Forest & Grassland Health

Yellow-Cedar Decline

Cause: Yellow-cedar decline is caused by fine-root freezing injury. It occurs on sites with shallow soils and low snowpack. Yellow-cedar is uniquely vulnerable to this form of injury compared to associated conifers.

Host(s) in Alaska:

Yellow-cedar (Callitropsis nootkatensis (D. Don) Oerst. ex D.P. Little; formerly Chamaecyparis nootkatensis

Topics

Climate Adaptation Strategy for Yellow-Cedar in Alaska

  Climate Adaptation Strategy for Conservationand Management of Yellow-Cedar in Alaska  

Several branches of the USFS (Forest Health Protection, Pacific Northwest Research Station, Alaska Regional Office, and National Forest System) worked together to develop 
A Climate Adaptation Strategy for Conservation and Management of Yellow-cedar in Alaska (2016). This report synthesizes the ecology, cultural and commercial values, taxonomy, and silvics of yellow-cedar; the mechanism and risk factors of yellow-cedar decline; guidance and opportunities for the active management of yellow-cedar; the development of models to estimate the distribution of yellow-cedar and the current and future risk factors for decline; and the current and projected future status of yellow-cedar in 33 management zones in Alaska.

Current Status & Distribution in Alaska (2018 Update)

  • In 2018, less than 18,000 acres of forest with actively dying yellow-cedar trees (trees with yellow-red crowns) were mapped during the aerial survey (see Map), down from a recent peak of 47,500 acres in 2017. This reduction in mapped acreage likely reflects a true decrease in decline activity following a colder, snowier winter, but also stems from more conservative mapping. In 2018, surveyors made a concerted effort to draw polygons tightly around areas of forest affected by decline. When surveyors draw larger polygons around affected forest and assign a lower value for the percentage of trees affected, it may inadvertently incorporate areas of forest only marginally affected by decline. Both styles of mapping are correct, but larger polygons can inflate the total area of mapped decline. Increases in overall affected acreage are expected to roughly coincide with the occurrence of decline events (early spring thaws followed by freezing conditions in the absence of snow, as occurred in 2015 and 2016). However, because trees and forests typically remain symptomatic for several years as trees gradually die, a significant decrease in mapped acreage is not expected from one year to the next, even when weather promotes cedar health.
  • Yellow-cedar forests along the coast of Glacier Bay and in Prince William Sound remain healthy. However, a 100-acre patch of yellow-cedar mortality with old snags was reported in 2016 alongside La Perouse Glacier (Glacier Bay National Park), tens of miles northwest of the northernmost mapped decline. Ben Gaglioti of the Lamont Doherty Earth Observatory confirmed that the dead trees/snags are yellow-cedar, and that adjacent healthy forest also contains yellow-cedar. The snags in this patch appear to have died around 1990 based on secondary branch retention (time-since-tree-death can be estimated based on snag charactaristics). In May 2018, Gaglioti and his team sampled 100 cedar snags from (a) the 100-acre mortality patch on the moraine and (b) adjacent glacier-buried forest that has been uncovered by glacial recession (their main project objective). They will cross-date the samples to pinpoint when they died and to understand their population structure. This yellow-cedar mortality, north of previously mapped decline, is of great interest, since cedar populations in Glacier Bay are considered healthy but at future risk of decline.
  • We continue to monitor young-growth stands for symptoms of yellow-cedar decline. Prior to 2012, young-growth stands were thought to be protected from freezing injury by relatively deeper rooting on productive sites, but 33 young-growth stands with decline symptoms have now been detected. In 2018, we installed 41 plots in five young-growth stands with the most severe decline crown symptoms and mortality to quantify impacts. Read more about decline in young-growth here and read the report from this plot installation effort here.
  • The U.S. Fish and Wildlife Service received a petition to list yellow-cedar as endangered or threatened under the Endangered Species Act in June 2014 and the listing decision is due in 2019. Read more here.

