Arboriculture & Urban Forestry 39(3): May 2013 Table 1. Timeline of significant events related to the 1997–2009 drought in the City of Melbourne. Date Event 1993 1997/98 summer 1999 2004 2006 November 2007 January 2010 autumn First appearance of elm leaf beetle in Melbourne tree population. First year of extended drought period. First applications of imidacloprid (Confidor® ) to treat elm leaf beetle. Evidence of crown death beginning in older elm trees. Application of Stage 2 Water Restrictions – irrigation of lawns banned. Application of Stage 3 Water Restrictions – first installations of drip irrigation. Drought “ends” with good autumn rain. 2010/11 summer Wet summer. BACKGROUND TO THE STUDY Melbourne is the oldest municipality in Greater Melbourne, a large urban area managed by approximately 40 independent local government bodies (Frank et al. 2006). The city manages a popu- lation of approximately 58,000 trees that are located primarily in parkland and streetscapes (Shears 2011). The street and park land- scapes of Melbourne are of great importance to the entire metro- politan area and contain a number of precincts that have heritage status. The tree population of Melbourne includes an important population of approximately 6,500 European elms (Ulmus pro- cera, U. × hollandica, U. glabra, and U. minor) that have never been affected by Dutch elm disease (Ophiostoma spp.). The oldest of these trees date to the period 1850–1860 (Spencer 1997). These elms, with London plane tree (Platanus × acerifolia) account for many of the large street trees in Melbourne and contribute charac- ter to many parks. London plane trees account for more than 75% of the trees in the Melbourne CBD (City of Melbourne 2011). The long-term average rainfall of inner Melbourne is 640 mm y-1 (1908–2011; Bureau of Meteorology 2011) with ap- proximately 50 mm falling each month throughout the year. Higher summer temperatures (January mean maximum 25.9°C, January mean minimum 14.3°C) and elevated evaporation during summer months result in a moderate level of summer water deficit. While the original tree plantings in Melbourne would have been established without fixed irrigation systems, techno- logical improvements from the 1950s meant that most parks and streets had irrigation systems installed to maintain green grass cover over summer and to assist trees to withstand dry periods. In Australia, year-to-year rainfall variability is a characteristic of the climate and recurring droughts are common (Gentilli 1971). Since records have been kept in Melbourne (from 1855), there have been a number of drought events, usually lasting for one or two years. However, between 1997 and 2009, an extended serious drought affected much of Australia, including the Melbourne area. Figure 1 shows annual rainfall for the period 1855–2011, with drought events evident, as is the protracted nature of the drought of concern in this paper. The drought period of 1997 to 2009 is the most severe on record for the Melbourne metropolitan area. Dur- ing the period covered by this paper the average annual rainfall was 515 mm (a reduction of 20% from the long-term average) and during the final four years of the drought, the average annual rain- fall was only 450 mm, a reduction of 30%. In the previous severe drought of 1982–1983, the City of Melbourne responded with radial trench cutting and flooding in an attempt to provide relief to water stress being experienced by ‘valued’ trees in iconic parks. This practice met with variable success and has not been repeated. The recent drought (1997–2009) depleted the water storage reservoirs in the hills to the north and east of Melbourne, result- ing in increasingly severe water-use restrictions, such that in late 2006, irrigation of parkland was banned (Table 1). Water restric- tions have been used in Melbourne in the past, in the summers of 1967–1968, 1972–1973, and 1982–1983 (I. Watson, Melbourne Water, pers. comm.), but the most recent restrictions have been exceptionally long. Parkland trees are normally irrigated with turf sprinklers, but these irrigation restrictions and the severity of the drought resulted in significant damage to the health of many trees, particularly poplars (Populus spp.), elms (Ulmus spp.), and plane trees (Platanus spp.). In the case of the elms, a concomitant infes- tation of elm leaf beetle (Pyrrhalta luteola) may have contributed to the decline in tree health. The City of Melbourne was able to negotiate a partial exemption from these irrigation restrictions but was only permitted to use potable water if drip irrigation was used. Accordingly, a program to retrofit drip irrigation into a number of parks and streetscapes began in 2007 (Table 1). The drip line used was primarily Techline™ (Netafim™ , Laverton North, Victoria, Australia), buried approximately 50 mm below the soil surface. In addition to the ongoing drought, the summer of 2008– 2009 had some of the highest temperatures ever recorded in Melbourne, which further increased tree stress. Plane trees were severely affected with significant defoliation in late Janu- ary 2009 after three consecutive days of maximum air tem- peratures >43°C, followed by one day >48°C one week later. Cooler temperatures, and higher than average rainfall, dur- ing the summer of 2010–2011 alleviated some of the effects of the drought and water restrictions were eased during 2011. 137 Figure 1. Annual rainfall (mm) for Melbourne CBD 1855–2010 (Bureau of Meteorology, Australia, www.bom.gov.au). ©2013 International Society of Arboriculture
May 2013
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