Climate Change: Tree Mortality

“Traffic, deadlines, health, car trouble…” What things in life causes you stress? For trees in Yosemite, that list might look more like “Water, bugs, disease, and my 100 closest neighbors.” California lost 142 million trees between 2012 and 2018 due to the combined impacts of many different stress factors, compounded by climate change. Forest health is particularly significant because growing vegetation serves the crucial role of removing carbon from the atmosphere and storing it in living ecosystems. Rising rates of tree death across the western US decrease the capacity of forests to store carbon, increasing the magnitude of global climate change.

A Domino Effect

When European settlers arrived in Yosemite in the 1850s, they began a policy of suppressing natural fires that had previously burned regularly on the landscape. They also ended the periodic burning of Yosemite Valley that had been conducted for hundreds of years by Yosemite’s first people. Without fire to thin the forest and open the canopy, forests became denser and more crowded. Today, fire is seen as essential for forest health and park fire managers use both natural and prescribed burns to help restore historic conditions. However, many areas of high-density forest remain, increasing competition for limited soil moisture during periods of drought.

 

Yosemite’s forests rely on regular fire to maintain space between trees.
Left: Forest burned in a prescribed burn in 1978 and by wildfire in 1996. (2007 photo).
Right: Forest unburned since 1936 (2008 photo). Figure from Lutz et al., 2009.

Drought in California is caused by complex long- and short-term climate processes that block winter storms from reaching the state, lowering winter precipitation and Sierra Nevada snowpack. Tree ring studies show that California has experienced repeated cycles of natural drought going back over a thousand years; recent significant droughts took place in 1929–34, 1976–77, 1987–92, and 2007–09. In 2012–16, record-high temperatures combined with dry conditions to create the most intense drought in centuries. Scientists refer to this event as a “hotter drought,” where unusually high temperatures magnify the threat to forests by increasing evapotranspiration (evaporation from the ground and from plant leaves), resulting in even less water availability.

Like dehydrated people, drought-stressed trees can develop other health problems. Bark beetles take advantage of drought to burrow into the living phloem (inner bark) without being ejected by pitch, the tree’s first line of defense. After invading, beetles carve galleries and lay eggs which hatch into larvae and continue to chew through the phloem, choking off the tree’s circulation. Pines in Yosemite are vulnerable to several different pine beetles (Dendroctonus spp.), which also transmit fungal spores that infect the trunk. Red and white fir trees can be attacked by fir engraver beetles (Scolytus spp.), while incense cedars host at least six different cedar bark beetles (Phloeosinus spp.). Like the final dominoes in a long cascade, these beetles (along with many other insects and fungal pathogens) can be the final cause of death for overcrowded, dehydrated trees.

But this domino effect isn’t always straightforward. Although all of these beetles are native to the Sierra Nevada, climate change is likely to affect their populations and phenology (biological timing) in ways that vary from species to species. During outbreaks when insect numbers are high enough, even healthy trees may be killed. At the same time, warming temperatures alone can create chronic stress on trees, making the effects of drought, insect attack, and fungal disease more severe.

Across the western US, the rate of tree mortality doubled between 1955 and 2007. Scientists identify the underlying cause as higher temperatures and water deficits associated with human-caused climate change. But how can we use the past to understand the future of Yosemite’s forests? Like forensic scientists at a crime scene, biologists use many different tools to determine the what, where, when, and why of tree death.

 

Forest mortality research takes place at both large and small scales.
Left: A photo taken during an aerial survey in 2018 shows mortality in mixed-conifer forest in near Yosemite West (USFS photo).
Right: A research team surveys a plot, gathering data on forest health over time (USGS photo).

When a tree falls in the forest...

Forests are vast ecosystems, difficult to understand fully from ground level. Every year, the US Forest Service use small planes to conduct aerial surveys, measuring the extent of mortality for different tree species throughout much of California (including Yosemite).

