Forest Health and Climate Change

View of dead pines on a slope in Sequoia National Park, October 2015. These trees died during the "hotter drought" of 2012-2016.

Photo courtesy of Nate Stephenson, USGS Sequoia-Kings Canyon Field Station

Tree death from disease, insects, severe weather, or fire is a natural process in forests. The death of older or unhealthy trees makes room for young trees, provides nesting sites in standing snags for cavity-nesters like woodpeckers, and adds nutrients to the soil as the older trees decay or burn. But if many trees die in a short amount of time, over a large area, there can be long-term impacts to forest health. Dead trees near developed areas pose hazards as they may be at greater risk of falling on people or infrastructure. It’s important we better understand large-scale tree mortality and develop strategies to increase forest resilience to changing climate.

 

Climate Change Impacts on Forests

Regional warming is implicated in the near doubling of annual tree-mortality rates measured in Sequoia National Park between 1983 and 2004. During the severe drought from 2012-2016, large numbers of trees died across the Sierra Nevada, particularly at lower elevations between 5,000 to 6,000 feet (see photo at top of page). This drought was severe because it was a “hotter drought”. Trees are already stressed from low levels of precipitation. The higher temperatures cause increased water loss from plants when the heat converts more fluid to water vapor (called evapotranspiration). Increased evapotranspiration made even less water available for the trees. This acute drought stress combined with bark beetle attacks resulted in greatly elevated mortality for several common tree species – ponderosa pine, sugar pine, incense-cedar, and white fir. During this hotter drought, scientists documented beetle kill of giant sequoias for the first time. Learn more on the Giant Sequoias and Climate page.

 

Scientist views dead or dying whitebark pine trees in Kings Canyon National Park.

NPS Photo - Sean Auclair

Even High Elevation Pines Showing Stress

In 2021 and 2022, local scientists starting noticing that increased tree death was also occurring at higher elevations. Even foxtail pine and whitebark pine, trees found at the highest elevations where trees can grow, are dying at higher numbers than previously observed. Some trees show signs of stress with browning foliage, and some have signs of bark beetles or disease. Survey efforts are underway to determine the spatial extent, severity, and causes of this tree death. Even trees at higher elevations may be showing vulnerability to warmer temperatures and recent hotter drought.

On December 15, 2022, the U.S. Fish and Wildlife Service (FWS) listed whitebark pine as threatened with potential extinction under the Endangered Species Act due to severe declines throughout much of its range. FWS determined that the primary stressor driving the status of whitebark pine is white pine blister rust, a fungal disease caused by the nonnative pathogen Cronartium ribicola. Whitebark pine is also negatively affected by the mountain pine beetle, altered fire regimes, and the effects of climate change.

 

View of extensive tree mortality in Redwood Mountain Grove, following 2021 KNP Complex Fire, June 2022.

NPS - Linda Mutch

Higher Severity Fires Kill Thousands of Trees

Much of the montane forest zone is at risk of severe fire because many areas have not burned in recent decades; warming, drying conditions also contribute to longer fire seasons and more extreme burning conditions. Since 2015, high-severity wildfires killed many trees in Sierran mixed-conifer forests, including an estimated 13-19 percent of all large giant sequoias. See the Giant Sequoias and Fire page for more information.

From the late 1860s to the early 1900s, grazing and decline of Native American use of fire reduced fire frequency, followed by fire suppression that excluded fire from most of these forests. This led to dense forests and heavy accumulations of fuels (sticks, logs, and other debris) on the forest floor. Starting in 1968, these parks began prescribed burning and managing some lightning-caused fires for ecological benefit in remote areas.These changes have helped make some forests more resilient to climate change and wildfire, but many forests have not had recent fire and remain vulnerable to burning at high severity (see photo).

 

When pine bark beetles feed on the inner living tissue underneath tree bark, they create patterns that help distinguish which type of beetle killed or damaged the tree. These are Dendroctonus ponderosae beetle galleries on a sugar pine.

Photo courtesy of USGS Sequoia-Kings Canyon Field Station

Bark Beetles and Pathogens

Warming temperatures can affect bark beetle life cycles in different ways, and the drought-stressed trees have increased vulnerability to beetle attack. They often cannot produce the pitch that helps protect them from beetles. Once beetles burrow through the bark and reach the inner bark (living tissue called phloem) that transports proteins, sugar, and nutrients, they create intricate patterns of galleries or tunnels when they feed. They lay eggs and produce larvae that continue this feeding and tunneling. Eventually, there are so many galleries that the flow of important “food” for the tree is cut off or greatly reduced. Bark beetle attack, along with other insects and pathogens, is often their final cause of death.

In addition to bark beetle impacts, sugar pine and western white pine have also declined from a non-native pathogen called white pine blister rust. This disease is often fatal as it damages and kills these pines by girdling branches and trunks, which restricts nutrient and water flow; it also causes wounds in the trees’ bark, making trees more susceptible to bark beetle attacks. Altered fire regimes, with changes in severity and frequency may also increase death rates. Learn more about declines in white pine species from this short article summarizing recent research.

 

Scientist measures the diameter of a dead foxtail pine in Sequoia National Park as part of a long-term monitoring project on foxtail and whitebark pine stands.

© Michael Durham

Understanding Tree Death

Park managers, local scientists, and partners are conducting research to better understand the different interacting factors affecting tree death. This knowledge informs management decisions for increasing forest resilience to stress from changing climate, hotter droughts, and more severe wildfires.

For current research related to giant sequoias, visit Giant Sequoias and Climate. And to learn about the effects of recent high-severity wildfires on giant sequoias, explore Giant Sequoias and Fire. Or learn more about park fire monitoring and research that informs the fire management program.

 

Managing Tree Hazards

 

What Are Tree Hazards?

The National Park Service defines tree hazard as any tree, either alive or dead, which due to outwardly visible defects could fall down (in part or entirety) and strike a person or property within any designated portion of a development zone.

 

Two dead pines growing near park buildings have been cut for removal by park foresters.

NPS Photo

Diseases and insects, along with wind, snow, fire, and local conditions like soil moisture, combine with human activities to create tree hazards. Dead trees, and live trees with structural defects, can present hazards in the parks’ developed areas, including campgrounds, roadways, visitor centers, and administrative sites. These defects contribute to tree failures (parts of the tree breaking off, or the entire tree falling). Because trees often contain structural defects that contribute to their failure and constitute a hazard to humans, their preservation must be balanced with the need for visitor safety.

As part of the tree hazard management program, foresters evaluate trees in developed areas for structural soundness and determine whether they pose a significant risk to people or buildings. Specially trained crews remove trees or parts of trees that are likely to fail and cause harm to life or property. During and after the 2012-2016 drought, thousands of trees had to be removed from developed areas. Unfortunately, trees without apparent defects also fail, thus tree hazards cannot always be immediately identified and abated or mitigated.

Visit our Tree Hazards page to find tips about staying safe from falling trees and branches.