By Melissa J. Fischer, Forest Health Specialist for Eastern Washington, Washington State Department of Natural Resources, firstname.lastname@example.org
Western redcedar (Cupressaceae Thuja plicata) is a beautiful, ecologically important and very valuable tree species found in eastern and western Washington. The full range of this species extends along the coast from northern California up to the southeastern Alaska. On the interior, its range extends from southcentral British Columbia down to western Montana and north Idaho (Figure 1). Western redcedar prefers moist growing sites with mild temperatures, but can be found on more harsh sites, particularly in the interior portion of its range.
What we’ve been seeing
For the past several years, forest managers, specialists and private landowners have noticed a change in their Western redcedar stands. Many trees have been exhibiting thinning crowns (Figure 2), heavy cone crops, discoloration (yellow or brown foliage; Figure 2), dead tops (Figure 3), and, sometimes, mortality (Figure 4). In comparison with the lush, green foliage typical of Western redcedar (Figure 5), these stands of dead and dying cedar have many people alarmed.
How extensive is the problem?
The problem is not specific to eastern Washington.
It is in fact quite widespread, with reports coming in from all over Washington, Oregon, Idaho, and Canada. Unfortunately, the full extent of the problem is unknown. The most effective tool we currently have for capturing large-scale tree mortality is the Annual Forest Health Aerial Detection Survey. The Aerial Detection Survey is used to detect newly dead or defoliated trees within the forested acres of Washington state. Although some of the Western redcedar damage has been mapped, much has not.
Damage caused by bark beetles and defoliators tends to be much easier to see from the air, as this acute damage results in bright red crowns or large, contiguous stands of defoliated trees. Damage to Western redcedar appears to be a much slower process: Trees slowly lose needles, change color, and die back – a signature which is much harder to see from the air.
What is causing Western redcedar decline?
As of this time, it is unknown what exactly is causing the problem. There are many pathogens associated with Western redcedar, including various heart-rotting fungi and root diseases, but none of these issues has been found to be a common denominator. Few insects attack Western redcedar, and those that do usually attack when the tree is already dead or dying. Given the extent of the issue and the lack of a common biotic presence, the problem is likely abiotic.
A very likely abiotic candidate is drought. A study by Seebacher (2007) found in a dendrochronological analysis (using tree rings and climate data) that Western redcedar dieback in British Columbia, Canada, was restricted to three hotter and drier sites out of four studied.
It is predicted that drought may become more prevalent in Washington state as the climate changes. Washington has had several recent droughty summers, but this may not be the singular issue given the fact that many of the Western redcedar stands exhibiting decline are located in rather wet areas. Looking at the climate data, the overall average precipitation in WA State from 1895-2018 actually shows a slight increase (+0.19ʺ/ decade; Figure 6).
If Washington is receiving more precipitation, how could drought be the problem? While the precipitation data does indeed show a slight increase in mean precipitation over time in the state, there are other factors to take into consideration, such as: Where in Washington state are these increases occurring? Is it throughout the entire state? What time of year is the precipitation occurring (winter, fall, spring, summer)? How is it falling (rain, snow, ice)? Perhaps the issue is the timing of precipitation. It’s possible that, regardless of the overall increase in mean precipitation overtime, that droughty conditions occurring during the hottest, driest parts of the year are what is negatively affecting Western redcedar.
What about temperature? A study by Gashwiler et al. (1971) found that of the mortality that occurred during their study of Western redcedar seedlings, 80 percent was caused by weather factors, with temperature being more critical than drought. Looking at the average temperature data from 1895 to 2018 in Washington state, there is a clear increase over time (+0.2°F/Decade; Figure 7).
It’s possible that Western redcedar are being affected by a combination of summer drought and higher temperatures. The summer months, part the growing season, trees are actively photosynthesizing, a process by which trees use sunlight, carbon dioxide, and water to produce glucose, i.e. their food. In order to obtain carbon dioxide, trees must open their stomata (microscopic cells on the underside of the foliage). When stomata are open, water is passively lost through transpiration. When it’s hot and dry, many species close their stomata to keep from losing too much water (isohydric adaptation). If a drought lasts for a long period of time, some trees may eventually begin to starve because they cannot photosynthesize and produce food with their stomata closed.
Other tree species leave their stomata open during drought conditions to continue photosynthesizing (anisohydric strategy). If the drought is prolonged, cavitation may occur. Cavitation is the breakage of water columns located in the trunk that move water from the roots to the foliage. If water is no longer reaching a twig or branch, that twig or branch will die. Species in the Cupressaceae family have an anisohydric adaptation to drought and thus are more likely to die due to cavitation (think gnarly dead trunks on Junipers in the dry pinyon/juniper woodlands of the Southwest).
Lack of snowpack?
