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Trees in the urban environment are subjected to a number of stresses which are very different from those suffered by trees in typical rural conditions. The stresses listed in this paper should be a basis for the selection criteria used in urban tree improvement programs. The basic properties of trees are climatic adaptation, disease resistance and a large phenotypic plasticity. Properties related to the urban situation are related to stresses caused by social factors, the restrictive soil volume and crown space, soil pollution, air pollution, de-icing salt, wind and drought. In addition, aesthetic factors, growth form and growth potential and resistance to breakage of limbs are important selection criteria. The priority ranking of the selection criteria depends on the environment wherein the plants are to be used. The need to broaden the range of species and cultivars planted in Nordic cities should have high priority, and selection programs should, therefore, include new species of urban trees. A practical selection should be made within the four major Nordic climatic regions, which are the northern maritime, northern continental, southern maritime and southern continental regions. The establishment of broad co-operation in the selection of plant materials for urban uses is discussed.
The Norwegian Crop Research Institute, Særheim Research Centre, Klepp, Norway 2:
Forest Research Branch, Reykjavík, Iceland 3:
The Danish Forest and Landscape Research Institute, Hoersholm, Denmark