Opening the seminar, BBC1 Panorama presenter and Woodland Trust ambassador Adam Shaw said of such valuations: "Widely different numbers are thrown around. You have to be able to say 'there is real value in putting these trees in' so councils know they aren't just a 'nice-to-have'. It's an important time to be talking about this. It's a powerful tale and the media loves it."
Birmingham Institute of Forest Research (BIFoR) director Professor Rob MacKenzie said of his institute, celebrating its second birthday this month: "The UK needs a rallying point for forested landscapes, otherwise it's too easy to say Britain doesn't do forestry, leave it to others. We are making the case for joined-up thinking - the 'triple bottom line' of economic, social and environmental benefits. We won't get traction in the public arena without that."
MacKenzie's University of Birmingham colleague, professor of economics David Maddison, explained: "'Excludability' is key to understanding the market failure in woodlands - you can't exclude the people who won't pay for the services they provide. Valuing those services is a precursor to determining the land's most beneficial use. But that's hard when these aren't services that are bought and sold."
He said "internalising" these external benefits would mean either subsidising woodland owners, regulating to prevent them switching to less beneficial land uses or to compel them to convert land to woodland, or "excluding those who benefit until they pay". He added: "There are subsidies for other land uses including agriculture. You have to question this."
Woodland Trust chief executive Beccy Speight posed the question: "There's a huge amount of privately owned woodland - how can this thinking lead to a profit for them?" Planting rates in the UK are in decline, with just 2,400ha of new woodland planted in England in 2014-15, 900ha down on the previous season, she said. But Speight added that the Government's recently outlined 25-year natural capital plan would be key to its efforts to influence policy.
On this, Professor Dieter Helm of New College Oxford said: "The Government accepts that trees and woodlands will be part of the plan. The benefits are big and multiple, though different at different locations. Trees are every bit as important in mitigating climate change as, say, offshore carbon storage."
On the use of woodlands for recreation, Helm said: "We know the price that people are prepared to pay. How to we capitalise that? We also know they have a health and educational value. How could money be spent to ensure people have access to woodland? Then there's tourism, water filtering - a multitude of benefits make up the sum total. We should look at assets from an accounting point of view. As with new factories, you pay up-front but you expect a return over time."
A report prepared for the Woodland Trust by consultancy Europe Economics earlier this year put a value of £270bn on the UK's woodlands, based on their "direct, indirect and non-use value". But the consultancy's principal and lead author of the report Dr Andrew Lilico pointed out a drawback in applying such costings to real-world situations. "If you are planning flood protection, you will overlook the other benefits of putting in trees when comparing that to the alternatives."
His colleague and report co-author Matthew Sinclair added: "There is potential to make it 'too economic'. You don't want to lose support of the public by over-monetising it."