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THE Broad Walk is one of London鈥檚 leafiest avenues, stretching between two corners of Hyde Park. On a cold but sunny spring day, it is a welcome escape from Park Lane鈥檚 noisy traffic, luxury hotels and car showrooms.

I鈥檓 taking a walk with Ian Rodger, who manages trees for the city鈥檚 Royal Parks. 鈥淭hese were planted in about 1860,鈥� says Rodger, gazing up at the giant London planes on either side of the path. Their canopies spread out high above him and fuzzy fruits fall occasionally from clusters in the branches.

The Victorians planted thousands of these trees because they thrive in any soil and their flaking bark and glossy leaves made them impervious to the soot and smoke of the industrial revolution. But as we have learned to our cost, their roots can spread widely, disrupting roads, pavements and buildings. 鈥淲hat the Victorians didn鈥檛 know was how bloody big they get,鈥� says Rodger.

Nor could they have appreciated the true value of the trees, beyond their hardiness and handsomeness. In fact, that is something we are only just coming to fully appreciate. Recently, a band of 鈥渢reeconomists鈥� have begun to put a fair price tag on trees, accounting for the services they provide, from keeping our buildings cool to preventing skin cancer. The results are sometimes startlingly large 鈥� and can help people like Rodger plead the case for our cities鈥� trees.

When trees hit the headlines, it is often in the context of deforestation. But in most developed countries, forests have actually been advancing over the past few decades (see 鈥�Green divide鈥�). And the trend shows no signs of abating. Take the Republic of Ireland: about 11 per cent of the country is covered by trees and the government aims to . In March, the UK began planting a , which will arc from Liverpool to Hull in the north of England. The UK government estimates it could generate 拢2 billion for the rural economy through tourism and employment.

However, those trees come at a cost. From sapling to old age, they must be constantly looked after. That is particularly true in cities, where ageing or diseased branches can fall on people and property, and roots can break up pavements. In some places, that financial burden is hard to bear. The local authority in the UK city of Sheffield has generated protests and national headlines with its chainsaw massacre of 5500 trees, with another 12,000 at risk of being felled. In March, a newly ordained vicar was among those arrested as she protested the felling of a mature lime tree nicknamed 鈥淭he Duchess鈥�.

The easy argument often made in favour of trees is that they are ineffably lovely. But that doesn鈥檛 cut much ice when set against the cost of upkeep and repairs. What is important is the trees鈥� economic benefits. Calculating those, it turns out, is a rather vexed question.

It鈥檚 not just disputes between councils and residents where the value of trees is important to know. Imagine a situation where a builder damages a tree during construction and is pursued in court for damages. Or where a tree falls and the owner wants to make an insurance claim.

It was situations like these that set Chris Neilan trying to value the benefit of trees more accurately. He began his career as a tree surgeon in the 1970s. Back then, councils counted the number of trees they planted, but not the number that survived long term. Neilan and his colleagues knew that most of the saplings died, often from under-watering or damage from mowers and strimmers. 鈥淭here was a joke at the time,鈥� he says, 鈥減lant a tree in 73; plant some more in 74.鈥�

When he started working for Epping Forest District Council just outside London in the late 1980s, Neilan was called to give evidence in a court case involving a young oak that a developer had failed to protect in front of a row of new houses. That entailed giving the oak a value.

He could have used an existing method developed by a British forester named Rodney Helliwell, which focused on a tree鈥檚 aesthetic contribution to a landscape. This typically generated values and thus fines in the hundreds of pounds. Neilan thought that seemed too low.

He began developing a new method, which attempted to capture trees鈥� worth as an amenity, taking into account their attractiveness and how they accentuate or diminish a sense of place. Now called Capital Asset Value for Amenity Trees (CAVAT), it starts by multiplying the cross-sectional area of the tree鈥檚 trunk by a unit price 鈥� currently 拢15.88 鈥� that relates closely to what the tree costs to buy, and which goes up with inflation. This gives a basic value based on its size, which is then augmented in several steps that take into account the tree鈥檚 species, visibility to the public, local population levels, the size and condition of the leaf canopy, the suitability of the species for its site and the tree鈥檚 life expectancy. 鈥淲hen I described the tree鈥檚 true value, the developer was fined exactly that sum 鈥� which was about 拢12,000,鈥� says Neilan.

But scientists have long known that trees have far more to offer us than pleasant feelings. Around the turn of the century, organisations like the World Bank and the UN Environment Programme pushed for a rigorous valuation of the merits not just of trees, but of rocks, rivers, soils and sediments too. The result was the landmark Millennium Ecosystem Assessment of 2005, which put a price on the services humans gain from nature, ranging from direct benefits like food and water to supporting services like the pollination of crops by insects and the absorption of carbon dioxide by plants.

Some say nature is priceless; that valuing it in financial terms ties it to an economic system that has only threatened it. Yet incentivising conservation with cash can work. In 1997, Costa Rica began a scheme that paid landowners to preserve forests rather than clear them to make way for profitable crops. The amount they received was based on the value of the services provided by the forests. This seemed to aid conservation: from around 1980, the country鈥檚 forest cover was either in decline or stable, but between 2000 and 2005 it increased by about 2 per cent.

