Forest & Grassland Health

Western Redcedar Topkill




Western redcedar with topkill on Prince of Wales Island in 2017.

  Western redcedar with topkill on Prince of Wales Island.

drought injury, especially where trees are shallow-rooted on wet sites
and/or bole-girdling caused by animal feeding

Host(s) in Alaska:

western redcedar (Thuja plicata)

Affected tree part(s): tree tops or whole trees



Click image for larger version. Photo credit for all: Molly Simonson, USFS.

Western redcedar with topkill on Prince of Wales Island.

Western redcedar with topkill on Prince of Wales Island in 2017. Older snags and topkill are yellow-cedar.

Western redcedar with topkill and obvious bole wound on Prince of Wales Island.

Western redcedar with topkill and an obvious bole wound (circled) on Prince of Wales Island.

Multiple topkilled western redcedar trees.

Multiple topkilled western redcedar trees in roadside young-growth on Prince of Wales Island.

Bole wounds below the dead top of western redcedar, probably killed the previous year.

Bole wounds on western redcedar just below the level of topkill; there are healthy lower branches below.

Current Status & Distribution in Alaska (2020 Update)

The cause of western redcedar topkill on Prince of Wales Island is under investigation. Widespread topkill of small and medium western redcedar trees and some full tree mortality has been reported in the southern Panhandle, with the most concentrated damage on central Prince of Wales Island (see Map). The damage is most concentrated along roads and has been detected in managed stands and in old-growth. Associated tree species are unaffected. Bole wounds with missing bark were common in damaged parts of the crown but were not found on all affected trees. Further assessment could help to define site factors associated with topkill and mortality. Drought is thought to be the most important abiotic cause, which may predispose trees to biotic stressors, such as animals that feed on inner bark, canker fungi, or bark beetles. Severe drought occurred on Prince of Wales Island in 2018 and 2019, followed by excessive rainfall in 2020. Interestingly, elevated mortality was first noticed in 2017, prior to excessively dry conditions. Retrospective climate analyses could help to identify climate trends in 2016 linked to the uptick in damage noticed in 2017.

Of 260 damage observations made in 2019, half were made from the air and half from the ground, almost always reflecting damage to one or a few trees. We were unable to ground check the mapped damage in 2020. We used high-resolution satellite imagery to identify 17 points of western redcedar topkill or mortality. We aim to assess these areas on the ground as soon as possible. This damage is more difficult to confidently detect from imagery compared to aerial surveys. Regardless of whether this damage is detected via aerial surveys or imagery scanning, it must be ground confirmed, since western redcedar topkill or dieback may look like young-growth yellow-cedar decline when yellow-cedar is killed rapidly by secondary bark beetles.

There is now a multi-regional and -agency effort to understand the abiotic and biotic causes of the notable increase of western redcedar mortality through the Pacific Northwest. The virtual 2020/21 Western Redcedar Summit focused on emerging western redcedar dieback research in both urban and wildland environments. Presenters and participants represented multiple government agencies, municipalities, and universities from Oregon, Washington, Alaska, and British Columbia. A collaborative survey form has been created by the Oregon Department of Forestry in the Survey123 application to facilitate widespread data collection.

See a description of other damage agents of western redcedar. Please contact Forest Health Protection if you have observations of western redcedar topkill or mortality to share, ideally with geographic coordinate information and clear photographs of affected tree parts and the tree bole immediately beneath the main area of damage.

Historic Activity

Active western redcedar topkill was observed at least once before on Prince of Wales Island during an aerial survey in the early 1990s. When the surveyors ground-checked the damage, they detected bole wounds associated with topkill that were tentatively attributed to northern flying squirrels based on teeth marks. When recently consulted about whether flying squirrels could girdle tree boles, wildlife biologists who have worked closely with the species suggested that the jaws and teeth probably are not strong enough to remove the bark in this way. Porcupines and Douglas squirrels, which could easily girdle conifer stems, are absent from Prince of Wales Island. Porcupines usually feed on spruce and hemlock, avoiding western redcedar and yellow-cedar. Teeth marks have not been observed during recent inspections of damaged trees.

Topkill response to drought has more commonly been observed in western Washington and Oregon. In 2015, multiple conifer species had topkill and mortality from drought, with greater impacts to young trees. Damage was most severe on sites with well-drained soils, such as gravelly glacial outwash around south Puget Sound. Similar damage was observed in western Washington in 2012 (see Forest Health Highlights in Washington- 2015). In western Oregon, 2013-2015 were drought years that resulted in significant injury to conifers, especially western redcedar. Dead western redcedar are pictured in this Forest Health Fact Sheet from the Oregon Department of Forestry on Drought Stress in Conifers.

