Trees in the urban environment: Response mechanisms and benefits for the ecosystem should guide plant selection for future plantings

It is well known that, at present, more than a half of the world population live in the built-up areas (in Europe almost 80%) and the urbanization trend will continue as the population increases and more people will live in large cities. In this scenario arboriculture and urban forestry should have a main role in the effort to manage urbanization and to create livable cities and it is certain that all green areas, starting from the single tree specimens to periurban forests which, in some way, recall the "concept of nature", can fulfil a paramount function in improving life quality and in reaching the minimal threshold for human well-being. What is going to happen next - over 10-15 years - will obviously be of the greatest interest to all people concerned with urban environment. This latter poses some constraints to tree growth and survival: drought, poor soil quality and compaction, pollutants, salinity, pathogens, light heterogeneity, and conflicts with human activities often cause premature plant death or limit plant growth, thus reducing the net benefit by urban green areas. It is therefore important to better understand the dynamics leading to tree decline in the urban environment, and to develop strategies and techniques aimed at improving the horticultural tolerance (i.e. the capacity to provide benefits, not only to survive, under stressful conditions) of urban trees. These include nursery pre-conditioning techniques and post-planting management techniques, but a key role is played by species selection. Hundreds of species are used in the urban environment, but selection criteria are frequently based upon aesthetics and whether the species are native or not, rather than on the tolerance to typical stresses imposed by the built environment and on the capacity to provide substantial benefits therein.