Western redcedar dieback

Western redcedar (Thuja plicata, also called arborvitae) is one of four tree species in Oregon and Washington that we commonly call “cedar” but are not true cedars. Other native false cedars include incense cedar (Libocedrus decurrens), Port Orford cedar (Chamaecyparis lawsoniana), and yellow or Alaskan cedar (Chamaecyparis nootkatensis).

All of these trees have similar foliage in that needles are arranged in a flat “spray”, but each have very distinct cones which aids in their identification. 

Pacific Northwest Tree ID:  https://treespnw.forestry.oregonstate.edu/conifer_genera/false_cedars.html 

Western redcedar is very shade tolerant. Trees can thrive in sunny locations with sufficient moisture, but they are more at risk during hotter droughts. This species requires moist conditions and thrives in coastal fog belts and moist inland areas up to about 4000 feet elevation. It tolerates most types of soils and outcompetes many other species in wet soils. Western redcedar has no taproot, and is shallow rooted in wet soils but roots can be just as deep as co-occurring conifers in well drained soils. Western redcedar may not do well in soil crowded by roots of other plants (including trees) that are competing for water. Western redcedar is often found in mixtures with western hemlock and Douglas-fir. Alternate species for redcedar on the westside can include incense cedar, sequoia, bigleaf maple in generally dry sites and western white pine, maple, alder, ash or cottonwood at wetter sites that do not dry out in the summer. 

WRC are commonly sold as ornamental varieties of arborvitae, and often planted as fence rows in sun-exposed yards which are not a preferred habitat. Drought stress and mechanical damage often makes these trees susceptible to opportunistic insects that would not ordinarily be able to infest these trees. 

In recent years, forest health specialists, forest landowners, and land managers have been observing western redcedar dieback from Oregon through western Canada.

Pockets of dieback have even been observed in areas where they should be thriving, such as along streams and in shaded areas. The cause for this sometimes sudden and expanding dieback is currently unknown and under investigation.

Specific symptoms include thinning crowns, branch dieback, topkill, chlorosis (yellow foliage), heavy cone crops and mortality.  

Seasonal dieback of older foliage (below) is normal and not symptomatic of impending mortality. Seasonal dieback is often most noticeable in the fall.

Western redcedar is fairly resistant to native insects and diseases. Insects and diseases that infest western redcedar are typically secondary, meaning that they are not aggressive tree-killing species that are direct tree killers but are opportunistic pests and can only attack dead and dying redcedar.

New pest alert: Japanese cedar longhorn beetle    https://www.oregon.gov/oda/shared/Documents/Publications/IPPM/JapaneseCedarLonghornBeetlePestAlert.pdf 

Report suspected invasives: https://oregoninvasiveshotline.org/

Redcedar contains terpenoid chemical defense compounds (thujone and thujaplicin) and also uses sap as a mechanical barrier. Redcedar can resist endemic levels of bark beetles (Phloeosinus spp.) and woodboring beetles (Semanotus amethystinus, Trachykele blondeli, Chrysobothris nixa, etc.). 

Western redcedar is susceptible to various root and butt rot pathogens (Postia sericeomollis, Phellinus pini, Coniferporia weirii, Armillaria ostoyae) and often has extensive heart rot, but pathogens have not been present at all sites. Novel insects or diseases have not been observed and are not considered the main causal agents of this dieback epidemic.

Given the apparent range-wide dieback and lack of consistent biotic factors, we hypothesize that the problem is abiotic. The western U.S. has been seeing increased droughty periods, especially during the summer months, and an increase in temperature. 

Climate change-induced drought is not all about setting records. Timing, duration and frequency of temperatures and precipitation are just as important as temperature highs and precipitation amounts.

Droughts can be most damaging when they occur during peak growing seasons such as spring, last for multiple days with no reprieve, or compound stress when repeated over multiple seasons.

In addition to chronic droughts, acute events can cause sudden pulses of damage and dieback. The 2021 heat dome event resulted in widespread reports of damage that was especially severe in drought-intolerant species such as WRC. Damage was highest on young foliage, along roads due to thermal radiation from below, and on south- and west-facing exposures where the angle of the sun at peak temperature was most intense.

