ABSTRACT
Globally, 54 per cent of the world’s population reside in urban areas. The potential of trees to mitigate the urban landscape in ways that are beneficial to the physiological and psychological health of the human population are now widely recognized (Konijnendijk et al., 2005; Nowak et al., 2014). Consequently, great expense is spent annually on the creation of urban treescapes worldwide. Trees planted and growing in urban landscapes are, however, routinely exposed to a range of environmental stresses that pose a serious threat to their biology. Prolonged drought, root deoxygenation (waterlogging, soil compaction), salt contamination of soil, atmospheric pollutants and high temperature episodes, singly and in combination can negatively impact tree growth, development and distribution (Hirons and Percival, 2012). Of concern is the fact that climate studies have shown significant summer warming on an annual basis and a trend towards an increased frequency of summer drought events throughout Europe (Schaer et al., 2004; Seneviratne et al., 2010). Moreover, climate models predict a stronger inter- and intra-annual weather variability, which will cause an increased risk of extreme heatwave, drought and precipitation events. Consequently an understanding of how urban trees tolerate and adapt to harsh environmental conditions is necessary in order to sustain ecosystem services (Pincetl et al., 2013).
