We already know of the pollutants that trees emit, which account for such things as the "smoke" in the Great Smoky Mountains. But it's worse than that - trees may be responsible for mass extinctions.
Before the tree huggers wet themselves let me note that I'm not down on trees. But the fact is that environments change, and it's a totally natural phenomenon, just like human beings are. Which makes environmentalists, in their efforts to freeze our environment as it is, are perhaps the most unnatural things on earth.