Wildfire disturbance shapes the physical and biological condition of Pacific Northwest stream ecosystems

Abstract

Wildfire constitutes one of the most influential processes affecting watershed conditions across the Pacific Northwest. Although the patterns and impacts of wildfire are well-described for terrestrial ecosystems, a comprehensive understanding of wildfire effects in aquatic ecosystems is still being developed. The occurrence of wildfire within stream catchments can greatly influence regimes in flow, temperature and organic material, but understanding how wildfire contributes to the natural variability of these processes at broader spatial and temporal scales is poorly understood. Additionally, multiple studies have documented biotic responses to wildfire within a single catchment or following an individual burn event, yet how these disturbances impact stream communities at the landscape scale is less well-documented. As wildfire activity continues to increase in the Pacific Northwest, there is a need to broaden our understanding of stream-wildfire dynamics and quantify how these disturbances interact with the habitat and biota of riverine ecosystems in the region. My thesis addresses these challenges by investigating the abiotic and biotic effects of wildfire for watersheds across the Pacific Northwest. In Chapter 1, we quantify the response of stream temperatures to wildfire using a pre-fire/post-fire comparison of annual thermal regimes collected at 18 watersheds across the region. Using a non-parametric effect size approach and constrained multivariate ordination, we describe the patterns and drivers of stream thermal response to wildfire in the first year following disturbance. Despite substantial variability at the reach scale in the response of most stream thermal metrics, we detected a regional increase in the number of warm events coupled with a decrease in the number of cold events in the first post-fire year, relative to historic conditions. Catchment precipitation, percentage of watershed burned and annual streamflow were identified as the most important predictors of responses across all facets of the thermal regime. Our results suggest that despite the importance of reach-scale conditions in driving the sensitivity of stream temperatures, regional-scale patterns are possible, indicating wildfire events can influence post-fire stream thermal dynamics across a wide variety of catchment conditions well into the post-disturbance period. In Chapter 2, we compare the functional diversity between burned and unburned streams in a large number of catchments across the Pacific Northwest experiencing variable habitat conditions and wildfire histories. Using regression analyses, we compare three indices of functional diversity in streams experiencing recent and extensive wildfire, and identify habitat- and watershed-scale conditions most responsible for describing patterns within these indices. Values in functional richness and functional dispersion were largely similar between burned and unburned streams, but total abundance was higher in streams that had experienced wildfire in the past year compared to unburned conditions. Values in stream temperature, substrate size and the amount of large woody debris were largely important in describing values in functional diversity, through which predictors did best depended on the wildfire history experienced at each site. Despite the potential for wildfire to play a major role in affecting community composition for individual watersheds, our analysis suggests that fine-scale habitat conditions play a more important role in structuring functional diversity across multiple catchments. Collectively, our analysis provides insight into the interaction between wildfire and stream ecosystems in the Pacific Northwest and helps identify factors affecting the physical and biological condition in streams within a fire-prone landscape.