The treasure is buried on the island with two non-parallel straight lines of trees.
Regarding the five spots on the island, we're looking for one where we can see a different number of trees each. That must mean that there must be something blocking the view for all but one of the spots. Since trees are the only things on the island, trees can only be obscured by other trees.
You would see fewer trees that there actually are if two or more trees are arranged in a straight line along your vantage point, so the island must be one where five different ponies could stand in five different spots and see different numbers of trees. That would narrow down the possibilities by quite a bit: virtually all the islands will have at least one point where all six trees can be seen, and most if not all will have a point where five trees can be seen, if two are lined up straight, one hiding behind the other. So you're looking for an island that has a lot of trees arranged in a line from a viewer's position. That's tricky: the hardest locations to find require more trees to line up with the view of the pony.
Which one, though, ticks all the boxes? Six trees arranged in one straight line is out: you CAN see one tree by standing on the far right or left of the line, two by standing right in the middle, and all six by planting yourself above the line, but nowhere on the island will let you see exactly three, four or five trees.
What if we had two lines of trees? Parallel lines of trees are right out, using similar logic above, but as long as they AREN'T parallel and their lines intersect over land, the point where they converge is where you can see two trees, and arranged properly, you can also see three, four, five or six trees, no matter how the trees are arranged or how many there are in a line - as long as they're grouped two and four, or three and three.