by Andre on 14 November 2005
Bill Tozier at Notional Slurry has been doing some seasonal posting about leaves, as has Carl Zimmer at The Loom, and it reminded me of a leaf phenomenon I noticed on campus the other day. Here’s the picture of the leaves next to a short wall that I took:
It seems like they pile near, but not against, the wall. Also, you can see that they make a sharp hook towards the wall where there is a break for stairs. Since there’s a building parallel to it sheltering it from cross-winds I think most of the flow is along the wall. So what’s the mechanism that clusters the leaves close to the wall, but not too close? What determines that distance? Maybe wall height, wind speed, leaf size… I’m sure an intrepid nonlinear scientist could find all kinds of power laws that don’t explain much about what’s going on.
My original hypothesis to account for the sparseness of leaves directly around trees that Bill Tozier claims to observe* was that grass tends not to grow very thickly there so the leaves just don’t stick as well as to the surrounding grass.** But given this wall effect where there’s no big change in surface roughness I’m starting to think that the wind may also play an important role…
You should do some simulations or, better yet, some hair-dryer-toilet-paper-role-little-bits-of-paper-experiments to figure it out. Heck, if you write good and you work at a university you could probably even get a grant for that high speed camera you’ve always wanted to misuse.
*I have yet to observe this effect. Not so many open fields in the city and all…
**Something to do with the allelopathics he mentioned?