Canary in the forest?-Tree mortality and canopy dieback of western redcedar linked to drier and warmer summers

Aim: Forest dieback is increasing from unfavourable climate conditions. Western redcedar (WRC)-a culturally, ecologically and economically important species-has recently experienced anomalously high mortality rates and partial canopy dieback. We investigated how WRC tree growth and dieback responded to climate variability and drought using tree-ring methods.Location: Pacific Northwest, USA.Taxon: Western redcedar (Thuja plicata).Methods: We collected tree cores from three tree health status groups (no canopy dieback, partial canopy dieback, and dead trees) at 11 sites in coastal (maritime climate) and interior (continental climate) WRC populations. From growth rates, we computed four growth indices that assessed the resilience to drought and estimated the year of death.Results: Warmer and drier climate conditions in May/June that extended the annual July-to-September dry season reduced radial growth in 9 of 11 sites (1975-2020). WRC trees recovered growth to pre-drought rates within 3 years when post-drought climate conditions were cooler/wetter than average. However, recovery from drought was slower or absent when warmer/drier conditions occurred during the post-drought recovery period, possibly leading to the recent and widespread mortality across the coastal population. WRC mortality was portended by 4-5 years of declining growth. Annually-resolved mortality in coastal populations predominately occurred in 2017-2018 (80% of sampled dead trees), a period that coincided with exceedingly hot temperatures and the longest regionally dry period from May to September (1970-2020). In interior populations, mortality was dispersed among years but associated with warmer and drier conditions from August to September.Main conclusions: Our findings forewarn that a warming climate and more frequent and severe summer droughts, especially in consecutive years, will likely increase the vulnerability of WRC to canopy dieback and mortality and possibly other drought-sensitive trees in one of the world's largest forest carbon sinks.

Automated summary

The study reveals that the increasing mortality rates of Western redcedar are mainly attributed to climate warming and drought. Additionally, the post-drought climate conditions significantly affect the recovery rate of trees, further exacerbating the mortality of some trees.