NOAA/WDS Paleoclimatology - Andrus - Ramirez Farm - THPL - ITRDB OR130

Aim: Forest dieback is increasing in many forest ecosystems from unfavorable 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 radial 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 as well as previously published climate and weather data. From growth rates, we computed four growth indices evaluating the resilience to drought. Results: Warmer and drier climate conditions in May and June that extend the annual July-to-September dry season reduced radial growth in 9 of 11 sites (1975-2020). WRC trees recovered radial growth to pre-drought rates within three years when post-drought climate conditions were average or cooler/wetter than average. However, recovery from drought was slower or absent when conditions were warmer/drier during the post-drought recovery period, which appeared to lead to the widespread mortality event across the coastal populations. WRC tree mortality was portended by 4-5 years of declining radial growth. Annually-resolved tree mortality in coastal populations predominately occurred in 2017-2018 (80% of sampled dead trees) and coincided with exceedingly hot temperatures and the longest regionally dry period from May to September (1970-2020). In interior populations, tree 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 carbon sinks.