A woodland site is considered to be ancient if it has been continuously wooded since at least 1600AD. This does not mean there has been continuous woodland cover over the whole site. Open spaces, either permanent or temporary, are important elements of ancient woodland.
It contains ancient semi-natural woodland (ASNW) mainly made up of trees and shrubs native to the site, usually arising from natural regeneration. When these sites are cleared and replanted, often with non-native conifers, they are known as Plantations on Ancient Woodland Sites (PAWS). As the name suggests, ancient woodland takes hundreds of years to establish and is defined as an irreplaceable habitat.