You can use scatter on alpha too. What you do is make a clump of about 10 or 20 alpha plains that work then scatter them. Doing that way though there is a posibiblity that you'll be wasting some. Like ones behind that will never be seen. I find it's far easier to place by hand but I sure don't do it one plain at a time. That's silly. I make a clump of about 20 plains then group them and then make a different clump and do that till I got 4 or more clumps. Then I group those clumps into groups. I grab whole clumps. move drag up in the air then drop them and then hit the down arrow once so they are firmly planted. On to the next clump. Doing it this way it takes about 1/2 hour to place thousands of trees perfectly. None are wasted.