'World first' measures benefits of trees

Study conducted at Manchester University shows that trees benefit city climates and that such benefits can be quantified.

Urban trees: benefits include reducing run-off and providing shade - image: HW
Urban trees: benefits include reducing run-off and providing shade - image: HW

The benefits of trees to city climates can be quantified, bolstering the case for their planting and maintenance to policy makers, a leading researcher has claimed.

Dr Roland Ennos of the University of Manchester said his research is a global first in actually measuring the behaviour of trees and their effects on their surroundings in real-world situations.

He told HW:"It turns out they are effective in several different ways."

First, he explained, they reduce runoff "more effectively than we thought they would", but principally through absorption in the planting hole.

Second, they provide shade and cooling to their immediate surroundings.

"They lower the radiant temperature from surfaces, by 5-7 degsC, rather than the air temperature directly," he said, explaining that the perception of temperature is a combination of the two.

Third, he said, they affect the climate of the whole city.

"We attached pyranometers to the leaves to measure how open the stomata were, from which you can work out how much water they lose, and so how much energy that evaporation is using up," Ennos said.

"From this we calculated that individual trees can provide up to 7kW of cooling, equivalent to two air-conditioning units. They can absorb up to 30 per cent of incoming radiation."

But this depends on the species and how well they are cared for, he stressed. The more densely canopied Callery pear (Pyrus calleryana) turned out to provide twice as much cooling as a crab apple.

"But the pears themselves could vary by a factor of five, depending on how well they were growing, which depended on how well they were looked after," he said.

The experiments were conducted with the help of local green space body Red Rose Forest, which planted some of the trees several years earlier and had records of their planting and care, and Barcham Trees, which supplied additional trees for planting.

Funding also came from Manchester City Council. "The council carries out green audits, so have an interest in the findings," Ennos added.

Tree study - Forty specimens under spotlight

As part of the research by Dr Roland Ennos, the growth and cooling ability of 40 trees planted by Red Rose Forest six years earlier were compared. These incorporated a wide range planting systems - tree pits filled with conventional topsoil, planting in Amsterdam structural soil (a compactable blend of sand and organic matter), and planting in grass verges.

Ennos said: "The trees in the Amsterdam soil grew about three times as fast as those in ordinary pits and provided five times as much cooling."

He ascribed this to the soil retaining a more open structure, which allowed better root aeration.

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