Post-fire riverscapes: exploring the ecological and behavioral responses of stream communities to wildfire in boreal streams

Abstract

Wildfire is the dominant disturbance in boreal forest ecosystems, and its frequency and severity are increasing with climate change. This dissertation examines how wildfire affects boreal stream habitats, food webs, and fish movement across spatial and temporal scales in interior Alaska. In Chapter 2, I compared burned and unburned streams 10 to 15 years post-fire using habitat survey, water chemistry, and aquatic community composition data. Burned sites had marginally significant lower canopy cover (-19.6%, p = 0.095), coarser substrate (+23.7 mm, p = 0.102), and higher macroinvertebrate abundance (626 ± 361 vs. 331 ± 349 individuals/m2, p = 0.03); fish biomass was more than twice as high in burned sites. In Chapter 3, I combined stable isotope analysis (δ13C and δ15N values) with simulations from the Aquatic Trophic Productivity (ATP) model to evaluate how wildfire alters food web structure and productivity for up to 50 years post-fire. Field observations revealed that burned streams had greater aquatic macroinvertebrate diversity, longer food chains (δ15N range = 6.2‰ vs. 3.4‰), and lower δ13C values in primary producers, suggesting increased algal energy inputs to food webs. ATP model outputs supported these patterns, predicting persistent increases in macroinvertebrate biomass and fish production for decades following fire. Bayesian isotope mixing models further indicated that juvenile Chinook salmon (Oncorhynchus tshawytscha) and Arctic grayling (Thymallus arcticus) in burned streams relied more on algal-based food resources and exhibited broader niche widths. Chapter 4 applied a spatial stream network model and strontium isoscape to estimate where Arctic grayling reared and moved throughout life. Approximately 20% of modeled fish locations occurred in burned areas, which is roughly proportional to their availability (~23% of the basin), suggesting no strong preference or avoidance at the basin scale throughout the lifetime of individual fish. Larger, mature fish with higher dry mass were disproportionately associated with burned habitats. Life-history clusters characterized by long­ distance movement were more common in burned sites, suggesting that post-fire habitats may provide key foraging opportunities within the boreal riverscape. Overall, my dissertation integrated empirical field data, modeling approaches, and spatial analyses to provide a broad assessment of how wildfire influences the structure and function of boreal freshwater ecosystems.