Arboriculture & Urban Forestry 39(3): May 2013 extensive ranges (Santamour et al. 1980), or from homoclime studies (matching against likely future climates rather than cur- rent conditions). The effects of the drought (see Table 2) have been considered by Melbourne city planners. In late 2011, a draft urban forest strategy was published (City of Melbourne 2011). One of the goals of the strategy is to increase tree species diver- sity, with a stated goal of having no more than 5% of the tree population represented by a single species. At present, three species [elms, London plane trees, and river red gums (Eucalyptus camaldulensis)] make up 35% of the city’s tree population. Irrigation If water deficit due to drought was the major cause of the health decline in Melbourne’s tree population, irrigation is the most logical solution, as no other soil or tree treatment is capable of overcoming sustained drought stress of mature trees. To improve the efficiency with which irrigation water is delivered, various approaches can be taken. These include improved soil moisture monitoring, use of alternative water sources, and high-efficiency delivery systems. Soil Moisture Monitoring To improve the quality of data provided by the soil moisture monitoring program described in this study, Melbourne has established a further network of 100 sampling sites for capac- itance dielectric soil moisture measurement to a depth of 1 m (Diviner 2000™ , Sentek Pty. Ltd., Stepney, South Australia, Australia) in both irrigated and un-irrigated parks throughout Melbourne. While this technology can provide useful soil mois- ture information for tree managers, it is recognized that the installation of the permanent access tubes for this technology is complex and quite expensive, which may limit its wider use. Alternative Irrigation Water Supplies It is unlikely that there will be a return to unrestricted irrigation of trees and greenspace with potable-quality water, although access to recycled sewage wastewater and/or desalinized water in the future may provide greater flexibility and improved tree health. These alternative water sources will require the monitoring of soil health indicators to detect potential salinization effects of these higher-salt water sources (e.g., Tanji et al. undated). Anoth- er promising alternative water source is the use of on-site (or near site) captured storm water for tree irrigation. This builds upon the water sensitive urban design concept with localized storage and distribution (passive or pumped) networks. Melbourne is beginning to install these facilities at a number of locations around the city. Point Source Irrigation Systems Because of their high efficiency, the continued adoption of drip irrigation, and similar point source systems, seems probable, but their efficacy for irrigation of parkland trees requires a clear understanding of water supply and demand. The tree water bal- ance model reported in this paper indicated that potential tree use of drip-applied water can be greater than the rate of supply, making it difficult to wet large volumes of soil or alleviate tree water stress. However, a study in California (Hickman 1993) showed that using drippers in mid-summer to irrigate drought- stressed oak trees led to improved growth that was evident up to four years after the irrigation event. In that California study, 143 the drippers were run for 30 hours at 2.5 mm h-1, delivering the equivalent of 75 mm of irrigation, which wetted the soil to field capacity to a depth of at least 350 mm. This is a much heavier application rate than that used by the City of Melbourne and it is worth investigating whether this level of irrigation is feasible with the infrastructure available within an urban context. The California study did not present information that allowed the irrigation application to be converted to L tree-1 for comparison. Irrigation for Root Growth Cockroft and Olsen (1972) and Richards and Cockroft (1975) found that in irrigated deciduous trees, fine root growth occurred in spring and was dependent on soil moisture content at that time. While irrigation could offset tree moisture deficit during summer, it had little effect on new root growth over summer unless soil was kept constantly wet. These findings suggest that apart from the obvious water deficit effects on tree canopy processes, many of Melbourne’s trees may not have been able to produce new fine roots in spring or sustain them for water resource acquisition through the summer months, possibly for several years. This may have resulted in a concurrent decline in root system health, in addition to the observed poor canopy health. It may be possible to address this issue through the timely operation of point source irrigation systems, to support and promote fine root growth in early spring, prior to the commencement of normal summer irrigation. Winter and spring irrigation with drip systems is one way of recharging larger soil volumes to field capacity at a time when evapotranspirative demand is low, and this will have great value in years when winter rainfall is below average and therefore soil water recharge is poor. The August 2009 study showed that in winter, soil could be brought to field capacity quite quickly with drip irrigation, but that most wetting occurred close to the emitter. Wetting to a depth of 1 m was possible but this would only be of benefit if there were roots at that depth to exploit the water. Urban tree root systems are often shallow (Gilman 1990), but deep roots can occur close to the trunk of many species (Stone and Kalisz 1991; Canadell et al. 1996). As such, it may be more effective to place drip irrigation lines close to the trunk for this reason and for the fact that potential evaporation will be less under the canopy of the tree. Heavy irrigation at the base of the trunk may also simu- late the effects of stem flow (water captured in the canopy and directed down the branches and trunk to the ground, where it is redirected along major roots) (Johnson and Lehman 2006). If access to tree irrigation water is limited in the future, drip irrigation and mulch are demonstrated to improve the efficiency of delivering that water. Further work is needed to investigate whether there is strategic value in being selective about where that water is placed. CONCLUSION The period of below-average rainfall that affected much of southern and eastern Australia between 1997 and 2009, and the changes in tree irrigation practices as a result of tighter restric- tions in urban water use in response to this drought, led to a decline in tree heath in the parks and streets of Melbourne, Victoria, Australia, especially in temperate climate species. The City of Melbourne retrofitted drip line irrigation systems in many park areas in an attempt to comply with tighter water restrictions while ameliorating soil moisture conditions experienced by val- ued tree populations. A study of soil wetting patterns under drip ©2013 International Society of Arboriculture
May 2013
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