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Yellow-Cedar Decline Summary

  • Yellow-cedar decline is linked to climate change. Yellow-cedar trees are killed by freezing injury to fine roots where there is insufficient snowpack to insulate roots from lethal cold temperatures (<-5°C, 23°F) during cold events in early-spring. Root and foliar tissue of yellow-cedar prematurely dehardens in spring, which makes it vulnerable to severe cold at this time.
  • As a long-lived tree, many affected yellow-cedar forests established under the colder, more favorable climate of the Little Ice Age (1400-1850). An abnormal rate of yellow-cedar mortality began in the late 1800s, spiked in the 1970s and 1980s, and continues today.
  • Yellow-cedar is relatively more abundant on sites with high water tables, where it faces less competition from western hemlock and Sitka spruce. Although yellow-cedar is more competitive on wet sites, shallow rooting translates to greater vulnerability to fine root-freezing injury. Research into root and foliar cold tolerance has shown that yellow-cedar roots are more sensitive to this type of injury than associated conifers.
  • From the time crown symptoms appear, it often takes 10 to 15 years for trees to die, making it difficult to associate observations from aerial surveys to weather events in particular years. Impacted forests tend to have mixtures of old dead, recently dead, dying, and living trees, indicating the progressive nature of tree death.
  • Yellow-cedar is extraordinarily decay resistant and trees often remain standing for 80 to 100 years after death, allowing for the long-term reconstruction of yellow-cedar population dynamics in unmanaged forests. Long-term retention of wood properties provides opportunities for salvage harvest to offset harvest from healthy forests.
  • Even in severely affected forests, it is typical for 20-30% of the yellow-cedar basal area to remain alive; residual healthy trees are thought to be protected by microsite factors (deeper rooting) or tree genetics (freezing tolerance). On a regional scale, excessive yellow-cedar mortality may lead to diminished populations (but not extinction), especially considering this species’ low rate of regeneration and recruitment in some areas. These losses may be balanced by yellow-cedar thriving in other areas, such as higher elevations and un-impacted parts of its range to the northwest.
  • Land managers can promote yellow-cedar through thinning to reduce competing vegetation or planting it on sites where it is expected to thrive into the future (deep soils and persistent snowpack). Yellow-cedar is preferred deer browse, and deer may significantly reduce regeneration in locations where spring snowpack is insufficient to protect seedlings from early-season browse.
Yellow-cedar snags and declining live trees in a forest near Peril Strait, Chichigof Island.

Forest with yellow-cedar decline near Peril Strait, Chichagof Island.

A planted yellow-cedar seedling on Prince of Wales Island.

A planted yellow-cedar on Prince of Wales Island.

A naturally-regenerated yellow-cedar seedling.

A naturally-regenerated yellow-cedar seedling.

 

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Historic Activity

Our information on tree ages indicates that most of the trees that have died within the last century, and continue to die, regenerated during the Little Ice Age (~1400 to 1850 AD). Heavy snow accumulation is thought to have occurred during this period, giving yellow-cedar a competitive advantage on low-elevation sites in Southeast Alaska. Trees on these low-elevation sites are now susceptible to exposure-freezing injury under warmer climate conditions. An abnormal rate of yellow-cedar mortality began around 1880, accelerated in the 1970s and 1980s, and continues today. These dates roughly coincide with the end of the Little Ice Age and a warm period in the Pacific Decadal Oscillation, respectively. On a finer temporal scale, recent analysis of 20th century weather station data from Southeast Alaska documented increased temperatures and reduced snowpack in late winter months, in combination with the persistence of freezing weather events in spring (Beier et al. 2008). From the time crown symptoms appear, it takes 10 to 15 years for trees to die, making it difficult to associate observations from aerial surveys to weather events in particular years. Although there is continued activity of yellow-cedar decline, mortality has subsided somewhat in the last two decades.

Recent mortality is most dramatic on the outer and southern coast of Chichagof Island (Peril Strait) and at higher elevations, indicating an apparent northward and upward spread that is consistent with the climate patterns believed to trigger mortality. At the southern extent of decline in Alaska (55-56° N), mortality occurs at relatively higher elevations, while farther north, decline is restricted to relatively lower elevations. Yellow-cedar forests along the coast of Glacier Bay and in Prince William Sound appear healthy, presumably protected by deeper and more persistent snowpack. In 2004, a collaborative aerial survey with the British Columbia Forest Service found that yellow-cedar decline extended at least 100 miles south into British Columbia. Since that time, continued aerial mapping around Prince Rupert and areas farther south have confirmed more than 120,000 acres of yellow-cedar decline in BC.

