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Science of Now: How recently burned hills prone to mudslides recover

Trees burned in the Colby Fire above Glendora, Calif.
Leslie Berestein Rojas/KPCC
Trees burned in the Colby Fire above Glendora, Calif.

On this rainy day, officials in L.A. County are keeping an eye on four areas burned by wildfire over the past couple of years.

Areas like the canyons above Glendora burned by January’s Colby Fire can produce mudslides during heavy rains.

That's in part because plants torched in a wildfire can leave behind an oily, waxy residue explained UC Riverside soil researcher Robert Graham.

That thin layer coats the soil about 5 to 10 centimeters below the surface, creating a water repellant barrier similar to the wax coat put on a car.

"So when the water comes down it just beads up," he said.

These rain drops pick up any loose dirt as they wash down the hillside, creating mudslides.

Dirt-digging insects like worms and ants are key to rehabilitating a burned zone, Graham said.

"They basically break holes in the water repellent layer," he said.

He says ants often start reestablishing their boroughs days after a fire, creating pores in the waxy layer where water can be absorbed. 

Within a few weeks, gophers and squirrels can show up tilling the soil as they dig holes or bury food.

This further breaks up the waxy, water resistant layer, allowing for more water absorption.

But Graham says a burn zone isn’t really safe until plants take hold since their roots keep loose soil in place and their leaves act like umbrellas shielding the ground from some of the rain.

However, it can take anywhere from 5 to 10 years before a healthy plant cover returns, he noted.