Shelterwood cutting is the progression of forest cuttings leading to the establishment of a new generation of seedlings of a particular species or group of species without planting. This silvicultural system is normally implemented in forests that are considered mature, often after several thinnings. The desired species are usually long-lived and their seedlings would naturally tend to start under partial shade. The shelterwood system gives enough light for the desired species to establish without giving enough light for the weeds that are adapted to full sun. Once the desired species is established, subsequent cuttings give the new seedlings more light and the growing space is fully passed to the new generation. What separates shelterwood cuttings from other regeneration systems, such as clearcutting or seed tree, is that the new seedlings are established before the mature trees are fully (or mostly) removed. This gives the forester more control over the species that are regenerated, and is more forgiving if the first regeneration effort fails. All mature trees may be removed, creating a young even-aged forest, or a considerable number of reserves may be kept to provide a two-aged structure. Small woodlot owners often prefer this method for the control it gives and also because the income from the harvest is spread out over a decade or more. Shelterwood systems may include preparatory cuttings, establishment cuttings, and one or more overstory removal cuttings: Preparatory cuttings are sometimes, but not always, the first step in a shelterwood regeneration system. The purpose of this cut is to remove species that are not desired so that they do not contribute seeds to the establishment cut. If this is not a problem, the preparatory cut is not necessary. When discussing desired and undesired tree species, foresters are combining their knowledge of the landowner's preferences with their education in the ecology of the trees in their area. A landowner may prefer oaks because they produce acorns that feed wildlife, or may prefer one species over another because it has a longer lifespan and tends to occur in mature forests. Most species can be desired in one place and undesired in another depending on the landowner's goals. Establishment cutting aims to establish the regeneration, and is done in a year when the seed crop is good. The intention is to provide a certain amount of light that is necessary for new seedlings to start, but not necessarily grow