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Windthrow is complex and may appear random at first glance. Windthrow researchers break down this complexity by separating damage caused by endemic and catastrophic winds, and by evaluating the role of environmental factors separately. The relative hazard assessment method presented here was developed for this workshop and uses environmental indicators with which managers are familiar. It has been adopted as the basis for the FS 712 Field Cards. It uses an ecological/physiological model of windfirmness rather than a mechanistic model. The underlying premise is that trees can adapt to endemic peak winds, and that lack of windfirmness results from some site/stand limitation (see Mitchell, 1998 for details).

The environmental factors which contribute to endemic windthrow risk can be broadly grouped into topographic exposure, soil and stand properties. These factors are integrated to yield an estimate of 'biophysical hazard.' Each component hazard (soils/topographic/stand) is assessed by asking a 'diagnostic' question (e.g., for soil hazard: 'is root anchorage restricted?'). 'Windthrow risk' is the combination of biophysical hazard and 'treatment risk.' Treatment risk refers to the change in wind loading on residual trees caused by a particular treatment. Treatments which result in major increases on residual trees are high risk. Both assessment of biophysical hazard and treatment risk require an estimate of the damaging wind direction(s). Historic windthrow patterns are useful indicators of damaging wind direction and site influences.

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