That’s in part because more trees means less water. Not only do trees use lots of water to carry out basic biological tasks, they also act like forest steam stacks, raking up water stored in the ground and expelling it into the atmosphere as vapor, where it’s inaccessible to humans and forest ecosystems until it falls back to earth as rain and snow.

Billions of Gallons of Water Saved by Thinning Sierra Forests

Jason Alvarez

U.C. Merced

There are too many trees in Sierra Nevada forests, say experts from UC Merced, UC Irvine and the National Park Service working at the National Science Foundation Southern Sierra Critical Zone Observatory (NSF SSCZO).

This comes as a surprise to those of us who see dense, verdant forests as a sign of a healthy environment. After all, green is good, right? Not necessarily. When it comes to the number of trees in California forests, bigger isn’t always better.

That’s in part because more trees means less water. Not only do trees use lots of water to carry out basic biological tasks, they also act like forest steam stacks, raking up water stored in the ground and expelling it into the atmosphere as vapor, where it’s inaccessible to humans and forest ecosystems until it falls back to earth as rain and snow. This process — by which plants emit water through tiny pores in their leaves — is known as evapotranspiration (ET). And according to experts, excessive ET may harm a fragile California water system, especially during prolonged, warm droughts.

Mechanically-thinned forest in the foreground with dense, unthinned forest in the background. The thinned forest resembles the Sierra Nevada forests of a century ago, before management policy encouraged forest to become unnaturally dense.

But new research published in the journal Ecohydrology shows that water loss from evapotranspiration has decreased significantly over the past three decades due in large part to wildfire-driven forest thinning — a finding with important implications for forest and water management.

A century of forest management kept wildfire to a minimum. But without fire, Sierra forests grew exceedingly dense. In recent decades, new policies have allowed nature to take its course, with wildfires helping to thin out overgrown forests. The new study shows that the impact on water supplies has been tremendous.

“Forest wildfires are often considered disasters,” said Richard Yuretich, director of the National Science Foundation’s Critical Zone Observatories program. “But in fact, fire is part of healthy forest ecosystems. By thinning out trees, fires can both reduce water stress in forests and ease water shortages during droughts. And by reducing the water used by plants, more rainfall flows into rivers and accumulates in groundwater.

In Yosemite, prescribed burning saved the Tuolumne giant sequoia grove from the Rim Fire, the largest-ever Sierra Nevada wildfire.

Using data from U.S. Geological Survey satellites and NSF SSCZO measurement towers, researchers found that over the period 1990-2008, fire-thinned forests saved 3.7 billion gallons of water annually in the Kings River Basin and a whopping 17 billion gallons of water annually in the wetter, warmer American River Basin — water that would otherwise have been lost through evapotranspiration.

Prescribed forest thinning has increased in recent decades in an effort to stave off disastrous wildfires fueled by dense forest. But this study shows that restoring forests through mechanical thinning or wildfire can also save the state billions of gallons of water each year.

See the full report here


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