Background Tree Mortality in Mature and Old-Growth Forests in Western Washington, 2008-2018

Increasing rates of tree mortality have been observed in recent decades for all major forest types worldwide. Elevated mortality has been linked to drought and high temperatures resulting from human modification of the atmosphere and climate. Older forests in the Pacific Northwest are globally significant as carbon sinks and may serve as refugia in the face of wildfires and climate change. We examined temporal patterns of tree mortality in mature and old-growth forests in national park units in western Washington, U.S.A, over the period 2008 to 2018, to evaluate whether the broader pattern of increasing tree mortality applies there. We also used individual-tree models of mortality to seek links between drought and tree mortality, while considering processes internal to forest stands such as competition. The annualized rate of tree mortality for the entire period generally did not exceed the expected, background rate of 1%. The mean was ≤ 1.0% for saplings (2.5 to 12.6 cm dbh) for 60% of plots, for small trees (12.7 to 76.1 cm dbh) for 66% of plots, and for large trees for 92% of plots. 95% confidence intervals wholly exceeded 1.0% yr-1 only for small size classes in a minority of plots (saplings, 7% of plots, and small trees, 11% of plots). Comparing the first and second 5-year segments of the observation period, there were no clear trends of increasing mortality for any of the size classes. We tested predictive factors, related to drought and internal processes such as competition, using generalized linear mixed-effects models and the Gompit link function, and found only an index of tree vigor to consistently predict probability of tree mortality. In one model, anomaly of vapor pressure deficit in winter months was significant, but the sign of the effect did not implicate drought (i.e., increasing mortality probability with higher than average moisture). These results show that, at least up to 2018, healthy trees within old-growth, protected forests in the Pacific Northwest may be resilient to current climatic changes, highlighting the importance of continued conservation and protection of these increasingly rare ecosystems and carbon stores. The occurrence of extreme climatic events following our study period emphasizes the need for continued observation and analysis to facilitate management for protection of refugia and migration corridors.