The scaling parameter you are looking for is the "zero-plane displacement height". It is defined as the mean height at which momentum is absorbed by the forest canopy.

See these nice notes written by D. Baldocchi:

http://nature.berkeley.edu/biometlab/espm129/notes/Lecture%2018%20Wind%20and%20Turbulence%20Part%203%20Surface%20Boundary%20Layer%20Theory%20and%20Principles%20notes.pdf

Hi Robert,

Thanks a lot. Baldocchi's notes helped.