And here is the chart to follow the progress:

taginfo.openstreetmap.org/tags…

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!

(1): nature.com/articles/nature1496…

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.