Windthrow

Cause: Heavy wind / predisposing tree, stand & site factors.

Host(s) 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 in Alaska (2023 Update)

When conducting aerial surveys, the specific year or season of abiotic damage can be difficult to determine. 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 2023 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.

Less than 1,500 acres of wind damage were mapped statewide during 2023 aerial detection surveys, with heaviest impacts in Southeast Alaska. Damage was scattered along the northern Lynn Canal northwest of Haines, on the east side of Zarembo Island, and on west Kuiu Island. Use of the GEE tool allowed us to confirm where heavy wind damage occurred in 2023 or the previous fall/winter, and also that extensive damage on Admiralty Island occurred during an earlier wind event in 2021 or earlier. In Southcentral, minimal damage was observed northeast of Willow and the southern tip of Susitna Flats.

More than 250 acres of landslides were mapped during our aerial detection survey, mostly in Southeast Alaska. We mapped a 160 acre slide on the western slope of Portage Mountain on Kupreanof Island. The largest slide in Southcentral Alaska occurred along the Chitina River near Lost Butte. After the completion of our aerial detection surveys, a devasting and deadly landslide impacted the community of Wrangell on November 20^th, 2023, disturbing over 37 acres of land and destroying personal property.

Historic Activity

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 the early-1880s. 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 S, SE and SW based on the direction of treefall.

Return to top

Disturbance Dynamics, Predisposing Factors & Predictive Models

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.

Survey Method

Distribution / Wind Damage by Region

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.

Return to top

Links to Resource & Publications

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. Available here.

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.

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 theTongass land management plan revision). Available here.

 

Content prepared by Robin Mulvey, Forest Pathologist, Forest Health Protection, robin.mulvey@usda.gov.

Contact us    Forest Health Protection Homepage