...gathering information in phases

In 2020, Oregon Department of Forestry and Washington Department of Natural Resources received funding through the U.S. Forest Service Evaluation and Monitoring Program to define the extent of western redcedar dieback and determine if there were variables associated with dieback. We developed an app using Survey 123. The app can be used to record locations where western redcedar dieback is occurring and also to take measurements at these sites. Data collected may include site data (e.g. location, acreage), stand data (e.g. basal area, aspect), and tree data (e.g. percent crown transparency, number of annual growth rings in core samples). 

Unfortunately, WRC dieback (topkill, flagging, yellowing, etc.) is not easily visible or distinguishable from damage in other conifer species during aerial detection survey, where this type of damage is typically recorded, therefore the extent is unknown, but it appears to be occurring range-wide.

Permanently marked locations across the PNW allow us to follow WRC symptoms over time and determine:

• What symptoms are present and where do these occur?
• How quickly do symptoms develop?
• Do dying tops or thinning crowns result in dead trees?
• How quickly do trees die?

• Site ownership and management type (e.g., forest vs. urban area, public vs. private)
• Acres of damage
• Diameter base height
• Estimated tree age
• Diameter growth (rings within core samples)
• Crown dieback and transparency ratings (none, low, medium, high)
• Crown classification
• Basal area (BAF10) of all trees in the plot as well as only symptomatic WRC
• Aspect
• Slope position
• Most common under and overstory species
• Photos to note dieback over time

As of this time, a total of 369 sites with western redcedar dieback in OR and WA have been recorded. At 148 of these sites, plot and site data have been collected. At a subset of these sites, western redcedar increment cores have been collected and are being stored for future dendrochronological analyses. 

Right: clickable GIS map

Because we did not collect location data where only ‘healthy’ WRC occurred, for some analyses we compared the environments of our known WRC dieback sites with locations of WRC distribution across OR and WA. WRC distribution data were obtained using a 10% subset of locations from Individual Species Parameter Maps (ITSP) across OR and WA ( https://www.fs.fed.us/foresthealth/applied-sciences/mapping-reporting/indiv-tree-parameter-maps.shtml ). We do not claim that all WRC distribution sites contain only healthy trees, but our goal was to define the environmental niche of our PNW dieback sites and compare with the environmental niche of the species’ PNW distribution.

We also divided sites into three ecoregions: western Washington, central/eastern Washington, and Oregon (WRC tends to occur within one ecoregion in Oregon). In several analyses of environmental data we compared environments between ecoregions (‘regions’) and WRC status (‘unhealthy/dieback’ vs. ‘WRC distribution’).

Data collection metrics were also co-designed with university extension and researchers for use in iNaturalist to engage citizen scientists and broaden collection efforts. Some analyses include both Survey123 and iNaturalist collected data when we included a subset of iNaturalist locations into our WRC dieback database (data source: Joey Hulbert, iNaturalist WRC Dieback Project:  https://www.inaturalist.org/projects/western-redcedar-dieback-map ).

The average basal area of WRC with dieback symptoms per plot varied little amongst the three ecoregions but ranged from 48 ft ² /acre in western Washington to 83 ft ² /acre in eastern Washington sites. 

Elevation at sites where WRC dieback occurred was generally lower than the WRC distribution across Oregon and Washington. This difference was most pronounced in Oregon and western WA.

Slope position was determined on a smaller stand-level scale versus a larger landscape-level scale to more accurately depict influence of microclimate in the immediate area.

Crown thinning was the most common symptom observed, followed by branch dieback.

To explore relationships of various weather/climate variables between dieback locations and the distribution of WRC in Oregon and Washington, we downloaded 30-year normal data (for the period 1991-2020) and yearly data from 2015, to determine if dieback was even higher at the driest sites during this unusually dry year (ClimateNA v7.20). We explored precipitation, temperature, vapor pressure deficit, and many derived variables by region and WRC health status.

Sites with WRC dieback generally had lower median long-term April, May, and June precipitation compared to the WRC distribution, but 2015 medians and variances were closer between the unhealthy/dieback sites and the WRC distribution sites. Long-term and 2015 maximum temperatures were both generally higher (and variance smaller) in sites where WRC dieback occurred compared to its normal distribution.

