Measure of simultaneous tree sways and estimation of crown interactions among a group of trees

We present a technique to measure the simultaneous sway of a group of trees and reconstruct the frequency of crown collisions and sway dynamics of individual or groups of trees. We placed a biaxial clinometer (tiltmeter) at the live crown base in each of ten neighboring 15-m-tall lodgepole pine trees in Alberta, Canada. Tree bole rotation at tiltmeter mount height was recorded during windy conditions at a rate of 10 times/s for the cluster of trees. Rotation angles were used in a bole curve calculation to estimate tree displacement in 2-dimensional (x, y) space. Collision reconstruction was done in Arc/Info by assigning asymmetrical crown area dimensions (polygons) to calculated bole displacement for each tree. Reconstruction of each time step measured any overlaps between crown polygon areas. Crown polygon overlaps estimated in this manner allowed assessment of collision frequencies, area of crown overlap during collisions, and identification of the tree(s) that a subject tree contacted. Collision statistics are only given for trees interior to the sensored cluster (n=3). For 15.0 min of data with an average wind speed of 4.5 m/s and a maximum of 10.0 m/s there was an average of 65 collisions/min for each tree, and an average collision overlap area of 24%. This frequency and depth of collisions supports the notion that wind-induced crown interaction inhibits lateral shoot extension and is an important mechanism for the development of crown asymmetry and crown shyness. Insight into dynamic tree sway behavior and crown interactions will allow estimation and cultivation of a forest stand structure that is more resistant to damage from wind. The techniques of recording multiple simultaneous bole sway and their reconstruction are applicable to a broad range of wind-forest interaction research.