Windthrow
Causes: Heavy wind combined with predisposing tree, stand, & site factors.
Tree Species Impacted in Alaska: All trees have the potential for windthrow/blowdown.
Damage(s): Top breakage, bole snap or uprooting of individual trees to large patches of forest; wounding from wind events provide entry points for stem decay fungi and windthrown trees create habitat for bark beetles.
Current Status of Landslides & Wind Damage 2024
Southeast Alaska is frequently impacted by fall and winter storms off the Gulf of Alaska. New areas of windthrow are mapped during aerial detection surveys (ADS) each year, but the year that wind damage took place can be difficult to determine from the plane. Using satellite-based tools that detect change from one growing season to the next, we identified the year wind events occurred on Admiralty Island and the adjacent shore of Chichagof Island. High-resolution satellite imagery, along with storm data (https://www.ncdc.noaa.gov/stormevents/), were used to determine the location and timing of two different wind events mapped in 2024: 70 acres of forest along the northern coast of Admiralty Island were blown down during the fall of 2023 and 540 acres of forest along the west coast were blown down at the end of January 2024. The windthrow on the west coast had a high density of dead western hemlock killed several years earlier by heavy hemlock sawfly defoliation. Defoliation by western blackheaded budworm likely occurred in the event area along the northern coast, but mortality from the defoliation was minimal.
Although both areas were impacted by the defoliation events from 2018 to 2023, defoliation did not contribute to windthrow in both areas. Patches of dead trees are easily toppled by wind years after mortality, as seen along the west coast. In general, dead trees are the most susceptible to windthrow, followed by shallow-rooted and highly decayed live trees. Live defoliated trees without other defects are considered the least susceptible to windthrow, since defoliated trees provide a reduced sail area to catch wind.
2024 Ground Detection Survey Observations: 1 area of windthrow was noted along northern Eyak Lake near Cordova.
2024 Aerial Detection Survey Observations: Most of the 1,180 acres of mapped windthrow in Alaska was in Southeast, though there was one very large windthrow event in the Interior along the Richardson Highway near Dot Lake (500 acres). Remote sensing methods were also used to evaluate windthrow events on Admiralty Island in Southeast Alaska.
Our team is utilizing satellite imagery and the LandTrendr RGB Pixel Time Series tool in Google Earth Engine (GEE) to validate the timing of landslide and wind events. This tool, developed by the Kennedy Lab at Oregon State University (https://emapr.github.io/LT-GEE/ui-applications.html#ui-landtrendr-pixel-time-series-plotter), accesses Landsat imagery from 1984 to 2024 in GEE and compares spectral values across years to detect change. Landslide or wind events often occur in the fall or winter and are detected as change the summer following the event. More accurate reporting on the timing of impactful disturbance events can improve our understanding of the relationship between landslides, topography, and weather events.
Tanacross Village in 2012 when a windstorm toppled spruce and left residual trees leaning.
(USDA Forest Service photo.)The most recent major wind damage occurred in 2012 in the upper Tanana Valley between the Little Salcha River (west of Delta Junction) and Tanacross (west of Tok) during a mid-September windstorm. The Alaska Department of Forestry conducted special aerial and ground surveys to evaluate the damage since it occurred after the annual aerial detection survey in July. Overall, it is estimated that 1.4 million acres of forest along approximately 55 miles of the Alaska Highway were damaged in this storm. Over much of the area, trees were tipped or severely bowed; toppled trees were less common. In a separate event in winter 2011-2012, strong gusts and heavy snow loads caused extensive damage along a 20-mile stretch of the Seward Highway on the Kenai Peninsula with top breakage of approximately 20% of the spruce in these stands, and greater damage to smaller trees. Beetle outbreaks did not occur in response to this damage, possibly due to poor weather during beetle dispersal.
In Southcentral Alaska, high wind events are a common source of disturbance and have resulted in large-scale, long-duration outbreaks of spruce beetle activity. For example, the 1987 Mallard Bay wind event (several hundred acres of blowdown) on the Kenai Peninsula set the stage for a spruce beetle epidemic that impacted the whole of Kachemak Bay. A similar event in the 1970s near Caribou Creek resulted in an epidemic that not only swept through Resurrection Valley, but also through the Big and Little Indian Valleys and beyond, into the Kenai National Wildlife Refuge. Wind has not been considered a major disturbance factor in Interior Alaska, but can occur as it did in 2012.
In Southeast Alaska, gale-force winds are strongest in fall and winter months, but can occur year-round (Harris 1999). Southeast gales (cyclones) are considered the most severe wind storms, consisting of counter-clockwise winds rotating around a central low-pressure core. Many unmanaged even-aged stands in Southeast Alaska originated from an extraordinarily severe and widespread cyclone in October 1880 (Mock and Dodds 2009, Kramer et al. 2001). Kramer et al. (2001) identified and subsequently ground-checked even-aged stands from aerial imagery on Kuiu Island. Of these presumably windthrown stands, 40% were ~110 years old, indicating that they originated following the 1880s cyclone. The other 60% of even-aged patches originated from at least four other major storms 50-400 years ago. The most common storm direction was from the South, Southeast and Southwest based on the direction of treefall.
