Sunday, November 21, 2010

San Diego Climate and Chaparral Fires

If you haven’t noticed, San Diego doesn’t get much rainfall. In fact, average rainfall in San Diego only averages 10.8 inches over the past 48 years (which is about half the amount of precipitation San Francisco gets)! This means that plants in San Diego County are somewhat drought-resistant. Fog, however, adds a great amount of moisture to the air which increases humidity and lowers plants’ evaporation rate. If you are new to UCSD, you will discover that fog has been known to reside over our campus for days or weeks at a time, especially in June and July.

Whether you have traveled around San Diego under its sunny or overcast skies, you may have noticed dense shrubs covering hillsides. This plant community is known as chaparral. Chaparral is the most prominent plant population that spreads throughout Southern California. These plants form dense thickets of woody shrubs on drier and/or shallow soils where woodlands cannot survive. Chaparral originally came from the Spanish word chaparro, which was first used to describe evergreen oak trees in Spain. Mixed chaparral areas can be found throughout La Jolla and the rest of San Diego, but you can view them locally at UCSD’s ecological reserve park and Torrey Pines State Park. The photograph below shows a mixed chaparral community north of Geisel Library.

A view of the chaparral
Photo taken from Google Earth

The dry soils and dense, woody shrubs of chaparral communities are unfortunately famous for catching fire. The fall season’s Santa Ana winds dry out vegetation, which spreads the nearly annual San Diego intense chaparral fires. These fires frequently reach temperatures of 1,200◦F, making them some of the hottest fires in natural environments in the world. Fire intensity is determined by many factors including temperature, humidity, wind, and foliage and stem moisture content. The fires of October 2007 burned 400,000 acres across San Diego County. The Witch Creek Fire, the Poomacha Fire, and the Rice Creek Fire ended up destroying 1259 structures and cost 7.6 million dollars to exterminate. Ashes fell from the sky for days and UCSD was subsequently closed for a week due to poor air quality.

Luckily chaparral shrubs have recovery strategies that have evolved in shrubs throughout the world. Most have woody root crowns below the soil surface that are protected from the heat of the fire. These root crowns are able to quickly sprout multiple new stems because they store carbohydrates, thus rain is not required for post-fire recovery.

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