Collaborative Research: The Occurrence of Severe Pacific Northwest Windstorms: A Multi-Century Dendroclimatic Assessment of Their Ecological Impacts

Portland State University, Portland OR

Investigators

Abstract

Strong midlatitude cyclones in the Pacific Northwest receive far less attention than the hurricane-prone regions of the southeastern U.S., yet generate high wind events (HWEs) of similar magnitude and frequency. These HWEs, with documented wind speeds up to almost 180 mph, are arguably the most important disturbance agent shaping forests west of the Cascade Range crest, which means that they exert a regional-scale influence on forest regeneration, forest succession, and fire regimes. For example, since 1880 at least five HWEs have each caused forest blowdowns exceeding one billion board feet. More recently, HWEs in 2006 and 2007 caused electrical power outages for millions of residents in western Oregon and Washington. However, despite their ecological, social, and economic significance, little is known about the timing, extent, cause, and consequences of Pacific Northwest HWEs. This research places those HWEs within a broader historical context by using the information in annual tree rings to compare wind events with known decadal-to-century-scale climate variations and to link their occurrence to forest dynamics and regional fire cycles. These objectives will be met using a multi-century climatic reconstruction based on tree-ring data collected at 10 sites in the Pacific Northwest and by employing a research protocol that measures and compares post-HWE tree-growth responses between wind-snapped, old-growth trees to those of younger, nearby undamaged trees. Growth anomalies matching those found in trees damaged by documented HWEs can then be identified for the period before the historical record to: 1) establish a 300-year record of HWEs using dendroecological techniques; 2) quantify the frequency, severity, spatial distribution, and synoptic causes of HWEs; 3) examine the relationship between HWEs and global and synoptic-scale climate phenomena, the end of the Little Ice Age, and anthropogenic influences on climate; and, 4) estimate the ecological ramifications of HWEs related to blowdown and post-blowdown fire cycles. A multi-century reconstruction and analysis of HWEs will provide several public and scientific benefits regarding an important but poorly understood topic. Foremost will be the establishment of a scientific basis for: 1) estimating the probability and severity of these wind events before the initiation of historical and instrumental records; 2) assessing their ecologic and potential economic impacts; and, 3) aiding agency preparedness. These findings will benefit future studies seeking to integrate climatology and forest ecology by providing a methodological approach applicable to other temperate regions affected by HWEs such as the northeastern United States. These results also will promote the understanding of how windstorms influence fire regimes and aid in the development of future forest management practices in an era of climatic uncertainty. Finally, the research will demonstrate how HWEs are influenced by climatic variability at the decadal-to-century scale and identify possible links between HWEs and human-induced climate change.