Mild 2012 Fire Season Predicted for Southern California

Fire ecologist Richard Minnich says winter drought killed the region’s grasses

Photo shows a wildfire.

UC Riverside has a wide variety of faculty members and researchers available to talk about wildfires.

RIVERSIDE, Calif. — Because Southern California just experienced a drought winter, the fire hazard in the region’s grassy areas is greatly reduced this year, according to Richard Minnich, a leading fire ecologist and a professor in the Department of Earth Sciences at the University of California, Riverside.

“Some rain in the fall allowed the grasses to grow, but then the winter drought killed most of them before they could make seed,” he explained. “As a result, the hills show mostly bare ground today, greatly reducing the fire hazard, especially in the interior valleys from Riverside to Temecula.  We should therefore see no big outbreak of fire in low-elevation grasslands in Southern California this summer.  There might be small fires on shading north-facing slopes where grasses barely survived the drought, as in the recent fire at Lake Mathews.”

Grasses either burn or are decomposed by rains the following winter.  They therefore always represent only one year’s growth.  In contrast, the fire hazard in chaparral and forests in the higher mountains is related to how long it has been since that vegetation last burned.  The fuel energy of chaparral and conifer forests is the collective growth of vegetation over decades, if not centuries.

Shrubs and trees gradually build canopy.  Chaparral becomes ever thicker and taller (up to 20 feet). The probability of fire in chaparral varies from stand to stand, with fires preferentially burning older stands. Pine forests continually reproduce and become filled with dense understory trees.  Further, the denser the chaparral and forest, the more rapidly plants extract water from the ground.

Image is a map showing the time of last fire in the San Gabriel, San Bernardino and San Jacinto Mountains.

The map shows the time of last fire in the San Gabriel, San Bernardino and San Jacinto Mountains. Legend: Dark purple 2000-2009; hot pink 1990-1999; dark blue 1980-1989; turquoise 1970-1979; green 1960-1969; yellow green 1950-1959; dark green 1940-1949; olive green 1930-1939; light yellow 1920-1929; yellow 1910-1919; beige 1900-1909; orange 1890-1899; bright orange 1880-1889; and red 1863-1879. No color indicates no record of fire and is best interpreted as “last fire at least before 1880.” Image credit: Minnich Lab, UC Riverside.

“Plants drought themselves,” Minnich said. “While the drought last winter will lead to an earlier onset of the fire season this summer, the onset will be sooner where soil water has been depleted faster, namely, in the oldest brushlands and forests since the last fire.”

According to Minnich, the fire hazard is diminished in forest areas that have recently burned, such as the areas that burned in 2003 along the south front of the San Bernardino and San Gabriel Mountains — from Glendora to Redlands.   The San Gabriel Mountains that burned in the 2009 Station Fire also have minimal fire hazard.

“The most vulnerable areas to fire are chaparral and forests that have not burned in 60-100 years, such as the mountains north of Monrovia, Wrightwood, Crestline and Lake Arrowhead, from Big Bear to San Gorgonio and Mill Creek Ridge, Yucaipa Ridge and Oak Glen,” he said.  “Lake Arrowhead has not burned since 1879 and Big Bear since 1900.  There is also a massive fire hazard at Idyllwild and areas south that have not burned since the 19th century. Pine Cove has nearby chaparral that has not burned since 1914.

“Normal fire intervals are about 30-60 years, and these areas have gone past normal fire cycles from 30 to 100 years,” added Minnich, who also chairs the Department of Earth Sciences.  “There is, therefore, a tremendous build-up of fuel. The situation here, which could be quite explosive where fires are concerned, is exacerbated by fire suppression, which prevents fires except in the most extreme weather conditions, such as Santa Ana winds.”

Minnich explained that in the 19th century fires in Southern California spread slowly and were rarely threatening to society — even in Pasadena, the first suburb to place houses at the edge of the chaparral.

“But in recent decades we have witnessed a succession of catastrophic wind-driven fires that readily destroy homes and denude large areas of watershed, more than 100,000 acres at a time,” he said.  “We need to accept fire as part of the ecological process in Southern California.   Slow burning not only removes fuels, but poses reduced risk to land use.  We can learn from slow burning in the Sierra Nevada and, more locally, with the Day and Zaca fires which spread for 1-2 months each in recent summers.  Here, firefighters could mitigate slow-moving flames near towns.”

Minnich predicts that Southern California’s desert areas will face little threat from fires this year.

“The deserts have been very dry,” he said, “with hardly any grasses or flowers in sight.”

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Richard Minnich
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E-mail: richard.minnich@ucr.edu

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