Planning with trees in mind

Street trees provide the city with a wide range of social, economic and environmental benefits, with large, mature trees maximizing those benefits. Street trees can, in fact, be viewed as form of 'green infrastructure', delivering a range of human and environmental services to the city, alongside the 'grey infrastructure' of conventional engineering services. The requirements of street trees for survival and growth are the same as those of trees in the forest, however in the hostile environment of the city they often struggle to survive, a problem exacerbated by unsustainable design practices. When designing streets and other spaces for trees we should therefore try to 'think like a tree'. The concept of Tree Sensitive Urban Design (TSU D) is proposed as a more sustainable paradigm for the design of urban areas to accommodate trees. Environmental services provided by trees include amelioration of the urban climate and reduction of the heat island effect through shading and evapotranspiration, with associated energy savings (McPherson, Herrington et al. 1988). Trees improve air quality by oxygenation, pollutant removal, and sequestration of atmospheric carbon through photosynthesis (Nowak, Crane et al. 2006). Trees also play an important role in the urban water cycle, their canopies intercepting rainfall and modifying runoff, reducing demands on conventional stormwater infrastructure (Xiao 2002). Street trees, even exotic species, play a role in enhancing biodiversity and creating wildlife corridors. The most commonly recognised benefit of street trees relates to their visual and aesthetic role in making our streets and cities more welcoming and pleasant places. Trees, however, are not just simple 'aesthetic decoration', but can play a significant 'placemaking' role in cities. Street trees can enhance a sense of identity, legibility and spatial definition in a street, and provide a sense of human scale in urban environments. A well designed avenue of trees can also provide a unifying element in a visually diverse streetscape. According to Alan jacobs in his book Great Streets Given a limited budget, the most effective expenditure of funds to improve a street would probably be on trees. Trees can transform a street more easily than any other physical improvement. Moreover, for many people trees are the most important single characteristic of a good street (jacobs 1993) p293 The real economic value of trees is also being increasingly recognised. Street trees have been shown to increase residential property values, and increase visitation to business centres (Wolf 2004). The real economic value of the environmental services provided by urban trees is now also being measured, both for the individual tree, and the collective value of the total urban forest. In the United States, communities can now calculate the net economic benefits generated by their urban forest using online modeling tools such as STRATUM (Street Tree Resource Assessment Tool for Urban Forest Managers) and UFORE (Urban Forest Effects Model)'. In times of drought, water restrictions and climate change, we need trees more than ever, to make our cities more livable places. At the same time, it is becoming increasingly difficult to grow trees, especially large mature trees, in our urban areas. Street trees should be allowed to survive and grow to maturity to maximise their benefits. However they tend to be squeezed into whatever space is left over after all other functional and engineering requirements have been met. The consequences have included declining tree health, reduced tree life expectancy, and increased mortality rates. The biological needs of street trees are essentially the same as those in the natural forest. There is no such biological entity as a 'street tree', only trees that we have chosen to grow in streets. The three natural processes of photosynthesis, respiration and transpiration must still be sustained. These processes have six essential requirements: oxygen, carbon dioxide, light, water, nutrients and appropriate temperatures. To this should be added the provision of adequate space, both above and below ground. Street trees, however, face life in a very hostile environment. In her 1984 book on urban ecology, The Granite Garden, Anne Whiston Spirn, highlighted the challenges facing urban trees in cities (Spirn 1984). These include lack of space, above and below ground, infrastructure conflicts, vandalism and accidental damage, polluted air, temperature extremes, and either too little or too much water. Probably the main concerns are those below ground, due to the highly modified and degraded nature of urban soils (Craul1992).