Western redcedar (Cw) is native to the Pacific Northwest of North America, with a broad geographic range extending from northern California, through Oregon, Washington, and British Columbia, into southeastern Alaska. Eastward, it extends as far inland as the western slopes of the Rocky Mountains in Montana and Idaho.

Cw is best adapted to climates with abundant precipitation and high humidity but can tolerate a range of conditions. In BC, Cw is most commonly found in cool and cold mesothermal climates on the coast and in montane continental cool temperate climates in the interior wet belt (Klinka & Brisco, 2009). The species grows across a wide variety of soil types, but is most productive on fresh to moist, nutrient-rich soils (Klinka & Brisco, 2009).

In British Columbia, Cw is found in two distinct populations: a coastal population, and an interior population. Approximately 81% of the province’s mature Cw volume is found in the coastal population (Klinka & Brisco, 2009). These two populations are ecologically distinct due to differences in climatic regimes, topography, and associated vegetation, but genetically, they are quite similar (Klinka & Brisco, 2009). It is common for Cw to co-occur on mesic and wet sites with western hemlock (Tsuga heterophylla) throughout its range in BC.

In the maps below, each colour represents the species environmental suitability during the historical reference period* of 1961–1990 as determined by expert ratings. Green represents a high suitability (E1 rating), blue represents moderate suitability (E2), yellow represents low suitability (E3), and white indicates areas that are unsuitable for the species. See CCISS Documentation tab > Methods > Environmental Suitability Ratings for more detailed definitions and information on the expert rating process.

*Note on terminology: The historical reference period is sometimes referred to as the “baseline” or simply “historical” or “reference” period.

On this page, you will find a visual and interpretive synthesis of the environmental suitability projections calculated using the Climate Change Informed Species Selection CCISS tool, including:

1. Proportion of the historic range that the species is predicted to remain environmentally suitable (i.e. persist) or become newly suitable (i.e. expand) by site type (edatope).

2. Predicted mean changes in environmental suitability across its geographic range in BC, by future time period and site type (edatope).

3. Predicted shifts in total suitable area over time across the province and by biogeoclimatic (BGC) zone.

Western redcedar (Cw) is generally more resistant to pests and pathogens than other timber species (Klinka & Brisco, 2009). However, opportunistic biotic agents can take advantage of already stressed trees. Cedar leaf blight (Didyamascella thujina) is the most severe pathogen currently affecting Cw Infecting leaflets of seedlings and mature trees, it causes gradual decrease in growth through a loss of photosynthetic area, eventually leading to tree death (Aldana et al., 2023). This pathogen is most common in dense stands with high humidity (Klinka & Brisco, 2009). The prevalence of and damage associated with cedar leaf blight is projected to increase under future climate scenarios (Aldana et al., 2023). Since genetic factors appear to influence susceptibility, selecting or breeding for resistance could be important in future management strategies for cedar leaf blight (Aldana et al., 2023).

Several species of heart- and butt-rotting fungi have long been associated with mature Cw, including brown cubical pocket and butt rot (Poria asiatica), white ring rot (Poria albipellucida), white pitted trunk rot (Fomes pini), brown crumbly butt rot (Merulius spp.), and spongy white rot (Poria subacida, summarized in Seebacher (2007)). To a lesser extent, Armillaria ostoyae (Armillaria root disease) and Phellinus weirii (laminated root rot) can also afflict redcedar trees.

Few insects are known to attack Cw directly. These include the western cedar borer (Trachykele blondeli, also known as the powderworm), the western cedar bark beetle (Phloeosinus punctatus), and the hemlock looper (Lambdina fiscellaria lugubrosa), which feeds broadly on conifer foliage (summarized in Seebacher (2007)).

For more information see the Forest Guide to Tree Health.

Widespread western redcedar dieback has been documented throughout the southern portion of the species’ range (Andrus et al., 2023), particularly in Washington state and southwestern British Columbia (Georgia Depression). This dieback is, at least in part, driven by drought, extreme heat events, and exacerbated by consecutive drought events without adequate recovery time.

Dieback in Cw is typically identified through a range of visible canopy symptoms. The most commonly reported indicators include thinning or death of the live crown (top kill), foliage yellowing (chlorosis), branch die-off (flagging), thinning, and in some cases, complete tree mortality. Trees may also exhibit increased cone production, which has been interpreted as a possible stress response. These symptoms often occur in the absence of obvious biotic agents, and are instead linked to environmental stressors, particularly drought and heat.

Since 2015, a pronounced increase in reports and field observations suggests that dieback is becoming more widespread in low-elevation coastal forests. A significant pulse of mortality was reported around 2017-2018, affecting forests from the Willamette Valley in Oregon, through western Washington, and into southern British Columbia (McWilliams, 2024). Similar reports come from southeastern Vancouver Island and parts of the lower mainland (Tomaszewski, 2022).

The causes of Cw dieback are increasingly understood to be multi-factorial, with climate water stress emerging as the dominant driver across most affected regions. While exact mechanism may vary by site and context, evidence consistently points toward seasonal drought, elevated temperatures, and increased vapor pressure deficit (VPD)—particularly during late spring and early summer—as key stressors contributing to decline and mortality.

Cw mortality is often associated with multiple consecutive years of water deficit, which leads to compounded physiological stress. Cw can recover from a single year of drought if followed by cooler, wetter conditions in subsequent years, but recovery slowed or failed when drought persisted across years (Andrus et al., 2023; Goodrich et al., 2025).