Arboriculture & Urban Forestry 38(5): September 2012 in nutrient-rich and moist sites where they develop into large and tenacious trees, while they show noticeably weaker devel- opment in inferior conditions (Ellenberg 1988; Påhlsson 1998). Thus there are currently two important directions in the selec- tion of urban trees: 1) Increasing the knowledge and experience of site-adapted species useful for the varying growing conditions in urban environments; and 2) increasing the knowledge and ex- perience of new and/or untested tree species with the potential to complement the existing urban tree population in the region. Research on the selection and testing of alternative tree species, genotypes, and provenances is underway in several countries, but with different objectives. In forestry, the main focus in selection and breeding programmes continues to be on fast-growing and high-performing genotypes of mainly, already widely used for- est tree species and on resistance to different pests and diseases (Perry 1998; Lu and Charrette 2008; Buránek et al. 2011). In the selection programmes for urban environments, the criteria do not differ greatly, apart from omitting commercial aspects such as tim- ber production. Some examples of species selection programmes in Europe are presented by Sæbø et al. (2005). In this field, two main strategies are suggested. The first is to concentrate on the genetic aspect of already much-used species in order to iden- tify the best varieties and genotypes in relation to city habitats. The second strategy involves broadening the range of urban tree species through a focused selection of species found in habitats showing similar conditions to city habitats. In this latter direction, Sæbø et al. (2005) describe a project in Greece where species are recommended after analyses of their natural requirements of soil conditions and climate. Furthermore, Ducatillion and Dubois (1997) based their selection on information from bibliographies, archives, and observations of living plants in southern France. When using habitat studies in the search for potential species and genotypes, it is unlikely that the species-poor native dendro- flora of the northern parts of Central Europe and the adjacent milder parts of Northern Europe (abbreviated to CNE-region; El- lenberg 1988) can provide a large range of species with extended tolerance of the environmental stresses characterising, for ex- ample, paved sites within urban areas of the region (Duhme and Pauleit 2000). Other regions with a comparable climate but with a richer dendroflora may have the potential to provide new tree species, genera, and genotypes for this purpose (Takhtajan 1986; Breckle 2002; Burga et al. 2005; Roloff et al. 2009; Sjöman and Richnau 2009). The key question is thus how to identify—with limited means and in a relatively short time—the right species and genotypes for further testing. The wealth of potentially suit- able species from areas such as China is overwhelmingly large, which means that strategies for how and where to start search- ing, and how to make the right choice, need to be developed. The objective of this paper is to present and discuss the first step in a selection procedure based on dendroecological studies in natural habitats in order to identify promising urban trees for inner-city environments in the CNE-region. This approach is il- lustrated with two case studies, one in central China and the other in northeast Romania and adjacent parts of the Republic of Mol- davia, where habitat studies were carried out in natural habitats comparable to the growing conditions of inner-city environments in the CNE-region (Sjöman et al. 2010; Sjöman et al. 2012). 195 DEVELOPMENT OF THE SELECTION APPROACH The grand old man in the sphere of modern arboriculture, Alex Shigo (1991), once said about the use and maintenance of city trees: “…we must understand the tree as it grows in its natural site first. To try to treat a city tree without under- standing the tree as it grows in its natural site is like draw- ing a data curve with only an y axis; and no base line!” In natural habitats, trees have been stress-tested and selected over evolutionary periods of time. Some species have developed an extensive plasticity and tolerance to a range of environmental conditions, while others are specialized to certain habitat types (Rabinowitz 1981; Oliver and Larson 1996; Gurevitch et al. 2002). Investigating the ecological background, and performance of spe- cies growing in habitats with drought during the growing season and winter temperatures similar to those of inner-city environ- ments in a particular area, can assist in identifying alternative tree species and genotypes (Flint 1985; Ware 1994; Ducatillion and Dubois 1997; Broadmeadow et al. 2005; Sæbø et al. 2005; Rol- off et al. 2009). The selection approach presented here comprises four steps (Figure 1). The first step is to identify floristically rich regions where the climate is similar to the CNE-region (Breckle 2002). In a second step, habitats where trees experience warm, dry summers with periodical drought are identified. Then third, dendroecological studies are conducted in the identified habitats (Roloff et al. 2009) in order to identify specialists for this type of habitat. Fourthly, seeds from identified species and genotypes are then collected in order to obtain appropriate genetic materi- als to be included in controlled selection programmes (Gurevitch et al. 2002; Mijnsbrugge et al. 2010). Already after Step 3 it is possible to identify the species for further evaluation (Figure 1). The case studies were chiefly directed to follow Steps 1 to 3. Through this procedure, two levels of results were achieved. The first group of species identified are already in cultivation and available at tree nurseries. The second group of species have rarely or never been cultivated in the CNE-region (Figure 1). Figure 1. The first steps in a habitat-based selection of tree species suitable for urban areas in the CNE-region, including two result levels depending on earlier experiences of the species. ©2012 International Society of Arboriculture
September 2012
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