More than a half-million acres of decline have been mapped in Southeast Alaska through aerial detection survey since surveys began in the late-1980s (see table of cumulative decline acreage by land ownership in Southeast Alaska), with extensive mortality occurring in a wide band from the Ketchikan area to western Chichagof and Baranof Islands. The cumulative estimate includes forests mapped in the 1980s that were killed since the turn of the century. This augments what is traditionally mapped during aerial detection surveys: actively dying trees with yellow-red tree crowns.

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Unraveling the Complex Causes of Yellow-Cedar Decline

Yellow-cedar decline functions as a classic forest decline and has become a leading example of the impact of climate change on a forest ecosystem. The term forest decline refers to situations in which a complex of interacting abiotic and biotic factors leads to widespread tree death, usually affecting one tree species or genus over an extended period of time. It can be difficult to determine the mechanism of decline, and the causes of many forest declines worldwide remain unresolved.

Our current understanding is that yellow-cedar decline is associated with freezing injury to fine roots that occurs where snowpack in early spring is insufficient to protect roots from late-season cold events. Yellow-cedar trees appear to be protected from spring freezing injury where snow is present, insulating tree roots and preventing premature root tissue dehardening (tissue activation following winter dormancy).

Hennon et al. (2012) provides a detailed summary of the interdisciplinary research approach at multiple spatial and temporal scales, along with extensive evaluation of the role of biotic agents (insects and pathogens) (Hennon 1986, 1990a), that unraveled the complex causes of yellow-cedar decline. Temporal patterns include the timing of yellow-cedar forest development and favorable climate (based on tree age), long-term linkages between climate patterns and pulses of decline, and fine-scale study of air and surface soil temperatures in research plots to identify temperature thresholds and mortality events. Spatial patterns range from landscape level (latitude and elevation, patterns of snow persistence), to site level (mortality concentrated where hydrology or bedrock restricts rooting depth and snowpack does not persist in early-spring), to tissue level (sensitivity of yellow-cedar to freezing injury in spring).

The hypothesis that has emerged is consistent with the observed patterns: conditions on sites with exposed growing conditions and inadequate snowpack in spring are conducive to premature root tissue dehardening, resulting in spring freezing injury to fine roots and gradual tree mortality. The temporal patterns help to explain why yellow-cedar occurs on many sites where it is currently maladapted.

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Symptoms & Damage

Symptoms of yellow-cedar decline include yellow-red-brown foliage discoloration (affecting greater than 15% of the tree crown) and crown dieback, which results from damage to the fine roots. Root-freezing injury often kills individual trees slowly over more than a decade, causing the tree crown to gradually thin and discolor. Some trees remain alive for decades with only 5-10% of the original foliated tree crown, having lost most of the biological and ecological function of a live tree. Affected forests often contain trees at various stages of decline.

Trees weakened by freezing injury may die rapidly if they are attacked and girdled by secondary bark beetles. Comprehensive assessment of pathogens and insects associated with declining trees found that Phloeosinus bark beetles (Phloeosinus cupressi) and Armillaria root disease play only minor roles in yellow-cedar mortality, attacking trees stressed by other factors (Hennon 1986, 1990a). However, the presence of these agents can indicate that trees have been stressed by root injury. Although it is not possible to see dead fine roots with the naked eye, it is possible to see dark-colored necrotic (dead) tissue moving up from dead coarse roots when the bark around the roots and root collar is removed.

Research on seasonal cold tolerance of yellow-cedar has demonstrated that yellow-cedar trees are cold-hardy in fall and mid-winter, but are highly susceptible to spring freezing. Yellow-cedar roots are more vulnerable to freezing injury, root more shallowly, and de-harden earlier in the spring than other conifer species in Southeast Alaska (Schaberg et al. 2005).

Click on image for larger version.

A dark-colored lesion at the root collar of a declining yellow-cedar crop tree with bark removed.

A dark-colored lesion at the root collar of a declining yellow-cedar crop tree with bark removed.

Mycelial fan of Armillaria root rot beneath the bark of a declining yellow-cedar.