To narrow down the list of potential climate/weather variables associated with WRC dieback, we explored data using categorical and regression tree (CART) models to create dichotomous splits.

The first predictor variable to separate out WRC dieback sites from WRC distribution sites in the westside Washington and Oregon CART model was low spring (March-May) precipitation as snow (spring PAS) followed by higher mean temperature of the warmest month of the year (MWMT). Sites with WRC dieback were predicted to occur in areas that receive less than 3 mm of spring precipitation as snow and occur in areas with higher temperatures in warm months. These predictor variables highlight the elevation effect, where sites with WRC dieback in western Washington and Oregon were at much lower median elevations than the general distribution of WRC.

The predictive variables to separate WRC dieback sites from WRC distribution using only central and eastside Washington data were summer (June-Aug) climate moisture index (summer CMI), extreme minimum temperature (EMT), and high autumn (Sept-Nov) evaporation (Autumn Eref, Hargreaves). WRC dieback sites were predicted to occur in areas with higher summer CMI but lower autumn evaporation, or areas with low summer CMI but more extreme minimum temperatures. This CART model had a higher misclassification rate than the westside model and was generally less useful in highlighting important predictor variables, which is likely because the elevation of sites where WRC dieback occurred was very broad across eastern Washington locations. We suspect that smaller scale site and stand variables are important in contributing to dieback in eastern Washington.” The Climate Moisture Index (CMI) is the difference between annual precipitation and potential evapotranspiration (PET) – the potential loss of water vapor from a landscape covered by vegetation.

Hargreaves is a representative expression for potential evapotranspiration (or drying) based on air temperature and extraterrestrial radiation.

These maps confirm our CART results:

This map of western Washington and Oregon can be used to visualize westside CART model results and compare the environmental predictor variables of sites where WRC dieback occurs. Yellow markers illustrate WRC distribution locations where 30-year spring precipitation as snow is less than 3 mm in western Washington, and blue markers indicate locations where it is greater than 3 mm. Black markers indicate where WRC dieback occurs and was found during this study. This map illustrates that the majority of WRC dieback sites were predicted by this first variable. This visualization also illustrates the elevation effect, where most dieback is concentrated at low elevation sites in valleys and troughs inland but west of the Cascades. We do not believe that snow or a lack of snow is an issue contributing to WRC dieback, but that low spring precipitation as snow is a characteristic of low elevation sites.

These maps confirm our CART results:

This map of central and eastern Washington can be used to visualize eastside CART model results and compare the environmental predictor variables to sites where WRC dieback occurs. Yellow markers illustrate WRC distribution locations where 30-year summer CMI is less than -30.9 (i.e. hotter and drier), and blue markers indicate locations where it is greater than -30.9 (i.e. cooler and wetter). Black markers indicate where WRC dieback was found during this study. This map illustrates that only about half of the dieback sites were well predicted by this first variable. WRC dieback occurs from low to high elevations in eastern Washington, and was therefore more difficult to predict using climate variables.

1. WRC dieback was observed across the PNW distribution of WRC, with the exceptions of coastal and higher elevation mountainous regions of southern Oregon.
2. Frequency of WRC dieback locations were highest in low elevation, urban corridors in western Washington and northwestern Oregon
3. The most common symptom of WRC dieback was thinning crowns, followed by branch dieback.
4. No site factors were associated with higher severity of individual tree crown dieback or transparency.
5. At sites with dieback, symptomatic WRC made up approximately 30-50% of the total basal area.
6. No biotic damage agent (e.g., insect, disease, etc.) was found associated with WRC dieback across the region.
7. More WRC dieback sites were observed at lower elevations compared to the WRC distribution across the PNW.
8. More dieback sites were observed on westerly aspects in Washington, and areas with no slope in Oregon.
9. Dieback sites were not strongly associated with any one slope position.
10. In westside systems (western Washington and Oregon), low spring precipitation as snow was the first chosen predictor variable from a suite of climate variables, and appeared to be a fairly strong predictor variable separating sites with dieback from the WRC distribution.
11. In eastside systems (central and eastern Washington), low summer climate moisture index was the first chosen predictor variable from a suite of climate variables, but was not a strong predictor variable separating sites with dieback from the WRC distribution.

