In this dissertation, I investigated mechanisms that mediate crown shyness in lodgepole pine (Pinus contorta var. latifolia Dougl. ex Loud). To examine how tree bole slenderness mediates crown collisions and crown shyness in closed canopy stands, I developed a technique to measure the simultaneous sway of a group of trees and reconstruct frequency of crown collisions and sway dynamics of individual or groups of trees. I then applied this technique to investigate tree sway and crown collision behaviour of even-aged lodgepole pine stands of different structure in Central Alberta. I hypothesize structural factors regulate crown collision dynamics and therefore the development of crown shyness and loss of leaf area as stands mature. Three stand structures representing differences in bole slenderness coefficient (height/diameter - SC) and stand density were studied. Comparing the sway statistics between stands indicated that crowns of slender trees have greater mean sway displacements, faster mean sway speeds and a greater mean depth of collision; thus indicating a greater likelihood of crown damage and/or growth inhibition. Comparing pre-thinned and thinned stands revealed the thinned stand had increased mean sway displacements, mean sway speeds and mean depth of collisions. Stand structure, specifically slenderness coefficient, is therefore, thought to regulate crown collisions and crown shyness.

Spatial patterns of crown displacement indicate that trees have a generally circular sway pattern that is not aligned with mean wind direction. Tree sway patterns were dramatically affected by thinning with many trees producing an elongated pattern of displacement that in most cases were not aligned with mean wind direction.

My closing investigation into the biomechanical influence on crown shyness involved measuring the percent crown closure (%CC) in groups of three trees (tree triangles) then correlating this to the mean slenderness coefficient (SC) and relative density (RD) of the tree triangle. Tree SC is thought to be inversely related to the collision intensity, while the index of relative density is generally assumed to be an indicator of crown development in closed stands. Tree triangles within each (closed canopy) stand were examined separately to eliminate confounding site-specific influences.

Canopy closure in tree triangles in all stands shorter than 11.4 m were positively correlated to RD, while canopy closure in most stands taller than 15 m were negatively correlated to SC. I believe this indicates a change in the mechanism controlling lateral crown development in even-aged lodgepole pine from light availability to crown collisions. The mean stand %CC (from all triangles within a stand) was negatively correlated with mean stand height; suggesting crown shyness typically increases as stands grow in height.