Mycelium of the Armillaria root rot fungus beneath the bark of a declining yellow-cedar.

Phloeosinus beetle galleries and larvae beneath the bark of a declining yellow-cedar crop tree.

Phloeosinus beetle galleries and larvae beneath the bark of a declining yellow-cedar crop tree.

Phloeosinus beetles beneath the bark of a declining yellow-cedar tree.

Phloeosinus beetles beneath the bark of a declining yellow-cedar tree.

Phloeosinus beetle galleries with bark removed.

Phloeosinus beetles beneath the bark of a declining yellow-cedar tree.

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Ecological Impacts

Yellow-cedar is an economically and culturally important tree. The primary ecological effects of yellow-cedar decline are changes in stand structure and composition (Oakes et al. 2014). Snags are created, and succession favors other conifer species, such as western hemlock, mountain hemlock and western redcedar. In some stands, where cedar decline has been ongoing for up to a century, a large increase in understory shrub biomass is evident. Nutrient cycling may be altered, especially with large releases of calcium as yellow-cedar trees die. The creation of numerous yellow-cedar snags is probably not particularly beneficial to cavity-nesting animals because its wood resists decay, but may provide branch-nesting and perching habitat. On a regional scale, excessive yellow-cedar mortality may lead to diminished populations (but not extinction), especially considering this species’ low rate of regeneration and recruitment in some areas. These losses may be balanced by yellow-cedar thriving in other areas, such as higher elevations and parts of its range to the northwest. Yellow-cedar is preferred deer browse, and deer may significantly reduce regeneration in locations where spring snowpack is insufficient to protect seedlings from early-season browse.

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Salvage Logging

Salvage recovery of standing dead yellow-cedar trees in declining forests can help produce valuable wood products and offset harvests in healthy yellow-cedar forests. Cooperative studies between the Wrangell Ranger District, the USDA FS Forest Products Laboratory in Wisconsin, Oregon State University, the PNW Research Station, and Forest Health Protection have investigated the mill-recovery and wood properties of yellow-cedar snags that have been dead for varying lengths of time (Kelsey et al. 2005). Prior to this work, Hennon et al. (1990b) developed a snag classification system that relates snag characteristics to time since tree death. The mill recovery work has shown that all wood properties are maintained for the first 30 years after death. At that point, bark is sloughed off, the outer rind of sapwood (~0.6" thick) is decayed, and heartwood chemistry begins to change. Decay resistance is altered somewhat due to these chemistry changes, and mill-recovery and wood grades are reduced modestly over the next 50 years. Remarkably, wood strength properties of snags are the same as that of live trees, even after 80 years. Localized wood decay at the root collar finally causes sufficient deterioration that standing snags fall about 80 to 100 years after tree death. The large acreage of dead yellow-cedar, the high value of its wood, and its long-term retention of wood properties suggest promising opportunities for salvage. Read about a current project on the economic feasibility of salvage logging here.

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Time since death for yellow-cedar based on snag characteristics.

Appearance, characteristics, and mean time-since-death for the five dead tree (snag) classes of yellow-cedar (Hennon et al. 1990b).

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Young-Growth Yellow-Cedar Decline

Yellow-cedar decline was recently observed for the first time in young-growth. Before this, it was thought that the fine roots of cedars in young-growth forests were protected from decline freezing injury by greater rooting depth on more productive sites managed for timber. In 2013, we investigated dying yellow-cedars in two adjacent young-growth stands on Zarembo Island and determined that the cause of tree damage was yellow-cedar decline. The affected stands had been thinned to favor abundant yellow-cedar.

Forest Health Protection has worked with the Tongass National Forest Silviculture Program to compile a database of young-growth timber units known to contain yellow-cedar, currently 338 stands, to facilitate monitoring (see Map). Low-altitude aerial imagery and aerial detection surveys are used alongside the database to identify stands with discolored tree crowns and suspected decline, which are subsequently inspected on the ground. Decline has now been ground-verified in 33 young-growth stands on Zarembo, Kupreanof, Wrangell, Mitkof and Prince of Wales Islands. Most affected stands currently have a low incidence of yellow-cedar mortality (1-15 trees), but three stands on Zarembo Island, one stand on Wrangell Island and one stand on Kupreanof Island are heavily impacted. In severe cases, up to half of the yellow-cedar crop trees are estimated to be dead or dying. Affected stands are typically 27 to 45 years old and thinned between 2004 and 2012; decline has now been detected in 18% of stands in this age bracket. Of the stands in our database, one-half are in this vulnerable age range and one-third are younger.

