In Anno 117: Pax Romana, forests are a very critical and important resource! In some residential areas, you are allowed to plant new forests yourself, which will support new production.
How to quickly plant trees?
If you started building a house in a forest and changed your mind, deleting the house returns the resource to you — specifically trees. Simply remove the structures, and the forest will regenerate in its original place.

You need to explore territories extensively — this key aspect will allow your forests to expand. For example, “Forest Regeneration” allows Woodcutters to automatically plant trees within their working area. Coal Burners have the “Tree Planting” research, and Resin Gatherers have “Untouched Soil”. These methods greatly simplify resource management.
The trees planted thanks to research remain on the map even after the building that planted them is demolished. This allows strategies such as building a Resin Gatherer on a meadow and waiting until a new forest grows around him, then removing the building and using those trees for a Woodcutter or other forestry structures.

Be careful! Trees from the decorations menu are only decorative. They are not a usable resource, and no structures can be built from them! However, keep in mind that you should not remove them, as they add (+) to city prestige, which is important for the development of your empire!