Look back: Urban deforestation
Jeremy's paper on urban deforestation was presented at Sheffield Hallam Univertity Conference Flooding and Water Management in the Landscape 9 years ago. It is as relevant now as it was then. For the full paper please click on the pdf link below.
Urban deforestation; it's here and it's going to hurt!
Barrell Tree Consultancy provides specialist advice on trees in the UK planning system. We deal with over 500 new sites every year across England, which provides us with an experience-based perception of what is happening to our urban tree stock and how that is affecting landscape character. This paper summarises our subjective assessment of the state of the urban canopy, speculates on the reasons for its condition and sets out our suggestions for its future management. It is not a scientific paper in the sense that it is research based; it is experience based, but we are confident that research will confirm our observations.
Arboriculture is about managing trees near people; the common focus for arboriculturists is on minimising the conflicts arising through proximity whilst maximising the multiple benefits that trees have to offer. Typical conflicts include trees casting excessive shade, leaves blocking gutters and the mess from insects and falling debris causing inconvenience to people who live nearby. Out of sight below ground, roots are well known for causing structural damage, whilst above the ground, falling trunks and branches damage and injure in a more spectacular fashion. In their favour, trees provide a dramatic contrast to the harshness of the urban landscape and offer significant benefits to the wellbeing of city inhabitants. Very few would argue that trees are not good, indeed it seems to be an intuitive truth that they are, but the reality is that the closer trees and people cohabit, the more fraught the relationship becomes.
Set in the context of global warming, two of the most important emerging issues in urban sustainability are rainwater management and temperature regulation. Traditionally, rainwater has been treated as more of a problem than an asset, with the focus on draining it out of cities quickly rather than storing it locally as a resource. However, as the global warming induced extremes of droughts and floods become more frequent, the folly of this conventional wisdom is becoming obvious. Understanding the value of rainwater as a resource and the harm that rapid flow from urban areas causes is focusing attention on storing and using it where it falls to buffer its dispersal. Similarly, through the urban heat island effect, there are predictions that global warming induced temperature rises of 3–7° C are likely in many of our major cities during the next century (GLA,2006)). This is a dramatic increase that will have multiple impacts on all aspects of urban life, from increased bills for air conditioning to the decreased wellbeing and comfort of city inhabitants (Shaw et al., 2007).
In addition to the rather intuitive benefit that grass, parks and trees improve the ‘feel’ of urban areas, there is increasing tangible evidence that green space intercepts rainwater and slows its flow into our traditional drainage systems. More specifically trees, through their size and leaf surface area, are particularly effective at slowing the rate that water reaches the ground and how much of it flows away. Furthermore, their capability to shade and reflect heat, combined with their verticality and large surface area in contact with the air, makes them very efficient at reducing temperatures in the extremes of summer (GLA, 2006). Indeed, there is emerging research to suggest that they are so effective at temperature buffering that an increase of just 10% in our present urban tree canopy cover would offset all but the most extreme temperature rises predicted through global warming (Gill et al., 2007). Although not the answer to all urban sustainability problems in isolation, big trees are obviously part of the solution and there is an emerging body of opinion that we need more of them (Shaw et al., 2007).
THE UNCOMFORTABLE TRUTH
In the hotter parts of the world, people have long been aware of the obvious benefits of trees, with strong traditions of incorporating green space into their urban infrastructure. However, in the UK, mitigating the effects of hot summers has not been a familiar experience and other seemingly more pressing requirements such as increased housing densities and minimising costs has resulted in trees being given a low priority when allocating funds. UK residents have not had much experience at coping with the heat of hot summers, which has resulted in a public not particularly tuned into what a significant impact trees can have on temperature. Against that background, although there is resistance to the idea of tree loss, the reality is that it happens slowly with short-lived public outcry and is soon forgotten. This slow level of awareness of the importance of trees is fostering the gradual erosion of our urban canopy without a full public appreciation of the scale of the loss when considered in total. Urban deforestation is occurring before our very eyes, but the process is so slow that no one has noticed.
To read the rest of this article please click on the pdf