Effects of short-interval fires on shrubland communities: Utilizing aerial photographs to quantify vegetation conversion in Southern California

Background/Question/Methods
Climate change and increasing anthropogenic fire ignitions are expected to decrease fire return intervals in shrubland communities in Southern California. Short-interval fires (two fires within five years) may initiate a positive feedback, placing native chaparral species at risk of replacement by alien annual grasses. Many chaparral species are obligate seeders, such as Ceanothus megacarpus, which require more than five years to reach reproductive maturity. If vegetation conversion is occurring due to short-interval fires, we should be able to observe and quantify its effects within Ventura and Los Angeles Counties, where Santa Ana wind events positively influence fire occurrence and fire spread.  
For this study, locations that experienced short-interval fires in Ventura or Los Angeles County were selected. Aerial photographs were acquired and assessed for ground cover before the first fire and more than six years after the second fire. Identical 50 x 50 m plots, on northern and southern aspects, were analyzed for ground cover classes: grass, chaparral, coastal sage scrub, tree, and bare ground. Control plots are located in adjacent locations that burned in the second fire to capture the same number of regrowth years in the post-fire image. 

Results/Conclusions
Evaluated plots were composed of grass, chaparral, and coastal sage scrub cover with minimal tree and bare ground cover. Preliminary results indicate grass cover decreased on northern aspects and increased on southern aspects, and the overall amount of change (post-fire cover minus pre-fire cover) is similar when comparing short-interval fire plots and control plots (burned only in the second fire). Chaparral cover showed a larger decrease on northern aspects than on southern aspects and does not exhibit a strong correlation between short-interval fires and a decrease in cover. Coastal sage scrub cover increased more on northern aspects than on southern aspects and had an overall greater increase in control plots than in short-interval fire plots. Our results do not support the hypothesis that short-interval fires cause a decline in shrub cover. Instead, there could be a more subtle, long-term conversion occurring from chaparral to coastal sage scrub.