Complementarity of functional traits in urban trees and ecosystem services.

Urban tree diversity is an issue largely debated. The use of uniform planting material from commercial ornamental nurseries and the presence of alien invasive species are factors that depress the diversity. On the other hands gardens and other green infrastructures can act as refuge for endangered species. Great relevance is attributed to the ecological corridors and the exchange of genetic materials between the urban centers and the surrounding country side. The role of tree diversity to improve of the ecological condition of a city has been explored from many years, however the aspects related to the interaction between functional traits (functional biodiversity) and their importance in providing services and benefits for the urban ecosystem are less know. In this presentation we propose some example how the complementarity in the use of resources between tree species in an urban forest produces benificials effects on population (ecosystem services). Trees with different superficial and deep rooting systems can keep water from the different layers of soil, and the plants have different stomatal response to the drought stress period. In this way the transpiration rates at the stand level is balanced through the season, so providing a more effective benefit for cooling and climate regulation. The same stomatal behavior make the plants more effective also in absorbing gaseous pollutants. The coexistence of tree species with different leaf surface characteristics makes the canopy layer more effective in trapping particulate pollutants, whereas a mixture of deciduous, semi-deciduous and evergreen species plays an important role also during the winter season. Other examples are shown and discussed in the context of the ecosystem services.