  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.
