thought about both of those ideas, but changing priorities in the database would change ALL the trees, or ALL the cliff edges, and very likely make others on the map stop working properly. I've overcome the problem THIS time simply by moving the tree so the lower priority tiles are behind the cliff, and it's worked, but that means I'm limited in where I can put them.
Think I'll have to either learn to live with the limitation, or mod the tileset to have several trees, all with different levels of priority.
Thanks for the suggestion.