Plants in urban ecosystems: Essential role of urban forests in urban metabolism and succession toward sustainability

The urban forest has several positive effects on urban metabolism: reducing urban runoff, consequent sewer overloads and resulting water pollution; and reducing heat islands through shading and transpirational cooling by tree leaves, which also reduces energy required for cooling buildings. It is likely that urban trees can mitigate CO2 emissions from combustion, thus reducing overall emissions from power plants. This has been modelled, but has not been verified by actual measurements. Large computer models have been used to predict uptake of air pollutants by urban trees. There is, however, considerable uncertainty about the validity of the predictions from these models. Very few real uptake data are available. Urban trees emit small biogenic volatile hydrocarbons (BVOCs), with amounts varying by species. BVOCs can become part of the photochemical oxidant cycle, increasing ozone levels in cities. It is likely, but unverified, that the urban forest can improve the quality of life for city residents. There is much to be learned about the characteristics and ecophysiology of trees selected for large urban plantings. Additional research will help to elaborate more fully the role of urban forests in urban metabolism and succession toward sustainability.