112 Moore: Water Scarcity and Urban Forests Table 3. Australian Tree species with full or facultative deciduousness, usually in response to a dry period (Australian Plant Study Group 1980; Francis 1981; Boland et al. 1984; Snape 2002). Species Brachychiton rupestris Brachychiton discolor Brachychiton bidwillii Brachychiton australis Ehretia acuminata Erythrina vespertilio Ficus superba Ficus virens Ficus fraseri Common name bottle tree lacebark tree rusty kurrajong large leaf bottle tree koda bat wing tree deciduous fig white fig sandpaper fig urban trees growing within the urban environment (Misra and Sands 1993), despite an urgent need by tree and water resource manag- ers for quantification (Connellan 2008). There are better data on the irrigation required for establishing young trees (May 2004). Drought avoiders such as E. camaldulensis, E. regnans, and E. marginata are profligate luxury water-users that will grow rapidly and use significant volumes of water if it is available. They may be inappropriate for urban use where water is limited in supply or costly, while proving ideal for places where water is abundant or as part of water-sensitive urban design measures to control local flooding by holding and absorbing water during more intense rainfall events predicted under a changed climate (Killicoat et al. 2002; Moore 2009). The economic value of reducing localized flooding could be substantial (Moore 2009). Research shows trees to be effective in removing pollutants, such as nitrogen and phos- phorus, from stormwater run-off (Denman 2006), and may prove to be useful, long-term elements of water-sensitive urban design. Many tree species also possess physiological, anatomical, and morphological adaptations to growing in arid conditions (Kursar et al. 2009). Many eucalypt species seem to remain physiologically active, using water under conditions of mod- erate to severe water stress, reflecting their mesophytic evo- lutionary origins. However, not all eucalypts are equal in their capacity to cope with dry conditions. In Western Australia, E. calophylla has better stomatal control than E. marginata, which is a luxury water-user. Similarly, in eastern Australia, E. regnans is a profligate water-user with little capacity for stoma- tal control, while E. obliqua behaves similarly to E. calophylla. It is interesting to compare a hypothetical scenario where Pinus radiata and Eucalyptus rossii are planted in the same, low phosphorus Australian soil in an urban streetscape where rain- fall is low and there is no irrigation after the first year of estab- lishment. When soil water potential falls, the P. radiata closes stomata, reducing photosynthetic assimilation and growth. The E. rossii on the other hand keeps stomata open and tolerates a decline in internal water potential. When occasional light rain falls, the E. rossii resumes photosynthetic assimilation imme- diately and commences growth (Florence 1981). The P. radiata does not open its stomata and the soil dries, perhaps compounded by the opportunistic uptake of water by E. rossii. The E. rossii out grows and out competes the P. radiata under this scenario. Winter deciduous Australian native trees are relatively rare, with Melia azedarach, Nothofagus gunnii, and Brachychiton acerifolius being notable examples. Furthermore a few northern species, including some eucalypts, such as E. clavigera, E. gran- diflora, and E. brachyandra, are facultatively deciduous during the dry period (Williams et al. 1997). This characteristic is shared with a number of other tree species, some of which are suit- ©2013 International Society of Arboriculture Species Gmelina leichhardtii Lysiphyllum cunninghamii Lysiphyllum carroni Lysiphyllum hookeri Nauclea orientalis Peltophorum pterocarpum Sterculia quadrifida Terminalia catappa Toona australis Common name white beech native bauhinia native bauhinia white bauhinia leichhardt tree yellow poinciana peanut tree sea almond red cedar able for urban use (Table 3). However, there has been very little breeding and selection of these native species for urban use, and even less research on whether breeding might allow deciduous- ness to apply to southern winters, expanding the potential use of any of these or related species (Munne-Bosch and Alegre 2004). Some species have stomata that respond to the vapor pres- sure of the ambient air (Table 1). Stomata close in response to drier air and leaf moisture content increases as a result, but transpiration reduces accordingly. Species with this char- acteristic could prove very useful in cities where water is limited, but while the response has been observed in some species with potential for urban use, it is largely unresearched. Some species of Australian urban trees come from popu- lations that have wide and extensive natural distributions in environments where water availability varies (Wheeler et al. 2003). There are good data to inform provenance selec- tions for many forest species (Hamrick 2004; Broadmead- ow et al. 2005; Craft and Ashley 2007; Gouveia and Freitas 2009), but arboricultural data on Australian species of ame- nity trees are not so easily accessed. Studies on provenances of Lophostemon confertus (Williams 1996) and Tristaniopsis laurina (Looker 2001), from different climate and soil con- ditions, have been undertaken and would allow urban selec- tions for drier climates. Even if species’ ranges are limited, there may be the option of selecting different species from within a genus. This is the case with the genera Eucalyp- tus and Acacia within Australia, where there are large num- bers of related species occupying a broad range of habitats. Often in eucalypt-dominated forests it is common for dif- ferent species to occupy environments that become increas- ingly drier (Fensham and Holman 1999). This gives rise to the concept of a displacement series, of often-related species, which replace each other over an ecotone of increas- ingly arid environments (Pate and McComb 1981; Shepherd et al. 2008; Holman et al. 2011). As this happens, species have a tendency to show characteristics (Table 4) that better adapt them to the drier conditions. These characteristics could be used by urban forest managers as a guide for what species might be successful for urban planting in drier conditions, but very little research has been applied to the urban context. Good Australian data support the use of irrigation under sin- gular mulches in general, and mixed particle size organic mulches in particular (Connellan et al. 2000; Handreck and Black 2002). Early morning subsurface irrigation regimes that permit trees to open stomata early to maximize photosynthesis before water becomes limiting are based on sound tree physiology. In many species, stomata are often closed by about 2:00 pm, especially if soil water is limiting (Eamus 2006). Furthermore, for many tree
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