Recent tree mortality in National Parks in western Washington

Background/Question/Methods

Tree mortality has been documented as doubling every 17 years in old-growth forests of the Pacific Northwest, most likely due to a warmer and drier climate. Implications include fewer large trees, less carbon storage, and forests predisposed to abrupt dieback. Some of the most significant remaining old-growth forests are in national parks of the North Coast and Cascades Network in western Washington state. As part of a nation-wide program to monitor the condition of ecosystems in national parks, we are tracking tree recruitment, growth, and mortality in mature and old-growth forests in the network. We established 45 permanent forest plots at Lewis & Clark, Mount Rainier, North Cascades, and Olympic National Parks, at elevations from sea-level to 1800 meters, capturing the extremes and the middle of the temperature and precipitation gradients for forests in the network in three strata defined by elevation. The plots are 0.25 hectares at Lewis & Clark and one hectare at the other three parks. Tree mortality is assessed annually and tree recruitment and growth will be recorded every five years. Using data from the 35 plots established in 2008, we evaluate patterns of mortality through 2011.

Results/Conclusions

Species composition and stand structure varied markedly between the elevation strata and were relatively similar between parks within a stratum. Low-elevation plots were comprised mostly of Tsuga heterophylla and Picea sitchensis; plots in the mid-elevation stratum were comprised mostly of Tsuga heterophylla, Pseudotsuga menziesii, and Thuja plicata; high-elevation plots were mostly comprised of Abies lasiocarpa and A. amabilis. Stem density increased and tree basal area decreased with elevation. The overall average annual mortality rate was 0.77%, which is within the range expected for older forests in the region. Most of the 35 plots (25) showed no consistent trend in tree mortality, with five plots each showing decreasing and increasing trends. Five plots had no tree mortality between 2008 and 2011. The highest average annual mortality rate for a plot was 5.2%, mostly due to two years of multiple tree deaths attributed to the Douglas-fir beetle (Dendroctonus pseudotsugae).