To add, tree canopies also have the fun distinction in that the leaves move with wind speed so the resistance coefficients may also change with flow rate! I think Ben Gurion University or the Technion (Israel) did some work on this - I vaguely remember the presentations from around 15-20 years ago. The secondary conclusion of the work was to use blown wind tunnels as the type with the fan downstream didn't like the amount of leaves that came off the test samples....
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