I actually heard about this a long time ago, you can apparently make a crude battery out of potted plants by connecting Plant A to the pot of Plant B and B to the pot of C and so on. With a common ground, like a national park this likely wouldn't work. However, connecting 500 trees in parallel rather than series could provide a high current, which you could then run that high DC current through an inverter and transformer and pump it up to whatever voltage you need.

However, I'm not sure how practical connecting hundreds of trees in series would be as you're going to get the problem of animals and what not. Unless you wedged your electrodes in good, or found a way to allow the tree to grow through them and hold them in place, you'd get trouble with squirrels pulling them out trying to run on the wires and other crazy stuff.