Linking tree mortality and canopy dieback of Western redcedar (Thuja plicata) to drier and warmer summer conditions
Project Description
Western redcedar (Thuja plicata) has experienced unusually high rates of tree mortality and thinning of tree canopies (i.e., western redcedar dieback) in the last decade in the Pacific Northwest (WA, OR, ID). Public land managers, private commercial foresters, indigenous peoples, and small forest landowners are concerned about recent western redcedar dieback, because it is a culturally, economically, and ecologically important species. We investigated how tree growth and dieback of WRC trees were affected by climate conditions and drought events in the Pacific Northwest using tree-ring methods. From 11 sites, we collected tree cores from 280 trees in three groups (no canopy dieback, partial canopy dieback and dead trees) and in total measured more than 30,000 tree rings. We addressed the following research questions: (1) How did the radial growth of unhealthy or dead trees differ from neighboring healthy trees? (2) How did the radial growth respond to interannual variability in climate conditions and how did the climate-growth relationship vary by tree status and population? (3) When pre-drought conditions and drought severity were similar, did radial growth response to drought differ between different climate conditions and populations? (4) How was annually-resolved tree mortality affected by interannual variability in weather and climate conditions?
Our recent study of western redcedar contributed several key findings to help understand the climate impact on recent dieback. Western redcedar trees that died and exhibited canopy thinning symptoms experienced a period of declining tree growth for 4-5 years prior to death or thinning, an indicator of increasing tree stress and declining physiological function. As has been observed for other conifers in the PNW, warmer and drier conditions in late spring and early summer, which extend the typical dry season (July-September), decreased radial growth of western redcedar. Radial growth recovery from drought slowed or ceased when post-drought conditions were warmer and drier; such conditions led to a widespread mortality event across coastal populations (westside of Cascade Mtns in Washington and Oregon). In coastal populations, tree mortality mainly occurred in 2017 and 2018 and coincided with multiple, consecutive years of uncharacteristically hot and dry conditions. In interior populations, tree mortality occurred over the last three decades and coincided with warmer and drier conditions in late summer (August-September). It is important to note that WRC tree mortality is not occurring in all forests with WRC trees, and many WRC trees are still healthy. However, our findings are an early warning that warming climate and more frequent and severe droughts during the late spring and summer will likely reduce growth of WRC trees and increase the vulnerability of WRC trees to canopy dieback and mortality.
Project Updates/News
Check out these news articles regarding the dieback and mortality of western redcedar:
Has this iconic Northwestern tree reached a tipping point? (Seattle Times)
New study sounds alarm, provides hope for western redcedars (Columbia Insight)