1. Monitoring trees with larger diameter base height tended to be in stands with lower basal area of unhealthy WRC. Interpretation: larger diameter trees are more vigorous and can better overcome site or climate stress.
2. Stands with higher basal area tended to also have a higher proportion of symptomatic WRC basal area as well as a slower diameter growth. Interpretation: more competition would inherently result in a larger proportion of unhealthy WRC that gain less diameter growth each year.
3. Intermediate crown classes exhibited faster diameter growth in more recent years than did trees in other crown classes. Interpretation: trees growing at intermediate crown classes benefit from more shading and have fewer water needs than larger trees and thus have benefitted during some periods when it comes to diameter growth rate.
4. Trees growing in toe slopes yielded the slowest diameter growth in recent years. Interpretation: we do not have an adequate theory as to why but suggest a larger sample size to test this interaction.
5. Northeastern slopes contained the most trees with high dieback ratings and south slopes contained the most trees with a medium rating. Interpretation: these results were contrary to what exposures are thought to be most stressful to trees, and may indicate that our estimations of slope aspects may have been evaluated at too small of a scale (i.e. utilizing small-scale microtopography relative to a macro or landscape-level scale).

In our study, we did not record healthy western redcedar, but we did observe healthy trees. Sometimes healthy trees were located adjacent to unhealthy trees. The general observation was that the healthy trees were shaded out, often in the understory, sometimes by topography, whereas the unhealthy trees were overstory trees or open-grown trees; trees that had more exposure to sunlight and wind. Given the fact that many of these trees are located right along streams where they should be receiving plenty of water, we hypothesize that trees exposed to more sunlight and wind may be respiring more than trees that are shaded-out, resulting in drought stress driven by microsite differences in addition to hotter and drier conditions. Site factors that affect microclimate such sloped topography or soil water-retention capacity may be further compounding drought stress conditions.

We could be seeing the range of WRC shift or shrink due to changing conditions that reduce the suitability of some sites for this species.

Right: maps developed from models ( Crookston et al. 2009 ) predicting the range of WRC at various points in time under varying climate change intensities. (Note, red is preferred and green is less preferred).

• a. current range (circa 2010)
• b. predicted range in 2030 under a mild climate change scenario
• c. predicted range in 2030 under an intense climate change scenario
• d. predicted range in 2090 under an intense climate change scenario

The predictive models seem to agree that interior preferential habitat may move north and into higher elevation areas.

Locations of dieback (left) appeared to be especially high in urban heat islands (right) such as around the metropolitan areas around Portland and Seattle (Heat island data source:  The Trust for Public Land non-profit using thermal signatures from 2018 and 2019).

Retain WRC where it can withstand changing climate

• If you have dying or symptomatic WRC at a site, it may be too dry and warm for the species during intense or long periods of drought, so consider diversifying and/or reforesting with more drought tolerant species. Opt for more drought-tolerant native species at dry sites. Depending on the region, these may include: incense cedar, Willamette Valley pine, ponderosa pine, western larch, Oregon white oak, Douglas-fir, etc.
• Adjust planting according to updated seed zones:  https://tinyurl.com/seedzone 
• Maintain climate-adapted tree density:  https://catalog.extension.oregonstate.edu/em9206/html 
• Increase invasive species (e.g., non-native blackberry and ivy) control efforts in WRC stands

Washington State University

Key findings:

1. Core sample analysis between living and dead WRC and correlated with climate data indicates that repeated droughts are causing tree mortality
2. WRC can recover from a drought within 3 years under cooler/wetter than average conditions
3. WRC mortality occurs after 4-5 years of reduced growth (reduced growth correlated with drought)

 https://labs.wsu.edu/meddenslab/projects/western-redcedar-mortality/ 

 https://pdxscholar.library.pdx.edu/cgi/viewcontent.cgi?article=1280&context=geog_fac 

Oregon State University

Rapid Retreat of the Pacific Maritime Forest (https://doi.org/10.1101/2020.08.31.273847)

WRC dieback sampling extended to Idaho