196 Sjöman et al.: Selection Approach of Urban Trees for Inner-city Environments Step 1: Identification of Floristically Rich Regions There are numerous geographical regions in the world with a climate regime similar to the microclimate of streets, squares, and courtyards in the CNE-region (Takhtajan 1986; Ellenberg 1988; Breckle 2002; Burga et al. 2005). Information on such regions can be gained partly from botanical, ecological, and dendrological literature combined with local climate data, and partly from earlier experiences of species in cultivation from these regions. Figure 2 presents examples of geographical re- gions with a rich dendroflora and species used in the CNE- region with the potential to tolerate warm and periodically dry conditions. These species can hence serve as markers in the search for interesting and promising habitats where further suitable tree species and genotypes can be expected to grow (Kollmann and Fischer 2003). The steppe forests of northeast Romania and Moldavia and the species-rich mountains of cen- tral China are used as model areas. These two regions include species that have been in use in the CNE-region for a long time and have been observed to perform well in paved spaces. For example, Quercus frainetto Ten., native to northeast Romania, and Styphnolobium japonicum (L) Schott., native to the moun- tains of central China, have both been used and recommended for urban environments in the CNE-region (Bengtsson 1998). In order to identify the potential of a region to host ap- propriate tree species for particular areas, such as inner-city environments, Sjöman and Richnau (2009) presented an ap- proach based on climatological comparisons and calcula- tion of the potential water net differences. For these calcu- lations the regression presented by Thornthwaite (1948) was used, with monthly potential evapotranspiration based on the values of temperature, precipitation, number of sun- shine hours per day, estimated water runoff, and cloudi- ness. These results indicate seasonal temperatures and show similarities and regards where, when, and how much water stress occurs during the growing season (Figure 3). Step 2: Search for Habitats Similar to Inner-city Environments Even if it is possible to identify a region’s potential to host appro- priate tree species for inner-city environments based on climate data, it is also important to find appropriate sites and microcli- mates because they can differ considerably within short distanc- es, particularly in mountain regions (Barry 2008). For example, the climate and water availability for plants can differ signifi- cantly between a south-facing slope and a north-facing slope at the same altitude, influencing the vegetation structure and species compositions in the stands (Figure 3; Sjöman et al. 2010). The di- rect exposure to sunlight on steep south-facing slopes causes low air humidity and rapid drying of the soil, which is comparable to the situation at many urban paved sites (Sieghardt et al. 2005). In order to gain such local site-related information, lit- erature and field measurements are necessary, including soil samples for evaluation of soil texture, humus content, and pH value (FAO 2006). This soil-related data gives valuable guid- ance in matching urban sites in terms of soil water-holding ca- pacity and other important variables (Craul 1999; Sieghardt et al. 2005). In mountainous regions, slope steepness is an im- portant factor since water runoff data are crucial in calculat- ing the net water differences in stands (Thornthwaite 1948). Step 3: Evaluation of Tree Growth and Performance After identification of appropriate regions and habitats, where plants are exposed to cold winter temperatures in combination Figure 2. Areas around the world with examples of interesting species and genotypes, when searching for regions with woody species for urban plantings. ©2012 International Society of Arboriculture
September 2012
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