Short-interval fires and vegetation change in southern California

Abstract

Questions: In southern California, shortened fire return intervals may contribute to a decrease in native chaparral shrub presence and an increase in non-native annual grass presence. To test the hypothesis that short-fire return intervals promote a loss in shrub cover, we examined the contribution of single short-interval fires and abiotic conditions on the change of shrub cover within Ventura and Los Angeles counties. Through evaluating pre- and post-fire historical aerial images, we answered the following questions, 1) How has vegetation type cover changed after repeat fires? and 2) What landscape variables contribute the most to the observed change? Location: Ventura County and Los Angeles County, California, USA. Methods: We assessed the impact of a single short-interval fire by comparing vegetation recovery in adjacent once- and twice-burned fire burn polygons (long- and short-interval respectively). Pixel plots were examined within each polygon and vegetation cover was classified to vegetation type. We determined the best predictor of vegetation type cover with a linear mixed effects model comparison using Akaike Information Criterion. Results: Pre-fire and post-fire community type cover was highly correlated. Burn interval was the best predictor of tree cover change (lower cover in twice-burned pixel plots). Aspect was the best predictor of sage scrub cover change (greater cover on north-facing aspects). Years since fire was the best predictor of chaparral cover change (positive correlation) and sage scrub cover change (negative correlation). Conversion of chaparral to sage scrub cover was more likely to occur than conversion of chaparral to annual grass cover. Conclusions: Our study did not find extensive evidence of a decrease in chaparral shrub cover due to a single short-interval fire. Instead, post-fire cover was highly correlated with pre-fire cover. Chaparral recovery, however, was dynamic suggesting that stand recovery may be strongly influenced by local scale conditions and processes.

Fire‐driven vegetation type conversion in Southern California

“…Although both Meng et al (2014) and Storey et al (2021) have questioned the role of short‐interval fire in explaining VTC in chaparral, and Lucero et al (2021) found weak evidence for it, the results here overwhelmingly point to short‐interval fire and the degree of departure from historical fire return intervals as most important—regardless of the modeling method used or spatial extent of analysis. It is noteworthy that we used here a variable that has not been explored in other work, including our own previous studies—the measure of fire interval departure (vs. minimum fire interval).…”

“…Another possible reason for the differences in results between our studies and those of Meng et al (2014) and Lucero et al (2021) is that our approach used a historical view of changes over time and theirs relied upon paired plots and a space-for-time substitution. That is, we directly tracked change at one site over time, whereas the other studies inferred change by comparing two sites with different fire histories and then attributed the change to the fire.…”

“…Understanding landscape changes requires a multivariate approach that considers factors other than fire interval. For example, Meng et al (2014) found a reduction in vegetation cover after a short interval between fires at low elevations, and this likely was due to changes in community composition (i.e., chaparral is replaced at lower elevations by the smaller-stature sage scrub), as revealed by Lucero et al (2021) and Syphard et al (2006) and by the fact that human ignitions are inversely correlated with elevation (Keeley & Syphard, 2018a). Additionally, given the association of elevation with different plant communities and a range of physiological factors associated with plant growth or postfire recovery (Franklin, 1996), there could be multiple reasons for this pattern.…”

“…Until recent years, empirical documentation on landscape scales of where and to what extent chaparral is being locally extirpated and replaced with herbaceous vegetation has been lacking. The few landscape‐scale studies that have been conducted have only covered parts of chaparral's range in Southern California and have used different methodologies (e.g., Lippitt et al, 2012; Lucero et al, 2021; Meng et al, 2014; Storey et al, 2021; Syphard, Brennan, & Keeley, 2019a & b). Complicating our understanding is that these studies have found variation in the relative importance of factors most strongly correlated with woody chaparral decline and conversion to grass, particularly the role of short‐interval fire.…”

“…In contrast, Meng et al (2014) failed to find a fire‐interval connection using Landsat remote‐sensing indicators of plant biomass and took a so‐called space‐for‐time approach in which, instead of demonstrating VTC at a single plot over time, they inferred it based on comparisons of paired plots with different fire histories. Lucero et al (2021) also used paired plots, but instead of using Landsat data, they used fine‐scale aerial imagery. These space‐for‐time studies assume that the only important difference between sites is the fire history; they control for certain types of environmental variation but cannot discern species composition.…”