Perhaps a new forest, but plants in general (current conditions may favor grassland, or something else). It's "re" because there was an ecosystem there before,

Perhaps a new forest, but plants in general (current conditions may favor grassland, or something else). It's "re" because there was an ecosystem there before, and now it's returning. Contrast with, say, a brand new island that is the result of volcanic action. Then it'd be just plain old colonization by way of wind-blown seeds, or seeds that survived a bird's digestive tract.