Arboriculture & Urban Forestry 39(3): May 2013 species evapotranspiration cools them, reducing the risks of heat damage, especially on hot windy days, the frequency of which is likely to increase under climate change. Such irrigation also captures at least some of the general and environmental benefits that the urban forest provides in terms of transpirational cooling. Table 4. Characteristics of a eucalypt displacement series from wetter to drier environments (Pate and McComb 1981). Characteristic altered as environment dries • Greater root:shoot ratio • Increasing root:shoot ratio in response to water stress • Slower stomatal response to decreasing xylem water potential • Slower decline in leaf turgidity with increased water stress • Lower rate of transpiration in wetter soils ROOT ARCHITECTURE AND WATER USE When a tree seed germinates in natural soils, the radicle emerges and usually develops into a tap root. In Australian native tree species, such as Eucalyptus and Acacia, it is not uncom- mon to find a seedling of 20 mm height with a primary root of 150–200 mm in length (Moore 2008). This root then rap- idly develops as a tap root, anchoring the young tree, provid- ing necessary water and nutrients and the framework from which lateral roots develop (Awe et al. 1976). In most urban trees, however, the tap root should be considered a juvenile characteristic, which only persists for the early establish- ment phase of the tree’s life cycle (Ashton 1975; Moore 1990). The root systems of mature trees have a tendency to be spread- ing and relatively shallow (Watson and Neely 1994). The typical urban forest tree root system consists of a shallow spreading root plate of lateral spreading roots complemented by the presence of descending (or vertical or sinker) roots, which usually occur around the base of the tree or close to the trunk, where oxygen is more readily available and where nutrients and organic matter are being actively recycled (Coile 1937; Perry 1982). While the lateral roots are often within 200–300 mm of the soil surface, descending roots may grow to depths of 1000 mm or more. There are also descending roots farther out along the root plate, which have a tendency to be smaller in diameter and shallower in their descent. These roots may persist for a number of years before they die back and are replaced (Moore 1995; Smith and Moore 1997). This common pattern of urban tree root architecture has profound implications for the application of water. However, there are few data on the variations in root architecture for native and exotic trees and almost none comparing Australian native species. Many irrigation regimes assume that roots are close to the trunk and under the drip line of canopies. This seems to be the case for species such as elms, but is not necessarily the case for eucalypts and other species where exposure of root systems with an air knife shows the presence of major structural roots within the drip line but very few, if any, fine absorbing roots (Moore 2008). The absorbing roots are often 10 m or more from the trunk and concentrated where moisture levels are higher. There is an urgent need for data on the root architecture of Australian urban tree species. It is vital to know where roots are, why they develop where they do, and how much wa- ter they are capable of removing from soil in their vicinity. It is also essential to know where, and at what depth, water should be supplied for efficient and effective irrigation (Con- nellan 2008). There is a popular view that trees absorb water 113 from deep in the soil profile and that only “deep soaking” is effective irrigation over summer. Current knowledge of root architecture suggests that this is not the case for urban forest trees, but there is little research to inform the debate. Con- sequently, water restrictions that limit irrigation of urban trees have been imposed rather than allowing an occasional irrigation of the absorbing root plate near the soil surface. This has resulted in higher levels of stress and the deaths of many mature trees in the urban forest over the past decade. CONCLUSION There has been great public interest in efficient and effec- tive water use and conservation. However, the debate has often been fuelled by anecdotal information rather than be- ing informed by data on water use by different plant species. There have been debates about whether trees—native or ex- otic—should be irrigated over the summer, and suggestions that perhaps nature should take its course and trees left to die. In many parts of southeastern Australia, restrictions to wa- ter use have been applied to gardens, parks, and streetscapes without data to support the impositions. Does restricting ir- rigation actually save water, and what are the consequences of the restrictions on trees and society as a whole? It has been argued that the use of water during days of extreme high temperatures could reduce ambient temperatures by both surface evaporation and transpirational cooling (Nicholls et al. 2008; Loughnan et al. 2010), thereby reducing the num- ber of excess human deaths that occur during heat waves. Australia’s major cities are not only urban forests but bio- diversity hot spots (Daniels and Tait 2005). The parks, gar- dens, streets, and front and backyards constitute an urban forest that is very diverse in its range of species that gener- ate myriad habitats and niches. High-density urban develop- ments and inner city renewal make it virtually impossible to grow trees in places that were once green and leafy. Water scarcity is exacerbating the loss of urban vegetation cover, but there are many alternate planting options available to ur- ban tree managers, if they are prepared to use the data that are available, largely from forestry research, on the root, foliage, and physiological adaptations of many Australian trees species to arid environments. There is an urgent need to obtain similar data for tree species commonly planted in urban environments. The costs of such research would be more than offset by improved water use efficiency and the benefits that effectively managed urban forests provide. At a time of climate change, it is concerning that trees in the urban forest—in both private and public open spaces—are threatened by a scarcity of water that is not just imposed by rain- fall decreases and climate change but by water restrictions as well. Water is a valuable commodity in limited supply, but by using the knowledge and data provided by research on the ad- aptations that many Australian trees have to water stress, much can be done in selecting and managing tree species for use in the urban forest that will allow amelioration of the heat island effect, reduction in wind speed, provision of shade, and reduc- tion in energy use. Such outcomes should ensure enhanced eco- nomic viability, capture the health and social benefits that trees in the urban forest provide, and offer valuable green infrastruc- ture that will contribute to the long-term sustainability of cities. ©2013 International Society of Arboriculture
May 2013
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