110 silent assets to cities for decades and even centuries. They are major and essential urban infrastructure (Daniels and Tait 2005). Cities are biodiversity hot spots due to the variety of habi- in public and private open space, especially tats available the diversity of plantings in domestic front and back yards (Daniels and Tait 2005). The requirement for tree manag- ers is to establish a priority for the urban forest in the alloca- tion of a precious and valuable, rather than scarce, commod- ity (Connellan 2008). Society will allocate water to items for which there is an economic and political imperative. For most of its history, the price of water in Australia has been subsidized, however, it does have a real economic value and in most States increasing water prices are moving toward that value (Victorian Department of Sustainability and En- vironment 2004; Victorian Department of Sustainability and Environment 2006; Victorian Department of Sustainability and Environment 2007). To maintain the urban forest, water must be used effectively and efficiently. There can be no going back to the days of profligate water use and year-round emer- ald green lawns (Moore 2009). The environment and economy cannot sustain such an approach (Water Resources Strategy Committee 2002). How well informed are the practices gov- erning the use of water in the urban forest and what are the research needs that would enhance best management practices? ADAPTATIONS RELEVANT TO WATER STRESS Trees in the urban forest face the dilemma of all terrestrial plants: the need to balance the interaction of carbon and wa- Moore: Water Scarcity and Urban Forests ter cycles to allow survival and growth. If water is limited and stomata close, carbon assimilation through photosynthesis is reduced (Cowan 1981; Curran et al. 2009; Martin St. Paul et al. 2012). Thus in the urban environment, restricting water avail- ability to trees in the urban forest may also restrict the benefits that they provide, such as their capacity for carbon sequestration (Jonson and Freudenberger 2011) and transpirational cooling. The performance of different trees species in minimizing water loss, but at the same time maintaining carbon dioxide gain, is defined as water-use efficiency: Carbon gained Water-use efficiency = Water lost The value of water use efficiency varies for different species and can be used to select trees that are more produc- tive for use in cities of drier climates (Ladiges et al. 2005). Australian tree species possess many and varied adaptations to growing in arid environments (Table 1). One of the defining characteristics of many Australian plant genera is sclerophylly. Sclerophyllous trees possess large amounts of sclerenchyma tissue, which maintains cellular volume as conditions dry. It is often assumed that sclero- phylls are low water users, but paradoxically many have poor stomatal control and will use whatever water is avail- able until they wilt (Ladiges et al. 2005). Many have the capacity to survive in environments where water is limited, and managers could proactively minimize the supply of water in low-water environments using sclerophyllous trees. Table 1. Adaptations of Australian tree species to aridity (Ashton 1975; Moore 1981; Pate and McComb 1981; New 1984; Moore 1990; Knox et al. 1994; King 1997; Atwell et al. 1999; Ladiges et al. 2005). Adaptation Sclerophylly Altered leaf anatomy Phyllodes/cladodes Vertically hanging leaves Leaf/pinnule movement Cuticular adornment Stomatal crypts Cuticular ledges Stomatal closure in response to atmospheric vapor deficit Facultative deciduousness Lignotubers/basal burls Epicormic buds Deep tap root High root:shoot ratio Mechanism Maintains cellular volume Reduces leaf surface area Reduces surface area; reduces evapotranspiration Reduces absorption of radiation Reduces exposed leaf surface area Reduces evapotranspiration Reduces evapotranspiration Reduces evapotranspiration Reduces transpirational water loss Reduces growth but allows survival over tropical dry period Rapid regrowth after foliage loss Rapid regrowth after foliage loss Allows access to deeper soil water profile Increases soil volume accessed for water supply ©2013 International Society of Arboriculture Examples Many Australian genera, such as Acacia, and members of the Proteaceae and Myrtaceae families Hakea and Acacia species with rolled needle like leaves Most Australian Acacia species Many eucalypt species Bi-pinnate Acacia species; Lophostemon confertus Many genera, such as Eucalyptus, Acacia, and Casuarina, with hairy, spiny, or glaucous leaves Banksia species, Hakea species Eucalyptus preissiana, E. obliqua Eremophila macgillivrayi, Myoporum floribundum, Myoporum platycarpum, Pittosporum phylliraeoides, Geijera parviflora Some Blakella eucalypts, such as E. clavigera, E. grandiflora, and E. brachyandra Most eucalypts; Acmena smithii Most eucalypts E. camaldulensis E. camaldulensis
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