When Green Infrastructure Turns Grey: Plant Water Stress as a Consequence of Overdesign in a Tree Trench System

Abstract

Green infrastructure (GI) systems are often overdesigned. This may be a byproduct of static sizing (e.g., accounting for a design storm’s runoff volume but not exfiltration rates) or may be deliberate (e.g., buffering against performance loss through time). In tree trenches and other GI systems that require stormwater to accumulate in an infiltration bed before it contacts the planting medium, overdesign could reduce plant water availability significantly. This study investigated the hydrological dynamics and water relations of an overdesigned tree trench system and identified factors contributing to, compounding, and mitigating the risk of plant stress. Water in the infiltration bed reached soil pits only once in three years, with that event occurring during a hydrant release. Moreover, minimal water was retained in soil pits during the event due to the hydraulic properties of the soil media. Through a growing season, one of the two tree types frequently experienced water stress, while the other did so only rarely. These contrasting responses can likely be attributed to roots being largely confined to the soil pits vs. reaching a deeper water source, respectively. Results of this study demonstrate that, in systems where soil pits are embedded in infiltration beds, overdesign can raise the storm size required for water to reach the soil media, reducing plant water availability between storms, and ultimately inducing physiological stress.

Supporting Growth and Transpiration of Newly Planted Street Trees With Passive Irrigation Systems

“…Although we cannot separate the effects of passive irrigation and change in substrate, the most likely reason trees in drained pits did not sustain greater growth was due to the trees “outgrowing” their small pit of imported biofilter sand. Hence, we suggest passive irrigation systems that are larger pits or a continuous trench and therefore replace a greater volume of native soil, with high infiltration substrates (particularly when the native soil is clay dominated) may be preferable to small pits with regard to sustaining greater annual growth rates for longer (Jim, 2019) and retaining greater volumes of stormwater (Tu et al., 2020).…”

“…The annual growth rates of trees in drained pits decreased from +44% year −1 in the first 2 years, to +18% year −1 in the third and fourth years, which was similar to the growth rates of trees adjacent to pits and control trees throughout the study. The decrease could be attributable to a change in the timing and volume of runoff received by passively irrigated pits (Taguchi et al., 2020; Tu et al., 2020). For example, July (winter) rainfall at our site from the third year was half that of the first 2 years (BoM, 2020), which may reduce growth rates in A. campestre (Rozas & Olano, 2017).…”

“…The impact of passive irrigation systems on tree growth is highly variable (Caplan et al., 2019; Scharenbroch et al., 2016; Szota et al., 2019). To support tree growth and transpiration, passive irrigation systems must ensure trees have adequate soil moisture between rainfall events (Tu et al., 2020), while limiting waterlogging (Grey et al., 2018a). To minimise the risk of waterlogging, designs can include an underdrain, or trees can be planted adjacent to stormwater infiltration pits (Grey et al., 2018a; Szota et al., 2019; Thom et al., 2020).…”

“…To minimise the risk of waterlogging, designs can include an underdrain, or trees can be planted adjacent to stormwater infiltration pits (Grey et al, 2018a;Szota et al, 2019;. However, such designs may limit soil moisture between rainfall events (Caplan et al, 2019;Tu et al, 2020). When waterlogging was avoided, Grey et al (2018a) demonstrated that passive irrigation systems can double the growth of newly planted trees, relative to control trees in the first 2 years of establishment.…”

Implications of Distinct Methodological Interpretations and Runoff Coefficient Usage for Rational Method Predictions

“…This figure shows examples of bioretention basins in Western Australia, which tend to be chronically oversized (Urbaqua et al 2018). For this kind of infrastructure, when insufficient water is available to vegetation, oversizing can result in a loss of amenity, reduced water treatment performance (Blecken et al 2008), reduced ecological benefit associated, for example, with poor vegetation growth (Brown et al 2015;Tu et al 2020), increased maintenance burdens, and increased imposition on public open space (which is often sacrificed to provide green infrastructure). High maintenance burdens can fall on residents, resulting in an active disservice (Keeler et al 2019).…”