A September 1999 satellite image shows two large, category 4 hurricanes (Floyd and Gert: up to 155 mph winds) approaching the US coast. Wind and rain damage from Floyd extended from the Carolinas to New England.

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Agents of Natural Change in Watersheds

Natural Change Processes: Windstorms
Windstorms are one of the many factors responsible for maintaining the spatial mosaic of different vegetation communities that exists across the landscape. Extreme windstorms, such as tornadoes in the Central US and hurricanes in the Southeastern US, occur regularly. In the Pacific Northwest, ten known storms with hurricane force winds have hit the coast in the last 200 years. Two of these storms had winds in excess of 150 mph. Many hurricanes, as well as simply high winds over sustained periods of time, have deforested areas of hundreds to thousands of square miles. In 1999, for example, 25% of the Boundary Waters Canoe Area of Minnesota was deforested in a massive blowdown not associated with a hurricane.

The effects of a windstorm can depend greatly on the local topography and the vegetation present. Disturbance associated with wind can thus be quite patchy. The ability of a given tree to withstand a windstorm depends on not only the energy of the wind, but the exposed surface area of the tree, its root mass, and the characteristics of the soil it is rooted in. Small groves of old-growth trees found in second growth forests can be especially susceptible to windstorms. Blowdown patches form open areas that become habitat for edge-preferring plant and animal species. Synergy between windstorms and pest outbreaks is evident when a disease weakens a stand of trees that later become victims of high winds. There can also be interactions between human land use and the effects of windstorms, as when removing natural windbreaks makes remaining vegetation more susceptible to windthrow.

Wind can affect water quality and aquatic ecosystem health, for example, by adding significant quantities of windblown soil to the water. Wind events are often responsible for transporting debris and in some cases organisms to different areas of the lake. Even non-extreme winds play an important role in determining when a lake stratifies or destratifies and the length of time that it retains an ice cover. The timing of lake turnover in the fall, for example, depends on the relative strengths of wind energy and the water's buoyant resistance to mixing.