When it comes to cities, sometimes we can’t see the forest for the urban trees, says Bill Gething
The Institute of Chartered Foresters’ forthcoming two-day conference, ‘Trees, People and the Built Environment’, is modestly billed as a national event.
In fact, in addition to eminent UK speakers, including Angela Brady, president in waiting of the RIBA, the programme includes contributions from leading international speakers from the USA, Canada, Italy, Denmark, France, Australia and Ethiopia.
The programme covers a correspondingly wide range of topics from the ‘soft’; the social and health benefits of urban greening, to the ‘hard’; dealing with the urban tree’s unnatural predators: highway engineers, insurers and utility companies. A session entitled ‘Resolving Conflicts with Urban Infrastructure’ will hopefully throw up some intelligent alternatives to the all-too-common practice of sweeping away substantial traditional street trees and replacing them with diminutive substitutes that contribute little or nothing either to the urban scene or to the wider environmental benefits provided by the urban forest.
The conference also showcases a study of Torbay to establish the magnitude and distribution of its urban forest resource using i-Tree software, an analytical technique developed in the USA that promises to provide hard economic and environmental ammunition to demonstrate the value of urban forests. Carried out as a partnership between Torbay Council and consultants Treeconomics, with the assistance of Natural England and Forest Research – the research arm of the recently reprieved Forestry Commission – this is a pilot to establish a UK benchmark against which other urban areas can be compared.
It aims to quantify a whole range of statistics, including species and number of trees, density of cover, air pollution removed, air quality improvement when the trees are in leaf, and the total amount of carbon sequestered in the urban forest and how much is sequestered in addition each year. To justify investment in tree programmes and their ongoing maintenance, the study aims to put a monetary value to the functions of the urban forest.
Some cities have already developed specific policies on urban greenspace, including Stuttgart, which has had one in place since the 1930s, and Berlin, where planners have devised a Biotope Area Factor to define how much green space is needed for each development site.
These policies are no doubt effective as a vehicle for setting aside a proportion of new development sites as greenspace. However, one wonders whether this will have a significant enough effect on overall urban green cover in the timescales in which we need to act. It would be a brave planner who insisted that sites in dense urban areas should be retained as, or even ‘converted’ to, urban forest rather than lucrative built assets.
In a future where summertime water resources are likely to be less abundant, the role of landscape is not simple. Many of our current native tree and plant species are likely to become increasingly stressed, and choices will have to be made; whether to provide sufficient irrigation so that they can survive – and provide corresponding cooling of the urban environment – or if more drought-tolerant species be selected that are hardier, but may not provide such effective cooling. Bluntly, is maintaining large areas of urban greenspace the most effective way of using water to keep urban temperatures down?
As far as I am aware, most of the work in this area has been focused on quantifying the gross impact of large areas of greenspace. In existing hot climates, the alternative approach of exploiting a more intimate relationship between built form and water and/or planting, such as the small courtyards of Seville, has proved particularly effective.
Either way, our built responses to a changing climate and the way we think about urban form and its associated landscape is going to need to progress quickly.
Bill Gething is the founder of consultancy, Sustainability + Architecture. For daily updates on sustainability, visit ajfootprint.com