YOU may not have realised it, but York’s Museum Gardens are home to some of the tallest and finest trees in Yorkshire – if not the country.

There is a beautiful, narrow-leaved ash that, when last measured in 2004, was the seventh tallest ash tree in the whole country. Nearby is a graceful weeping lime that has never been measured – but is far taller than the ash. “I wouldn’t be surprised if it is the tallest lime in Yorkshire,” said Alison Pringle, the garden manager at Museum Gardens.

We’ll soon know. Because yesterday, both trees were “invaded” by a rare species of climber – arboriculture students from Askham Bryan College.

With the aid of ropes, they were climbing into the very tops of the trees, giving breathtaking demonstrations of branch walking - and clearly having great fun.

There was a serious side to what they were doing, however. When last measured in 2004, the narrow-leaved ash was 23 metres tall. But the students were measuring it again, to make sure it was still among Britain’s ten tallest, and so qualified for the status of “champion tree”.

They were also giving it a health check: clearing away dead wood, and looking for signs of rot.

Down on the ground Adrian Whitmore, a lecturer in arboriculture at Askham Bryan, said establishing the tree's height and making sure it was healthy were important, because it could be used as breeding stock to help ensure healthy future generations of narrow-leaved ash.

The tree, which is about 150 years old, must have fought off disease several times to grow so tall, he said – so is clearly a strong tree.

“It’s like racehorses. If you take two fast horses and breed them, you get a fast horse. If you take two strong trees and breed them, you get a strong tree.”

This ash could help ensure the health of future generations of ash trees, in other words.

Meanwhile, if you missed the tree climbing students yesterday, they will be back in Museum Gardens for a further demonstration of their skills – and to measure and check more trees – on Thursday May 30.