142 May et al.: Managing and Monitoring Tree Health and Soil During Extreme Drought for the elevated soil moisture content below and around the drip line. Across all eight study sites, the operation of drip irrigation for two weeks in August 2009 wetted the soil to at least the depth of the subsoil, and in most cases some way into the subsoil. The volume of wetted soil varied from site to site but ranged between 0.5 and 1.4 m3 soil m-1 drip line. The irrigated zone extended to depths of up to 800 mm and distances of up to 1 m on either side of the drip line. Between 30% and 90% of the water irrigated during the two-week period could be accounted for as stored in the wetted soil volume. By contrast, un-irrigated soils at most parkland sites had no available water within their topsoil, except for a shallow (0–25 mm) surface layer wetted by recent rainfall. This study showed that drip lines typically wetted a horizontal column of soil along the line and suggested that winter drip ir- rigation can be used in dry, or below-average rainfall winters to help recharge soils to alleviate tree water stress, encourage appro- priate fine root development and provide a resource for tree water use in the coming spring. The volume of soil that can be effec- tively wetted through this approach is as yet unknown, but longer run times or staggered run time schedules may be able to wet larger volumes of the tree root zone. Obviously, installing mul- tiple drip lines would also enable wetting of a larger soil volume. DISCUSSION During the drought period from 1997 to 2010, many trees in the streets and parks of Melbourne suffered health declines and in some cases death. The health and survival of European elms and London planes within the City of Melbourne are of particular interest because they are such an important element of the city landscape. The causes of tree health decline are not completely understood and may vary with species; however, the extended period of drought and associated restrictions on tree irrigation are undoubtedly major contributors. Extended tree water stress is recognized as one of the most common contributors to tree mortality, but tree mortality is often multi-factorial in nature (McDowell et al. 2008). The effects of an elm leaf beetle infestation during this period of drought undoubtedly added another level of stress to elms and contributed to mortality levels (Kuhlman 1971). There is a commonly held view that the years of sprinkler ir- rigating parkland has led to the development of trees with shallow root systems that are subsequently more vulnerable to water stress when irrigation is reduced or restricted. However, not all parkland tree species in Melbourne experienced a decline in health during the drought or in response to irrigation restrictions. Figure 4 and Table 2 show the health status of trees surveyed in The Domain. The trees shown in the table are those species where there were more than 15 specimens present. This data clearly shows that many of the temperate zone species are in poorer health than most of the Australian native trees or trees from other drier regions. In fact, it may rather be the case that years of lawn sprinkler irrigation allowed the continued growth and survival of species that have become, or always were, marginal under a Melbourne climate. The process of tree health decline, where trees gradually lose canopy volume (leaf thinning followed by branch death and even- tually tree death), has been described many times. Various contrib- utory factors can include drought, acid rain, disease, insect pests, changed soil physical conditions, and root loss. An assessment of the stages of a tree decline provided by Heatwole and Low- man (1986) state that if a tree’s energy resources are exhausted by epicormic shoot growth in an unsuccessful attempt to replace ©2013 International Society of Arboriculture crown loss, epicormic growth then ceases and the tree eventually dies. Melbourne canopy health surveys employed in this study use a similar series of stages to categorize tree condition. Surpris- ingly, the mechanisms of drought-induced tree health decline are not universally accepted and debate continues as to the dominant mechanisms involved. McDowell et al. (2008), in a review of drought and plant death, stated that drought-induced tree injury or mortality had two possible mechanisms. In one mechanism, trees ultimately perish as a result of “hydraulic failure” and desicca- tion, and in the other they perish through sustained “carbon star- vation,” whereby carbohydrate reserves are exhausted by ongo- ing metabolism and respiration demands that are not adequately replenished by photosynthesis because of stomatal closure from associated water stress (Waring 1987; McDowell et al. 2008). Regardless, it is apparent that tree water stress plays a role in both scenarios as tree health declines towards mortality, and the domi- nant mechanism probably varies according to the species, plant functional group, and their suite of stress adaptation strategies. For example, more drought tolerant species, able to maintain low levels of carbon assimilation, may be more likely to suffer “hydraulic failure” where soil moisture availability (or atmo- spheric vapor pressure deficit) drops so low that the continuum of water between the soil, roots, stem, and canopy is broken, result- ing in the death of crown tissues. However, as summarized by Mc- Dowell et al. (2008), “our current understanding of the causes of tree mortality is surprisingly limited, even though a rich literature exists on plant responses to stress. Essentially, we cannot address questions such as: how severe must a drought be to kill a tree; and during drought, which trees will die and which will survive?” In the 1997–2010 drought, the soil moisture condi- tions presented in this paper and the heat wave temperatures experienced in January 2009 can be considered as a foretaste of future climate change conditions. The environmental conditions that the trees in Melbourne experienced, as a result of drought and water restrictions, and the efficacy of subsequent manage- ment interventions, need to be assessed, considered, and dis- cussed to inform future urban greenspace management. The development of tree and green space management strategies for drought preparation and response should be central to any city’s overall climate change adaptation strategy and should consider some of the following issues and management options. Plant Selection The recent drought in Melbourne resulted in several con- secutive years where rainfall was reduced to two-thirds of the long-term average, which adversely affected some species more than others, with temperate deciduous species in particu- lar being badly affected. Tree managers should be consider- ing the species composition of their tree population renewal programs to accommodate the possibility that extreme and extended drought events become more common in the future. While tree population diversity is regarded as desirable (Muller and Bornstein 2010), diversity reflecting increased toler- ance of environmental stresses is rarely specifically addressed. Trees that are most likely to be successful under the envi- ronmental conditions forecast under climate change will pos- sess physiological attributes that endow both tolerance of water stress and heat stress (Moore 2011). Potentially useful species may be found in examination of published tree lists from other regions, wider ranges of provenance for species with
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