Why is my Tree Dying? How Drier Summers Affect Native Trees

The problem first gained attention in bigleaf maple trees, which have been dying off since 2011 in some parts of Washington and Oregon.

The Washington State Department of Natural Resources (DNR), the University of Washington, the U.S. Forest Service, the Oregon Department of Forestry, and Oregon State University tried to find out why. They investigated a wide variety of insect-related diseases and disease-causing fungi to determine if any of those could be the cause of the sick and dying bigleaf maples.

Despite many attempts to uncover the cause and treat the trees in Western Washington, by the end of 2018 there were no signs of recovery. The most recent decline symptoms include partial or entire crown dieback, discoloration and reduced size of the leaves, crown thinning, and death.

A University of Washington study suggests that increased human development, higher summer temperatures, and severe summer droughts are linked to the declining health of bigleaf maples.

Now scientists fear that other Pacific Northwest tree species are suffering a similar fate. Across Washington, people are reporting diebacks not only in big-leaf maples, but in Douglas-fir, western red cedar pines, and western hemlock, Washington’s official tree.

Damage and mortality in Douglas-fir and western red cedar was immediately noticeable during the drought in 2015. Symptoms included entirely red crowns, red tops and scattered red branches, with symptoms becoming more severe during record-breaking heat in spring 2016. It was more difficult to notice in western hemlock, because many dying hemlocks dropped foliage without color change.

DNR has received a number of calls from people who have seen tree dieback on their property. Read on for more information about why it’s happening and how to help your trees.

Droughts and tree health

drought stress on a Douglas-fir
Branch flagging in Douglas-fir in Thurston County caused by drought stress.

Droughts occur when average temperatures increase and average precipitation decreases. As explained by the Oregon Department of Forestry, drought conditions can “create water stress inside the tree and can reduce growth or cause mortality.”

In Washington, drought conditions have reached moderate to severe levels since 2012, leaving many of Washington’s native trees struggling to survive because they can’t get enough water. And drought conditions are expected to get worse with climate change.

“Under extreme water stress, the water column (in a tree) breaks at some point in the stem,” said Kevin Zobrist, associate professor with Washington State University’s Extension Forestry program. “This can result in an air bubble in the pipe, which is called an embolism. The chain of water molecules is now disconnected between the roots and the leaves. If the tree is unable to repair the disconnect, that water pathway no longer functions and everything above it dies.”

It is also important to care for trees that are exposed to drought conditions, as they become more vulnerable to pathogens, insects, and diseases. The Oregon Department of Forestry suggests the following 10 tips to mitigate drought stress on trees:

  1. Select native and local drought-tolerant species that are appropriate for your site and soil conditions.
  2. Thin stands during normal years, not within a drought if possible, as thinning can cause a short-term increase in water stress. Remove damaged, stressed or overly mature trees.
  3. Control vegetation (especially grasses) that compete for soil moisture.
  4. Remove or destroy freshly dead or dying trees and slash or blowdown created in the previous year to prevent insect infestations and outbreaks.
  5. Avoid damaging and compacting soil around tree root zones from vehicles, grazing animals, etc. – especially during the wet season.
  6. Irrigate landscape trees during dry weather. Apply water slowly over many hours so it penetrates to tree roots, or use drip irrigation.
  7. Apply mulch to landscape trees to retain soil moisture.
  8. Do not alter drainage patterns near established trees.
  9. Do not fertilize during droughts. Fertilizer stimulates foliage production and can increase water requirements.
  10. It may be less effective to use systemic pesticides, which are absorbed into a plant’s tissues, on drought-stressed trees because these pesticides rely on water translocation within the tree.

Research on bigleaf maple decline

A bigleaf maple tree showing a loss of leaves
Many bigleaf maple trees in Washington have exhibited symptoms of decline, including partial to entire crown dieback, discoloration and reduced leaf size, loss of leaves and death.

In 2011, more and more bigleaf maples in Western Washington seemed to be sick or dying. Some of the most prominent symptoms were yellow flagging of large branches, small leaf size, and partial or entire crown dieback. Sixty-one sites were sampled to determine if Verticillium wilt was the causing factor.

In 2014 and 2015, DNR, the U.S. Forest Service, Oregon State University, and the Oregon Department of Forestry conducted a survey to determine if microscopic root diseases were the cause of the bigleaf maple’s dieback. By mid-2016, many causes had been investigated such as armillaria root disease, verticillium wilt, fungal pathogens neonectria and nectria, and other small pathogens. All of these causes were ruled out.

By the end of 2018, there was no improvement in the condition of the bigleaf maples in Western Washington, but increased drought conditions correlated with the declining health of the trees.

Climate change, drought, and the forest

Seven out of the past 10 years have been warmer than average in Washington state, supporting projections by the Climate Impact Group at the University of Washington.

The group’s report found that “the Pacific Northwest is projected to warm rapidly during the 21st century, relative to 20th century average climate, as a result of greenhouse gases emitted from human activities.”

Regarding precipitation patterns, Washington has seen a mix of wetter, normal, and drier years over the past 10 years. This mixed precipitation pattern is also supported by the Climate Impact Group’s report, which suggests that seasonal precipitation changes will be mixed but “most models project drier summers.” Precipitation patterns are expected “to be primarily driven by year-to-year variations rather than long-term trends, but heavy rainfall events are projected to become more severe.”

Unfortunately, heavy rain events caused by climate change are not going to offset the effects of drought conditions on trees. As explained by DNR scientist Glenn Kohler: “Increases in rainfall are coming in spring and fall, not during the hot, dry summer months when trees are experiencing the most moisture stress during these severe droughts.”

Washington’s iconic native trees aren’t the only things that climate change is threatening. If emissions continue to increase, temperatures will continue to rise, which will have a negative impact on Washington’s economy, agriculture, habitats, and water resources.

In 2018, the Climate Impact Group updated its report on how climate change will affect the state, and the far-reaching consequences of inaction.

Learn more about what DNR is doing to combat climate change in this video: