I do a lot of hiking (do I have to add “barefoot”?).
Many of the trails that I hike on have a specific focus: some are principally horse trails, some are designating as trails for mountain bikes, and some are for foot traffic only. It is interesting to see the different kinds of wear that each gets, and I think my observations can be used to consider how bare feet affect the trails. (These observations regard the eastern United States. They may not apply to areas with drier climates.)
Horse trails are the absolute worst. There are many areas where I hike in which the horse trails have been dug a full three feet into the ground. Wherever the trail crosses damp ground, the soil is horribly churned up and the trail becomes a morass of foot-deep mud. Not only that, since the horses don’t like walking through that stuff, the trail usually gets very wide there, as more and more users try to walk around the morass. (One advantage to bare feet is that, if you are forced to walk through the mud, you will not have any boots sucked off your feet. I’ve seen that happen with my shod friends.) This sort of trail damage is caused by two things: the weight of the horse and rider, and the sharp edges of horseshoes.
Mountain bikers sometimes seem to have a bad reputation. This, however, seems to come from the irresponsible ones. The mountain bike trails that I hike on were built by and are maintained by the Athens Bicycle Club. They do a really good job. The trails are designed with plenty of switchbacks, and users are heavily discouraged from cutting through. This really reduces erosion. In fact, the wide bicycle tires really don’t seem any worse, erosion-wise, than any hiking boot.
On regular foot trails I do still see erosion, particularly, again, in the softer, muddier spots. This erosion seems to be caused by the sharp edges of waffle-stomper hiking boots. They cut into the mud and soil, not only at the edge of the footprint, but even in the interior.
Bare feet just don’t do that. When hiking, I’ll often check to see what sort of footprints I am leaving, and I usually have a hard time seeing them, even in wetter soils. My weight is spread out over my whole sole, instead of concentrated at specific pressure points, so I just don’t dig in and cause damage. Also, the edge of a foot is rounded, and that too minimizes erosion. Hiking barefoot really is friendlier to our trails.
I should add one caveat, though. Bare feet will still cause the creation of trails. Trails through the woods are created not only by erosion, but simply by the pressure of a footstep. The pressure compacts the soil making it harder for any plant to grow there. However, a trail created by bare feet only will not erode away soil the way any other of the other trails users will.