This is a common misconception of LOTR. Tolkien was not an environmentalist or a left-winger of any stripe but, in his own term, an "anarcho-conservative" (probably similar to a libertarian over here). His views on the environment would fall into the conservationist camp—he would prefer to preserve privately what we have (and he would have loathed getting the government involved, especially if it meant higher taxes!). I'm sure many American conservatives like to camp or take a hike through natural woodland, and Tolkien would have wanted those woodlands to be available for just that purpose—not to worship nature by any means.
He did, however, have a decidedly anti-industrial bent. He despised machines, for some reason, feeling that the machine had led civilization down a road toward oblivion. In many ways, the ring itself represents the machine and the evil ways it can be used to enslave and corrupt free people. The fellowship, then, is an effort by those free people to remain free and live as they are accustomed, that is, in their agrarian-based societies.
As for the trees (Ents) getting into the action, Tolkien was very fond of trees, and it was this fondness that probably spurred the idea of the Ents. Their coming in on the side of the free peoples and fighting with them is "natural," in that the Ents primary enemies were ax-wielding orcs commanded by Saruman. I don't think it was any kind of environmental statement.