Understanding catastrophic wind disturbance 21 Concepts

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Despite extensive previous work on catastrophic wind disturbance and subsequent ecological effects, there has been no specific definition of wind disturbance. Defining catastrophic wind disturbance is difficult because wind varies within/between events in intensity, size and frequency. In this review, we refer to catastrophic wind disturbance as including high wind events, mainly hurricanes, tornados, downbursts, gales and severe windstorms that may result in substantial tree damage or mortality. In most cases, the catastrophic wind disturbances we focus on in this synthesis represent a form of large, infrequent disturbance (LID) such as described by Turner and others (1998) as natural, catastrophic events that are 'large in spatial extent and infrequent in occurrence'. Catastrophic wind disturbances can be identified from their high wind intensity and extreme maximum gusts (Foster & Boose, 1995; Everham & Brokaw, 1996; Peterson, 2000; Lugo, 2008). The strongest winds (maximum wind speed about 125 m/s and average speed about 100 m/s) characterize tornadoes. A hurricane is a tropical storm when its wind speed is higher than 35 m/ s and a typical hurricane has an average wind speed of 70 m/ s. Gales (average wind speed about 50 m/s) and severe windstorms (average wind speed about 3050 m/ s) more often produce winds of only moderate intensity, but in some cases, they can generate winds as destructive as tornadoes. A downburst is a straight-direction catastrophic surface wind in excess of 17 m/ s caused by a small-scale, strong downdraft from the base of convective thundershowers and thunderstorms (Fujita, 1985), and can exceed 50 m/s (or even 75m/s) and cause tornado-like damage.

Location

Forest type

Windstorm type

Reference

Asia

Taiwan

hardwoods

typhoon

Lin et al., 2003

China

northern forests

wind and snow

Zhu et al., 2005

Japan

Subtropical forests

typhoon

Xu et al., 2003b, 2004

Australia-New

Zealand

Australia

Australian rain forests

cyclones

Bellingham, 2008

New Zealand

planted forests

windstorms

Moore & Quine, 2003

Europe

United

spruce forests

windstorm

Quine, 2003

Kingdom

Finland

boreal forests

wind and snow damage

Pellikka &Jarvenpaa, 2003

North America

Canada

boreal forests

windstorm

Mitchell et al., 2001

United States

Alaska

coastal temperate rainforests

severe windstorm

Kramer et al., 2001

Colorado

Picea-Abies-Pinus subalpine forests

severe windstorm

Veblen et al., 2001; Lidemann & Baker, 2002

Massachusetts

hardwoods

hurricanes

Wilson et al., 2005

Michigan

hardwoods

windstorms

Woods, 2000, 2001

Minnesota

hardwoods

severe windstorm

Peterson, 2004

New England

hardwoods

hurricanes

Boose et al., 2001

Wisconsin

hemlock-north hardwoods

severe windstorm

Schulte & Mladenoff, 2005

New York

mixed hardwood forests

severe windstorm

McMaster, 2005

Pennsylvania

hemlock-hardwood forests

tornados

Peterson, 2000

North Carolina

Piedmont pine-

hurricanes &

Elliott et al., 2002; Xi

hardwoods, Appalachian

downburst

et al., 2008a, 2008b

hardwoods

Florida

slash pine savannas; mixed-hardwoods

hurricanes

Platt et al., 2000, 2002; Batista & Platt, 2003

Texas

southern mixed hardwood forests

hurricane & severe storm

Harcombe et al., 2002

Table 1. Example case studies of catastrophic windstorms in temperate forests since 1998 by geographic locality, forest type, and windstorm type

Table 1. Example case studies of catastrophic windstorms in temperate forests since 1998 by geographic locality, forest type, and windstorm type

Occurrences of catastrophic wind events vary greatly in frequency and return times among windstorm types and localities. Hurricanes are tropical, high-wind events and can be common in near-coast tropical regions, but are less frequent in the inland tropics. Catastrophic hurricanes (defined as Saffir-Simpson as category 4 or 5) reoccur for a particular area of the coastal tropics on average every 20-60 years (Brokaw, 1991). The frequency of hurricanes decreases from tropical coasts to inland temperate regions. Major hurricanes only occasionally achieve landfall in temperate areas, and rarely reach the inland temperate areas (Webb, 1999).

The reoccurrence intervals of major hurricanes in temperate forests vary greatly from less than 20 years in the Southeastern US coastal regions (Gresham et al., 1991; Doyle, 1997; Platt et al., 2000), to about 50 years in the temperate Piedmont of the Southeastern US (Xi, 2005), to about 70-100 years in the Northeastern US (Foster & Boose, 1986, 1995). Storms like the 1938 hurricane (Category 5) that caused disastrous forest damage in the Northeastern US typically occur in the region only once per century. The reoccurrence rates of major hurricanes on a geological time scale for a specific location in a temperate region might be even longer. A sediment core study used to quantify hurricane activity in the Lake Shelby region of coastal Alabama showed a recurrence interval of about 300 years for catastrophic hurricanes during the last 5,000 years, and about 600 years during the last 10,000 years (Liu & Fearn, 1993). Compared to hurricanes, the frequencies of other types of catastrophic wind events (e.g., tornado, gales, downburst and severe storms) are highly variable in the temperate zone. Tornadoes have been reported widely in temperate North America, especially in the central Great Plains of the United States, where they can be particularly violent. In the Tornado Alley region (Oklahoma-Kansas, USA), the number of tornadoes can reach 38 per 100 km2 per year (Fujita, 1985). Downbursts are more frequent than tornadoes, but due to their isolated and sudden nature (lasting several minutes to half an hour), their recurrence rates are rarely reported in the literature. To date, few studies have reported the occurrences of gales, although Gallagher (1974) and Fraser (1971) reported 34 years and 75 years for return times in forest regions of Ireland and Scotland respectively. For windthrow, Zhang and others (1999) reported the average rotation period over the Upper Peninsula of Michigan to be 541 years. In the northern temperate forests of Wisconsin, severe windstorm return periods vary greatly with estimates ranging from 450 to 1200 years (Canham & Loucks, 1984; Schulte & Mladeoff, 2005).

The spatial extent or magnitude of catastrophic wind disturbances, which can be expressed as mean affected area per disturbance event, varies significantly among windstorm types. Sizes of hurricanes are generally large. The eye of a hurricane is normally 30-60 km in diameter and its influence often extends over an area 300-500 km in diameter along its path (Baldwin, 1995). One example that illustrates the potential large size of a hurricane is the 1989 Hurricane Hugo, one of the most catastrophic windstorms in United States history and which significantly damaged more than 18,210 km2 of timberland in South Carolina (Sheffield & Thompson, 1992). Along its path in North Carolina, Hugo damaged more than 10,926 km2 of forests, with almost complete destruction of 275 km2 (Barnes, 2001). Another example was the 1938 hurricane, which blew down more than 2,400 km2 of forestland in central New England (Spurr, 1956). In contrast, a tornado usually causes substantial damage only along its long and narrow path. A typical tornado path is normally several dozen to several hundred meters wide, and 15-20 km long (Ruffner & Bair, 1984). The actual surface damaged by a tornado may be much less than its path owing to the way tornadoes skip across the landscape (Peterson, 2000).

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