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Blog Post · May 30, 2017

How Unhealthy Forests Affect Water Supply

California’s mountain forests have been badly stressed by years of drought and fire suppression practices that encourage overly dense stands of trees. We talked to Scott Stephens―a forestry and wildfire expert at UC Berkeley and a member of the PPIC Water Policy Center research network―about how unhealthy forests affect the watershed.

PPIC: What is the status of the state’s forests?

Scott Stephens: California now has more than 100 million dead trees, mostly in the central and southern Sierra. This has implications for water supply, wildfire management, local economies, and many other issues. The die-off is a symptom of unsustainable forest conditions. Drought is part of the California landscape, but why are we seeing such profound mortality? The drought caused more trees to compete for less water and increased infestations of bark beetles, which kill trees. Droughts are getting warmer, which stresses the trees more. But the underlying cause is unsustainably dense forests. Forest management in the past century increased forest density by removing the most common ecosystem process that once thinned the forest: fire. We need to reduce tree density in Sierra forests so they are more resilient to drought.

Fires start more easily in recently killed forests because embers have more dead foliage to land on. Fighting fires is much harder because there’s an increased risk that standing dead trees will fall and kill people. And since most of these dead trees will never be removed, in 10 to 15 years they’ll be on the ground. This will increase the fuel load substantially and make for hotter fires.

PPIC: What are the consequences to our water supply from dying or burning forests?

SS: It’s useful to compare the effects of a large uncontrolled wildfire—such as the 2013 Rim Fire in San Francisco’s Hetch Hetchy watershed—with what is going on in a part of Yosemite that has had regular fires over the past 40 years. In the Rim Fire zone, trees were killed over a vast area, and sediment and fire debris has moved into streams and reservoirs. By comparison, there’s an area in Yosemite called Illiloutte Creek basin, where small natural fires have been allowed to burn—unlike in most forests, where all fires are extinguished as quickly as possible. We’ve seen a jigsaw puzzle of fire-changed landscapes develop in Illiloutte basin over this time and a change in forest density. For example, an area that was once a solid pine forest is now a wetland after fire cleared about 10 acres of trees. Now we’re doing research to see if water in that basin is increasing. One thing is certain: fewer trees use less water. Also, in denser forests some snow stays in the canopy and is more likely to go back into the atmosphere rather than seep into the ground.

PPIC: Given the scope of the problem, what are our management options?

SS: There are three potential options that would improve on current practices. The first is to manage lightning fires in some areas—such as the remote upper watersheds―so we can naturally reduce forest density. This option can be scaled up relatively fast. We’d need to monitor these fires carefully and allow them to work―just as they’ve done in Yosemite for the past 40 years. Three national forests have proposed allowing managed lightning fires on about two-thirds of their land―Inyo, Sierra, and Sequoia. A second option is to do more prescribed burns. These will be smaller in scale than lightning fires. Lastly, we can use ecologically based mechanical thinning of forests, which can be combined with prescribed burns.

These fixes will be expensive in some areas. But fire suppression currently costs us about $2.5 billion a year just on federal lands—and these methods can help bring down that cost. Shifting the federal firefighting budget so more money goes for forest management could help.

Learn more

Read California’s Water: Protecting Headwaters (from California’s Water briefing kit, October 2016)
Watch our 3-minute video “Headwaters”
Read “Managing Wildfires Requires New Strategies” (PPIC Blog, September 23, 2015)
Visit the PPIC Water Policy Center’s drought resource page


climate change Drought Forests and Fires Water Supply Water, Land & Air wildfires