Role of Crown in Tree Resistance Against High Winds

TL;DR

This study investigates the role of tree crowns in resisting high winds by using real tree samples and wind tunnel experiments, revealing how pruning affects drag force and modeling leaf behavior under wind stress.

Contribution

The paper introduces a novel experimental approach with real tree samples and develops a model for leaf shape and drag force variation under wind conditions.

Findings

Pruning reduces drag force by up to two thirds.

Power-law relations for leaf cross-sectional area are confirmed.

The model's exponents differ from empirical values but can be refined.

Abstract

Rather than using wooden sticks to simulate the breakage of trees in high winds as in most research, we employed fresh samples with branches and leaves to certify the crucial role played by the tree crown. By using the blowdown wind tunnel with a maximum wind speed of 60 m/s, we purposely reduce the number of leaves and show that the drag force will drop by as much as two thirds when half pruned. Based on real observations, we model the leaf by an open and full cone in the presence of light and strong wind, and calculate how their corresponding cross-sectional area and drag force vary with wind speed. Different power-law relations are predicted and confirmed by experiments for these properties before and after the formation of a full cone. Compared to the empirical value of 1/3 and 3/4, our simple model gave 2/5 and 2/3 for the power-law exponent of cross-sectional area at low and high winds. Discrepancy can be accounted for by including further details, such as the reorientation of open cones and the movement of branches.