Arboriculture & Urban Forestry 46(2): March 2020 Arboriculture & Urban Forestry 2020. 46(2):135–147 URBAN FORESTRY ARBORICULTURE Scientific Journal of the International Society of Arboriculture & The Effect of a Heat Wave on Urban Tree Pests in Melbourne, Australia: Examples that May Inform Climate Change Tree Management By G.M. Moore and G. Lefoe Abstract. Climate change will have profound deleterious effects on many trees in urban environments; however, as in any biological system undergoing change, there will be benefits. On 7 February 2009, the Australian city of Melbourne experienced its hottest day on record (46.4 °C [115.5 °F]) after a heat wave. In the days that followed, the foliage of native Australian mistletoes, Amyema miquelii and A. pendula, growing on Eucalyptus camaldulensis were observed to lose their green color and turn gray. In large numbers, the mistletoes can cause significant stress, leading to tree death. In the aftermath of the record hot day, large numbers of mistletoes died, and 5 years later the level of mistletoe infestation remained low. On the afternoon of 7 February 2009, tens of thousands of elm leaf beetles, which heavily graze the mature elms of Melbourne (Ulmus procera and U. × hollandica), were found dead under the canopies of street trees, and numbers remained low for at least 5 years there- after. Similarly, psyllids, Mycopsylla fici, and infestations of Ficus macrophylla, which can seriously defoliate trees, fell from high to undetect- able levels in the month following the heat wave. The effects of heat waves and very high temperature days have significant implications for those managing pests in urban forests. Pest control programs were unnecessary in the immediate aftermath of the heat wave and hot days and for up to 5 subsequent years. This has positive implications for tight tree management budgets, but could also lead to a discontinuation of pest monitoring and control programs. Such an approach could see a return to high levels of infestation. Keywords. Amyema miquelii; Elm Leaf Beetle; Eucalyptus camaldulensis; Ficus macrophylla; High Temperatures; Mistletoe; Mycopsylla fici; Ulmus. 135 INTRODUCTION There is a paucity of information on the effect of extreme weather events, such as heat waves and very hot days, on urban tree populations (Jentsch et al. 2007). Burrows defined a “hot wave” as a spell of three or more days on each of which the maximum shade temperature reached or exceeded 90 °F (32.2 °C)(Glickman 2000), but the Australian Bureau of Meteorology defines a heat wave as three or more days of unusually high maximum and minimum tem- peratures in any area (Bureau of Meteorology 2016). Under climate change, the frequency and magnitude of heat waves and hot days is predicted to rise in Mel- bourne, with the number of hot days (> 35 °C) and very hot days (> 40 °C) predicted to double by 2050 and almost triple by 2070 (Commissioner for Envi- ronmental Sustainability 2012). Tree managers will have to plan for a warmer, drier climate and consider the impact of extreme weather events on tree health and pest and disease management. The urban heat island (UHI) effect causes higher temperatures in large cities than in the surrounding countryside, and could favor the development of ectotherms such as insects (Meineke et al. 2013). However, pest insect abundance varies with the patchiness of the UHIs, which adds greater uncer- tainty for urban tree managers already dealing with climatic uncertainty (Bennett and Gratton 2012; Meineke et al. 2013). Climate change will affect urban forests and street trees and while there will be adverse effects, there will also be benefits (Grace et al. 2002; Salzer et al. 2009; Yang 2009; Nathan et al. 2011; Ken- dal and McDonnell 2014; Kendal et al. 2014; Brandt et al. 2016; Pretzsch et al. 2017). The challenge for ©2020 International Society of Arboriculture
March 2020
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