At what scale do disturbances matter for terrestrial to marine carbon fluxes? Comparing undisturbed aquatic carbon fluxes to disturbance frequency, extent, and associated carbon export at fine and regional scales

Brian Buma, Natural Science, University of Colorado, Denver, Denver, CO, Allison Bidlack, NOAA, Juneau, AK and Sarah M Bisbing, Department of Natural Resources and Environmental Sciences, University of Nevada - Reno, Reno, NV

Background/Question/Methods
Disturbances cause rapid and substantial change in ecosystems, and many disturbances – especially events like fires, landslides, and winds – are becoming more common with climate change. The impacts are felt strongly at the watershed scale, altering the flux of nutrients and energy through watersheds and into marine environments. But the scale-dependent importance of disturbances to broad-scale carbon fluxes is poorly characterized. Certainly fires or other events can have substantial impact at the watershed scale, but basal flux of carbon as a result of leaching, background erosion, and other mechanisms in undisturbed areas are also significant. We pose two relatively simple questions: How does the rate of carbon flux driven by disturbance compare to basal flux rates at a regional and subregional scales? At what scales does disturbance play a major role in aquatic carbon flux? Using continental scale, 30m resolution characterization of disturbance frequencies, recently developed carbon maps, and literature-based estimates of carbon fluxes associated with different disturbance types, we compare basal flux estimates to disturbance-drive flux estimates in a model system of the North Pacific Coast of North America, a landscape with high terrestrial to aquatic carbon fluxes and immense carbon stocks.
Results/Conclusions
Terrestrial stocks of C range from 552-603 Mg C/ha in disturbed locations. Baseline flux rates of carbon in the region are relatively high (e.g., 12-33 g/m2/year depending on estimate), and current disturbance rates relatively low (0.03% - 0.1% of the region/year). At watershed scales, disturbances (especially fire and landslides) have considerable impact on overall carbon export for decades (fluxes of 50-100+ Mg/ha possible). However, when scaling to the region, disturbances are likely responsible for only a small fraction of annual carbon export. Disturbance frequency would need to increase a minimum of 4-5x to approach carbon flux rates similar to baseflow at the regional scale or increase in terms of their impact on carbon flux but an equivalent amount, regardless of cause. Overall, the regional carbon flux appears relatively insensitive to disturbance-driven flux variability at present, as opposed to fine-scale fluxes which are highly sensitive. Given the current disturbance regime, it appears that climatic variation in flux drivers (e.g., via precipitation) is much more significant, although major shifts in disturbance type (e.g., mass increases in fire or landslides) should still be considered. This method and associated datasets, which are available at the continental scale, should be valuable for other systems as well.