Western Redcedar Dieback Linked to Warmer, Drier Summers

By Robbie Andrus, PhD, School of Environment, Washington State University,  robert.andrus@wsu.edu

Figure 1.  Stages of western redcedar decline (Photo:  Robbie Andrus, WSU)

Western redcedar dieback has been widely reported in recent years, from Oregon to southeast Alaska and from the Pacific Coast to the Northern Rocky Mountains, prompting questions about the likely causes of dieback. Western redcedar (WRC) is an iconic species in the Pacific Northwest with a rich ecological, cultural and economic value. The very large and old WRC trees found in the region are a testament to their longevity and persistence for centuries, including surviving through many drought periods. The term dieback refers to partial thinning of tree canopies and tree mortality (Fig. 1), and dieback of conifers has been attributed to one or multiple agents, such as droughts, insects, or disease. Recent observations of WRC dieback and speculation about drought and warm temperatures as the likely cause of dieback has raised concern about the capacity of WRC trees to persist under climate change.

In a recent study, we investigated how tree growth and dieback of WRC trees were affected by climate conditions and drought events using tree ring methods. Tree rings are an indicator of tree health, with wider rings often indicating more vigorous trees, and the widths of tree rings can help identify potential causes of dieback. We examined both growth and dieback in hopes that we would find more evidence to support the likely cause of dieback and understand the potential implications of our results in the context of climate change. Our study area was focused on the westside of the Cascade Mountains where most WRC tree mortality has been observed, but we also included sites in the eastern Cascades and Northern Rocky Mountains (northeastern Washington and northern Idaho).

WRC trees are adapted to the PNWs seasonal drought. WRC typically put on most of their growth in May/June when ample soil moisture and moderate temperatures support photosynthesis and tree growth. As the hot and dry conditions of July and Aug deplete soil moisture, WRC conserve water by growing less. However, what happens to growth when May/June are unseasonably dry and hot?

In our study, we found that weather conditions during May/June were critical to WRC tree growth and likely their overall health. WRC trees grew less (narrow tree rings) in years when temperatures were warmer and less rain fell in May/June. Such conditions likely reduce soil moisture below critical levels earlier in the growing season (compared to average years) and lengthen the period of low soil moisture. Very low soil moisture places immense stress on water transport systems, potentially creating air bubbles (embolisms) in water transport structures, that greatly reduces a trees capacity to maintain basic physiological functions, such as water delivery to the tops of trees and photosynthesis.

Many of the WRC trees with dieback starting around 2015 had signs of stress prior to dieback, we just hadn’t looked! We compared annual growth rings of healthy trees with trees that died in 2017 or 2018 (peak years of WRC mortality west of the Cascades). We found evidence of declines in growth for multiple years to decades prior to tree death (compared to surviving trees), indicating that stress was accumulating in the trees that died.

A common response to severe drought is for treetops to die (top kill) and branches to dieback from the tips to the trunk. Less foliage reduces the amount of water needed, and we observed many barely living trees with very little remaining foliage. While this strategy is helpful for surviving shorter-term droughts, it indicates a tree is at risk of dying and we expect many of the dead trees that we sampled likely were experiencing topkill and branch dieback prior to death.

WRC trees have recovered from many past droughts, but the climate conditions associated with recent dieback event were different. Growth of 80 percent of the dead trees that we sampled on the westside of Cascades abruptly terminated in 2017 or 2018 (i.e., death year), indicating that the morality event occurred synchronous in time from the Willamette Valley to the greater Puget Sound. In 2015, the PNW experienced a severe regional drought, and it was followed closely by multiple, consecutive years with extended periods of exceptionally hot and dry conditions in summers from 2016 to 2018.

Importantly, the longest summer dry period (consecutive days with <1 mm of precipitation) in more than 50 years occurred in 2017 or 2018. Such conditions had likely not occurred since the 1930s, and the trees that died, predominately trees < 80 years, may not have been alive or were shaded by larger trees during the period with similarly hot/dry conditions. The lack of a biotic agent, such has lethal bark beetles or disease, associated with recent dieback further supports hot drought as the likely cause of dieback.

Our findings contribute important new knowledge that helps inform the potential implications of climate change for a tree species that is culturally important to Indigenous peoples of the region and an integral component of Pacific Northwest forest resources and ecosystem services. In our study, we were not able to directly attribute the WRC mortality event to climate change. However, our findings are an early warning that future droughts and the warmer and drier summers expected under continued climate change will likely lead to more WRC dieback.

WRC tree mortality is not occurring in all forests with WRC trees, and many WRC trees are still healthy. The WRC trees that died were growing immediately next to many healthy trees, suggesting that suitable habitat still exists in most areas of PNW where WRC trees are growing. Additionally, the largest and older trees generally survived the recent unfavorable conditions, possibly due to genetic differences, better site location with access to water deeper in the soil profile, or other factors. Under future climate conditions, WRC trees are expected to persistent in wetter and cooler locations on the landscape, such as north-facing slopes or in valley bottoms. Taking action to limit climate change and adapt forests to a warmer future is essential for sustaining forests.

Figure 2.  Visit https://foresthealth.org/redcedar/ to learn more about the Forest Health Watch community science efforts to learn more about western redcedar decline.

Additional research and long-term studies about the health of western redcedars are needed to further understand the impacts of warmer and drier summers on our forests. You can get involved by signing up as a community scientist and/or adding observations of healthy and unhealthy western redcedar trees to WSU’s Western redcedar dieback map on iNaturalist (Fig. 2). Community scientists are encouraged to share observations and take note of tree health issues to help monitor western redcedar dieback across the Pacific Northwest.

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