Arborist Christopher Neilan created a Capital Asset Value for Pleasant Trees (CAVAT) which was used to determine the value of a Gap sycamore tree.
He said it was difficult to estimate the value of the sapling because of its intangible sentimental value, likening it to a piece of “stolen art.”
“In one sense it is just paint on a canvas, in another it may not be worth anything because it cannot be sold on the open market.
“But collectors can pay a huge amount for it.”
Cumbria Police said the theft is believed to have happened between June 9 and June 16.
The force is also appealing to anyone who visited the castle and its grounds to provide information that could help narrow the time frame.
In July 2025, two men who felled the Sycamore Gap tree, which was valued at £450,000, were jailed for four years and three months.
Daniel Graham, 39, and Adam Carruthers, 32, have never revealed their motives.
Follow BBC Cumbria on Xexternal, Facebookexternal, Next door And Instagramexternal.