200 Sjöman et al.: Selection Approach of Urban Trees for Inner-city Environments teration, with a peak negative net water balance (Figure 3) due to its southerly location with a warmer summer climate and low lev- els of precipitation (Table 2). In early autumn Galaţi experienced the highest water deficit, followed by Bârlad and Iaşi (Figure 3). With its lowest net water balance in July onwards and with no abrupt changes, Corneşti deviated from the other study areas (Figure 3) due to higher precipitation (637.2 mm) and lower mean annual temperature (8.7°C) (Table 2). Codri differed from the other five study areas due to higher precipitation and lower mean annual temperature, whereas the other sites had rather similar water stress status over the season. Rădeşti had a some- what more slowly developing negative net water balance early in the season compared with Bădeana, Bursucani, Repedea Hill, and Roşcani, but had a much more rapid negative trend in August. In the comparison between the study sites and ur- ban environments in Copenhagen, Codri showed a closer match with park environments, while the other study sites showed a closer match with paved sites (Figure 3; Sjöman et al. 2012). A total number of 1159 trees, representing 23 species, were found in 30 plots throughout the six different areas (Table 3). The following 13 species had 25 individuals or more occurring in the study plots: Acer campestre, A. tataricum, Carpinus betulus, C. orientalis, Cornus mas, Crataegus monogyna, Fraxinus excelsi- or, Quercus dalechampii, Q. frainetto, Q. pubescens, Q. robur, Sorbus torminalis, and Tilia tomentosa (Table 3). Of these, seven species, Acer campestre, Carpinus betulus, Fraxinus excelsior, Quercus dalechampii, Q. frainetto, Q. robur and Tilia tomento- sa, had an annual mean height growth of 25 cm or more (Table 3). Four oak species, Q. dalechampii, Q. frainetto, Q. pubescens, and Q. robur, together with F. excelsior, were mainly present in the canopy layer, while A. tataricum, C. mas, C. monogyna, and S. torminalis were mainly present in the understory layer (Table 3). The remaining species were present in high numbers of individuals in both structural layers of the stands (Table 3). DISCUSSION As has been suggested by a number of authors, investigating the ecological background and performance of species grow- ing in habitats that naturally experience drought during the growing season and winter temperatures similar to those of inner-city environments provides a sound and reliable selec- tion method (Flint 1985; Ware 1994; Ducatillion and Dubois 1997; Broadmeadow et al. 2005; Sæbø et al. 2005; Roloff et al. 2009). The dendroecological studies described here in the Qinling Mountains of China and the steppe forests of Romania and the Republic of Moldavia present an approach for identi- fying alternative trees for future urban use in the CNE-region. As noted by Roloff et al. (2009), dendroecological descriptions are seldom or never available for most species. By starting the selection process with dendroecological studies in natural habitats with similar climate and site situations as inner-city environments, first-hand information can be gained on spe- cies growth and performance in these climates and habitats. This first stage in the selection process can thus screen out species showing slow and/or underdeveloped growth in habi- tats similar to urban inner-city environments. This allows the focus to be directed towards the species in these natural sites that develop rapidly into large trees. This first stage conse- quently identifies genotypes of the species that ought to be ©2012 International Society of Arboriculture included in the following steps at an early phase of the pro- cedure. In the Qinling Mountains of China and the steppe forests of Romania and Moldavia, a total of 27 tree species (genotypes) were identified as specialists for warm and peri- odically dry habitats (Rabinowitz 1981; Gurevitch et al. 2002). The trees at the study sites all experienced drier condi- tions than those in park environments in Copenhagen. In the match between the study sites and paved environments in the cities, all the sites in Romania and Moldavia except Codri showed a comparable water net difference. Hence, the 27 spe- cies identified as developing rather rapidly into large trees at the field sites are all potential trees for tough sites experi- encing drier conditions than traditional park environments. The 27 tree species can be divided into two groups depend- ing on their use background in the CNE-region (Table 3). Acer campestre, A. tataricum, A. altissima, Carpinus betulus, Cor- nus mas, Crataegus monogyna, Fraxinus excelsior, Koelreuteria paniculata, Quercus frainetto, Q. robur, Sorbus torminalis, and Tilia tomentosa are all more or less commonly used in urban environments of the region, mainly as park trees (e.g., Bengts- son 1998; Forrest 2006), and are also available from tree nurs- eries in Europe (e.g., Lorenz von Ehren 2004; Lappen 2007; Bruns 2009). For Carpinus orientalis, C. turczaninowii, Celtis bungeana, Fraxinus chinensis, Morus mongolica, Ostrya ja- ponica, Quercus aliena var. acuteserrata, Q. baronii, Q. dale- champii, Q. pubescens, Q. wutaishanica, Sorbus folgneri, Ostrya pekinensis, Ulmus glaucescens, and U. pumila prior ex- perience of the performance and plasticity is limited or non- existent for the CNE-region (Nitzelius 1958; Bengtsson 1998). In the selection model presented here, two groups of spe- cies are discernible. Group 1 comprises species already in use in the urban environment where prior knowledge and experi- ence of pest resistance, propagation issues, and wood stabil- ity already exists. However, it is essential to use the appropri- ate genotypes in order to select specimens with the properties identified at study sites (Jones et al. 2001; Mijnsbrugge et al. 2010). However, even if it is necessary to re-evaluate these well- known species (genotypes), collected from the study sites, their introduction will be a rather quick procedure due to the prior knowlege of the same species already used in the urban setting. Group 2 includes species for which prior experience of per- formance in urban environments is limited or non-existent in the CNE-region. Evaluation of these 15 tree species identified in the case studies in China, Romania, and Moldavia would be a much more time-consuming procedure before any full-scale trials could be performed in urban settings. The offspring would have to be planted in controlled field trials, for further evaluation, in order to obtain knowledge and experience concerning hardiness, growth and development, wood stability, pest resistance, inva- siveness risks, and propagation issues. Species must then be test- ed in full-scale trials in urban environments of the CNE-region. The obvious advantage of studying habitats and evaluat- ing well-known species is that the subsequent selection process is much shorter and this tree stock can be introduced immedi- ately once proper genotypes are available and tested. This pro- cess also considers trees that are currently used exclusively in park environments but could possibly be transferred into street environments. However, it is equally important to have long- term approaches where many new and untested species and genera can be identified in order to increase the diversity of
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