Symptoms, Biology & Impacts

Dead tops and multi-forked dead tops of western redcedar are common in old-growth forests. It is possible that the active damage observed recently is caused by the same factors as this common, older damage. Synchronized active mortality of western redcedar tree tops and whole trees is not frequently observed in Southeast Alaska as it was on Prince of Wales Island in 2017. Scattered, damaged trees were generally open-grown and vigorous before damage occurred. Affected trees ranged from 5 to 40 feet tall and up to 12 inches in diameter. Most had 5 to 60% of the tree crown killed, while a few trees were killed outright. Sections of missing bark/bole wounds were common in the mid and upper tree bole immediately below and interspersed throughout dead parts of the crown. Individual wounds did not usually completely encircle stems, but were aggregated together on and around the bole such that girdling could occur. Dead tops were full and bright red, indicating rapid onset within weeks rather than progressive damage over months or years.

Further assessment of damaged trees could reveal common site factors associated with the damage, and could also help to evaluate the role of animals in wounding tree boles. It is possible that animal feeding on western redcedar was triggered by a reduction in the availability of normal food sources, and it is curious that other conifer species were apparently unaffected. The indeterminate growth of western redcedar might make it more valuable as an early food source; more flexible release from winter dormancy could provide animals with earlier access to photosynthetic products in the phloem tissue compared to associated conifers. Special thanks to Molly Simonson, Tongass National Forest Silviculturist in Thorne Bay, for sharing thorough observations of this damage.

On wet sites, western redcedar can be shallow-rooted, making them more vulnerable to drought injury during abnormally dry periods. In March 2017, late-winter drought resulted in pronounced green needle drop from Sitka spruce and western hemlock at many locations in Southeast Alaska; trees recovered quickly with normal budbreak. Rainfall throughout the 2018 growing season was far below average.

Other damage agents of western redcedar: Drought-stressed trees are vulnerable to further damage (secondary injury) from insects and pathogens. Another mortality agent of western redcedar is the cedar bark beetle, Phloeosinus sequoiae, which typically only attacks trees stressed by other factors. Western redcedar can also be severely browsed by deer, elk and rodents. Tip or shoot dieback may be caused by fungal diseases, such as cypress canker (Seiridium cardinale) or Pestalotiopsis tip blight (Pestalotiopsis funerea). Cedar leaf blight (Didymascella thujina) causes foliage discoloration and premature foliage loss; find out more about this damage in the Pacific Northwest Plant Disease Handbook or in Insects and Diseases of Alaskan Forests (page 130).

Tip dieback of western redcedar. Fungal fruiting structures associated with western redcedar topkill.

Tip dieback of western redcedar. Credit: Molly Simonson, USFS.


Stained stem tissue and fungal fruiting structures associated with western redcedar tip dieback. Credit: Molly Simonson, USFS.

Survey Method 

Western redcedar topkill information came from informal observations in 2017. It was reported that 200-300 topkilled and dead western redcedar trees were observed from the road, scattered across Prince of Wales Island. In August 2018, data was collected from 46 trees at 30 observations points concentrated near Thorne Bay and along the road between Thorne Bay and Klawock. Collected information pertained to the proportion of the tree crown damaged, the presence and severity of bole wounds, the health status of nearby western redcedar, and the tree species around affected trees. In 2019, observations were made through road-based surveys and aerial surveys.


Western redcedar topkill and mortality in the southern Panhandle 2017-2019.

Western redcedar topkill and mortality in southern Southeast Alaska 2017-2019 detected through ground and aerial surveys. Ground surveys were not possible in 2020 due to the COVID-19 pandemic.

Links to Resources & Publications

Drought Stress in Conifers, Forest Health Fact Sheet from the Oregon Department of Forestry

Forest Health Highlights in Washington- 2015

Antos, J.A., Filipescu, C.N., Negrave, R.W. 2016. Ecology of western redcedar (Thuja plicata): Implications for management of a high-value multiple-use resource. Forest Ecology and Management 375: 211-222. Abstract available here.

Minore, D. 1983. Western redcedar—a literature review. Gen. Tech. Rep. PNW-GTR-150. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 75 p. Available here.

Tesky, J. L. 1992. Thuja plicata. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available here.



Content prepared by Robin Mulvey, Forest Health Protection,

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