Theoretical and Empirical Analysis of Crown Collisions in a Forest Canopy

Background/Question/Methods

Crown collisions have the potential to protect a forest stand against wind breakage and wind throw by reducing the oscillation amplitudes of the trees in the stand. Crown collisions have also been implicated as one of the factors determining the degree of crown shyness in mature stands. A close examination of the nature of crown collisions is needed to understand the mechanical stability of forest stands and the ecological effects of these interactions. We present a theoretical analysis of how two types of collisions – elastic spring-like collisions and inelastic dashpot-like collisions – affect the behavior of an idealized canopy that is modeled as a finite one-dimensional lattice of intermittently colliding oscillators driven by a moving wind pulse.

Results/Conclusions

Both types of collisions reduce oscillation amplitudes near the leading edge of the lattice. Dashpot collisions have the additional benefit of reducing oscillation amplitudes along the entire length of the lattice. Spring collisions, however, tend to increase oscillation amplitudes further away from the leading edge. Elastic and inelastic collisions also produce frequency shifts in the oscillator power spectra, albeit in opposite directions.

Tree sway was monitored at 10 Hz using biaxial clinometers mounted at the base of the live crown in 16 trees (Pinus resinosa) in a 45 year old plantation on the Massabesic Experimental forest in Southern Maine. Comparing our theoretical findings to empirical tree sway data indicates that after increasing the spacing between trees their sway behavior is consistent with spring collision model but not with dashpot collision model. This finding is also consistent with the possibility that the post-thin increase in wind loading was accompanied by increased aerodynamic damping, which would reduce the oscillation frequency. Models suggest that both mechanisms spring collisions and increased aerodynamic

damping should have larger effects as wind speed increases. However, the analysis does not show a strong correlation between oscillation frequency and wind speed.