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'.
Ancient woodland is formally defined on maps by Natural England and equivalent bodies. Many ancient woodlands have legal protection of various types, but it is not automatically the case that any ancient woodland is protected.
The definition of ancient woodland includes several sub-types. Ancient semi-natural woodland (ASNW) is composed of native tree species that have not obviously been planted; features of ancient woodland often survive in many of these woods as well, including characteristic wildlife and structures of archaeological interest.
Species which are particularly characteristic of ancient woodland sites are called ancient woodland indicator (AWI) species, representing a type of ecological indicator. The term tends to be applied more usefully to desiccation-sensitive plant species, and particularly lichens and bryophytes, than to animals, as they are slower to colonise planted woodlands, and are thus viewed as more reliable indicators of ancient woodland sites. Sequences of pollen analysis are also indicators of forest continuity.
Lists of ancient woodland indicator species among vascular plants were developed by the Nature Conservancy Council (now Natural England) for each region of England, each list containing the hundred most reliable indicators for that region. The methodology involved studying the flora of known woodland sites and analysing patterns of occurrence to determine which species were most indicative of sites from before 1600. Although ancient woodland indicator species can and do occur in post-1600 woodlands, and also in non-woodland sites such as hedgerows, it is uncommon for a site which is not ancient woodland to host a double-figure AWI species total.
Ancient woods were very valuable properties for their owners, as a source of wood fuel, timber (estovers and loppage) and forage for pigs (pannage). Hazel was grown for coppicing, the branches being used for wattle and daub in buildings, for example. Such old coppice stumps are easily recognised for their current overgrown state, now that the practice has largely disappeared. Ancient woods were frequently Royal Parks and hunting grounds and given special protection against poachers, for example. The forest law was very strictly enforced, although various ancient rights to collect firewood are still extant. In English law, it was illegal to assart any part of a royal forest. This was the greatest trespass that could be committed in a forest, being more than a waste: for whereas waste of the forest involves felling trees, they can grow again; assarting involves completely rooting up all trees — the total removal of the forested area. Ancient woods were well-defined, often being surrounded by a bank and ditch, so that they could be easily recognised. Such indicators can still be seen in many ancient woodlands. Many ancient woods are described in the Domesday Book, as well as the Anglo-Saxon Chronicles, such was their value to early communities.
Ancient woodland inventories 
Ancient woodland sites over 2 hectares (5 acres) in size are recorded in Ancient Woodland Inventories, compiled in the 1980s and 1990s by the Nature Conservancy Council in England, Wales, and Scotland; and maintained by its successor organisations in those countries. There was no inventory in Northern Ireland until the Woodland Trust completed one in 2006.
Britain's ancient woodland cover has declined greatly. Since the 1930s almost half of ancient broadleaved woodland in England and Wales has been planted with conifers or cleared for agriculture. Only 3,090 square kilometres (760,000 acres) of ancient semi-natural woodland survive in Britain – less than 20% of the total wooded area. More than eight out of ten ancient woodland sites in England and Wales are less than 200,000 square metres (49 acres) in area, only 501 exceed 1 square kilometre (250 acres) and a mere fourteen are larger than 3 square kilometres (740 acres).
Most ancient woodland in the UK has been managed in some way by humans for hundreds (in some cases possibly thousands) of years. Two traditional techniques are coppicing (harvesting wood by cutting trees back to ground level) and pollarding (harvesting wood at about human head height to prevent new shoots being eaten by grazing species such as deer). Both techniques encourage new growth while allowing the sustainable production of timber and other woodland produce. During the 20th century, use of such traditional management techniques has declined while there has been an increase in large-scale mechanised forestry. These changes in management methods resulted in changes to ancient woodland habitats, and a loss of ancient woodland to forestry.
- Bedgebury Forest, Kent
- Bernwood Forest, Bucks and Oxon
- Bradfield Woods, Suffolk
- Cannock Chase, Staffs
- Cherry Tree Wood, London
- Coldfall Wood, London
- Edford Woods and Meadows, Somerset
- Epping Forest, Essex
- Forest of Dean West Gloucestershire
- Foxley Wood, Norfolk
- Grass Wood, Wharfedale, Yorkshire
- Hatfield Forest, Essex
- Hazleborough Wood, Northants, part of Whittlewood Forest
- Highgate Wood, London
- Hollington Wood
- Holt Heath, Dorset
- King's Wood, Heath and Reach, Beds
- Lower Woods, Gloucestershire
- New Forest, Hampshire
- Parkhurst Forest, Isle of Wight
- Queen's Wood, London
- Ryton Woods, Warwickshire
- Salcey Forest, Northants
- Savernake forest, Wiltshire
- Sherwood Forest, Nottinghamshire
- Titnore Wood, West Sussex
- Vincients Wood, Wiltshire
- Wentwood, Monmouthshire
- Whinfell Forest, Cumbria
- Windsor Great Park, Berkshire
- Whittlewood Forest, Northants
- Wormshill, Kent: Barrows Wood, Trundle Wood and High Wood
- Wyre Forest bordering Shropshire and Worcestershire
- Yardley Chase, Northants
See also 
- What is ancient woodland? The Woodland Trust website
- Forestry Commission Wales Reclaiming our Forgotten Inheritance (RoFI) project
- G. F. Peterken, "A Method for Assessing Woodland Flora for Conservation Using Indicator Species", Biological Conservation 6 (1974:239-245).
- Mild and moist sites in Ireland at Connemara and in the Killarney Valley have shown AWIs to be occurring in secondary woodland: Centre for Earth and Environmental Science Research, Kingston University: Dr Michael Grant and Dr Petra Dark, "Re-evaluating the concept of woodland continuity and change in Epping Forest"
- Spencer, J. and Kirby, K. (1992) An inventory of Ancient Woodland for England and Wales. Biological Conservation 62, 77-93
- Walker, G.J. and Kirby, K.J. (1989) Inventories of ancient, long-established and semi-natural woodland for Scotland. Nature Conservancy Council: Research and survey in nature conservation No. 22
- The Woodland Trust's search for Northern Ireland's oldest woods
- The Woodland Trust page on ancient woodland loss