In addition to documenting the what and where of tree death, scientists work to discover the when and why. Some forest plots in Yosemite have been studied continuously since the early 1990s, with researchers visiting every year to track the number and species of living and dead trees. These studies provide a real-time record of conditions that lead to death and new life in a forest. But how do you figure out what killed a tree, decades after the tree’s death? Using tree rings to determine the year of death paired with historical data on drought severity and snowpack levels, scientists can look back into a forest’s past (see chart below).

Like forests themselves, the study of forest health is large-scale and long-term—just a handful of trees or a few years of observation can’t tell the whole story. Fire or insect outbreak can devastate some patches of forest while leaving others untouched; a single wet year can be followed by a longer drought. Large-scale studies that combine data from across the western US over many decades provide important context as we work to understand how changes seen in Yosemite’s forests.

 

A tree ring study shows that high mortality is linked to periods of low snowpack and high drought. Models predict Sierra Nevada snowpack to drop 64% by 2100 as winter temperatures continue to warm, with unknown implications for forests. (Figure from Guarín and Taylor, 2005.)

What changes are we seeing?

• Increasing mortality: Red fir and mixed conifer plots in Yosemite that have been studied continuously since the 1990s show trends of increasing mortality. This reflects a larger pattern across California’s forests, where mortality rates nearly doubled between 1983 and 2007.
• Snowpack matters: Tree ring analysis in Yosemite shows that high mortality is triggered by multi-year periods of low spring snowpack and corresponding high drought conditions.

• Disappearing giants: Between the 1930s and 1990s, the density of large-diameter trees in Yosemite declined by 24%. The researchers attribute this change to water stress associated with climate change, making survival more difficult at sites that had historically been optimal.

What’s next?

• Hotter droughts: Unusually high tree mortality associated with the 2012–16 drought is likely to become more common as “hotter droughts” become the norm. Even if precipitation patterns remain unaltered, increased evapotranspiration caused by warmer weather and a longer growing season may increase demand for limited water.
• Novel pests and pathogens: Species not currently found in Yosemite, such as the fungus responsible for sudden oak death (Phytophthora ramorum), the invasive shot-hole borer beetle (Euwallacea sp.), and others may arrive in coming decades, leaving outsized impacts on forests already stressed by climate-related factors.
• Uncertain futures: Some scientists project tree mortality to continue to climb, with up to a 15–20% increase in tree death for each additional degree of warming. However, as survival becomes harder in historically favorable areas, it may grow easier in places that were previously inhospitable. In other words, as young trees expand into new zones, other stands of old-growth forest may be lost.

 

Left: A giant sequoia in the Mariposa Grove shows thinning foliage.
Right: Beetle galleries are visible on the limb of a sequoia.

Threats to Yosemite's Sequoias

Giant sequoias are famously long-lived trees, with many “monarchs” reaching 2,000 years of age or more. Scientists have long believed them resistant to low-intensity fire, drought, and insect attack; in the past, the main cause of death in sequoia groves was high winds, bringing the shallow-rooted giants crashing to the ground. However, over 2014–19 (during and after the historic drought of 2012–16), Sequoia and Kings Canyon National Parks lost 28 standing monarchs to beetle infestation. Several sequoias in Yosemite’s Mariposa Grove also died. These unexpected losses have prompted concern about the future of the big trees as hotter droughts become more common; experts fear that more large sequoias could be lost in coming years.

Carbon Storehouses

What is a forest worth? Apart from its value to human visitors and local ecosystems, trees play a critical role in sequestering (storing) carbon that would otherwise enter the atmosphere as a greenhouse gas. As plants grow, die, decompose, and are burned in fires, carbon cycles in and out of the atmosphere. According to a 2015 study, at any given time vegetation in Yosemite stores around 550,000 metric tons of carbon—roughly equivalent to the annual emissions of 2.6 million Americans. However, Yosemite’s carbon storage capacity has fallen in recent years largely due to vegetation lost in large wildfires, and researchers project the park’s capacity to drop to 351,000 metric tons by 2050. Increasing wildfire and slower vegetation growth rates across the western US raise the possibility that western forests could transform from carbon “sinks” to carbon “sources”—producing more carbon than they absorb. Rising atmospheric carbon, in turn, may increase forest mortality and fire activity, further decreasing carbon storage capacity in a feedback cascade with long-term impacts.

