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.
Mid-latitude cyclones wielding windspeeds comparable to category 1, 2, and 3 hurricanes occur as near annual events along the Pacific Northwest coast of the United States, yet have received little national attention despite their widespread and severe socio-economic and ecological impacts. As a result, little is known about the historical frequency and severity of these high wind events and their relationship to short-term climate oscillations and global warming. Our research sought to close this knowledge gap by creating a 300+ year tree-ring history of hurricane-force winds with the goals of characterizing their spatio-temporal variability and their influence on the region’s forest ecology. There were three distinct outcomes from our research. First, we developed a new method of using growth anomalies embedded in the tree-ring record to document historic windstorm events. This method allowed us to detect approximately 80 percent of the known windstorms between 1895 and 2003 along the Oregon Coast. We then applied our method to document the severity of a 1706 windstorm recorded in journal accounts by the Lewis and Clark expedition. This application illustrated the potential of our method to quantify and compare the severity of a major historical windstorm relative to those documented during the 20th century. Our second research outcome was the discovery that Pacific Northwest windstorms exhibit a 20 to 30-year periodicity corresponding with the warm and neutral phases of the Pacific Decadal Oscillation (PDO) and atmospheric pressure differences as measured by the North Pacific Index (NPI). This relationship appears to be related to sea surface temperatures not directly correlated with the El Niño/Southern Oscillation (ENSO). Using an expanded data set from seven coastal sites, we also: 1) identified several large and previously undocumented windstorms that impacted multiple sites along the entirety of the Oregon coast and 2) documented a historically recent (post-1880s) and significant northward latitudinal shift in the frequency and severity of historical windstorm events. Our third research outcome shows how codominant tree species uniquely respond to windstorm events. This research revealed that Sitka spruce is more growth sensitive to the frequency of windstorm-induced changes in canopy conditions than is Douglas-fir, but that both species accurately record the occurrence of high-magnitude windstorms. We attributed the different sensitivities of these species to their physiological tolerances, habitat requirements, and windstorm exposure. Unveiling the history, frequency, and severity of windstorms along North America’s west coast has several important implications related to the extreme hazard and profound influence these events have on the local and regional economies causing severe property damage, large areas of forest blowdown, economic challenges associated with decreased work productivity, and secondary spending to repair and replace infrastructure. Our research provides an important historical perspective of these extreme weather events improving our understanding of their frequency, severity, and future occurrence. This information should thus contribute to a better foundation for promoting local, state, regional, and national preparedness.
