232 Sjöman et al.: Hunting for a Larger Diversity of Urban Trees in Western Europe be combined with good design of planting pits and the use of structural soils combined with stormwater management. The results from this study should thus be seen as a part of a holistic approach, where cli- mate-oriented design and construction are of equal importance. Moreover, it is important to bear in mind that this study focuses on water stress, which is the main constraint for trees in urban environments and especially in paved environments. However, there are further constraints for trees in urban environments which are important to include in the selection pro- cess in order to create a sustainable tree population, such as pollution, high and low temperatures, and tol- erance for flooding, since we can also expect heavy rain events with large and intense rainfall levels in a future climate. Tolerance for these stressors needs its own unique perspective and is not included in this study. The next step in the research process from this study will be to collect ecotypes of the species identi- fied that best match the conditions in the particular city for which they are intended. Since a diverse tree population for paved urban sites and high density res- idential areas is most urgently needed due to much more extreme growing conditions than in park envi- ronments; finding species that can develop into large, healthy trees at these tough sites should be prioritized. This makes areas such as Akhmeta and Mtskheta very interesting, especially for a future climate situation in Western Europe. Species such as Carpinus orientalis, Celtis caucasica, Quercus colchica, Q. hartwissiana, Sorbus torminalis, and Zelkova carpinifolia can be particularly significant, since they occur naturally in habitats where they are exposed to water stress regimes similar to those occurring in many dense cities. When desirable ecotypes have been collected and estab- lished, more detailed evaluations must be performed to determine specific leaf area and wood density (e.g., Greenwood et al. 2017), plant vulnerability to cavita- tion (e.g., Cochard et al. 2013), determinants of leaf turgor loss point (e.g., Bartlett et al. 2012), etc., as evidence of the plant material’s capacity to cope with drought. These evaluations will need much more controlled environments, which impedes their imple- mentation in the field. They will also be expensive and time consuming. However, if a preliminary screening of the potential of the plant materials for a specific site situation can be done prior to these evaluations, the focus can be directed towards highly promising species and ecotypes, which would undoubtedly will limit the time lag before they can be released to the market. ©2019 International Society of Arboriculture LITERATURE CITED Allen, C.D., A.K. Macalady, H. Chenchouni, D. Bachelet, N. McDowell, M. Vennetier, T. Kitzberger, A. Rigling, D.D. Breshears, E.H. Hogg, P. Gonzalez, R. Fensham, Z. Zhang, J. Castro, N. Demidova, J.H. Lim, G. Allard, S.W. Running, A. Semerci, and N. Cobb. 2010. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecology and Management 259:660–684. Allen, R.G., M.E. Jensen, J. Wright, and R.D. Burman. 1989. Operational estimates of reference evapotranspiration. Agron- omy Journal 81:650–662. Allen, R.G., L.S. Pereira, D. Raes, and M. Smith. 1998. Crop evapo- transpiration—guidelines for computing crop water require- ments. Irrigation and Drainage Paper no. 56, FAO, Rome, Italy. Alvey, A.A. 2006. Promoting and preserving biodiversity in the urban forest. Urban Forestry and Urban Greening 5:195–201. Bartlett, M.K., C. Scoffoni, and L. Sack. 2012. The determinants of leaf turgor loss point and prediction of drought tolerance of spe- cies and biomes: a global meta-analysis. Ecol. Lett. 15:393–405. Bauerle, W.L., T.H. Whitlow, T.L. Setter, T.L. Bauerle, and F.M. Vermeylen. 2003. Ecophysiology of Acer rubrum seedlings from contrasting hydrologic habitats: growth, gas exchange, tissue water relations, abscisic acid and carbon isotope dis- crimination. Tree Physiology 23(12):841–850. Breckle, S.W. 2002. Walter’s Vegetation of the World. 4th edition. Springer. Cochard, H., E. Badel, S. Herbette, S. Delzon, B. Choat, and S. Jansen. 2013. Methods for measuring plant vulnerability to cavitation: a critical review. Journal of Experimental Botany 64:4779–4791. Cowett, F.D., and N.L. Bassuk. 2014. State wide assessment of street trees in New York State, USA. Urban Forestry and Urban Greening 13:213–220. Craul, P.J. 1999. Urban Soil—Applications and Practices. John Wiley & Sons, Canada. Deak Sjöman, J., and S.E. Gill. 2014. Residential runoff—the role of spatial density and surface cover, with case study in the Höjeå river catchment, southern Sweden. Urban Forestry and Urban Greening 13:304–314. Dirr, M.A. 2009. Manual of Woody Landscape Plants, 5th edition. Stipes Publishing L.L.C, Champaign, IL. DMI. 2017. Danish Meteorological Institute. Accessed December 19, 2017. Gill, S.E. 2006. Climate change and urban greenspace. Doctoral Thesis submitted to the University of Manchester, School of Environment and Development, UK. Gill, S.E., J.F. Handley, A.R. Ennos, and S. Pauleit. 2007. Adapting Cities for Climate Change: The Role of the Green Infrastructure. Built Environment 33(1):115–133. Gómez-Muñoz, V.M., M.A. Porta-Gándara, and J.L. Fernández. 2010. Effect of tree shades in urban planning in hot-arid climatic regions. Landscape and Urban Planning 94(3-4):149–157. Greenwood, S., P. Ruiz-Benito, J. Martínez-Vilalta, F. Lloret, T. Kitzberger, C.D. Allen, R. Fensham, D.C. Laughlin, J. Kattge, G. Bönisch, N.J.K. Kraft, and A.S. Jump. 2017. Tree mortality across biomes is promoted by drought intensity, lower wood density and higher specific leaf area. Ecology Letters 20:539–553.
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