A forester can be placed in an already established forest to start harvesting trees immediately, or in an open plain where the foresters will start planting trees.
If a few trees remain in an area not maintained by foresters, the forest will still grow naturally. Trees will die, and seeds will take root to grow into new trees. However this process takes more time and the forests are less full than an area with a foresters lodge.
Foresters will also slowly harvest any iron and stone that lies in their radius, planting trees instead.