Introduction
Effects of wind on trees and stands– seed and pollen dispersal
– influence on mass exchange (i.e., CO2 and water vapor)
– stem form and growth patterns– damaging effects
• uprooting, stem breakage• loss of foliage
Damaging Effects of Wind
Uprooting and breakage of trees are important stand replacing disturbances.
In managed forests they are generally viewed in a negative light.
In many areas of the world, wind is a major constraint on forestry
Modeling Wind Damage
Empirical approaches– analysis of sample plot data– retrospective studies
Mechanistic approaches– static– dynamic
Dynamic Response
Trees are dynamic structures which respond to the fluctuating wind forces acting on them
Two approaches to modeling dynamic response of trees to wind:
• mechanical transfer function• dynamic structural model of tree
1. Mechanical Transfer Function
Measure both wind speed and tree response
Transfer function
Wind speed Tree movement
u(t) y(t)
)()()( HUY )(
)()(
U
YH
2. Dynamic Structural Model
Predict response of trees to an applied wind force
Dynamic motion of tree described by the following equation:
)()()()( tFtkytyctym
2
)()(
2 ACtutF d
Model Requirements
• Need information on the following:• Natural frequency of tree• Damping ratio• Applied bending moment
• These can be predicted from a knowledge of the within-stand wind speed profile and tree size
Factors Affecting Applied Bending Moment
• Within-stand wind speed
• Crown size and shape
• Foliage drag coefficient
• Stem stiffness
• Height to center of mass
• Presence of snow or ice
Crown Size and Shape
• Drag force acting on crown is a function of crown frontal area
• Damping is also a function of crown mass and its distribution
Crown Shape Models
• Engineering theory in mechanistic models is quite advanced
• Mensurational information is quite primitive!
Conclusions
• Mechanistic models can provide useful information on the risk of wind damage
• Present models may suffer because the mensurational information they use does not match their level of engineering sophistication