The 2007 Zaca Fire burned 972 square kilometers near Santa Barbara, making it the fourth largest fire in California history. One of the species most affected is bigcone Douglas-Fir (Psuedotsuga macrocarpa), a rare and endemic conifer limited to mountains of southern California. Given how little is known about effects of the fire itself versus several years of intense drought which followed, our goal was to examine the relationship between burn severity and losses of P. macrocarpa. We used fire severity maps from remote sensing, which classify the change in greenness soon after fire on a scale of 0-5. We also used aerial imagery from Google Earth from before (2006) and after (2013-14) the fire to digitize polygons of P. macrocarpa stands; we were thus able to map both current living stands and areas of loss since the fire. Each stand was given a rating, ranging from 0 (no loss) to 5 (complete loss). After overlaying spatial fire severity and P. macrocarpa patterns in a GIS, a chi square test of independence was used to assess how well the fire severity maps match the actual stand loss amounts observed in aerial imagery.
Results/Conclusions
The chi square test showed highly significant evidence for associations between mapped fire severity patterns and observed P. macrocarpa losses (χ² = 333, df = 40, p < 0.001). In particular, we found the closest associations on the extreme ends of the spectrum (i.e., areas in low burn severity classes tend to have the smallest rates of loss, and vice versa). Extreme loss (>50%) impacted 39% of the landscape in our study, and is strongly associated with high severity fire patches. Intermediate loss (10-50%) impacted 9% of the landscape, and did not correlate as well with fire severity. Areas of the lowest and highest fire severity have the closest associations with P. macrocarpa loss, which suggests that those parts of the landscape were most impacted due to fire alone. Meanwhile, in intermediate severity areas where patterns are not as clear, losses may show a stronger signal of water stress caused by the subsequent drought. Findings will help us refine our field sampling to identify fire severity versus drought effects and should help predict P. macrocarpa loss in other fires using fire severity maps.