Same with all terrains, the trees are randomly generated by the terrain tile type, and cannot be individually placed/moved. The reason they only show up at the ends of runways is because they "fill" the entire runway area, but when you put in an airfield tile on top, they are removed, that is removed until the tile ends (the end of the runway) then they pop back up again just like they had been before the tile was placed on the map.
There's really no way to remove them, aside from changing the way the terrain immediately around the field looks. Even then, that other terrain might have trees, too!