Around the same time, David Nowak was developing a piece of software that catalogued the benefits of trees in more detail than ever. Nowak, who works for the United States Forest Service in Syracuse, New York, released . Unlike CAVAT, with its focus on amenity value, iTree attempts to price up the environmental services trees provide. It combines local weather and pollution data with tree metrics 鈥� including trunk girth, species type, canopy size and sunlight exposure 鈥� to calculate the value of the services the trees provide. These range from the pollution they remove from the air to the carbon they store, and the run-off into the sewage system they prevent by soaking up rain. Where it is relevant, more data can be included in those calculations, for example to take account of where ultraviolet radiation is highest, and which trees are best able to filter it out and reduce the risk of skin cancer.

Nowak imagined iTree would only be used by forest managers, but it seems to have caught the public imagination. In Milwaukee, an influential 2011 advertising campaign included billboards that put iTree-generated price tags on trees, challenging passers-by to think differently about their surroundings. These days there is a version of iTree that anyone can use to estimate the value of any tree in the US (see 鈥�Price my tree鈥�).

Over the past few years, Nowak has helped value the trees in many US cities, with armies of volunteers measuring trunks and recording species. But his most recent work is more ambitious. It used algorithms to crunch data from satellites and aerial photography and estimate .

The study focused on four core benefits trees provide: soaking up air pollution, storing carbon, saving money on energy by shading buildings in summer and cooling them in winter, and avoiding the emissions associated with the production of that energy. Totting these up, Nowak found the grand total was $18.3 billion a year 鈥� about $3 a tree on average. One reason the figure is rather low is that rural trees provide less valuable benefits to humans, simply because there are fewer people around.

Elsewhere, it is a different story. Back in Hyde Park, Rodger and I are joined by Kenton Rogers, a consultant at , a social enterprise based at the University of Exeter, UK. He pulls a report out of his satchel. Working with Forest Research, part of the UK鈥檚 Forestry Commission, Treeconomics has used iTree and CAVAT to calculate the worth of urban trees across the UK and Rogers has got the results for Hyde Park.

A crew of park staff and volunteers spent weeks during the three summers to 2015 auditing the park鈥檚 3174 trees, including 1188 planes, the dominant among 104 species. According to the iTree estimates, their environmental services are worth 拢208,916 per year. The bulk of this 鈥� about 拢183,000 鈥� is the 鈥渟ocial damage鈥� cost of the pollution the trees prevent, a standard metric that accounts for trees鈥� benefits to human health and the environment.

鈥淭ree valuations can help us plan the urban forests of the future鈥�

But that is before you include the CAVAT valuations. Because the park is in such a densely populated area, and because the trees are large, visible and highly suited to their setting, the amenity value of all the trees there is huge: 拢52,378 on average for each London plane. 鈥淎nd it鈥檚 worth every penny,鈥� says Rodger as he looks up.

Tree valuations are starting to spread. As well as several cities in the UK, groups akin to Treeconomics have used iTree to value urban trees in Toronto, Barcelona and M茅rida in Mexico among other places. This is giving us a richer picture of urban trees than ever before (see 鈥�It grows on trees鈥�).

The next step, says Nowak, is to use the valuations as a way to forecast the value of services different trees will provide in the future. That gives us a tool to help plan tomorrow鈥檚 forests.

This is already possible, to an extent. An offshoot of Nowak鈥檚 program, called , allows an urban forester to rank the environmental services desired in a new planting scheme, including reducing air pollution, wind and ultraviolet light. After plugging in these characteristics, alongside the hardiness required to cope with local geography and weather, and the predicted mature size of the tree, the software produces a ranked list of recommended species. 鈥淚t鈥檚 becoming a more intelligent system,鈥� says Nowak.

Taken further, systems like this could be used to ensure any new planting scheme delivers a clear return on investment. Get that right, and disputes over the value of trees like the ones in Sheffield could be a thing of the past.

In the meantime, arborists hope that tools like this will improve trees鈥� status. 鈥淚f a lamp post falls down or is damaged, people expect it to be replaced 鈥� it should be the same for a tree,鈥� says Kieron Doick, head of the Urban Forest Research Group at the UK Forestry Commission. The difference between the two is that trees get larger and provide more value as they get older, meaning they need to be preserved, not replaced.

In Hyde Park, where the 拢52,000 planes cast long shadows over tourists cycling past, Rodger knows that not everyone likes the idea of valuing nature. 鈥淚 do think it鈥檚 a sad indictment of today鈥檚 society that everything seems to have a value,鈥� he says. Trees have been his professional passion for 25 years. But like the other tree lovers seeking to evaluate them, he has come to believe in the practice.

鈥淵ou can talk about the loveliness of trees until you鈥檙e blue in the face,鈥� says Neilan. 鈥淏ut it doesn鈥檛 get you extra money when some wretched contractor trenches through the root system.鈥�

It grows on trees

They say you know a tree by its fruit, but perhaps its price tag is more instructive still. A recent report valued all the ways London鈥檚 8.4 million urban trees benefit us. Click on the image below to find out more

This article appeared in print under the headline 鈥淭reeconomics鈥