Typical tree planting terrain is one of the least robotics-friendly environments imaginable. Logs, brush, loose sticks and rocks, swamp, sand, hills, you name it.

Here's a photo of a planting site I took during one of my planting contracts a few years back: http://i.imgur.com/tITCW03.jpg

It is often possible for logging companies to prepare the land better (clearing out slash, sometimes scarifying the land), but the expense here is really substantial. Currently the most effective technological tree planting solution is aerial seeding.