194 Sjöman et al.: Selection Approach of Urban Trees for Inner-city Environments Arboriculture & Urban Forestry 2012. 38(5): 194–204 Selection Approach of Urban Trees for Inner-city Environments: Learning from Nature Henrik Sjöman, Allan Gunnarsson, Stephan Pauleit, and Roland Bothmer Abstract. High diversity of species and genera and site adaptation are two important factors in achieving a healthy and sustainable urban tree population. This paper presents and discusses a selection procedure for the identification of trees adapted to inner city environments. The procedure is based on den- droecological studies of trees in natural habitats, with similarities in climate and site conditions as inner city environments. By studying trees in such habi- tats, firsthand information can be gained on the growth and performance of a wide range of species and genotypes. Two field studies were conducted, one in central China and another in northeast Romania and the adjoining Republic of Moldavia with the aim to identify tree species and genotypes adapted to inner city environments in the northern parts of central Europe and the adjoining milder parts of northern Europe. In total, 27 tree species were identified as specialists for warm and periodically dry habitats. Of these tree species, only four are currently used frequently or used to some extent in northern Europe, meaning that 23 other tree species identified in the case studies can be potential supplements for diversification of the urban tree population. Key Words. China; Dendroecology; Moldavia; Romania; Selection; Urban Environments; Urban Trees. Urban trees have aesthetic, socio-cultural, environmental, and economic values that are beneficial for the sustainabil- ity and quality of life in urban areas (Nowak 2002; Tyrväinen et al. 2005). Greenspaces and particular areas with trees can also play an important role for adaptation of cities to climate change (Gill et al. 2007). However, trees need to be healthy, well-growing, and long-lived to provide these services (Gómez- Muñoz et al. 2010). The average lifespan of street trees and of other trees living in paved environments exposed to high stress is often rather short (Sæbø et al. 2005), and can be as low as only 10 years (Foster and Blaine 1977; Gilbertson and Brad- shaw 1990; Nowak et al. 1990). Surviving trees are often in a poor condition. The main reason for this decline is the differ- ence in environmental conditions provided in the urban land- scape compared with that of the habitats within which most trees in the urban landscape originate (Sieghardt et al. 2005). The modern city has changed dramatically due to the ef- fects of the urban heat island (UHI), restricted rooting space, water-impervious surfaces and poor growth substrates. This makes trees planted along streets and in paved sites suf- fer particularly due to lack of water and oxygen, as well as unbalanced provision of nutrients (e.g., Craul 1999; Hoff 2001; Sieghardt et al. 2005; Nielsen et al. 2007). A recent study showed that even in a Nordic city with a cool climate, such as Copenhagen, Denmark, street trees suffer from lack of water even in summers that are actually considerably wetter than average in Copenhagen (Bühler et al. 2007). Creating good growing conditions is therefore impera- tive for sustainable development of the urban stock of trees (Trowbridge and Bassuk 2004). Recent research has con- centrated on the development of methods and techniques ©2012 International Society of Arboriculture to improve habitat conditions in the city. In particular, the soil and rooting zone have been in focus (e.g., Grabosky and Bassuk 1996; Kristoffersen 1999; Pedersen et al. 2000; Trowbridge and Bassuk 2004; Roberts et al. 2006; Morgen- roth and Visser 2011). However, improved methods for tree planting in paved urban sites will not provide similar grow- ing conditions as in forests and parks. Therefore, the selec- tion of a broad range of hardy species and genotypes for urban paved sites remains an important task, even more so if climate conditions become more adverse for urban tree life through climate change. For Central and Northern Eu- rope, air temperatures are predicted to be higher, and peri- ods of drought during summertime will increase (EEA 2008). With climate change, the future urban tree stock also needs to withstand such challenges as an increase in pests and dis- eases. It has thus been suggested that the urban forest should be diversified. Some researchers have indicated that no species should represent more than 10%, no genus more than 20%, and no family more than 30% of the total population (Santamour 1990). However, surveys have shown that the reality is differ- ent and dictated by traditions rather than based on rules ground- ed in science. This narrow choice of species and genotypes is particularly apparent in Northern Europe. In a compilation by Sæbø et al. (2005, based on a survey by Pauleit et al. 2002), eight tree species were reported to be the most common species in street environments in Northern Europe: Acer platanoides L., A. pseudoplatanus L., Aesculus hippocastanum L., Betula pendula Roth., B. pubescens Ehrh., Populus trichocarpa Torr., Sorbus spp. [e.g., Sorbus × intermedia (Ehrh.) Pers.], and Til- ia × europaea L. However, the main natural habitats for these species, except Betula pendula and Sorbus × intermedia, are
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