I do not see you setting a radius for your sprite entities. Sprites should behave in the same way as entities as far as type assignment goes. They will take it alright. However a sprite is not a conventional mesh so forget relying on poly picks\collisions. The best you can do is to set a radius (or try setting a box, though this will not follow sprite orientation).
Maybe the simplest option is to just define a box or an elipsoid that contains the main trunk of the tree sprite and some of the foilage as RiverRatt just said.
Maybe the simplest option is to just define a box or an elipsoid that contains the main trunk of the tree sprite and some of the foilage as RiverRatt just said.