Arboriculture & Urban Forestry 39(3): May 2013 The leaves and phyllodes of many Australian species (Table 1) are isobilateral and often hang vertically, thereby reduc- ing the surface area that is exposed to the sun (King 1997). Species such as Eucalyptus preissiana (Knox et al. 1994) and E. obliqua have prominent cuticular ledges, which overarch their stomata, creating a stomatal antechamber that reduces transpi- rational water loss (Moore 1981). However, the stomatal anat- omy of many common street trees species remains unknown. In Australian tree species, the number of stomata rang- es from about 28 mm-2 in Persoonia (geebung) to between 100–350 per mm2 in eucalypts. The number often varies inversely with size with fewer larger stomata contrasting many smaller stomata (Knox et al. 1994). In Eucalyptus globulus, there are 300 stomata mm-2 , but the leaf area occupied by sto- matal apertures is only about 1%. However, with stomata open, the rate of transpirational water loss is the same as for evapo- ration from an open wet surface; water and gaseous movement through open stomata is remarkably efficient. Thus, knowledge of stomatal rhythms and behavior is essential to understand- ing tree water use and survival in water-limited environments. Trees such as Casuarina littoralis, Eucalyptus calophylla, Eremophila macgillivrayi, Pittosporum phylliraeoides, and Myoporum floribundum show effective stomatal control and so more efficient water use, but if water is limited then their growth rates may be slowed to the point where they are ineffective for planting in the urban forest. Similarly, species such as Acacia melanoxylon or Eucalyptus grandiflora, which reduce water use through reduc- tion in leaf surface area, may lack the canopy characteristics and density that would make them attractive for urban forest planting. For most Australian tree species planted in urban environ- ments there are almost no data on basic physiological pro- cesses, such as stomatal behavior, let alone whether they are stress avoiders or tolerators in relation to water (Table 2). Which trees have good stomatal control as soil moisture dimin- ishes (Eamus et al. 2001; Prior et al. 2005), which keep their stomata open and so are luxury water-users, and which spe- cies can tolerate low internal water potentials are largely un- Mechanism(s) 111 known (Atwell et al. 1999), except for those few species that are of interest for forestry, timber, or agricultural research (Pate and McComb 1981; Meier and Leuschner 2008). Such basic research would not take large amounts of funding, and simple data gathering using basic porometry would not take long, but this has not attracted the interest of the research funding bodies. Acacia is Australia’s largest indigenous genus with over 900 woody species ranging from shrubs to large trees. They are generally sclerophyllous and Australian species are typically phyllodenous in contrast to the Acacia species of Africa and South America (Thukten 2006). Many arid zone Acacia species are known for their extreme avoidance of desiccation (New 1984; Broadhurst and Young 2006; Page et al. 2011). While A. harpophylla is more drought resistant than A. aneura, even the latter has phyllodes that can lose a large proportion of their water content without harm. Many species maintain cell turgor despite high levels of moisture stress. In some species, phyllode size reduces in drier areas (Thukten 2006; Deines et al. 2011). The size and shape of A. melanoxylon phyllodes are affected by both aridity and seasonal rainfall patterns (Farrell and Ashton 1978). Several Acacia species have very deep roots that may reach depths of 12 m or more (Table 2). A. mearnsii may have roots that penetrate to 6 m, but 75% of the root system is within 600 mm of the soil surface. The closure of pinnules as soils dry is easily observed in A. mearnsii—a bi-pinnate leafed species—growing in the basaltic clays of the western plains near Melbourne. This reduces transpirational water loss. In plantations, A. mearnsii could lose 261 kg of water per day compared to A. decurrens’ 44 kg, but this was largely due to a difference in foliage density with A. mearnsii having a foliage mass of 69 kg, while A. decurrens had a foliage mass of 9 kg (New 1984). In an urban forest, a choice between these species may come down to a decision about canopy appearance, density and impact versus water use. There are major research gaps in the use of Australian native species, as well as exotic species, growing under Australian envi- ronmental conditions. Few studies are available on water use by Table 2. Avoidance and Tolerance Mechanisms for coping with low water environments. Strategy Growth Drought avoidance Drought tolerance by improved water status Grow where and when water is available Increased rooting volume Increased root density Good stomatal control Capacity for osmotic adjustment Reduced leaf surface area Larger root:shoot ratio Drought tolerance by maintaining cell volume Dehydration tolerance More elastic cell walls Cells and physiology unaffected by reduced water content Unaffected until water is limiting Improved Improved Usually reduced Usually reduced Usually reduced Usually reduced Usually reduced Usually reduced or restricted Examples Eucalyptus regnans, E. camaldulensis, E. marginata Acacia mearnsii, E. camaldulensis, E. clelandii, E. trivalvis E. camaldulensis, Acacia mearnsii Casuarina littoralis, E. calophylla, Eremophila macgillivrayi, Pittosporum phylliraeoides, Myoporum floribundum Atriplex nummularia, E. viminalis Acacia melanoxylon, Acacia mearnsii, E. clavigera, E. grandiflora, E. brachyandra E. camaldulensis, E. marginata, Acacia mearnsii Acacia aneura E. rossii, E. viminalis, Acacia aneura Note: Columns 1–3 of this table are extended and modified from Atwell et al. 1999. Column 4 is based on the author’s experience with these Australian species. ©2013 International Society of Arboriculture
May 2013
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