The effects of wildfire on stream ecosystems in Southern California

Wildfire is a frequent and pervasive feature of most ecosystems in Mediterranean climates. Further, the intensity, extent, and frequency of wildfire in many of these areas is predicted to increase with climate change. To evaluate the current and projected impacts of wildfire on stream ecosystems in southern California, we monitored a suite of physical, chemical, and biological variables at 12 to 21 stream sites in basins that were and were not burned by two fires in fall 2008 and spring 2009, both just before and for up to 3 years after the fires occurred. Fire effects on riparian vegetation were variable, with some stream sites with burned upland areas having burned riparian vegetation (open sites) and others with riparian vegetation remaining intact (shaded sites). Large amounts of sediment were deposited at sites within and below burned areas during the rainy season after fires, but deposited sediment returned to pre-fire levels within 2 – 3 years. Stream nutrient concentrations were much higher at sites in burned than unburned basins during the first major storms after fires. Open sites in burned basins had higher temperatures, lower CPOM inputs, higher algal biomass, and different algal assemblages than shaded sites in burned and unburned basins a year after the fires. Stream benthic bacterial communities showed quicker responses to fire, being different in burned and unburned sites one month after one of the fires. Although vagile, opportunistic invertebrate species were relatively more abundant at sites in burned vs. unburned basins one year after fires, invertebrate assemblages at shaded sites in unburned and burned basins were similar after 2 – 3 years. On the other hand, invertebrate assemblages at open sites in basins where both upland and riparian vegetation burned were still different from those at other sites after 3 years, being dominated by warmwater species. Trout populations in burned basins nearly disappeared during the rainy season after fires. Stable isotope and gut analyses showed that algal-based food webs dominated in streams in basins where both upland and riparian vegetation burned whereas detrital-based food webs dominated in streams draining unburned basins or burned basins where the riparian vegetation remained intact.