Arboriculture & Urban Forestry 39(3): May 2013 mates (Kozlowski and Pallardy 1997; Hawkins et al. 2008). Some recent models applying a global temperature increase of 2°–3°C are projecting that over the next century, up to 50% of vascu- lar plant species could be threatened with extinction (Bramwell 2007). This risk should also be contemplated for urban forests. In Victoria, climate change over the coming decades is anticipated to result in increased temperatures; drier conditions and increased frequency in severe events, such as extreme rainfall, bushfires, and droughts (CSIRO 2008), and most of these events are also common to the rest of southeast Australia (Suppiah et al. 2006; CSIRO 2008; CSIRO 2010). For Melbourne, it is expected that by 2070 under a lower greenhouse gas emission growth sce- nario, it will be 1.3°C warmer with 6% less rain, while under a higher greenhouse gas emission growth scenario it will be 2.6°C warmer with 11% less rain (out of a range of -6% to -24%) (CSIRO 2008). The greatest increases in temperature are expected during summer, while the greatest rainfall reductions are projected during winter to spring, from -11% to -21% respectively (CSIRO 2008). The potential impact of reduced rainfall on urban vegetation is well illustrated by considering the cumulative deficiency in rainfall, relative to long-term averages, over extended dry periods (Figure 1). Melbourne’s mean annual average temperature is 15°C. Many trees species grown in Melbourne (e.g., Acer, Betula, Platanus, Prunus, Quercus, and Ulmus) commonly occur in cities around the 117 temperatures. The influence of the urban heat island effect, increased thermal mass and reduced surface permeability of urban sites will contribute to temperature extremes (Coutts et al. 2007). Rainfall is projected to change, in Melbourne region, by 2070 with average reductions of 11% to 21%, for winter and spring respectively (CSIRO 2008). This can impact on the volume of stormwater harvested for irrigation purposes. There appears to be an amplification relationship between rainfall reductions and runoff of up to 1:3 (Howe et al. 2005). For example a 21% reduction in winter rainfall may translate into a 63% reduction of stormwater flow. Or in another case, the projected 7% reduction in summer rainfall may return a 21% reduction in stormwater harvest at a time of year when it is most needed (CSIRO 2008). Figure 1. Cumulative monthly rainfall anomalies for RBG Mel- bourne January 1997 to July 2012. Note: This shows a cumulative trend of monthly rainfall anomalies compared to average monthly values from 1997 to 2012. There was steady decline during what is known as the Millennium Drought in Australia, from 1997 to early 2010. The cessation of the drought followed two La Niña events (often result in above average rainfall for eastern Australia) dur- ing 2010–2012, but these were not adequate to return the status to an equilibrium, even though the 2010–2011 La Niña event was unprecedented in its high strength and high amounts of rainfall since records began in Australia. world, with mean annual temperatures ranging from about 10°C to 13°C (Kendal 2011). It is conceivable that some of these taxa are already experiencing significant heat stress, particularly with summer extreme temperatures. It is likely that an overall increase in annual average temperature by 1°–3°C (notwithstanding tem- perature extremes compounded by urban heat island effects will place many of these species outside their viable cultivation range. The impact of climate change and urbanization is likely to expose some plantings, for example street trees, to elevated Tree and Landscape Microclimate Microclimate mapping within the landscape is one approach that can assist with informed tree selection and the development of urban forests. This includes establishing the characteristics of both the edaphic (soil) and atmospheric environments throughout the year. For example, the edaphic environment for a deciduous arboretum will likely contain a higher moisture status during the tree’s dormancy. The converse may occur during the tree’s active growth period. Microclimate mapping is useful for establishing generic zones within the landscape. Yet, there is still even greater variation involved, even at small units of area. To illustrate, the study of the amount of rain penetrating through overhead tree canopy (throughfall) and corresponding soil moisture levels in the RBG Melbourne has revealed significant variation even at sub-meter spacing. In natural habitats, plants would only typi- cally establish in niches suited to their recruitment and growth. However, in contrived landscapes, the establishment period and planting site is often chosen to match amenity and functional criteria and this may not be the best match to the environmental conditions. Seasonal soil moisture or the soil water balance is one of the critical factors, and researchers need to develop and improve specific ap- proaches of examining and monitoring point levels of soil moisture in respective landscape zones. This can be achieved in a technologically advanced way by using soil moisture sensors, or by physically exam- ining soil cores or excavating pits, to compare moisture status against standardized methods. While this can be effective, it is usually less practical and more resource intensive, especially when regularly sur- veying sites across an entire urban forest planting. A simple matrix can be generated using variables—such as sun–shade, dry–moist, or cool–warm—to classify and map areas within the urban forest, to then guide tree selection and planning for the future (Figure 2). Figure 2. Simple microclimate matrix. Note: Plant selection for the RBG is becoming more focused towards the dry/warm quad- rant of the matrix. Some trees from natural habitats in the moist/ cool quadrant, such as wet montane forests of southeastern Australia, are already showing signs of stress and some have been removed due to irreversible decline. ©2013 International Society of Arboriculture
May 2013
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