Ancient woodland is a term used in the United Kingdom to refer specifically to woodland that has existed continuously since 1600 or before in England and Wales (or 1750 in Scotland). Before those dates, planting of new woodland was uncommon, so a wood present in 1600 was likely to have developed naturally. The analogous American term is “old growth forest”.
For many species of animal and plant, ancient woodland sites provide the sole habitat, and for many others, conditions on these sites are much more suitable than those on other sites. Ancient woodland in the UK, like rainforest in the tropics, is home to rare and threatened species, more than any other UK habitat. For these reasons ancient woodland is often described as an irreplaceable resource, or ‘critical natural capital’.