 

 

References

• Banasiak, Adam, Linda Bilmes, and John B. Loomis. “Carbon Sequestration in the U.S. National Parks: A Value Beyond Visitation.” SSRN Electronic Journal, 2015. https://doi.org/10.2139/ssrn.2577365.

• Das, Adrian, John Battles, Phillip J. Van Mantgem, and Nathan L. Stephenson. “Spatial Elements Of Mortality Risk In Old-Growth Forests.” Ecology 89, no. 6 (2008): 1744–56. https://doi.org/10.1890/07-0524.1.

• Dorminey, Bruce. “Giant Sequoias Face Looming Threat from Shifting Climate.” Yale Environment 360. Yale School of Forestry & Environmental Studies, March 21, 2013.

• Gonzalez, P., John J. Battles, Brandon M. Collins, Timothy Robards, and David S. Saah. Aboveground live carbon stock changes of California wildland ecosystems, 2001-2010. Forest Ecology and Management 348 (2015): 68–77. https://doi.org/10.1016/j.foreco.2015.03.040
• Griffin, Daniel, and Kevin J. Anchukaitis. “How Unusual Is the 2012-2014 California Drought?” Geophysical Research Letters 41, no. 24 (2014): 9017–23. https://doi.org/10.1002/2014gl062433.

• Guarín, Alejandro, and Alan H. Taylor. “Drought Triggered Tree Mortality in Mixed Conifer Forests in Yosemite National Park, California, USA.” Forest Ecology and Management 218, no. 1-3 (2005): 229–44. https://doi.org/10.1016/j.foreco.2005.07.014.

• Jonest, Jeanine. “California's Most Significant Droughts: Comparing Historical and Recent Conditions.” State of California: California Department of Water Resources, 2015.

• Lutz, J.A., J.W. Van Wagtendonk, and J.F. Franklin. “Twentieth-Century Decline of Large-Diameter Trees in Yosemite National Park, California, USA.” Forest Ecology and Management 257, no. 11 (2009): 2296–2307. https://doi.org/10.1016/j.foreco.2009.03.009.

• Moore, Jeffrey, Jacqueline Pope, Meghan Woods, and Adam Ellis. “2018 Aerial Survey Results: California.” USNFS Pacific Southwest Region Forest Health Monitoring Program, 2019.

• Moran, Emily V., Adrian J. Das, Jon E. Keeley, and Nathan L. Stephenson. “Negative Impacts of Summer Heat on Sierra Nevada Tree Seedlings.” Ecosphere 10, no. 6 (2019). https://doi.org/10.1002/ecs2.2776.

• Stephenson, Nathan L., Adrian J. Das, Nicholas J. Ampersee, Beverly M. Bulaon, and Julie L. Yee. “Which trees die during drought? The key role of insect host‐tree selection.” Journal of Ecology 107, no. 5 (2019): 2383–2401. https://doi.org/10.1111/1365-2745.13176.

• van Mantgem, P. J., N. L. Stephenson, J. C. Byrne, L. D. Daniels, J. F. Franklin, P. Z. Fule, M. E. Harmon, et al. “Widespread Increase of Tree Mortality Rates in the Western United States.” Science 323, no. 5913 (2009): 521–24. https://doi.org/10.1126/science.1165000.

Learn More

• Check out an interactive map showing forest mortality across California with the Tree Mortality Viewer.
• Read more about Yosemite's "Ghost Forests."
• Learn about safety in areas of high tree mortality.