Is there any way to balance the interests of off-road vehicles and environmental concerns?
The short answer to this is no. Off-road vehicles are extremely damaging to the environment, especially because the physical terrain favored by ORVs is that of the desert (which lacks trees and rocks to get in the way of the vehicles, which can then speed around faster). The desert is, of course, an extremely fragile environment, one that can stand very little intervention. There is simply no way that ORVs can use desert lands without harming them. And this damage is made worse by the fact that the kind of people who are inclined to want to ride ORVs have a culture that is at its heart scornful of the environmental movement. Off-roaders think that the earth is theirs and that they have no obligation to save any of it for the next generation. It is a millennial culture, in which people risk their personal futures and so do not see why they should not rid that of the earth as well.
Is tree-sitting a legitimate way to stop clear-cutting?
The answer to this is probably both yes and no. The recent success of saving an old-growth sequoia through the efforts of a single woman show that an individual can make a difference in the life of a single tree if she or he is willing to undergo great personal sacrifice. But clear-cutting of old-growth forests cannot be stopped by such tactics alone. They do draw attention to the problem of clear-cutting, and this is probably their most important function. Once attention has been drawn to the issue of clear-cutting, then more enduring and legally binding efforts can be put into place to protect other trees. Tree-sitting is an act of desperation, and acts of desperation are certainly legitimate. But, by their nature, they are not particularly well thought out and may be too far outside the mainstream to be entirely effective. Legislative mandates work better -- but treesitters can get legislators to the tab...