Why not "final tree"?
It is accepted now, it was possibly added right after your comment.
Is this Duolingo telling me that this is the last exercise with a tree?
No, it could only be Duolingo telling you that you won't get another tree.
If "strom" is masculine why do we use "í" at the end of the adjective and not "ý"?
Пишешь last ,выдает правильное past ,пишешь past выдает правильное last.В чем прикол?