Current management recommendations are to maintain tight spacing between cedars (6-8 ft) during pre-commercial thinning to account for potential loss to crop trees, and to avoid thinning in wet portions of stands; thinning provides little tree-growth payoff on these sites and may contribute to greater soil temperature fluctuation. Key management treatments include promoting yellow-cedar through planting and thinning in areas suitable for the long-term survival of this valuable species. Understanding the risk factors for yellow-cedar decline in young-growth is a major research need. Predicting where decline is likely to occur in young-growth could allow managers to prioritize other conifers during thinning in some units or portions of units expected to be vulnerable to decline.

Bioevaluation reports are available from work on young-growth yellow-cedar decline on Zarembo Island (Mulvey et al. 2013, updated 2015) and Kupreanof Island (Mulvey et al. 2015) and from our effort to quantify decline impacts in the most severely affected young-growth stands on Kupreanof, Wrangell and Zarembo Islands (Mulvey et al. 2019).

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A young-growth stand on Kupreanof Island with yellow-cedar decline, seen by float plane.

A young-growth stand on Kupreanof Island with yellow-cedar decline, as seen by float plane.

 

Recently-killed crop tree on Zarembo Island that appears to have died rapidly.

Recently-killed crop tree on Zarembo Island that appears to have died rapidly due to its full red-brown crown.

Forest pathologists examine a declining yellow-cedar crop tree with a thin crown.

Forest pathologists examine a declining yellow-cedar crop tree with a thin crown on Kupreanof Island.

A yellow-cedar crop tree on Zarembo Island with a thinning discolored crown.

A yellow-cedar crop tree on Zarembo Island with a thinning, discolored crown. 

A healthy yellow-cedar crop tree on Zarembo Island in a stand above the main area of decline.

A healthy yellow-cedar crop tree on Zarembo Island above the main area of yellow-cedar decline in this stand.

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Recent Work

Yellow-Cedar Petitioned for Endangered Species Act Listing: The U.S. Fish and Wildlife Service received a petition to list yellow-cedar as endangered or threatened under the Endangered Species Act in June 2014. The initial finding was that a review of the science and status of yellow-cedar is warranted. As part of the scientific review of yellow-cedar, the Yellow-Cedar Biology, Ecology, and Emerging Knowledge Summit was held at the University of Alaska Southeast in October 2017. The meeting was attended by experts from many disciplines from the United States and Canada and covered the best available science and information needs regarding yellow-cedar. The Species Status Assessment is expected in spring 2018 and the listing decision is due in 2019.

Permanent Monitoring along the Northern Margin of Decline (2011-2015): Lauren Oakes completed her PhD at Stanford University in 2015 researching yellow-cedar decline in Southeast Alaska. She installed permanent monitoring plots along the outer coast of Chichagof Island, the northern margin of yellow-cedar decline, and in healthy yellow-cedar forests in Glacier Bay, to quantify changes in forest community structure in decline-affected yellow-cedar forests (Oakes et al. 2014). Key findings were that succession favors other conifer species, especially western hemlock, and that understory functional plant diversity and composition changed. She evaluated the current and expected future status of yellow-cedar in Alaska under climate change, and the cultural and social perspectives around active management of yellow-cedar on lands with varying protection status (Oakes et al. 2015a). Lauren explored the relationship between knowledge of, and adaptation to, widespread, climate-induced tree mortality (Oakes et al. 2015b). She has written a non-fiction book based on her graduate work, The Canary Tree, see her website for more details.

Natural Yellow-Cedar Migration in Alaska (2014-2016): University of Alaska Southeast/Fairbanks and the Forest Service undertook a project to understand the establishment, migration and spread of yellow-cedar populations near Juneau. John Krapek mapped all known yellow-cedar populations and established plots at their edges to examine regeneration success and stand expansion. Despite large areas of suitable habitat, yellow-cedar only occupies < 1% of its potential niche near Juneau, indicating an ongoing migration. Recent stand expansion appears limited, with the last major pulse of establishment during the Little Ice Age (1100–1850). Yellow-cedar migration in the region appears episodic, and tied to climate and/or forest conditions different than today (Krapek and Buma 2017).

