Arboriculturist Christopher Neilan created the Capital Asset Value for Amenity Trees (CAVAT) which was used to determine the value of the Sycamore Gap tree.
He said estimating the value of the sapling was difficult, because of its intangible sentimental value, likening it to a piece of “stolen art”.
“In one sense, it’s only paint on canvas – in another, perhaps it isn’t worth anything because you can’t sell it on the open market.
“But collectors might pay a huge amount for it.”
Cumbria Police said the theft was believed to have taken place between 9 and 16 June.
The force is also appealing to anyone who visited the castle and its grounds to come forward with information which might help narrow down the timeframe.
In July 2025, the two men who chopped down the Sycamore Gap tree, which was valued at £450,000, were jailed for four years and three months.
Daniel Graham, 39, and 32-year-old Adam Carruthers have never revealed their motive.
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