Well, with regards to your neighbors’ yards, make it clear to your aggron that this is quite literally crossing a line. You may need to compromise with him by taking him to a forest and allowing him to engage in “renovations” there or by restricting him to your own back yard. Simply explaining to him that he may not enter a neighbor’s property should be sufficient. Aggron are more intelligent than most people think—at least to the point where one can understand a “no to this but yes to this.” Establishing literal boundaries can be a surprisingly simple task.
Beyond that, if your concern is that your aggron is planting trees in an area that may be susceptible to wildfire, have him dig a trench around the edge of your property and instruct him to plant trees as far away from that boundary as possible. This is the first step of the compromise: a physical boundary with a clear reason behind why it must exist. It will also help your aggron to understand the risks the drought is presenting to the trees it’s planting, which in turn will prepare him to defend his forest from a blaze. Teaching him Rain Dance or Water Pulse will further help him prepare (not to mention give him the means to take care of his forest himself).
That having been said, aggron rarely plant trees that cannot survive on their own, and they rarely plant trees that they themselves have no intention of caring for. (Note that aggron don’t generally display an interest in water beyond what they can incorporate into their moves, but if yours attempts to water their trees through a means other than either Rain Dance or Water Pulse, you may need to hide your garden hose and instruct your neighbors to do the same.) Once you establish what is and isn’t its territory, you can let it be, believe it or not. The trees it chooses will essentially take care of themselves and require no effort on your part, and should they die, your aggron will understand that the trees it’s chosen are ill-suited to the dry conditions it lives in. At that point, he may give up on his own. Alternatively, should the trees survive, they may actually provide excellent shade, which in turn may assist the local wildlife.
In short, the challenge is really in telling your aggron not to wander into other people’s yards. The trees themselves may not entirely be of any concern unless your aggron is actually stealing the ornamental trees from your neighbors. At which point apologize to your neighbors, return the tree, and consider investing in a stern psychic- or water-type to keep your aggron from doing that again.