Get out and hug a tree, and you could help save a little part of Britain's ancient natural history.

The Woodland Trust will today launch its Ancient Tree Hunt in London, where it already has records of about 500 specimens including planes, oaks, yews, silver birch and cherry trees. Some were saplings centuries ago, when the whole world had fewer people living on it than live in the capital today, and organisers hope that Londoners can help identify many more of the venerable trees, in public spaces or gardens.

"There may be many more out there," said the trust's spokeswoman Fiona Moss. "That's why it's a real public call to action: there could be a tree in your local park, or [walking] past a shopping centre, you have never even noticed."

The campaign aims to identify 100,000 ancient trees by next year. To qualify as "ancient", some trees can be at least 100 years old, while some species must have lived several more centuries: oaks are ancient when they are about 600.

The best way for ordinary people to tell is to hug them, says the trust. The broad rule of thumb is an ancient tree will be 5m or more around the trunk – about two and a half adult hugs – though some younger qualifiers like the cherry and silver birch are only one adult reach.

Likely suspects will be provisionally added to the charity's database before they are verified by one of its experts.

The scheme will be launched under a large London plane tree in Barnes, south-west London – believed to date back to 1685 when James II, England's last Catholic king, was on the throne and there were only 6 million people in England.

Jill Butler, the Trust's conservation policy advisor, said: "The older trees are, the more valuable they become for wildlife. They are literally nature reserves on people's doorsteps, and once removed or fragmented, the ecology associated with them is isolated and cannot survive. Even when a tree has actually died, the deadwood can go on providing a valuable habitat for decades for thousands of insects, birds, fungi and lichen. Many can't live anywhere other than in trees in this condition."

Since the national launch of the Ancient Tree Hunt in 2007, 63,000 ancient trees have been reported and recorded.

The trust has also published a borough by borough guide of London's most popular ancient tree species (PDF).