Wildfire effects on stream metabolism: Aquatic succession is mediated by local riparian succession and stream geomorphology

Abstract

As climate change shifts and intensifies fire regimes, it is important to understand stream ecosystem responses to fire. How stream metabolism responds remains largely unexplored. We investigated effects of fire severity and watershed geomorphology on stream ecosystem metabolism at multiple spatial scales in an Idaho wilderness watershed. We measured dissolved oxygen, temperature, and irradiance in 18 streams varying in fire history and watershed characteristics in order to model diel oxygen dynamics, from which we estimated rates of production (P) and respiration (R), then used P:R as an index of stream metabolic state. We found that post-fire riparian canopy recovery strongly influenced stream metabolic state. Severely burned streams with dense riparian regrowth were heterotrophic, whereas streams with less canopy recovery were autotrophic. Fire effects on stream metabolic state were highly mediated by watershed geomorphology, with the strongest long-term changes observed in low-order, narrow, steep streams. Effect sizes of fire and watershed geomorphology on stream metabolism changed from fine spatial scale (500-m riparian buffer) to coarse scale (watershed), and were strongest at fine scales. These results indicate that the physical habitat template mediates aquatic ecosystem response to disturbance, and that context and scale should be explicitly considered in assessments of ecosystem response to fire.