Do smaller trees play any real role in tackling carbon and pollution?

Tree leaves

The effectiveness of some smaller tree species in absorbing carbon and pollutants is questioned by a new study 

A study in Torbay is the first time i-Tree software has been used in the UK to establish the true value of the 'urban forest' and raises questions over the effectiveness of smaller trees in absorbing carbon and pollutants

Big trees in towns and cities should be valued more highly because of the all-round benefits they bring to urban communities.

Veterans such as ash, sycamore, evergreen oak and native oaks absorb more carbon and filter more pollution from the air, according to the findings of a major study on urban trees being unveiled later this week. 

The value of big trees is one of the key findings of the project, which attempts to measure the economic, social and environmental benefits of the so-called urban forest. Kenton Rogers, of Exeter-based forestry consultants Hi-Line, and one of the people behind the study, said: 'It confirms what a lot of people have been saying but have not had the evidence for.

'In the Seventies, Eighties and Nineties big trees were being taken out and replaced by smaller ones, such as cherries and rowan. Small trees have their place but it is the big trees that are more meaningful.

'Big trees, the ones the Victorians planted for us, are what we need to maintain but they are few and far between.'

Torbay experiment

The study, at Torbay, in south Devon, is the first time the revolutionary i-Tree software developed by the United States Forest Service has been used in the UK. It attempts to establish a pound-value for the urban forest, in order to give local authorities a tangible way of calculating the value of their trees.

The report finds Leyland cypress, the suburban favourite, and cause of many neighbourly disputes, to be Torbay’s most common species, representing 15 per cent of the total, but one which in many cases has negligible benefits.

Ash, the second most common, at 12 per cent, is ranked the most important because it is the most widespread of the big trees, and in common with the other veterans has a correspondingly large leaf canopy.

The ten species with the highest ‘importance value’, which combines their environmental benefits set against the number of trees, ranked in descending order are: ash, sycamore, Leyland cypress, hazel, beech, evergreen oak, elm, Lawson cypress, hawthorn and English oak.

Leyland cypress scores highly because of the number of trees. English oak, at only two per cent of the total is consequently lower down the table.

The survey found Torbay’s forest cover to be 12 per cent, slightly higher than the UK national average, and containing roughly 818,000 trees. For comparison, tree cover in Chicago and New York, where i-Tree has already been used successfully, is 17 per cent and 24 per cent respectively.

One hundred and two different species were recorded in Torbay, from all six continents. The dominant continent of origin was Europe, with slightly more than half of those being native UK species – natives representing 35 per cent of species.

Tree worth millions

The study calculates Torbay’s urban forest represents a cash asset worth more than £280 million, and provides ‘eco-system services’ worth an additional £346,000 annually.

Fifty tonnes of pollutants, chiefly those linked to road transport, are removed from the air each year, equating to an annual value £281,000. An estimated 98,000 tonnes of carbon is stored in Torbay’s trees, enough to offset the emissions of 592 residents.

Although the tonnage is at first glance minuscule, the report estimates the monetary value of carbon stored to be £1.4 million, while additional carbon sequestration is valued at £64,000 annually.

Rogers said trees and woodland in the countryside rather than urban areas were better suited for storing carbon, and although the storage figure was small the urban forest still made a contribution. He said it would be better if there was more to compare the Torbay findings against on this side of the Atlantic, and urged other local authorities to follow suit.

He said: 'Finally we have got an evidence base on the urban forest. Valuing it is one thing but having the information on the composition and the structure is probably worth more when it comes to justifying budgets.

'We need to be looking towards trees, particularly big trees, as an asset rather than a liability, which is how they are normally managed.'

Woodland Trust spokesman Chris Hickman echoed his comments: 'All trees, especially native trees, are of huge value but big trees in particular are of great importance for a host of reasons.

'Not only do big trees provide greater assistance in the carbon cycle but they can provide homes and sustenance for literally thousands of mammals and invertebrates which small trees cannot.'

The results of the i-Tree survey will be unveiled at the Institute of Chartered Foresters’ ‘Trees, People and the Built Environment’ conference in Birmingham on Wednesday. A workshop on field techniques associated with i-Tree will take place at the Royal Geographical Society, London, in June.

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