Western Redcedar Topkill

drought injury, especially where trees are shallow-rooted on wet sites
and/or bole-girdling with the cause under investigation

Host(s) in Alaska:

western redcedar (Thuja plicata)

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

Current Status & Distribution in Alaska (2023 Update)

Two causes of western redcedar crown dieback and topkill have been identified in Southeast Alaska: drought injury and stem feeding damage. A multi-regional and -agency effort is underway to investigate western redcedar mortality and crown dieback throughout the Pacific Northwest. A collaborative survey has been created by the Oregon Department of Forestry in the Survey123 and iNaturalist applications to facilitate range-wide data collection. To submit observations from healthy or damaged trees, please use the Western Redcedar Dieback project in iNaturalist.

Western Redcedar Suspected Drought Injury

Western redcedar is known to be susceptible to drought impacts, which could have greater influence on western redcedar health in Alaska in the future if droughts become more frequent or intense. We detected a few locations on Prince of Wales Island in 2022 and 2023, and along the Ketchikan road system in 2023, with thin western redcedar tree crowns and no stem wounds. These trees appear to have been damaged during the severe drought in 2018 and 2019. This form of damage had apparently subsided with the return to higher precipitation levels during recent growing seasons; however, this year, some previously damaged trees experienced another pulse of crown dieback extending down from the top of the live tree crown. A collaborative survey effort to track western redcedar health is ongoing throughout its range in the Pacific Northwest.

Western Redcedar Stem Feeding Damage

In June 2022, we conducted a roadside survey on Prince of Wales Island to map the incidence of western redcedar topkill, an issue initially reported in 2017. During this survey, we mapped 120 western redcedar trees with recent topkill and bright red to brown crown discoloration, as well as 36 trees with older, more difficult-to-detect damage due to the loss of discolored foliage. Topkill damage was consistently associated with stem wounds that fully encircled stems. Small toothmarks covered wound surfaces. Wound size was variable, with some long linear wounds extending several feet along the tree bole. Based on our information from Prince of Wales Island, and observations elsewhere in North America, the Prince of Wales flying squirrel (Glaucomys sabrinus griseifrons) is the most likely culprit. Although this damage was relatively common in 2017-2022, significantly less topkill was observed in 2023.

We destructively sampled 15 wounded trees in 2022 to measure the size of wounds and the distance of wounds from the ground. Wounds occurred 7 to 31 feet from the ground, 18.5 feet on average. Wounds affected parts of stems under 4 inches in diameter and caused girdling injury on stems 2.6 inches in diameter or less. This year, we processed the wound samples collected in 2022 by creating half-inch thick cross sections of each wound to date the year and seasonality of wounding events. There were 227 wound events across 15 sample trees, ranging from a single wound to 52 wounds per tree. Individual trees were usually wounded across multiple years, and sometimes wounds were adjacent to those inflicted the prior year. Wounds on our sample trees spanned 2014 and 2022, increasing from 2018 until a peak in 2021. Wound seasonality was categorized as early (0-35% through annual growth), middle (36-70% through annual ring growth), or late (>70% through annual ring growth) in the growing season based on the relative thickness of ring growth that occurred before versus after wounding. Most wounds were inflicted early to midway through the growing season, while only 5% occurred late in the growing season. More work is needed to understand the annual growth rate and duration of western redcedar in Alaska to precisely establish the timing of injury, but we estimate that most damage occurred in May and June. In the Puget Sound, the growing season for western redcedar is defined as a 120-day season from April through September but varies with latitude and elevation (Silvics of Western Redcedar, Boyd 1959). In western Washington, diameter growth, as measured with electronic dendrometers, begins in March (Personal Communication, Dr. Connie Harrington, Jan 2024). In general, both diameter growth and shoot growth of western redcedar begins earlier in the growing season and continues at a steadier rate compared to other studied conifers (Phenology of Pacific Northwest Tree Species, Harrington et al. 2016).

In 2023, we observed a freshly girdled western redcedar tree in a managed young-growth stand near Rush Peak outside of Thorne Bay. Strips of bark that had been removed, creating a large wound, were still hung up in the branches and scattered around the base of the tree. Collection of the wound tissue and bark pieces provided ample material for genetic testing, which will occur in 2024. The bark chips remained at the tree and the cambium had been consumed, indicating that the cambium is used as a food source rather than nesting material. Overall, topkill associated with girdling stem wounds does not appear concentrated or common enough to cause substantial economic or ecological impacts. Affected trees recover with new leader development, but it is possible that the damage impacts future wood quality of trees used for timber or totems.

Historic Activity

Active western redcedar topkill was observed at least once before on Prince of Wales Island during an aerial survey in the 1980s. 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.

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. Rainfall throughout the 2018 and 2019 growing seasons were 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).

 

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.

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.

In June 2022, we conducted roadside surveys on Prince of Wales Island to map the incidence of western redcedar topkill, an issue initially reported in 2017. We mapped 120 western redcedar trees with recent topkill and noticeable crown discoloration, as well as 36 trees with older, more difficult-to-detect damage due to gradual loss of discolored foliage. Topkill damage was consistently associated with stem wounds that fully encircled stems. Destructive sampling of wounded trees allowed us to measure both the size of wounds and the distance of wounds from the ground. Wound samples were retained for further analysis.

Maps

Links to Resources & Publications

Drought Stress in Conifers, Forest Health Fact Sheet from the Oregon Department of Forestry, February 2019.

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, robin.mulvey@usda.gov.

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