222 Urban Tree Diversity In attempts at making the urban forest more diversi- fied in its species makeup, an important consideration that is often overlooked is the value of choosing spe- cies with the right adaptation qualities. Simply order- ing new tree species and genotypes that are untested for the region is not appropriate, as careful testing of adaptability and longevity in stressful urban habitats must weigh heavily in the selection (Raupp et al. 2006). Poor or incorrect choices may result in increased mortality, reduced lifespan, and ultimately greater costs when failed or failing trees must be removed or replaced (Tello et al. 2005; Raupp et al. 2006). Important information to consider is the origin of the species, in order to assess whether the prove- nance matches the site climate and to secure winter hardiness. Another determinant is the kind of ecosys- tem from which the genetic material originates, which may reflect its capacity for different growing habitats. An illustrative example of this is given in a study by Bauerle et al. (2003), where a selection of red maple (Acer rubrum), originating from dry habitats, showed significant capacity to withstand dry conditions in urban areas compared with genotypes originating from wet habitats. In a recent evaluation by Sjöman et al. (2015), different genotypes of red maple and sugar maple (A. saccharum) showed marked differences when turgor loss point was used to rank drought tol- erance. This indicates that it is not possible to treat a whole species group as either sensitive or tolerant to e.g., water stress, since tolerance in this regard can differ widely between different genotypes. This is especially the case for species covering a wide natu- ral distribution and growing in different climates and site conditions. As regards the tree species most commonly used in cities today, there is a substantial understanding of the capacity and limits of their tolerance to various stresses depending on species and genotype (e.g., Trowbridge and Bassuk 2004; Dirr 2009). However, this understanding is much more incomplete as regards rare or unconventional species and geno- types, where new perspectives are highly needed. A rather new approach is to use the concept of locally adapted ecotypes, as suggested in a review by Mijns- brugge et al. (2010). This method can assist in finding appropriate ecological traits and characteristics in dif- ferent species and genotypes correlating to challeng- ing urban situations where water stress is common. This also introduces new plant hunting profiles, ©2019 International Society of Arboriculture Sjöman et al.: Hunting for a Larger Diversity of Urban Trees in Western Europe where new and alternative species can be targeted in order to increase knowledge of specific traits depen- dent on varying growing conditions. Arboreta and botanical gardens offer a sound platform for such studies, where correlations can be made to analyze the potential of specimens for urban paved environ- ments. Such analyses can include specific leaf area and wood density (e.g., Greenwood et al. 2017), plant vulnerability to cavitation (e.g., Cochard et al. 2013), and determinants of leaf turgor loss point (e.g., Bart- lett et al. 2012), which provide a solid base to analyze water stress and various means to cope with drought. However, the genetic plant material in the tree collec- tions of botanical gardens may not be the best assort- ment when selecting species for dry urban sites, since the best genetic material may still be out in the wild. In the plant hunting literature with the focus on trees, attention to date has mainly been botanical, i.e., intro- ducing new species to science, and horticultural, i.e. introducing species with extraordinary flowers, autumn color, leaf texture, trunk structure, etc. (e.g., Musgrave et al. 1998; Lancaster 2008; Kilpatrick 2014). Past interest in finding trees tolerant to the var- ious stresses that occur in urban environments has been almost non-existent. This gives the impression that, when analyzing plant material from famous tree collections such as Gothenburg Botanical Garden in Sweden, Kew Gardens in London, UK, or Arnold Arboretum in Boston, U.S.A., the best genetic mate- rial is not being analyzed. Therefore, new approaches where stress tolerance and capacity for delivering ecosystem services are included in the plant hunting process, together with traditional horticultural and botanical interests, are needed to search for future urban trees. Aim of the Study The aim of this study was two-fold. The primary aim was to devise and assess a method for locating natural habitats where trees grow under conditions that are comparable to those in urban environments in terms of water stress. A secondary aim was to identify spe- cific ecotypes in the wild demonstrating great poten- tial to handle growing conditions similar to those in urban environments based on the natural habitats identified in step one. These aims were pursued in a case study of five forest reserves in the southern Cau- casus (Republic of Georgia), where calculations of water net differences and forest inventories were car- ried out in order to assess the potential for locating
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