Short-term Effects
The most notable short-term effects of wildfire on salmonids include a loss of riparian habitat (and subsequent shading) and an influx of fine sediment. Riparian habitat loss affects both stream water temperature and increases sediment delivery to rivers. In turn, significant increases in fine sediment to watersheds affected by fire have been shown to reduce salmonid spawning and rearing habitat principally through a loss of viable egg habitat. Local hydrology and water chemistry can also be greatly impacted by fire as runoff and overland flow increase with an abrupt loss of riparian vegetation. For instance, Oliver et al. (2012) found that runoff occurred earlier in recently burned areas in California and that the delivery of nutrients, such as nitrate and phosphorous, increased. In general, intense wildfires adjacent to streams can alter local hydrology and change sediment delivery patterns and in some cases even lead to the direct mortality of fish. Bozek and Young (1994) found that a combination of intense rainfall immediately after a wildfire and increased sedimentation lead to the direct mortality of salmonids in the greater Yellowstone ecosystem. Yet, even in these instances, recolonization of recently
impacted habitats can be relatively quick and largely depends on the location of viable source populations. For instance, Rieman et al. (1995) found that most stream reaches affected by wildfire were repopulated within the first year and approached densities of unimpaired stream reaches in one to three years.
Wildfires can also positively affect salmonid habitat. Flitcroft et al. (2015) recently found that large wood recruitment increased in recently burned watersheds and improved overwintering habitat for juvenile salmonids through the creation of complex pool habitats. Increased delivery of nutrients, as documented by Oliver et al. (2012) and others, may also improve ecosystem productivity by stimulating stream food webs and improving the quantity and quality of prey resources. Changes in hydrology and increases in overland flow may also lead to large debris flows in upstream tributaries which, as a result, may deliver larger, cobble-size sediments to spawning areas to help improve spawning habitat. Similar to the effects of flooding, wildfires can also promote and even enhance instream processes known to benefit salmonids, yet this often occurs over longer time periods. (continued on page 54 )
From his perch aboard the International Space Station, Rand Bresnik caught these incredible views of the Thomas Fire on December 7. Photo: Rand Bresnick/NASA via Twitter