We should see a saturation arround 3 trillion trees. (1)
You can easily help achivieving this goal by reworking natural=tree_row and adding single natural=tree into natural=wood areas :) I know, some say this is discouraged, but we really need to map EVERY TREE!
Ah! I would assume the imported sources were decent enough, at least!
I think mapping them might be decent with SCEE. (Might be in normal StreetComplete as well.) You have to enable the quest type as it is disabled by default, but then it offers the quest and even displays recently used values.
That is if you know your stuff, me I would have to have a tree identification book in my other hand, so I guess a specialized app with that functionality would be neat.