Arboriculture & Urban Forestry 36(6): November 2010 the lack of young trees among the majority of the species indi- cates the pioneer strategy among these heat- and drought-tolerant tree species, which have high demands for sunlight and therefore difficulties in establishing under tree canopies. This pioneer strat- egy also explains the age distribution of some of the species. In the case of Ailanthus altissima and Koelreuteria paniculata, for example, the age distribution was more uneven, since their es- tablishment took place during a specific time period (e.g., when a light ‘window’ opened after a landslide). This can be com- pared with the more shade-tolerant Carpinus turczaninowii, in- dividuals of which were spread among the age classes (Table 3). There was great variation between a single-stemmed and multi-stemmed growth habit for approximately 50% of the 25 tree species observed in the plots (Table 3). This might be at- tributable to genetic variation within the species, but is also possible that the multi-stemmed growth is a result of previ- ous coppicing. This is much more likely to be the case among the oak species, the wood of which is used in great quanti- ties for local mushroom production. Therefore the data re- garding this habit need to be interpreted with caution. The majority of the specialist species identified in the study are generally not used for urban paved sites in the CNE-region. In other parts of the world too, experience on the use of these species at paved sites within cities is still very limited except for Ailanthus altissima and Koelreuteria paniculata, which have proven long-standing adaptation to the harsh conditions at urban paved sites. In southern Europe, Ailanthus altissima is a widely used street tree for urban paved sites and Koelreuteria panicu- lata is used to some extent as well (Sæbø et al. 2005). Despite these experiences in regions of the world differing from the CNE- region, they can be seen as an indication of the potential useful- ness of the remaining habitat specialists identified in this study. CONCLUSIONS The two important challenges in the planning and manage- ment of trees in urban streets and at other paved sites include the need for more knowledge and practical experience about site-adapted use of species, and the need for a greater variety of species and genera with natural adaptations for surviving and developing well at such sites. The problem for urban tree plan- ners at present lies not in finding a great variety of species that are well adapted for the favorable growth conditions that often exist in urban woodlands and parklands, but in finding species that can withstand the harsh conditions at urban paved sites. In order to achieve a closer match between the study plots in the Qinling Mountains and paved sites of Copenhagen, new strate- gies such as increased planting pit size, including an allowance for stormwater infiltration in tree plantations, must be includ- ed when planning for trees at urban paved sites in the future. This study in the Qinling Mountains identified 14 tree spe- cies (Group 2) for urban paved sites in the CNE-region. These can be considered a starting point in a possible selection process aimed at increasing the range of tree species and genera that are site-adapted for urban paved sites. However, aspects such as hardiness, health status, wood stability, allergy risks, propa- gation issues, establishment, and management problems must be tested before these new species are introduced into public places. Another important aspect is the possible invasiveness of these candidate species. The next stage of the process should be 269 closed growing trials. Such trials should be started immediately, as tree selection is a long-term process, and the focus should be on promising, high-performing species instead of random testing. Acknowledgments. Thanks to Zhang Wenhui and Kang Yongxiang at the Northwest Agriculture and Forestry University in Yangling, China, and Johan Slagstedt for supporting in the field work. We would also like to extend our gratitude to Kenneth Lorentzon for sharing his dendrol- ogy expertise and to Roland von Bothmer for fruitful comments on the manuscript. LITERATURE CITED Balok, C.A., and R., Hilaire. 2002. Drought responses among seven south-western landscape tree taxa. Journal of the American Society for Horticulture Science 127(2):211–218 Brady, N.C., and R. R., Weil. 2002. The nature and properties of soils. Upper Saddle River, NJ: Prentice Hall. 960 pp. Breckle S.W. 2002. Walter’s vegetation of the world. 4th Edition. Spring- er. 527 pp. Breda, N., R. Huc, A. Granier, and E. Dreyer. 2006. Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences. Annals of Forest Science 63:625–644. Bühler, O., P. Kristoffersen, and S.U. Larsen. 2007. Growth of street trees in Copenhagen with emphasis on the effect of different establishment concepts. Arboriculture and Urban Forestry 33:330–337. Corcuera, L., J.J. Camarero, and E. Gil-Pelegrin. 2002. Functional groups in Quercus species derived from the analysis of pressure- volume curves. Trees: Structure and Function 16(7):465–472. Craul, P.J. 1999. Urban Soil – Applications and Practices. John Wiley & sons, Canada. 366 pp. David, T.S., M.O. Henriques, C. Kurz-Besson, J. Nunes, F. Valante, M. Vaz, J.S. Pereira, R. Siegwolf, M.M. Chaves, L.C. Gazarini, and J.S. David. 2007. Water-use strategies in two co-occurring Mediterrane- an evergreen oaks: surviving the summer drought. Tree Physiology 27:793–803. DMI. 2009. Danish Meteorological Institute. Ducatillion, C., and E. Dubois. 1997. Diversification des plantes orni- mentales méditerranéennes: estimation des besoins qualitatifs des villes en arbres et arbustes (Diversification of ornamental mediter- ranean plants: assessment of the qualitative needs of cities concern- ing trees and shrubs), In: INRA (Ed.) La plante dans la ville. Angers. 139–149 pp. (French). Duhme, F., and S. Pauleit. 2000. The dendrofloristic richness of SE- Europe, a phenomenal treasure for urban plantings. Mitteilungen aus der Biologischen Bundesanstalt für Land- und Forstwirtschaft Berlin- Dahlem 370:23–39 Ehrlich, R., and B. Weinberg. 1970. An exact method for characteriza- tion of grain shape. Journal of Sedimentary Research - March 1970 40(1):205–212. FAO. 2006. Guidelines for soil description. Food and agriculture organi- zation of the united nation, Rome 2006. 98 pp. Flint, H.L. 1985. Plants showing tolerance of urban stress. Journal of Environmental Horticulture 3(2):85–89. Gale, M.R., and D.F. Grigal. 1987. Vertical root distribution of northern tree species in relation to successional status. Canadian Journal of Forest Research, 1987 17(8):829–834. Grissino-Mayer, H.D. 2003. A manual and tutorial for the proper use of an increment borer. Tree-Ring Research 59(2):63–79. ©2010 International Society of Arboriculture
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