DISCONTINUED - every man and their dog has made a tree spreading mod, so download one of those. Trees spread naturally over time, they'll grow around anywhere the player visits, but only on grass and dirt areas, not concrete or any man-made tiles or sandy deserts etc. This originally was msakuta's mod, I'm simply making it work on newer versions of Factorio. Also I'll add a few new things too.
Small changes concerning balance, gameplay, or graphics.
Discontinued mod, just search for "tree" to find an assortment of mods that do this sort of thing.