Postfire stream responses to spatial fire patterns in riparian and upland zones

Abstract

Wildfire frequency and intensity are increasing in many parts of the world, often with substantial repercussions for stream ecosystems. The severity, scale, and patterns of wildfire burning can differ between riparian and upland zones due to differences in environmental conditions and vegetation. However, little research has focused on differences in burn patterns in riparian versus upland zones and their implications for stream characteristics. We studied fire patterns and postfire vegetation trajectories in riparian versus upland areas across 26 stream sites in coastal southern California over 12 years, as well as the relationships of burn patterns to stream variables after fire. Upland environments burned more severely and more extensively than did riparian woodlands, and this difference was magnified for perennially flowing streams, along which riparian woodlands burned less severely than those along intermittent streams. Burned vegetation returned to prefire canopy cover and greenness in both riparian and upland areas in about 8 years with regrowth delayed by a prolonged drought. Additionally, we observed differences in spatial burn patterns and vegetation regrowth in upland versus riparian zones, with greater local patchiness in upland settings, likely due to differences between upland and riparian areas in the spatial patterns of hydrologic, topographic, and vegetative drivers of fire and vegetation reestablishment. We then assessed relationships between spatial fire patterns and geomorphological, physical, hydrochemical, and biological conditions at creek sites. We observed relationships that were generally attributable to either the local destruction of riparian woodland along the streams or to changes in the basin-scale movement of water, nutrients, and sediment following fire and postfire floods. Basin-scale burn patterns were associated with increases in sediment deposition, reduction in pool and riffle habitat extents, and changes in particulate organic matter during the first and second years after fires; impacts subsided about a year after the first substantial postfire flows. In contrast, loss of riparian canopy cover was associated with increases in light level, temperature, algal cover and biomass, density of algivorous invertebrates and amphibians, and water conductivity, as well as decreases in particulate organic matter and invertebrate shredders. In contrast to impacts from basin-scale burn patterns, impacts from the local loss of riparian canopy cover persisted for longer periods of time, paralleling riparian woodland reestablishment and growth.