Degradation and type conversion of native shrublands to alien-dominated grasslands has been noted by numerous investigators, some of whom contend that increased frequency of disturbance is the primary factor that favors non-native annuals over woody native species. In the absence of fire, seeds of non-natives have a low residence time in the soil; thus, the presence of these species on burned sites is more often due to colonization after fire. Typically a repeat fire within the first postfire decade is sufficient to provide an initial foothold for aliens. In addition to outcompeting native plant species, non-native grasses alter the fire regime from a crown-fire regime to a mixture of surface- and crown-fires, where highly combustible grass fuels carry fire between shrub patches. This increases the likelihood of fires and ultimately increases fire frequency. As fire frequency increases there is a threshold beyond which the native shrub cover cannot recover.
Fire management practices potentially conflict with natural resource needs. These landscapes currently experience an unnaturally high frequency of fire; thus, much of it is at risk for alien invasion. When fire managers add to this by using prescription burning and other fuel manipulations, they open up these shrublands and expose them to invasion and potential type conversion to non-native grasslands. In managing these landscapes it might be helpful to consider the fact that the vast majority of alien species in California are opportunistic species that capitalize on disturbance. Adding additional disturbance through prescription burning (or grazing) will only exacerbate the alien problem.
Very little chaparral landscape is protected in parks or wilderness areas. Much of it is in private hands or under federal jurisdiction. Historically, it has largely been managed as rangeland by frequent burning to destroy the chaparral cover, or burned to reduce fuels perceived to be hazardous to more desirable forests or urban environments. Today the expansion of urban development has resulted in large portions of urban communities being juxtaposed with watersheds of potentially dangerous chaparral fuels. Historical studies show that large high-intensity crown fires are a natural part of this ecosystem and there is little reason to believe there will not be more such fires in the future. Fire management has always worked under the philosophy that they can change the vulnerability of communities to wildfires through manipulation of fuels. However, over the past century of such management, every decade has been followed by one of increasing losses to wildfires. Californians need to embrace a different model of how to view fires on these landscapes. Our response needs to be tempered by the realization that these are natural events that cannot be eliminated from the southern California landscape. We can learn much from the science of earthquake or other natural hazard management. No one pretends they can stop earthquakes; rather, they engineer infrastructure to minimize impacts. In the future, living safely with fire is not going to be achieved solely by fire management practices, but will require close integration with urban planning.
See also: Ecological Niche; Fire; Invasive Species; Mediterranean; Plant Demography.
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