Using stormwater to promote the growth and transpiration of street trees

© 2021 Jasmine Krystel ThomCreation of cities leads to extensive impervious surfaces as part of the built environment, which alter the natural hydrological cycle and local climate. Rainfall on impervious surfaces can generate large volumes of stormwater runoff which degrades receiving waterways, while also reducing the volume of rainfall infiltrated to urban soils. Trees provide numerous benefits, which may mitigate these negative impacts of impervious surfaces. However, growing conditions may lead to drought stress that limits the benefits trees can provide, such as cooling. Irrigation can support the growth and transpiration of trees, two processes that are intrinsically linked to their benefits. Directing excess stormwater runoff to street trees (passive irrigation) therefore has the potential to support transpiration and growth, while reducing the volume of stormwater runoff received by downstream waterways. This thesis quantified growth and transpiration of trees to assess whether passive irrigation systems can be designed to i) support transpiration and growth, and ii) reduce the volume of stormwater received by downstream waterways through transpiration losses and species selection. To do this, growth and transpiration of establishing and established trees were assessed for several system designs in the field, and the transpiration behaviours of young trees were measured in a glasshouse experiment. Further, key factors influencing the water balance of passive irrigation systems to support future design and species selection were investigated. Drained passive irrigation systems doubled the growth of establishing trees relative to control trees in the first two years, which meant they transpired twice the volume of water. Higher growth rates were not sustained in years three and four, likely because the trees outgrew the small volume of growing media. Conversely, passive irrigation systems had no impact on the transpiration of established trees, likely because they already had access to sufficient water resources. Hence, when passive irrigation systems support greater growth and water availability, they can support higher transpiration, thereby providing greater transpiration-related ecosystems services, such as urban cooling and stormwater runoff reduction. Designing passive irrigation systems with a larger volume of growing media may sustain greater growth rates and associated benefits for longer. The potential contribution of transpiration to the reduction of stormwater volumes depended on canopy and catchment sizes, species selection, and rainfall. For established trees in the field, transpiration was equivalent to 17% of annual runoff, much greater than typically reported for other vegetation types. The glasshouse experiment identified several species that had the potential to maximise transpiration losses from passive irrigation systems and improve stormwater outcomes by balancing moderate to high transpiration, moderate drought avoidance, and high transpiration recovery. Several other critical factors that should be considered in the design of passive irrigation systems include internal water storage, drainage, and soil volume. This thesis demonstrates that passive irrigation systems have the potential to improve urban greening and stormwater management outcomes by establishing greater canopy cover, leading to greater transpiration, and reduced volumes of runoff received by downstream waterways. Delivering on this potential depends on good design, which takes into account the impacts of species selection, canopy area, catchment area, and design considerations influencing the soil moisture regime