If you put it in an already existing default folder, like MyTrees for example in any way, then you might want to place it in a new group or out of game, into My_Objects. Delete the one existing in the default group and replace the trees in the map, even if it sounds hard.. At least, for a downloaded mesh. If it isn't a download mesh then just try to load and re-make and add it in, I guess.
Hope this helped.~
