Does anyone know where I could get a pdf of these studies:

"A Guide to the Impacts of Non-Motorized Trail Use" - Don Weir and Assoc - Alberta Canada (2000)

Assessing and Understanding Trail Degradation: Thurston, Eden and Reader, Richard L. 2001. Impacts of experimentally applied mountain biking and hiking on vegetation and soil of a deciduous forest.

The first doesn't appear to be online anywhere and the second is only available on some kind of books-online site.

You may want to try your local library. The Cobb Co. system has access to a lot of the online references.