Makes me wonder though if it optimizes it at all, since it has to be tested on all the trees, and calculating square roots isn't exactly all that fast. And surely collision detection starts with bounding box testing, so that if bounding boxes do not overlap, no per-pixel calculation is done. And I'd think bounding box overlaps would be a lot faster to calculate for all trees compared to their distance to player.
But if you really want to do some sort of distance checking, I think something like this (using Is number between) would be more optimized:
Is tree.X between player.X-ScreenWidth/2*Zoomlevel and player.X+ScreenWidth/2*Zoomlevel
Is tree.Y between player.Y-ScreenHeight/2*Zoomlevel and player.Y+ScreenHeight/2*Zoomlevel
-> Check for overlap