Arboriculture & Urban Forestry 47(2): March 2021 In rural and forested environments, rainwater is stored within the forests and wetlands and then slowly infiltrates into the ground. By contrast, urban infra- structures such as buildings, roads, and footpaths channel water very quickly away as runoff with min- imal water flowing into unpaved soils (Pitt et al. 2008). This is expected to be exacerbated by increased rainfall intensities and rainfall volumes resulting from climate change (Mullaney et al. 2015). There is also the problem of waste and pollution transported by stormwater that poses quality issues (Barbosa et al. 2012). Also important is the need for sustainable yet flexible urban stormwater management approaches— for example, poorly maintained SuDS can pose a flood risk themselves (Annicka et al. 2013). SuDS are built to mimic nature and manage localised precipita- tion. SuDS can be more sustainable than traditional drainage methods and can be built and designed to carry and slow down runoff before it reaches the city’s drainage network. In addition, they store water while allowing some to infiltrate into the ground, while more water can be evaporated or lost via vege- tation through evapotranspiration (Grey et al. 2018). SuDs can therefore be regarded as environmentally beneficial drainage systems that urban cities can use to efficiently and sustainably drain surface water while reducing pollution and improving the water quality of local water bodies (Nowak et al. 2008). Urban Trees in Stormwater Management In urban environments, urban forests are an effective stormwater management strategy where precipitation is intercepted by the canopy. Likewise, trees direct the flow of water into the ground through trunk flow (Armson et al. 2013), and roots absorb stormwater (Bartens et al. 2009). For example, in a report by Xiao and McPherson (2003) it was concluded that individual tree canopies can intercept approximately 80% of a 24-hour rainfall (20 mm) under full foliage conditions (Xiao and McPherson 2003). However, the urban conditions may not permit such optimum canopy growth given the harsh growing environments (e.g., reduced rooting volume). Likewise, bioswales and rain gardens require frequent maintenance. But the inclu- sion of trees and structural soils under pavement can effectively function in confined, impervious conditions. This is because the structural soil profile normally occupies about 0.6 m of depth and is compacted to meet engineering standards that typically hold streets and car parks (Grabosky and Bassuk 1998). Despite 73 the high levels of compaction, the structural soil pro- file is still able to possess high porosity of approxi- mately 30% to 40% (Grabosky and Bassuk 1995). Hence, structural soil profiles can store stormwater and maintain tree growth alongside root extension. In turn, the improved growth conditions may improve urban runoff through rainfall capture in large, overlapping can- opies even before the rain reaches the ground surface. Roots and Hydraulic Conductivity The flow of water along tree roots has been previously documented in reports by Johnson and Lehmann (2006) and Bejan et al. (2008). In addition, earlier work by Bramley et al. (2003) using containers confirmed that hydraulic conductivity was 15 times faster when trees were present as compared to those without trees. Rosolem et al. (2002) and Clark et al. (2003) also highlighted the importance roots conferred to hydrau- lic conductivity, where water flow was decreased in compacted soils as a result of limited root penetration and smaller roots. Therefore, this study was set up to evaluate the potential of roots to direct the flow of water through compacted soils and how this might increase hydraulic conductivity. Compared to canopy or trunk interception of water, the ability for tree roots to improve infiltration rates has received much less attention, hence, less is known about their benefits, especially in compacted soils, which is an area covered in this study. The intent was to establish an understanding of hydraulic conductiv- ity, not just within, but also alongside the stormwater control systems. Conversely, much more is known about the benefits of structural soil for enhanced root- ing in spite of limited growing spaces (Ow and Ghosh 2017a, 2017b). Therefore, the aim of this study was to use a SuDS reservoir and structural soil to increase the rooting volume for the tree and at the same time examine if the flow of water through compacted soils could be improved via rooting channels. If tree roots were observed to benefit infiltration, the incorporation of trees within bioswales and rain gardens could be regarded as a critical element during the planning and design stages. Therefore, the outcomes of this study will contribute to current as well as future stormwater retention best management practices. The experi- ments were conducted under controlled conditions and they took place concurrently across a 1-year period with the objective of improving our under- standing of the effects of trees and roots on the hydrau- lic conductivity of urban soils. ©2021 International Society of Arboriculture
March 2021
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