Wind is a common and important disturbance in Alaskan forests, contributing to stem breakage (wind/bole snap) or complete uprooting of trees. In the coastal rainforest of Southeast Alaska, where wildfire is rare, blowdown is an important stand renewal process. Small canopy gaps facilitate the development of uneven-aged forests and increased growth of understory vegetation, while large canopy gaps create even-aged forest stands. Stand-scale windthrow may occur on exposed sites when heavy rain is followed by extreme wind. The direction of treefall from past wind disturbance can be determined long (at least 150 years; Kramer et al. 2001) after a wind event based on the pit and mound topography created by large, overturned root systems.
Windthrow occurs when the force of the wind exceeds a tree’s stem or anchor strength (Harris 1999). Shallow rooting depth and soil saturation increase vulnerability to windthrow from uprooting (i.e., the wind force is greater than the anchor strength). In contrast, stem, butt and root decays increase vulnerability to windthrow from bole or root collar breakage (i.e., the wind force is greater than the stem strength). Trees that are uprooted or broken by wind are typically aligned with one another and the prevailing wind direction.
Stand characteristics, such as tree height-to-diameter-ratio and tree density, and tree mechanics (height, diameter, crown size and rooting depth) are important predictors of windthrow potential. Wind firmness decreases with increased height growth and crown size, and increases with deeper rooting depth and tree diameter. Although larger diameter trees are more wind firm, the probability of stem decay, which decreases the structural integrity of tree boles, also increases with tree diameter (age) and varies by species. Stem fluting of western hemlock, which typically occurs on exposed coastal sites, is thought to contribute to wind firmness.
Topographic conditions and stand management activities influence windthrow potential, because wind accelerates as it moves over and around landscape obstacles. Depending on landscape position, thinned stands or stands adjacent to clearcut harvests may experience increased susceptibility to windthrow. Nowacki and Kramer (1998) used exposure, landscape position and topography to define distinct wind disturbance regimes; model results show that recurrent, large-scale wind events are common on exposed landscapes, while small-scale canopy gaps predominate on wind-protected landscapes. They suggest that land managers may seek to mimic natural wind disturbance regimes through silvicultural treatments, or use wind disturbance patterns as a template to evaluate past and current land management practices.
Kramer et al. (2001) found that the best windthrow prediction model contained the variables exposure, slope, soil stability and elevation, as well as three interactive terms (exposure x slope/soil/elevation). The model correctly classified forests as storm-prone or protected with 68% accuracy on Kuiu Island and 72% accuracy on Zarembo Island. As predicted, forest structure in study plots was more homogenous in windthrow-prone areas. DeGayner et al. (2005) found that large-diameter trees were twice as abundant in wind-protected forests compared to wind-prone forests, and discuss the significance to bear-den habitat selection in large diameter trees with large hollows from stem decay.
Windthrow is mapped during the aerial detection survey in summer.
In Southeast Alaska, fall and winter cyclones are an important form of forest disturbance, creating small canopy gaps or stand-replacing, catastrophic disturbance. Several publications below focus on wind disturbance in Southeast Alaska.
Storm activity is typically most severe along the coastal fronts of western and Southcentral Alaska, which can also impact broad areas as storms move inland. Dispersed damage from seasonal storms provide ample brood material to maintain low, sustained, bark beetle populations until conditions are favorable for the next large outbreak.
Wind has not been considered a major disturbance factor in Interior Alaska, but can occur as it did in 2012.
DeGayner, E. J.; Kramer, M. J; Doerr, J. G.; Robertsen, M. J. 2005. Windstorm disturbance effects on forest structure and black bear dens in southeast Alaska. Ecological Applications 15:1306-1316. Abstract available here.
Harris, A.S. 1999. Wind in the forests of southeast Alaska and guides for reducing damage. Gen. Tech. Rep. PNW-GTR-244. Portland OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 63 p.
Kramer, M. G.; Hansen, A. J.; Taper, M. L.; Kissinger, E. J. 2001. Abiotic controls on long-term windthrow disturbance and temperate rain forest dynamics in Southeast Alaska. Ecology 82(10): 2749-2768. Abstract available here.
Mitchell, S.J. 2012. Wind as a natural disturbance agent in forests: a synthesis. Forestry: An International Journal of Forest Research 86(2): 147-157. Available here.
Mock, C. and S. Dodds. 2009. The Sitka Hurricane of October 1880. DOI: 10.1007/978-90-481-2828-0_7. Abstract available here.
Nowacki, Gregory J.; Kramer, Marc G. 1998. The effects of wind disturbance on temperate rain forest structure and dynamics of southeast Alaska. Gen. Tech. Rep. PNW-GTR-421. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 25 p. (Shaw, Charles G., III, tech. coord.; Julin, Kent R., ed.; Conservation and resource assessments for the Tongass land management plan revision).
Content prepared by Robin Mulvey, Forest Pathologist, U.S. Forest Service, robin.mulvey@usda.gov.