By Mark Kinver
Science and environment reporter, BBC News
The UK has a high proportion of important ancient trees, like Newton's apple tree
One of the UK's biggest landowners is embarking on a comprehensive survey to identify previously unrecorded ancient trees on its properties.
The National Trust hopes to find 40,000 of them during the three-year project.
Ancients trees provide unique habitats that support a wide range of rare species, which will be at risk if the trees are allowed to die, say experts.
The data will be fed into a national record, managed by the Woodland Trust, which is available online.
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"Ancient trees can be thought of as the cathedrals of the natural world," said Ray Hawes, the National Trust's head of forestry.
"This new survey will provide us with the opportunity to understand more about these special trees in our care and map their exact location."
Volunteers will be used to survey the Trust's 25,000 hectares of woodlands, 200,000 hectares of woodland, and 135 parks.
A number of famous trees will be listed in the findings, including the apple tree that was said to have inspired Isaac Newton to develop the "notion of gravitation" in 1665, and a yew tree that featured in one of William Wordsworth's poems.
One of the "fraternal four" yews mentioned in Wordsworth's 1803 poem
Apart from the stars of the aboricultural collection, the Trust's newly appointed ancient tree adviser Brian Muelaner said that it was an area that had been overlooked.
"Lots of individuals, particularly property staff, will know where their biggest trees are, and which ones are considered to be locally important," he told BBC News.
"But there has never been a co-ordinated assessment of all of them within the National Trust."
The UK has one of the highest proportion of ancient trees in Europe, and Mr Muelaner said that it was vital to effectively manage the unique habitats provided by the multi-centurions.
"These trees are remnants of our primeval forests," he explained.
"They are a direct link back to that time, so the biodiversity that is associated with those cannot be found anywhere else.
"If they are lost, then the dependent ecology - lichen, fungi, deadwood invertebrates - will suffer."
It can take about 250 years for a tree to become a suitable host for some lichens.
Once an ancient specimen has been identified, a management plan will be put in place to ensure it lives for as long as possible.
Mr Muelaner said the main threat facing these trees was often from farming.
"Too often, arable fields are ploughed right up to the base of the trunk," he observed.
"Ploughing greatly damages the roots, while fertilisers will damage the mycorrhizal fungi, which are essential for a tree's uptake of nutrients and water.
"Even pesticides and the use of pharmaceuticals in cattle can have an impact".
As well as protecting ancient specimens, the Trust will also look at ways to ensure the dependent ecology is not lost.
"We need to get succession planting going very quickly if none is underway," Mr Muelaner stated.
"It takes hundreds of years to get the trees in the right conditions to support the rare and endangered species."
He said that the survey would also be recording "veterans" that show characteristics of an ancient tree - such as deadwood, holes and cavities that support a diverse range of species - and have a higher conservation value.
Trees that are considered to be culturally important will also be listed - for example, ones that were carved by US soldiers shortly before the D-Day landings in June 1944.
"Or it could be the Tolpuddle martyrs' tree, where they first met," Mr Muelaner explained.
"On its own, the sycamore tree is not that significant but because of its historical context, it is very important."
One of the challenges facing the team of volunteers will be identifying what trees are deemed ancient, as there is no fixed criteria.
The main working definition is an individual specimen that is very old in comparison to others of the same species.
For example, oaks in excess of 600 years are considered to be ancient, yet beech trees older than 300 years would also qualify.
However, calculating age based on appearance is not as simple as it sounds because factors such as location, soil, access to daylight, management techniques, affect the characteristics.