The marked seasonal change in climate is conducive to massive wildfires, which are spawned by the very dry shrub foliage in the summer and fall and spread by the dense contiguous nature of these shrublands. Fires have likely been an important ecosystem process since the origin of this vegetation in the late Tertiary Period, more than 10 Ma, if not earlier. Until relatively recently the primary source of ignitions was lightning from summer thunderstorms. Fires would largely have been ignited in high interior mountains and coastal areas would have burned less frequently and only when these interior fires were driven by high winds with an offshore flow. In many parts of California such winds occur every autumn and are called Santa Ana winds in southern California and Diablo winds or Mono winds in northern California. When Native Americans colonized California at the end of the Pleistocene Epoch around 12 000 years ago, they too became a source of fires, and as their populations greatly increased over the past few thousand years humans likely surpassed lightning as a source of fire, at least in coastal California. Today humans account for over 95% of all fires along the coast and foothills of California.
Chaparral fires are described as crown fires because the fires are spread through the shrub canopies and usually kill all aboveground foliage. Normally, following a wet winter, high fuel moisture in chaparral shrubs makes them relatively resistant to fire. The amount of dead branches is important to determining fire spread because they respond rapidly to dry weather and combust more readily than living foliage. As a consequence, fires spread readily in older vegetation with a greater accumulation of dead biomass. However, there is a complex interaction between live and dead fuels, wind, humidity, temperature, and topography. In particular, wind accelerates fire spread primarily by heating living fuels and often can result in rapid fire spread in young vegetation with relatively little dead biomass. Fires burning up steep terrain also spread faster for similar reasons.
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