And here is the chart to follow the progress:
https://taginfo.openstreetmap.org/tags/natural=tree#chronology
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!
Unfortunately most of the species tagged trees are mass imported and not mapped by a real mapper.
I try often to map species or genus but there is no good mobile app for this task yet.