Quantifying effects of short-interval fires on dominant cover in Southern California wildlands using historical aerial photographs

Background/Question/Methods

Climate change and increasing anthropogenic fire ignitions are expected to decrease fire return intervals in shrubland communities across Southern California. These shortened fire intervals may initiate a positive feedback, placing native chaparral species at risk of replacement by alien annual grasses. This conversion is predicted because many chaparral species are obligate seeders, requiring more than five years to reach reproductive maturity, and if a fire occurs before seed set, chaparral species could be extirpated. To assess if long-term vegetation change is occurring, sites that experienced a short-interval fire (two fires within five years) within Ventura or Los Angeles County were compared with adjacent locations that experienced only one fire within the same five-year period. Historic aerial photographs were acquired and assessed for percent vegetation cover before the first fire and more than six years after the second fire. Study sites on north or south-facing aspects were selected. Georectified prefire and postfire aerial images were used to classify vegetation cover to life form: invasive grass, chaparral, coastal sage scrub (css), woodland or bare ground/exposed rock. 

Results/Conclusions

There was no strong indication that vegetation cover differed following a short-interval fire in Southern California through analysis of historic aerial photographs (1956-2003). No significant differences were found in “percent change in cover” when comparing vegetation regrowth at sites that experienced one fire or two fires. Considering aspect, invasive grass cover significantly increased on south-facing aspects and css strongly increased on north-facing aspects while all other classes did not have a significant difference. A site’s distance from the coastline, elevation, and the number of years since fire were also analyzed as explanatory variables although further investigation of full fire history, community composition or weather conditions the year following fire may offer more insight to the variability observed among vegetation responses. Overall, this study did not find evidence for marked loss of shrublands strictly as a result of a single short-interval fire occurrence. Instead aspect was found to be a strong indicator of vegetation change, especially for invasive grass. Thus, a single short-interval fire event may not be strongest predictor of vegetation change and other factors, such as aspect, may be important to consider when predicting vegetation change after a short-interval fire in Southern California.