Another abiotic factor to consider is snowpack. Figure 8 shows trends in April snowpack (measured as snow water equivalent or the amount of water contained within snowpack) from 1955-2016. April snowpack is particularly important because this will provide a slow supply of water throughout the spring, maybe even into the summer, as snow melts from the mountains down into the lowlands. As can be seen, snowpack has declined at more than 90 percent of the sites measured. The average change across all sites is a loss of about 23 percent. Large and consistent decreases have been observed throughout the western U.S., but have been especially prominent in Washington, Oregon, and the northern Rockies.
Let’s Delve a Little Deeper:
An interesting hypothesis: Yellow cedar decline (D’Amore et al. 2009)
As much as we would like things to be simple (i.e. there’s one specific disease or insect affecting our Western redcedar and it’s a manageable problem), in the world of ecology, problems and solutions tend to be much more convoluted. Let’s take a look at the case of another Cupressaceae species, Alaska yellow cedar (Cupressus nootkatensis). This species is believed to have been on the decline for 75-plus years in southeast Alaska and British Columbia. Drought and lack of snowpack have been related to the decline, but an interesting hypothesis for this decline shows that the specific causes can be much more complex …
A distinctive trait of the Cupressaceae family, under which both yellow cedar and Western redcedar fall, is that they accumulate calcium in their tissues. Why calcium? As most everyone knows, trees, plants in general, need nitrogen. Many conifers uptake ammonium (NH4+), but cedars preferentially uptake nitrate (NO3-) as a competitive adaptation on marginal sites. A byproduct of nitrate assimilation is oxalic acid, which is a potentially toxic compound. Calcium neutralizes oxalic acid to form calcium oxalate; in other words, cedars are using the calcium they are accumulating as a detoxification process.
Cedars have two features that help with uptake of both nitrate and calcium: early dehardening (plant tissues lose hardiness and are ready to resume growth early in the season) and shallow rooting. Early dehardening of yellow cedar allows fine roots near the soil surface to access soil rich in ammonium and calcium in early spring. This strategy only works if roots are protected from freeze/thaw events that induce root injury. Historically, consistent snowpacks have limited soil freezing and protected roots from extensive damage, but depths of snowpack, particularly at lower elevations in northern latitudes, have declined during the 20th century. The shallow depth and early spring activity of roots that historically enhanced the competitive status of yellow cedar on wet soils may now be the reason behind its decline.
Could this hypothesis for yellow cedar decline be applied to Western redcedar as well? It certainly is a possibility and would explain why we are seeing Western redcedar dieback in areas that appear to have plenty of water year-round.
What can be done?
Given the fact that we don’t know exactly what is causing Western redcedar dieback and mortality, there is currently no sound management advice available. We often suggest thinning stands to allow more availability of resources to the residual trees, but in the case of yellow cedar, open canopies were found to lead to greater exposure and associated temperature changes that in turn lead to increased soil-freezing events and root freezing injury. Clearly more research is necessary, but funding for research in the field of forest health is hard to come by and quite competitive when available.
Figure 9 shows the predicted future range of Western redcedar in green; red denotes the area where Western redcedar might be found, but under stressed conditions. Gray denotes a large portion of the current range, which is predicted to become the historic range. This is a bleak future for such an important species.
For more information, please refer to the resources listed below.
D’Amore, D.V., P.E. Hennon, P.G. Schaberg, G.J. Hawley. 2009. Adaptation to exploit nitrate in surface soils predisposes yellow cedar to climate induced decline while enhancing the survival of western redcedar: A new hypothesis. Forest Ecology and Management. 258 (2261-2268).
Gashwiler, J.S. 1971. Emergence and mortality of Douglas-fir, western hemlock and western redcedar seedlings. Forest Science. 17: 230-237.
Mathys, A., Coops, N.C. and Waring, R.H. 2014. Soil water availability effects on the distribution of 20 tree species in western North America. Forest Ecology and Management 313: 144-152.
Mote, P.W., A.F. Hamlet, M.P. Clark, and D.P. Lettenmaier. 2005. Declining mountain snowpack in Western North America. Bull. Amer. Meteor. Soc. 86(1):39–49.
Mote, P.W., and D. Sharp. 2016 update to data originally published in: Mote, P.W., A.F. Hamlet, M.P. Clark, and D.P. Lettenmaier. 2005. Declining mountain snowpack in Western North America. B. Am. Meteorol. Soc. 86(1):39–49.
Mote, P.W., A.F. Hamlet, M.P. Clark, and D.P. Lettenmaier. 2005. Declining mountain snowpack in Western North America. B. Am. Meteorol. Soc. 86(1):39–49.
Seebacker, T.M. 2007. Western redcedar dieback: possible links to climate change and implications for forest management on Vancouver island, B.C. Thesis Master of Science. The University of British Columbia.