Abstract
Increased fire frequency has been shown to promote alien plant invasions in the western United States, resulting in persistent vegetation type change. Short interval fires are widely considered to be detrimental to reestablishment of shrub species in southern California chaparral, facilitating the invasion of exotic annuals and producing “type conversion”. However, supporting evidence for type conversion has largely been at local, site scales and over short post-fire time scales. Type conversion has not been shown to be persistent or widespread in chaparral, and past range improvement studies present evidence that chaparral type conversion may be difficult and a relatively rare phenomenon across the landscape. With the aid of remote sensing data covering coastal southern California and a historical wildfire dataset, the effects of short interval fires (<8 years) on chaparral recovery were evaluated by comparing areas that burned twice to adjacent areas burned only once. Twelve pairs of once- and twice-burned areas were compared using normalized burn ratio (NBR) distributions. Correlations between measures of recovery and explanatory factors (fire history, climate and elevation) were analyzed by linear regression. Reduced vegetation cover was found in some lower elevation areas that were burned twice in short interval fires, where non-sprouting species are more common. However, extensive type conversion of chaparral to grassland was not evident in this study. Most variables, with the exception of elevation, were moderately or poorly correlated with differences in vegetation recovery.
Highlights
Fire is an important ecological process with dynamic interactions between vegetation, biogeochemical cycles, and human activity [1,2,3]
Given widespread predictions of increasing fire frequency with ongoing climate change [9,10], it is important to explore whether altered fire return intervals can drive long term vegetation type change, and which environmental variables predict where such vegetation type change is likely to occur
Positive changes in DMN values most consistent with type conversion were at lower elevation in proximity to communities of coastal sage scrub, in accordance with previous results found by Lippitt et al [17]
Summary
Fire is an important ecological process with dynamic interactions between vegetation, biogeochemical cycles, and human activity [1,2,3]. Dry summer climates make Mediterranean ecosystems, like southern California chaparral, prone to wildfire [4,5]. Four years later many of the same areas were burned in the Harris, Witch and Poomacha fires. It has been suggested that in ecosystems dominated by fire adapted species, either very long or very short fire return intervals will drive long term compositional change [7,8]. Given widespread predictions of increasing fire frequency with ongoing climate change [9,10], it is important to explore whether altered fire return intervals can drive long term vegetation type change (often referred to as ‘‘type conversion’’), and which environmental variables predict where such vegetation type change is likely to occur
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