Yellow-Cedar Salvage Potential (ongoing since 2014): The Alaska Coastal Rainforest Center, Forest Service, State of Alaska, and University of Alaska Southeast are conducting a project to evaluate the economic feasibility of salvaging dead yellow-cedar. Monitoring plots have been installed on Kupreanof and Prince of Wales islands to compare forest composition, structure, seedling regeneration, and damage to residual trees from logging equipment in salvaged and unsalvaged stands impacted by yellow-cedar decline. Logging and milling costs will be tracked to assess the economics of this harvest against the value of the final products. Salvage recovery of standing dead yellow-cedar trees in declining forests may provide new opportunities for small mill operators, and can offset harvests in healthy yellow-cedar forests.

Rangewide Vulnerability Assessment (ongoing since 2015): Specialists from the US and Canada initiated a project in 2015 to evaluate the climate vulnerability for yellow-cedar throughout its entire range, from northern California to Prince William Sound in Alaska (Buma et al. 2016). Future work will incorporate climate models related to freezing injury, drought, and fire disturbance events.

Common Garden Studies (ongoing since 2009/10): A yellow-cedar common garden study has been established at the Héen Latinee Experimental Forest near Juneau and at several sites on Prince of Wales Island to evaluate differences in growth and survival between seedlings of different genetic sources and collection locations. Heavy deer browsing pressure on Prince of Wales caused notable mortality of seedlings. Seedlings near Juneau have experienced very high survival and growth, presumably because snow protected them from early-spring browse at this site. In 2009, 3,300 one-year-old plugs of yellow-cedar were planted across 30 total acres at three sites in Yakutat. Seedling survival was very high (90%) in post-planting measurements, but within the last two years survival has dropped to an estimated 20-30%. Planting sites were visited by Tongass silviculturists and Forest Health Protection staff in August 2017. Restricted rooting depth and seasonal flooding at planting sites in the Yakutat forelands likely increased vulnerability to fine root freezing injury in the absence of insulating snowpack. Survival was noticeably higher along skid roads, where equipment had churned the soil. The use of plugs may have also resulted in compromised root structure. A canker disease was also detected on some seedlings and collected for diagnosis. The causal fungus was cultured and genetically sequenced and is thought to belong to the genus Allantophomopsis, but an exact species-level sequence match was not found.

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Maps

Click on image for larger version.

Yellow-cedar decline active and cumulative 2018.

Active and cumulative yellow-cedar decline mapped by aerial detection survey in Southeast Alaska as of 2018 with the distribution of yellow-cedar.

Young growth stands that contain yellow-cedar and relative yellow-cedar decline severity.

Young-growth stands that contain yellow-cedar and the severity of yellow-cedar decline in 33 affected stands (2018).

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Resources & Publications

Click here to access more than 50 yellow-cedar publications. Also see the annotated bibliography of yellow-cedar (Hennon and Harris 1997), and Literature Cited sections of Hennon et al. (2016, 2012).

Hennon, P. E ; Harris, A. S. 1997. Annotated bibliography of Chamaecyparis nootkatensis. Gen. Tech. Rep. PNW-GTR-413. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 119 p. Available here.

Hennon, P. E.; D'Amore, D. V.; Schaberg, P. G.; Wittwer, D. T.; Shanley, C. S. 2012. Shifting climate, altered niche, and a dynamic conservation strategy for yellow-cedar in the North Pacific coastal rainforest. BioScience. 62: 147-158. Available here.

Hennon, P. E.; McKenzie, C. M.; D'Amore, D. V.; Wittwer, D. T.; Mulvey, R. L.; Lamb, M. S.; Biles, F. E.; Cronn, R. C. 2016. A climate adaptation strategy for conservation and management of yellow-cedar in Alaska. Gen. Tech. Rep. PNW-GTR-917. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 382 p. Available here.
 

Content prepared by Robin Mulvey, Forest Pathologist, Forest Health Protection, robin.mulvey@usda.gov.

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