The selection of plant materials for street trees, park trees and urban woodland

Abstract

This chapter focuses on the plant materials being used in urban forestry. It intends to be an introduction to the choice of species and a guide to the identification of better plant materials for different urban situations. However, the chapter is not aiming at giving advanced guidelines for plant breeders, although some of the different methods of tree improvement are mentioned briefly. The methodology of tree improvement programs needs to reflect national and regional aims. The chapter also includes an overview of genotypic resistance to stresses, which trees need to withstand in urban situations. Those stresses are mainly dealt with elsewhere (Chap. 11). The status of tree selection is also illustrated by some European examples. In Europe the recent focus has been on the relationship between the location in which trees are planted and the relative resulting stresses, which they experience. Street trees are exposed to a relatively high stress level and the average lifespan of the trees is short. The stresses include different polluting agents, mechanical damage, high and low ambient temperatures, de-icing salt, restricted space for crown development, small root volumes, low quality growing substrates and insufficient access to water and oxygen to roots (see also Chap. 11). Street trees may also be negatively affected by shading, local wind gusts and the impact from recreational users of green areas. Park trees are exposed to moderate stress and the average lifespan of trees increases. However, the trees are often exposed to pollutants and to stresses from people and animals. Finally, trees in urban woodland are subjected to relatively low stress levels and may reach a considerable lifespan. In urban woodland, the level of stress depends more on climatic growing conditions, long distance air pollution, soil conditions, recreational patterns with increased fire risk and biotic damages (Sæbø et al. 2003). Today, urban forestry relies mainly on the selection and breeding efforts done in species of interest in conventional forestry. Plant materials, which have been selected for commercial forestry, can probably be used with some success in urban areas. However, for many of those species of interest to urban forestry, selection of improved trees, and related research, such as the mapping of the genetic structure of the species, has mostly not been carried out. Thus, there is a strong need to increase systematic work on selection and even breeding of trees and shrubs adapted to urban conditions, with the aim of finding genotypes that can easily be propagated, produced and established in urban green areas. This is especially the case if planners and practitioners are to succeed in increasing the range of species used in cities.