A Theoretical and Experimental Investigation of Crown Collisions in a Forest Canopy

Date of Completion

January 2011


Agriculture, Forestry and Wildlife|Atmospheric Sciences




A comprehensive analysis of the wind-canopy system requires quantitative descriptions of at east three phenomena: the dynamic characteristics of the canopy elements, including the collision interactions between those elements; the dynamic characteristics of the overlying atmospheric shear layer: and the interactions that occur between the dynamically active canopy and atmosphere. This dissertation summarizes the results of theoretical and experimental investigations of each of those canopy and atmospheric phenomena.^ In the first part of this dissertation. I describe several theoretical models of crown collisions in an idealized forest canopy. I also describe the modeled effects of those collisions on atmosphere-canopy dynamics. The primary conclusions of the this theoretical analysis are: that the probabilities and intensities of collisions between adjacent Forest trees depend in specific, quantifiable ways on the mechanical properties of the trees, the spacings between the trees, the intensities of the forcing gusts of wind, and the durations of the forcing gusts of wind; that the energetics of the crown collisions (i.e., energy-conserving or energy-dissipating) determine the specific effects of collisions on canopy behavior; and that crown collisions have very little effect on the shear instability characteristics of the coupled canopy-atmosphere system, as predicted by a simple linear stability analysis.^ The second part of this dissertation describes the results of a stand-thinning experiment in a pine plantation forest. The data indicate that collisions among the crowns of a group of adjacent trees produce observable effects on the oscillation frequencies of those trees. In the study plot, crown collisions were observed to cause upward shifts in tree oscillation frequencies which. I theorize, reflect increases in the effective mechanical stiffnesses of the trees. Crown collisions thus helped to increase the collective stability of the forest. ^