[VIPS] Crown ideotype for hybrid poplar

Frank W. Telewski telewski at cpp.msu.edu
Fri Oct 6 09:21:17 EST 1995

Speed specific drag will be a function of the total crown area,
and the flexability of the crown components, leaves, branches and 
main axis.  As wind speed increases, drag can actually drop as the
leaves and branches are forced together.  Gusts will be the most
destructive since the force applied is fairly instant and the
crown does not have time to respond in an elastic fashion.

Leaves of the poplar group are actually very well adapted to deal
with wind stress.  The flat petiole allows the leaf to 'flutter'
freely, dispersing or shedding wind energy as turbulance in the wake 
of the tree.  Smaller leaves should be less likely to be mechanically
torn by the wind, but the total leaf area will still influence the
drag.  The smaller leaves should also move more freely, possibly 
decreasing the drag. A wind tunnel comparison would be interesting.
Dr. Steve Vogel, Duke University has looked at leaf fluttering in
wind tunnels and drag, but I don't beleive he has conducted a comparison
for hybrid poplars. Steve is in the Zoology Dept. and author of the
book on life in moving fluids.

The flexural stiffness of branches and the main stem will also influence
the drag.  Usually wind increases flexural stiffness by producing
thicker stems, however the elastic modulus (the second component of
flexural stiffness) has been observed to decrease, allowing the tree
to absorb more of the wind energy as deformation in the woody tissues.
More flexible branches should reduce drag, but I'm not convinced that
the branches of wind-blown trees are more flexible.  Actually, I think
it is the inverse.

Frank W. Telewski 
Assistant Professor and Curator
W.J. Beal Botanical Garden
Michigan State University
East Lansing, MI  48824

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