Do we possess the tools to prove trees' benefits?

Further evidence needed to fully value functions, according to Forestry Commission's study.

Trees: role in water quality and availability, flood alleviation and biodiversity as well as physical and mental health
Trees: role in water quality and availability, flood alleviation and biodiversity as well as physical and mental health

The availability of conceptual tools for valuing services provided by trees and woodlands remains patchy, according to a report published by the Forestry Commission. The study identifies trees' role in water quality and availability, flood alleviation, biodiversity and physical and mental health as areas where further evidence is most needed for these functions to be fully valued.

"A substantial evidence base has developed, particularly in relation to (trees' role in) open-access recreation and climate change mitigation," while "a firmer evidence base is also emerging for their contribution to improving air quality", the authors, from the University of Exeter, state.

But to address the evidence gaps they identify, the authors propose viewing nature as a "natural factory" contributing to economic outcomes, with trees seen as a form of capital enabling production processes. They add that the benefits of trees and woodlands on farms, such as shelter, soil stabilisation and pollinator habitat, have been poorly quantified and "biophysical data are lacking on the impacts of woodlands on agricultural production".

With ongoing concern over the impact of exotic pests and diseases on UK tree stocks, there is a "substantial need" for better economic evidence of how these impact on timber yield, carbon sequestration and the quality of other ecosystem services, they add.

The authors welcome "an emerging class of integrated ecosystem service mapping tools, which incorporate state-of-the-art models to capture interactions and trade-offs between multiple ecosystem services at different spatial and temporal scales" and note the UK's "leading role in the development of new economic valuation models".

A set of Great Britain woodland ecosystem accounts has been produced by environmental consultancy EFTEC, while Forest Enterprise England published its first set of corporate natural capital accounts in July last year. Earlier analyses of the non-market social and environmental outputs of UK woodlands put their value at more than £2bn a year.

Proposing research priorities to target each of the deficient areas they identify, the authors add that these "show the importance of a cross-disciplinary approach between natural scientists and economists to meet future evidence needs", but add that this is "a complex and challenging task".

University of Exeter senior lecturer in environmental economics Dr Amy Binner, who led the study, says: "Many of the social and environmental benefits of trees are wholly or partly unvalued in markets and GDP calculations. This doesn't mean that these benefits have no value. There is strong evidence to show that nature plays a major role in generating economic activity and well-being, and awareness is increasing of the environmental and economic risks of undervaluing this role."

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Policy - Better balance recommended between physical and social benefits

Urban forestry policies tend to focus on the physical benefits of trees rather than "less intuitive" social ones, but better balancing the two could not only strengthen the case for investment but would also affect where trees are planted, according to Dr Geoffrey Donovan of the US Forest Service's PNW Research Station.

Writing in the journal Urban Forestry & Urban Greening, he compares the biophysical benefits of urban trees in reducing residential heating and cooling costs and storm water management costs, with the social benefits of improved public health, specifically improved air quality, reduced stress, increased exercise and improved social connections.

"A biophysical strategy emphasises planting close to houses and over impervious surface," he notes. But as access to parks correlates with both increased exercise and reduced stress, investment here would better achieve these outcomes, he suggests. "Concentrated areas of publicly accessible green space have the strongest relationship with increased social connectivity," so reducing loneliness, he says.

"Showing that we are currently underinvesting in urban forestry is of little practical use unless urban foresters are able to attract additional funding to correct this underinvestment," he concludes, and suggests that "health care institutions may be willing to support tree-planting programmes because they are cost-effective public health interventions".

He admits that quantifying social rather than physical benefits of trees "is more challenging" but urges greater use of randomised controlled trials that are a mainstay of public health research.

Donovan will be among speakers at Kew Gardens' State of the World's Plants symposium in May.

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