... of course the nurseries don't have seedlings to plant one trillion trees. Growing seedlings of the appropriate species would have to be part of that effort.
There are volunteer efforts in some places to spread the load in starting seedlings. Not sure how effective it would be though in contributing to an effort of that scale where you need commercial scale to make a dent.
It can be done though, if anyone that has a balcony or a back yard were to look after a tray of seedlings (25 or so trees?) until they're ready to be planted, it'd quickly add up. The challenge then becomes logistics though. But again, can be done with volunteers.
