"Forest" surface growing should take place only on blank (cleared) spots around a tree when its growing.
Dependent on tree size, the area which gets new "Forest" surface grown will increase, so you can benefit
when you let a tree grow to full size, that you get more resources from it.

Regrow of "Forest" surface (when you only harvest part of the 4 resources) should take place at the speed of 1 resource an ingame day.

"Grass" surface growing should work by introducing grass seeds (you get it random when foraging). You can plant this seed only on blank spots and it grows in size 1 - 2 times so it covers a larger area until it finally converts to normal grass ressource which can be harvested.

Regrow of "Grass" surface should work same as for "Forest" surface.

greets,
Tybor