A regional increase in tree mortality rates associated with climate change will influence forest health and ecosystem services, including water quality and quantity. In recent decades, accelerated tree mortality has occurred in some, but not all, forest types of the western USA. At four long-term study sites, trees > 5 cm dbh were mapped and measured within < 1 ha plots located in mature (~100-400 years) Douglas-fir dominated forests along an elevation gradient in the western Cascade Mountains, Oregon, USA: Falls Creek (530 m), Moose Mt. (658 m), Soapgrass (1190 m), and Toad Creek (1198 m). We report results from multiple forest health and mortality surveys conducted since the mid-1990s as part of an effort to evaluate biotic and abiotic factors influencing forest health and subsequent impacts on ecosystem services.
Overall average annual mortality rates were < 1% (0.47-0.72), and were dominated by small understory tree death. Occasional dominant over-story tree death occurred through bole breakage or uprooting; predisposing factors were root, butt, or heart rot. At Moose Mt. tree mortality involved mostly bigleaf maple and at Falls Creek mostly seedlings and a few sapling western hemlock. At these two lower elevation sites, even though mortality rates of dominant Douglas-firs remained low, growth rates from tree-ring analyses showed a steady decline since ~1990, synchronous with increasing air temperatures measured locally. In the open understory at the mesic high-elevation site, Soapgrass, deaths of hemlock and silver fir saplings were infrequent, but included some larger silver firs. At the xeric high-elevation site, Toad Creek, mortality was higher than at other sites, but numerically deaths were still largely sapling true firs and hemlock while over-story treefall from root rot opened some large canopy gaps. Thus, while we have not detected an overall increase in tree mortality rates at any site, we did find a differential mortality rate between Douglas-fir and late successional species: unlike Douglas-fir, the overall annual average mortality of smaller, shade-tolerant, understory trees exceeded 1% at all sites, and often included periodic rates of 2-3%. Nevertheless, Douglas-fir susceptibility to tree mortality at lower elevations is likely increasing as indicated by the multi-decadal declining growth rate trends in combination with rising regional temperature.