302-5 Monitoring Process Cycles to Understand Riverscape Resilience to Wildfire

Abstract:

Wildfires alter both watershed-scale fluvial regimes of water, sediment, and wood, and local valley bottom ecogeomorphic context (i.e., biota, material properties, etc.). This can test a system’s resilience, or ability to sustain a substantial change in form without losing its fundamental character in response to disturbance. Despite our general desire for healthy, resilient riverscapes, resilience can be difficult to discern from empirical observations collected over short timescales, and the processes that generate resilience can exhibit equifinality. Here, we present paired restoration monitoring and historical analysis from multiple post-fire riverscapes across the western United States to illustrate both the complexity of evaluating resilience and the nature of the processes that generate it. Specifically, we focus on the resilience of water, sediment, and wood retentiveness driven by channel-floodplain lateral connectivity, in-channel obstructions, and partial avulsion.

We analyze riverscapes using a process cycle framework in which we hypothesize the processes that must repeat over varying timescales to create and maintain retentiveness. We determine historical range of variability through our examination of historical aerial imagery. This historical range of variability interpreted through the lens of process cycles establishes an empirical and mechanistic basis for evaluating whether riverscape change constitutes evolution along a trajectory (i.e., a lack of resilience) or simply variability in form despite maintenance of character (i.e., resilience). We evaluate modern monitoring observations in the context of this historical variability to determine whether, why, and for how long these sites can be said to be resilient to wildfire in terms of their retentiveness.

From monitoring results, we show that resilience derives from the presence of a fully functional process cycle in which key riverine constituents are replenished by both local and non-local processes as they are depleted. We show how systems can evolve towards greater resilience through both fire-driven change (e.g., simultaneously increasing influxes of water and wood) and interventions by biota (e.g., humans, beavers). The mechanistic and temporally explicit conceptualization of post-fire resilience provided by a process cycle framework helps illustrate that continued resilience in many watersheds will very likely require restoration, or, rather, stewardship, of watershed-scale regimes over decades to centuries.

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