transfer of OC from the terrestrial environment to the atmosphere. As disturbance events may increase in the future, this process has the potential to release large quantities of stored OC to the atmosphere, affecting global climate. We present data quantifying OC storage in wood and sediment from a basin in the Olympic Mountains of Washington, USA as well as from a basin in the Wind River Range of Wyoming, USA. Rivers in the Olympics tend to be in higher relief basins with greater primary productivity and precipitation, and OC storage tends to occur dominantly in wood. However, there are few longitudinal discontinuities in these river networks to trap sediment. In contrast, rivers in the Wind River Range experience lower annual precipitation, primary productivity, and basin-scale relief. They tend to have numerous longitudinal discontinuities that effectively trap large quantities of sediment, and wood in the Wind River Range decays very slowly. We use field and GIS data to explore these basins and develop a conceptual model of how basin-scale carbon dynamics are reflected in both the quantity of OC stored in each basin as well as the potential for that OC to be respired during a major disturbance event. These two basins represent end members of a spectrum of exhumation rate, gross primary productivity, basin longitudinal profile, wood dynamics, and climate. Thus, they provide an excellent comparison for understanding where and how OC is stored in the basin. Such a comparison allows us to better predict how each basin may respond to future extreme disturbance events and inform new land management paradigms that seek to sequester OC on the landscape.