Or rather - aren't there some kinds of mushrooms and other flora that glow in the dark? Why not just splice that plant with a tree. I know, I use the term splice like its an easy task.
I've always thought that was a good idea, but two problems: First, I don't think it would be that easy of a task. I think it involves different proteins reacting with each other, luciferase breaking down luciferin in some sort of reaction, or something. I don't know how easy it would be to get a plant to produce both of those (I wouldn't think it would be too hard to get them to simply produce the proteins), but getting them to emit enough light to matter at the right time all night, if the plant would have enough energy to actually go that, the light not goofing up it's leaves' sensence in the fall, not sure how easy that would be to do Getting something to glow is much different than, say, inserting the gene for green florescent protein, which is what you usually see 'glow,' like when the news talks about glowing kittens or pigs or whatever, and is a relatively simple task, but that only works under a blacklight. I don't know much about those types of things though, so I could be wrong, all I really know is that the tobacco plant that could glow (maybe you saw that in a science book) could produce the enzyme but was watered with the substrate, so I'm (baselessly) assuming there is some sort of catch, otherwise they would have just made it express both. I hope there's some way to do it though, that would kick ass. Course, you're going to have a problem in the winter, and I highly doubt an evergreen could work up the energy to glow in the dead of winter (at least not without some massively advanced genetic engineering that quite frankly we are no where near seeing).
Second, however, may be an even bigger problem. You will probably be sued for every other headache, upset stomach, and mild case of the sniffles that occurs within a five mile radius of each tree. Most people don't understand anything about this sort of stuff except what some clueless fearmonger told them, and like the radio tower that could cause headaches before it was turned on, you can bet your bottom dollar people will attribute their mental illnesses and physical problems and erectile dysfunctions to the trees with the spliced gene, and they'll probably get chopped down, if they're ever planted at all. On the other hand, people might realize how cool and useful they are and embrace them, striking a blow to fear and giving science a win, but perhaps that may be an overly optimistic scenario.