City Trees

soil volume, vulnerable to carbon starvation, or lose their leaves very early in the growing season (Figure 1). Even though these strategies may allow trees to survive successive periods of stress, none of these avoidance traits really improve tree per- formance in the delivery of ecosystem services, such as cooling. It is much better to select species with an inherent tolerance to water deficits as they will maintain physiological function for longer in the drying cycle; they do not metaphorically bury their head in the sand at the first sign of trouble. The challenge from a tree selection point of view is to distinguish between species that are able to tolerate a low water potential and those that are not. Interestingly, this is not as simple as observ- ing the change in species composition along precipitation gradients, as both avoidance and tolerance mechanisms may be at play in any sin- gle forest ecosystem. It is, therefore, necessary to select a trait that will help characterise the extent of a species' tolerance to water deficits. One of the best candidates for this is the leaf tur- gor loss point since it can quantify the "drought" tolerance of a species. Although this was once a highly laborious value to determine (typically through the use of pressure-volume curves), a strong linear relationship between the leaf turgor loss point and the leaf osmotic potential at full turgor means that the relatively rapid screening of species is possible (Bartlett et al. 2012a, b). For example, a study of 27 Acer species revealed large differences in the leaf turgor loss point of closely related species. In summer, the leaf turgor loss point varied from around – 2MPa (-290 psi) to less than – 4 MPa (-580 psi), with species segre- gating broadly in line with their preferred habitat niches: Acer species specialising in the shady, humid understorey being much less tolerant to a low water potential than those found in drier, warmer habitats (Sjöman et al. 2015). This technique has now been used by our group to evaluate a wide range of temperate trees (~200 species to date). Another trait that is particularly instructive with regards to drought tolerance is the stem water potential at 50% loss of hydraulic conductivity. This is a quantitative measure of the level of water stress that is required for the sapwood to lose 50% of its ability to transport (conduct) water, and is an important trait in many ecological studies evaluating drought-induced tree mortality (Choat et al. 2012). These data are highly instructive for those selecting trees for paved urban sites which are characterised by frequent, and often severe, water deficits throughout the growing season. Despite trait data being of inherent value to those specifying trees for urban environments, it can be somewhat complex to interpret for those without a background in plant sciences. Therefore, in our new digital guidance on species selection, Tree Selection for Green Infrastructure: A Guide for >> Figure 3: New, free digital guidance on tree selection is available from the Trees & Design Action Group website. www.urban-forestry.com 35

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