Climate, lightning ignitions, and fire severity in Yosemite National Park, California, USA

Abstract

Continental-scale studies of western North America have attributed recent increases in annual area burned and fire size to a warming climate, but these studies have focussed on large fires and have left the issues of fire severity and ignition frequency unaddressed. Lightning ignitions, any of which could burn a large area given appropriate conditions for fire spread, could be the first indication of more frequent fire. We examined the relationship between snowpack and the ignition and size of fires that occurred in Yosemite National Park, California (area 3027 km2), between 1984 and 2005. During this period, 1870 fires burned 77 718 ha. Decreased spring snowpack exponentially increased the number of lightning-ignited fires. Snowpack mediated lightning-ignited fires by decreasing the proportion of lightning strikes that caused lightning-ignited fires and through fewer lightning strikes in years with deep snowpack. We also quantified fire severity for the 103 fires >40 ha with satellite fire-severity indices using 23 years of Landsat Thematic Mapper data. The proportion of the landscape that burned at higher severities and the complexity of higher-severity burn patches increased with the log10 of annual area burned. Using one snowpack forecast, we project that the number of lightning-ignited fires will increase 19.1% by 2020 to 2049 and the annual area burned at high severity will increase 21.9%. Climate-induced decreases in snowpack and the concomitant increase in fire severity suggest that existing assumptions may be understated – fires may become more frequent and more severe.

Differences in wildfires among ecoregions and land management agencies in the Sierra Nevada region, California, USA

“…In forested areas, our high-severity category therefore generally describes a conversion of forest to a nonforested condition. For Yosemite, we augmented the severity data we had previously developed to match the size (.80 ha) and time period ) of the FS data (Collins et al 2009, Lutz et al 2009). The Yosemite fire severity database includes data for management ignited fires and wildfires that were either lightning or human caused.…”

Fire and the Distribution and Uncertainty of Carbon Sequestered as Aboveground Tree Biomass in Yosemite and Sequoia & Kings Canyon National Parks

“…However, fires do not uniformly reduce carbon over the area within a fire perimeter-except in the most extreme high severity scenarios. Rather, there is a range of severities that produce a range of carbon loss to the atmosphere and transfer to the standing dead pool [31,[75][76][77]. Figure 6 presents a basic model of carbon fluxes immediately following wildfire and provides a computational framework for post-fire carbon loss accounting using fire severity data.…”

“…However, these results do not suggest either a large or statistically significant effect of low-to moderate-severity fire. A more comprehensive assessment of the effects of fire on the landscape will require a consideration of fire severity [32,75], the proportionate area within a fire perimeter that remained unburned [78,79,82,83], and details on pre-fire and post-fire carbon accumulation rates [41,46,76]. .…”

Estimating Risk to California Energy Infrastructure From Projected Climate Change

“…Several studies have shown that climate change will increase the size and frequency of wildfires in California, which is problematic for a state that already leads the nation in wildfire-related economic losses (Flannigan et al 2000;Lutz et al 2009;Westerling et al 2009 While the physical effects of fire on transmission lines is widely noted, it is more difficult to estimate the length of time that a line would be down in these cases, as these impacts can interrupt the line's service either momentarily or for an extended time period (CAISO 2009). It is even more difficult to relate these events to an actual outage, since the transmission system can often cope because of the redundancy intentionally built into the power system.…”