August Fuel Moisture: Live, Dead, and Combined

Post date: Aug 30, 2015 6:37:35 PM

While I’ve written many times about fuel moisture, an email from and subsequent conversation with Sarah Collamer begs yet another exposition.

One of Sarah’s duties as a forester at CAL FIRE is take monthly measurements for the National Fuel Moisture Database at four reference sites in Santa Cruz and San Mateo counties. 

She reports that, due to the unusual light rains this summer, the live fuel moisture actually increased in August as new growth emerged well outside of the usual growing season. At the same time, dead fuel moisture continues to fall as the foliage on our stressed conifers thins drastically and our drought deciduous hardwoods are losing their leaves a month or so early.

The effects are particularly striking in our chaparral and in our forest understory shrubs. If grass is the spark plug for wildfires, then scrub is the carburetor. With a dense thicket of dead twigs in the interior, fallen leaves covering the ground, and a canopy of live foliage sparse enough to allow the sun to bake the ground and thick enough to trap the heated air, scrub provides a prime situation for heat, fuel and oxygen to come together and yield explosive wildfires with enough heat to rapidly dry and ignite the new growth.