California chaparral exhibits regional differences in burning patterns and largely due to regional variation in winds. In much of coastal California autumn winds create severe fire conditions. These occur every year and result in 5-10 days of strong offshore flow with windspeeds of 100 kph or more. These winds result from a high-pressure system in the interior West, and are known as Santa Ana winds in southern California and Diablo or Mono winds in northern California. As these air masses move from the high-pressure cell in the interior to a low-pressure trough off the coast, the air descends and dries adiabatically, resulting in relative humidity below 10%. The fact that these winds occur every year and arrive at the end of an extended drought results in one of the most severe fire conditions in the world. As a consequence only a small portion of southern California landscape has escaped fire during the last century, and much of the lower-elevation chaparral has burned at an unnaturally high frequency.
In contrast, Santa Ana winds are absent from the southern Sierra Nevada and parts of the central coast, in part to mountain barriers that fail to funnel these winds coast-ward. This, coupled with lower human population density, has resulted in many fewer fires. As a consequence nearly half of the landscape in the southern Sierra Nevada has not had a fire for well over a century. This condition places these landscapes at the upper end of the historical range of variability. Nonetheless, these older stands of chaparral appear to maintain natural ecosystem processes and exhibit no sign of dying out or replacement by other vegetation types. This is particularly evident, following fires in ancient stands of chaparral from the region, that it exhibits vegetative recovery in cover and diversity indistinguishable from postfire recovery in younger stands.
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