The mutual interference of spruce canopy structural elements

Abstract

The drag force on fourteen three-prong spruce shoots (Picea rubens) has been measured in a wind tunnel in velocities ranging from 0.4 to 4.5 m s−1. The influence of wind velocity and shoot needle density on momentum transfer and the mutual interference to momentum transfer are evaluated. The influence of the shoot needle density (ranging from 0.6 to 1.7) on momentum transfer is velocity dependent. The shoot needle density does not influence the momentum transfer to the shoot when the wind velocity is less than about 1.5 m s−1. However, a slight trend of increased momentum transfer to the shoot with decreased shoot needle density was found for wind velocities between 1.5 and 4.5 m s−1. This trend is not statistically significant. The shelter factor, a measure of the mutual interference to momentum transfer to the shoot structure, is shown to be dependent on the model used to describe the flow around and through the shoot. This suggests that any apparent velocity dependence of the shelter factor may actually be due to a change in flow conditions. It is concluded that: (1) the needles are the vegetative component involved in mutual interference within the shoot for wind speeds between 0.4 and 1.5 ms−1, and (2) the shoots themselves are the vegetative component involved in mutual interference within a shoot array for wind speeds between 1.5 and 4.5 ms−1. The shelter factor at the low wind speeds is estimated to be 2.1 The shelter factor at the higher wind speeds should be based on interference between shoots and not between needles within a single shoot.