Windfirmness of Southeastern Piedmont oak and pine trees, tested by static winching

Background/Question/Methods

    Wind is one of the primary causes of natural disturbance in many forests.  However, mechanistic understanding of wind disturbance lags far behind analogous understanding of fire as an agent of disturbance.  Fundamental to a mechanistic understanding of wind effects is knowledge of the windfirmness or resistance of trees.  While theoretical approaches can be used to estimate trunk strength, overall resistance can only be measured through static winching.  Such tests have been common in Canada, the U.K., and Scandinavia, but to date only one study has been performed in forests of the U.S.  To gain insight into differential interspecific patterns of damage in natural windstorms, we are using static winching tests to quantify tree windfirmness for several common Georgia tree species.  We hypothesized that 1) windfirmness is a curvilinear function of tree size; and 2) that oaks are more windfirm than pines.  Static winching employs a strap and winch apparatus; the strap is attached as high as is practical on the pulled tree, and to the front of the winch; the back of the winch is in turn attached to the base of an anchor tree.  The winch is tightened until the pulled tree fails by trunk breakage or uprooting.

Results/Conclusions

     We winched trees ranging in size from 34 to 43 cm diameter at 1.3 m (dbh).  All loblolly trees winched to date failed by trunk breakage.  The cable force required to cause failure (critical cable force) of loblolly pines ranged from 1032 kg to 2786 kg.  There was a strong quadratic relationship between critical cable force, and trunk diameter:  Critical force (kg) = -92.09(dbh) + 3.64(dbh²).  Static winching is underway for oak species (Quercus alba and Quercus falcata) in the same study sites.  The critical forces necessary to cause tree failure can be combined with size-specific parameters of crown size and streamlining to estimate critical wind velocities that may cause tree failure; such extrapolation will allow mechanistic prediction of levels of wind damage from given wind velocities in storms such as hurricanes, tornadoes, and derechos.  Only a knowledge of tree windfirmness will allow understanding of wind disturbance to move from a phenomenological basis to a mechanistic basis.