Sierra Nevada Forests: Climate Hero or Villain?

Forests are identified as California’s largest carbon sink by the California Air Resources Board. However, today many Sierra Nevada forests are overgrown and are suffering from insect attacks, drought, and large, damaging wildfires. They are no longer the reliable carbon sink that California has depended on, but with help, they can become our climate heroes once again. 

Overgrown, Unhealthy Forests: California’s Climate Villain   



“Trees in California should absorb CO2, not generate huge amounts of black carbon and greenhouse gas as they do today when forest fires rage across the land.” Governor Edmund G. Brown Jr.   

Historically, Sierra Nevada forests have helped regulate our climate by removing carbon dioxide from the atmosphere and storing it as carbon in the soil, branches, and trunks of trees.  

However, research shows that between 2001 and 2010, California’s forests emitted more carbon than they sequestered, and since then, conditions in Sierra forests have gotten worse. Between 2010 and 2017, over 200 million trees in the Sierra Nevada were killed by wildfire, bark beetles, and drought. Once killed, these trees stop absorbing carbon dioxide and begin to release their stored carbon back to the atmosphere, turning them from a net sink to a net source of greenhouse gas emissions.  

Overgrown, unhealthy forests and their resulting large, damaging wildfires and tree mortality are causing serious problems for California’s greenhouse gas reduction goals.


The situation in the Sierra Nevada is cause for alarm, but it’s not too late. If we acknowledge that large, damaging wildfires and tree mortality are major problems for our climate, we can apply the most effective solutions to address them.    

Wildfires and Greenhouse Gas Emissions     

Wildfires release stored carbon back to the atmosphere as greenhouse gas and particulate matter as plants and trees burn, and the larger and more severe the fire, the more significant the emissions. Emissions from large, damaging wildfires can equal in just a few weeks what other industrial greenhouse gas sources produce in a year. For example, the smoke plume of the 2013 Rim Fire emitted what 2.57 million cars emit in a year.   

An increase in high-severity fire in the Sierra Nevada is having long term implications on carbon storage.

However, the most significant impact on the atmosphere from these large, damaging wildfires occurs after the event. When areas burn at high severity, most of the trees are killed. Once killed, these trees stop absorbing carbon dioxide and begin to release their stored carbon back to the atmosphere. In the Sierra Nevada, larger and larger patches of area are burning at high severity, and these large patches can shift Sierra forests from a carbon sink to a carbon source. One study found that a forest burned at high severity was still a net source of carbon to the atmosphere 15 years after the fire occurred, and was expected to continue to be so for more years to come.

We also can’t count on post-fire regrowth to balance emissions from wildfires anymore.

In addition, vegetation that returns after a wildfire can take decades, if ever, to restore the same level of sequestration and carbon storage. Forests that burned at high severity in recent megafires are showing less post-fire regeneration than normal. This suggests that they are at risk of being replaced with shrub and grasslands, which frequently re-burn at high severity and store only ten percent of the carbon than the forests they replaced did.   

Tree Mortality and Greenhouse Gas Emissions

California has experienced an unprecedented rise in tree die-off during recent drought years. According to the U.S. Forest Service, 129 million trees have died across the state from drought, insects, and disease since 2010, and 85 percent of those dead trees are in the Sierra Nevada.

Recent tree mortality will have both immediate and long-term impacts on the stability of carbon in Sierra Nevada forests.

Bark beetles, the primary agent causing die-off, target the largest trees – our most effective carbon storage and sequestration tools. The carbon in these dead trees will slowly decay over the next few decades, or be quickly released in future fire events. The Sierra Nevada Conservancy estimates that 52 million metric tons of carbon dioxide equivalent moved to the dead pool due to tree mortality in the Southern Sierra in 2016. Those dead trees will decay, and their emissions will equal more than the emissions reductions California’s economy made over three prior years – combined.  


Healthy forests capture and store more carbon.

The good news is that the activities that reduce the risk of large, damaging wildfires and strengthen forests against drought and bark beetles also protect carbon storage and sequestration in the Sierra Nevada. Thinning and prescribed and managed fires help to reduce greenhouse gas emissions from Sierra forests over the long term by reducing fire severity, protecting forests from drought and insects, and lowering emissions.


Healthy forests, even during more challenging climate conditions, can continue absorbing carbon from the atmosphere at a significant rate, and the larger the tree, the more carbon it will pull from the atmosphere on an annual basis.

Photo credit: U.S. Forest Service, Pacific Southwest Research Station

Thinning overgrown forests and returning low-severity fire to the landscape can improve our forests’ ability to absorb and store carbon. The animation to the left shows tree growth before and after an aggressive thinning treatment in the northern Sierra. While this particular thinning approach may not be appropriate for all areas in the Sierra, research shows that thinning the forest frees up additional resources for the remaining trees, which allows them to grow more in the years following thinning efforts and absorb more carbon each year.  






Healthy Forests: California’s Climate Hero

California is currently making decisions on how best to meet our greenhouse gas reduction targets, and the opportunity exists now to elevate forests as a key program. If we don’t, Sierra Nevada forests will continue to offset California’s greenhouse gas reduction progress and make our current investments less effective.

WIP Logo


The Sierra Nevada Watershed Improvement Program – a partnership led by the Sierra Nevada Conservancy and the U.S. Forest Service – is working to stabilize forest carbon by increasing the pace and scale of ecologically sound restoration across the Sierra Nevada Region. The Sierra Nevada Conservancy has invested nearly $10 million in Proposition 1 funds in support of the Sierra Nevada Watershed Improvement Program, but additional funding, policy changes, and infrastructure are still needed to establish Sierra Nevada forests as a more reliable long-term carbon sink. To learn more about the Sierra Nevada Watershed Improvement Program visit   



The 2008 Climate Change Scoping Plan is the framework for implementing Assembly Bill 32, the California Global Warming Solutions Act of 2006. A Forest Climate Action Team (FCAT) was assembled in August 2014 to develop a Forest Carbon Plan, and the SNC is a member of this team. The overall goal guiding the Forest Carbon Plan is to firmly establish California’s forests as a more reliable long-term carbon sink, as opposed to a carbon source, and Sierra Nevada forests play a critical role in achieving that goal.   

The Final Forest Carbon Plan is now available and the SNC prepared this statement on the Plan.