An inventory of Wales' ancient woodlands is to be updated to ensure all sites are protected.
It will be the first time the list has been updated in 30 years and new technology means it will be more accurate.
It will identify former ancient woodlands that have since been planted with conifer trees to satisfy the demand for timber.
The Ancient Woodland Inventory may help restore them to their natural state.
The centuries-old woodlands, which date back to at least 1600, support many species of plants and wildlife that depend on the evolving environments created by dead and dying wood and broken sunlight.
Around 14% of Wales is covered in woodland according to the Forestry Commission Wales.
It said the updated inventory will guide decisions by organisations to remove non-native trees from some ancient woodlands and replace them with native species, such as oak, birch, rowan and ash.
Such work helps to increase the variety of different plant and wildlife species in the woodland by improving habitats and providing food and shelter, the commission added.
The new information will also be supplied to councils for their use when developing planning policy that affects woodland.
Michelle van Velzen, forestry and environment policy & programme manager at Forestry Commission Wales, said: "Ancient woodlands are a precious and finite resource that cannot be recreated.
"This update to the Ancient Woodland Inventory will ensure we have the most comprehensive and accurate information on the extent and nature of ancient woodlands in Wales.
"The new inventory will be used by various organisations to help protect and restore our ancient woodlands."
Wood pastures - ancient and veteran trees found on grazed sites - will also be recorded as part of the update to the inventory.
Despite the ecological value of wood pasture, it has no legal protection, so the forestry commission hopes that identification on the inventory may help protect these sites from damage or destruction.
The updated inventory, commissioned by Forestry Commission Wales, Countryside Council for Wales and the Woodland Trust, will be completed in March 2011.