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Monitoring tree mortality in mature Douglas-fir forests: size and species matter
Cline, S., EHenry Lee, P. Beedlow, AND Ron Waschmann. Monitoring tree mortality in mature Douglas-fir forests: size and species matter. Ecological Society of America, Portland, OR, August 06 - 11, 2017.
In the Pacific Northwest (PNW), forested watersheds cover about 47% of the land area and are the source of over 80% of Oregon’s water. Recent evidence indicates that forests throughout the west are sensitive to temperature and drought stress, making them more vulnerable to disturbance, widespread accelerated tree mortality, and subsequent impacts on ecosystem services, including water quality and carbon sequestration. As part of an effort to evaluate biotic and abiotic factors influencing forest health and identify indicators of climate change in forested watersheds, tree mortality rate was measured on long-term monitoring plots established in the mid-1990s in mature (~100-400 years) Douglas-fir-dominated forests in the western Cascade Mountains, Oregon, USA. We address key information gaps and scientific uncertainties regarding the ability to predict tree growth and mortality in response to climate change and climate-mediated forest disturbances. This work is intended to improve forest resource management decisions. This work contributes to a deliverable under ACE/CIVA 2.4.
Background/Question/MethodsA 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.Results/ConclusionOverall 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 tree fall 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.