I would suggest lowering the resolution of these trees if your tile size is 16x16 and you have restrictions to the amount you can use. They could be at least a third smaller than they are now and still carry with them all the detail and interest they have now. It would also make it easier to tile, given that there is less to go wrong.
I would pay attention to your light sources though. The tree looks like it is being lit up from the left at ground level. The top of the tree would catch a lot more light in typical daylight.
My suggestion for the tiling, is to craft decent looking tiles out of the perimeter of the tree, and the places that the perimeter happens to fall on when tiled. Which means, that you should edit the edges of the tree so that they mesh well with the areas they overlay when tiled repeatedly. Instead of it looking like a bunch of the same tree pasted over and over, you could create an organic, bushy, and lush forest coalescing in perfect harmony. Part of that is going to be loosening up on that strong lighting, the contrast is so high between the shadows and the highlights, and in such a strange position, that you will probably consistently get grid patterns until it becomes more subtle.