Arboriculture & Urban Forestry 38(5): September 2012 the urban tree population. Subsequent selection and searches for complementary tree species do not need to exclusively ad- dress new and untested species. Further information on spe- cies already used in the urban landscape, taken from their natural habitat, will increase the current body of knowledge and illustrate new use-functions for these well-known species. The approaches in evaluating species which are more or less commonly used in urban environments of the CNE-region and those where prior experiences of their performance and plas- ticity is limited or non-existing for the region were taken into consideration when selecting the different case study areas. The mountains of China host an impressive richness of species (Ying and Boufford 1998) and the potential to detect a large number of species and genera never used in cultivation before is large. Of 14 tree species identified in China, 12 have never been grown or grown to a very limited extent in the CNE-region. The Ro- mania and Moldavia cases were chosen on the basis that many well-known species occur in the region but knowledge about their site plasticity is sparse. Only three of 13 species were associated with a lack of prior knowledge on their performance (Bean 1980). Of the 27 tree species identified, only Acer campestre, Caprinus betulus, Crataegus monogyna, and Fraxinus excelsior are consid- ered much used or used to some extent in northern Europe (Sæbø et al. 2005). Thus, the 23 tree species identified in those field stud- ies can be potential supplements for diversification of the urban tree population in the CNE-region. Since the two case study areas differ from each other in terms of the number of individuals pres- ent of potential species, the analysis of the findings also differed between the two case studies. The Qinling Mountains host a large diversity of tree species (Ying and Boufford 1998) and few of each species are present in the forest stands, so the amount of rare species scattered in the forest systems is large. The results from the China study were therefore interpreted using a more qualita- tive approach (Sjöman et al. 2010). A quantitative approach was possible in the case study in Romania and Moldavia, since the forests there included more species that were present at the sites in large numbers and rare species were few (Sjöman et al. 2012). Distinguishing between species with their main position in the canopy layer and those in the understorey layer makes it pos- sible to pinpoint a particular use potential. Trees in the canopy layer modify the wind, humidity and temperature microclimate for species in the understorey layer. Canopy species, on the other hand, suffer from much higher transpiration due to their expo- sure to wind and warmer temperatures (Oliver and Larson 1996). In the two case studies tree species with their main position in the canopy layer were A. altissima, Celtis bungeana, Fraxinus excelsior, Koelreuteria paniculata, Morus mongolica, Ostrya japonica, Quercus aliena var. acuteserrata, Q. dalechampii, Q. frainetto, Q. pubescens, Q. robur, and Q. wutaishanica (Sjöman et al. 2010; Sjöman et al. 2012). Thus, these species experienced a more stressful environment, with much higher evapotranspira- tion rates compared with the species in the following structure. This distribution indicates that species only or mainly existing in the canopy layer in mature forest stands have difficulties in germinating and developing successfully under shading tree crowns (Oliver and Larson 1996; Gurevitch et al. 2002). These species have great use potential in warm and sun-exposed sites, such as town squares and courtyards. Acer campestre, Carpinus betulus, Caprinus orientalis, C. turczaninowii, Fraxinus chinen- sis, Quercus baronii, Syringa pekinensis, Tilia tomentosa, and 201 Ulmus glaucescens had individuals in both the canopy layer and the understorey layer (Table 3). Species capable of developing successfully in both the canopy and understorey layer may be useful in e.g. street plantations, where the light and tempera- ture conditions can differ greatly between the two sides of the street (Sieghardt et al. 2005). Moreover, Acer tataricum, Cornus mas, Crataegus monogyna, Sorbus folgneri, Sorbus tormina- lis, and Ulmus pumila were all found growing primarily in the understorey layer and can be useful either in warm courtyards and squares that are shaded for most of the day, or as understory vegetation underneath existing trees, as with paved sites (Sjöman et al. 2010; Sjöman et al. 2012). However, these latter species can probably tolerate sunnier and hence drier and warmer condi- tions, since they all exist in forest edge zones, but inside the forest stands they cannot compete with other much taller species and have therefore developed a tolerance for more shaded conditions. This study focused on trees that in their natural sites are ex- posed to warm and dry growth conditions, since water stress is argued to be the main constraint for tree growth and health in urban environments (e.g., Craul 1999; Hoff 2001; Sieghardt et al. 2005; Nielsen et al. 2007; Roloff et al. 2009). It is important to bear in mind that this process with dendroecological habitat studies in order to identify potential urban trees is just the first step in the selection process. Further research is necessary in order to evaluate the species tolerance towards warm and peri- odically dry growth conditions in another geographical area and towards other stressors, such as de-icing substrates or air pollu- tion. Nevertheless, this approach constitutes a faster and more effective route, since subsequent selection work can focus on species with high potential for the purpose instead of testing species randomly. Dendroecological studies, as presented in this paper, contribute to an ecological understanding that provides for a much wider knowledge base in the selection process, thus helping to evaluate the reaction, tolerance, and performance of different tree species to different stressors. Furthermore, dendro- ecological studies provide valuable guidance regarding the use potential of species, which can be of importance in their subse- quent evaluation in full-scale plantations in urban environments. It is also important to note the invasiveness risks of species included in these types of studies. Relevant research on these aspects in parallel with ongoing selection work can help pre- vent the introduction of invasive species and genotypes. Den- droecological studies in natural habitats, as presented in this text, can also provide some guidance on this issue. By identi- fying the ecological strategies, including disposal and reproduc- tive traits of species and genotypes in their natural settings, it is possible to determine their potential to escape from cultiva- tion and become an aggressive competitor in natural habitats in another region (Pyšek and Richardson 2007; Pyšek et al. 2009). LITERATURE CITED Barry, R.G. 2008. Mountain Weather and Climate. Cambridge University Press, Cambridge, UK. 506 pp. Bean, W.J. 1980. Trees and Shrubs Hardy in the British Isles. 4th volume, Eight Edition Revised. John Murray Ltd, London, England. Bengtsson, R. 1998. Stadsträd från A-Z (Street Trees from A-Z). Stad & Land nr. 154:1998, SLU, Alnarp, Sweden. 168 pp. (In Swedish) Breckle, S.W. 2002. Walter's Vegetation of the World. 4th edition. Springer. 527 pp. ©2012 International Society of Arboriculture
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