Trees and buildings are not normally found to be mutually supportive. Indeed, all too frequently they are mutually destructive. To appreciate the importance of a harmonious co-existence of trees and buildings it is necessary to examine the individual function of each and then try to find a solution to their interactive problems that is beneficial to all, particularly for those who insure the problems associated with the relationship.
To properly understand the overall function of trees and their increasingly important role in the environment, it is necessary to consider the predictions for global climate change.
At a minimum, average annual temperatures are expected to rise by 3ºC by 2100, and atmospheric carbon concentration to increase from 300 parts per million to 700 parts per million by the end of the century. By 2080, overall rainfall patterns, specifically in the south-east of England are predicted to increase by 5%, but with a 16% decrease in rainfall during the summer months. The effects of this climate change will be disproportionately felt in cities where the increases in temperatures will be particularly extreme.
Trees, plants and other vegetation are friendly to the environment by removing atmospheric pollutants, such as carbon dioxide, nitric acid vapour, ammonia and sulphur dioxide. The rate of pollutant removal is affected by environmental conditions, being lower, for example, in low light or where there is limited availability of water. While, clearly, the length of day cannot be influenced, planting particularly deep-rooting or drought-tolerant species of trees and plants can help maximise the removal of pollutants, particularly during summer months.
Latest American research shows that all cities have “carbon debt”. Trees, plants and forests play a particularly important role in the urban environment since carbon is a major pollutant of air, contributing to lung, throat and eye irritation, respiratory diseases, heart disease and cancer. Urban trees remove and store carbon which, classically, accounts for some 50% of the dry weight of most trees. The US has 737 million acres of forest land, storing 60 billion tonnes of carbon – approximately 40 times their annual carbon emissions. Simply planting more trees could increase carbon storage by up to 300 million tonnes a year, and could bring down city temperatures. Large trees in urban environments store 90 times more carbon a year than small ones. They act as natural air conditioners, cooling their microenvironment without the need to burn fossil fuels. Trees planted for shade, particularly to shade windows, can be effective in reducing the level of energy used by domestic air conditioning units by as much as 50%.
Other benefits of trees are less quantifiable but nonetheless real, including their amenity value and their value to wildlife. It is clear that trees, particularly in an urban environment, bring great benefits and should be preserved and maintained wherever possible, even if there is a cost.
A deeply rooted problem
The domestic dwelling's primary function is to provide accommodation but, in our social structure, goes further in expressing status and aspirations. Most people's largest single financial commitment is the provision of a roof over their heads. Mortgages tend to be long-term and the mental stress to the owner/occupier brought about by structural damage can be easily understood. When this damage is caused by an event covered by an insurance policy, for example subsidence of the site, the mental stress to the owner is no less, but the financial burden is passed to the insurer and it is up to the loss adjuster to find a practical solution.
There has been a long-standing problem where trees and their roots are located in close proximity to low-rise domestic dwellings. The problem is particularly acute in areas where the soil content is predominantly of shrinkable clay, which is highly susceptible during periods of low rainfall. This problem will become all the more profound given the predictions for climate change, especially in the south-east of England where there will be a much greater need to remove pollutants from the atmosphere by larger, deep rooted trees. The benefit of such trees in close proximity to buildings is clear, but effective solutions have to be found to the historic problems to enable trees and houses to live together in harmony.
During the 1980s, houses affected by subsidence were routinely underpinned so that the foundations of the building were re-established well below the influence of tree roots. While this stabilised dwellings and allowed trees and buildings to live together, the expense to the insurers was considerable. A more cost-effective solution was required.
More recently the tendency was to remove the trees and monitor any movement in the dwelling. Once stability had returned, repairs would be carried out to the structure. Householders and their neighbours who own trees have, for the most part put up with this solution, accepting, that trees and houses do not easily cohabit.
The pruning lobby
Where the owner of a tree is a local authority a considerable financial burden falls upon them in the form of recoveries sought by domestic insurers. In a number of cases this has led local authorities to devise and implement systematic pruning programmes which have, to an extent, been effective but at the cost of reduced pollutant removal. The maintenance budget of local authorities, particularly in a number of London boroughs, is considerable. This pays for an ongoing programme of pruning, a practice that is supported by the arboricultural world.
Traditionally, building professionals have been sceptical of tree route barriers. My experience, however, is that tree route barriers designed by arboriculturists have been very effective in enabling trees and buildings to live side by side. Where the local authority provides co-operation or even contributes to the cost of installing a tree route barrier, domestic insurers often take a sympathetic view by, say, not pursuing recoveries against the local authority.
“Ebony and Ivory, living in perfect harmony”. Paul McCartney's lyrics express the desire for a solution in human relations, but all of us must equally respect the important role of trees in our environment and, where they conflict with our basic need for shelter, find practical solutions to overcome that conflict and preserve the benefits.