Wildfire in western Oregon increases stream temperatures, benthic biofilms, and juvenile coastal cutthroat trout size and densities with mixed effects on adult trout and coastal giant salamanders

Abstract

Wildfire has become increasingly common and severe across forested landscapes. Shortly after wildfire, loss of riparian cover along streams and subsequent increases in light can elevate stream temperatures, a key control on metabolic rates of biota. Increased light can also increase autotrophic basal resource availability with potential bottom-up effects. We evaluated wildfire impacts on aquatic ecosystems in a replicated Before-After Control-Impact study 1 year after a severe wildfire in western Oregon, U.S. Stream temperature, chlorophyll a accrual, and age-0 coastal cutthroat trout size, density, and biomass increased in all three burned streams relative to changes in three unburned references. When streams were evaluated collectively, fire did not have an effect on larger vertebrate density or biomass. However, considering streams individually, two severely burned sites had substantial temperature increases and declines in larger vertebrate density and biomass, but the moderately burned site had modest temperature increases and adult cutthroat trout and Pacific giant salamanders increased. The loss of riparian canopies post-fire increased temperature and algae, but fish responses varied with age class and larger vertebrate responses were inconsistent.