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Page last updated at 09:55 GMT, Wednesday, 23 April 2008 10:55 UK

How is a tree valued?

The Magazine answers...

Trees are often blamed for subsidence

A plane tree in central London has been valued at 750,000 under a new system that puts a "price" on trees. How?

A six-foot-wide plane in Berkeley Square, Mayfair, is thought to be the UK's most valuable tree.

Large, mature, city trees like this one are being blamed - sometimes wrongly and often fatally - for damage to neighbouring properties.

But it is hoped a new valuation system will make it harder for "expensive" trees to be felled due to doubtful suspicions they are to blame for subsidence.

By considering its size, unit value, number of people living nearby, its benefits and problems and its life expectancy

So how are trees priced? Size is the biggest factor, followed by population density of the surrounding area (how many people enjoy the tree), the size of the canopy, its life expectancy, its impact (does it flower or drop annoying honeydew) and any special factors, such as Queen Victoria planting it.

The system has been trialled in London and is gradually being adopted by local authorities elsewhere, such as Bristol.

There was a previous method of tree valuation called Helliwell but it measured the visual worth of a tree, says Jon Stokes of the Tree Council. This new system, devised by tree officer Chris Neilan, is complementary but different.

"Ultimately the purpose is that more trees are saved, because you can say 'this tree has a relative value to this community'," says Mr Stokes.

"It's not just a tree, it's something that is part of the estate of this local authority or this person, therefore you need to protect it."

Putting a price on a tree changes people's attitudes and if developers think in financial terms, then a community asset must be valued in the same currency, he says.

So if a developer is in court for illegally destroying a tree, then the fine could be a reflection of the tree's value, says Mr Stokes. Or if a new development replaces a stock of trees then the builder could contribute to the community a sum equal to the value of that lost stock.

A regular part of the BBC News Magazine, Who, What, Why? aims to answer some of the questions behind the headlines

In London, where large trees often face the axe due to subsidence fears, Mr Neilan's system will enable councils to weigh up whether a tree is worth fighting for or not.

Mr Neilan, who works in Epping Forest, Essex, began working on the formula seven years ago, in his spare time, after becoming concerned by the number of large trees being removed from cities.

"It's a passion," he says. "I don't actually manage trees and I don't use it in my job but it's something I believe in. I've seen trees wasted, they're planted and then replaced three years later.

"I wanted to show the positive aspect of trees on the balance sheet and I wanted to make a difference."

Under this system, plane trees like the one in Berkeley Square are the most expensive, mainly because they retain their full crowns. Cedars, on the other hand, lose theirs very quickly.

His calculation tries to assess a tree's replacement value and its outline is as follows:

    • The basic value is the full cost of a newly planted tree in a given area, divided by its trunk area. There are two parts - the nursery gate price, expressed in terms of the cost of each square centimetre of stem (or unit area cost), and the planting cost (transport, planting, materials, immediate care and management costs, but not after-care)

    • The CTI value is the basic value adjusted to take account of the population density and degree of use of the location. It may be multiplied by a factor of up to 250%, or reduced by up to 75% according to these factors

    • This becomes the functional value when accounting for an expert assessment of the tree's functionality, including how big the crown is and what condition it is in

    • After a surveyor's assessment of any special factors like heritage importance and whether it causes problems to residents or parked cars, you get an adjusted value

    • The final value reflects the life expectancy. If it is greater than 80 years the tree retains 100% of its adjusted value; those with a life expectancy of less than 5 years lose 90%

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