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.