Arboriculture & Urban Forestry 47(2): March 2021 coarse and fine, fibrous root structures were evaluated for their effects on hydraulic conductivity aided by root penetration. Experiment 1 and the structural soil study both showed that tree roots can improve hydraulic conduc- tivity in compacted soils. Roots were found to have penetrated compacted soils, but it is important to note that the conditions of the soil were generally moist before measurements. This tended to have implications on soil strength, and root growth would be enhanced for species tolerant of moist conditions. Therefore, the outcomes may vary with tree species dependent on their tolerance of inundation (Ow and Ghosh 2017a). Future research should attempt to understand root colonisation and penetration capabilities in dry soils and in actual field environments across varying spe- cies and rooting types (e.g., woody vs. fibrous, etc.). However, it was clear from the data from Experiment 1 that trees with coarse and woody root systems gave rise to higher hydraulic conductivity when compared to those with fibrous roots. By contrast, the compari- son between woody roots and controls (without trees) indicated a twofold difference in hydraulic conduc- tivity. In addition, the magnitude of change was simi- lar across both compaction levels. This ran counter to our expectations, as the higher compaction levels were assumed to have affected drainage more exten- sively given the smaller pore sizes within the soil. This absence of change may reflect the need for a wider selection between bulk densities for differences to show. By contrast, differences were observed in Bartens et al. (2008) in which similar bulk density values were tested. The species and soil types used, however, were different. In addition, a longer experi- mental duration to facilitate root growth and varying root architecture may also be factors that will impact infiltration through compacted soils (Jin et al. 2017). Although there were some improvements observed in hydraulic conductivity across the 12-month period, the magnitude of improvement was limited. Values ranged between 7% to 13% for the lower level of compaction and approximately 3% to 9 % for the higher level of compaction. The small improvements may partly be explained by the short experimental duration, soil type, and root architecture (Tron et al. 2015; Ow and Ghosh 2017a; Zaibon et al. 2017). Nonetheless, as with hydraulic conductivity, root growth observa- tions (upon harvesting), alongside that of infiltration improvements over time, indicated a strong species 81 effect, whereby the species that possessed woody and coarse root systems exhibited significantly higher hydraulic conductivity. More importantly, such out- comes can provide urban municipalities and urban arborists with informed decisions on species choice when mitigating the ill effects of compacted soils in relation to drainage. Furthermore, the results also provide insights into the importance of an urban for- est in highly urbanised cities to mitigate the effects of urban surface runoff that has been exacerbated due to changing climatic conditions (Xiao and McPherson 2003; Armson et al. 2013). In the trial involving structural soils, Ksat for con- tainers without trees was found to be 50% less effi- cient in drainage. Noteworthy also was that the trees that grew in structural soil were approximately 30% larger than those in Experiment 1 (mean stem diameter and height were 6 cm and 2 m at harvest, respec- tively). Apart from confirming that all roots had grown through the structural soil and penetrated the geotex- tile, the number and size of roots in the structural soil study were approximately 12% to 20% greater than those observed in Experiment 1 (Figure 4c). These data, alongside visual observations, were supportive of an ideal growing condition provided by the structural soils for tree roots that facilitated lateral and down- ward penetration of roots. The ability for tree roots to penetrate compacted soil and increase hydraulic conductivity will, in turn, reduce surface runoff, enhance groundwater recharge, and improve water quality (Bartens et al. 2008). Sim- ilarly, we would expect species adapted to wet and dry soils to grow best in urban stormwater systems with structural soils, given the high porosity rates and reservoir features of these systems (Mullaney et al. 2015). Also important to note is the nutrient content of soils and the depth at which the nutrients are located. For example, Sæbø and Ferrini (2006) and Yang et al. (2014) have recorded that root penetration was reduced when nutrients were located in the top layers of the soil. However, more research into this area will be required to validate these findings. It is also worth noting that the effect of root devel- opment and drainage would also depend on other fac- tors, such as the level of the water table (Imada et al. 2008). Additionally, future work should also focus on the geotextile used in urban stormwater systems. Although the geotextile was found to be in a good condition in this study (e.g., unclogged), such material ©2021 International Society of Arboriculture
March 2021
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