Water use and drought responses of eight native herbaceous perennials for living wall systems
Graphical abstract
Introduction
Green infrastructure can mitigate some of the negative ecological and social effects of urbanization (Fischer et al., 2018). Green roofs and green walls have been especially popular as they can be introduced in already built-up areas as an integrated part of buildings, thereby increasing the green cover without restricting further development. While green roofs have become widespread, vertical greenery technology is still underexploited despite many ecosystem services it offers (Pérez-Urrestarazu et al., 2015; Bustami et al., 2018; Larcher et al., 2018). A possible explanation is that compared to the typical horizontal green roofs, green walls receive less precipitation, and therefore depend more on irrigation systems, making the whole construction more complex and costly (Perini and Rosasco, 2013; Riley, 2017).
Green wall systems are sometimes expected to contribute to urban stormwater management systems by serving retention and detention functions or to rely primarily or exclusively on stormwater through temporary storage of runoff, both of which can be difficult to achieve (Ostendorf et al., 2011). This is because it is difficult to design substrates and modules that can hold enough water for plant survival during rainless periods without compromising the temporary storage and retention effect of the system (Riley, 2017; Palermo and Turco, 2020). Additionally, the wide spectrum of living wall designs create variable conditions for plant growth depending on the used technology and growth substrates (Manso and Castro-Gomes, 2015; Medl et al., 2017; Brković Dodig et al., 2019). These are usually combined in a unique growth system, as indicated by Segovia-Cardozo et al. (2019), which makes water issues on living walls even more challenging.
When considering the global decline of native species caused by loss of habitats due to urbanization (Aronson et al., 2014), green walls can offer a perfect opportunity to serve as habitats for invertebrates and support urban-dwelling birds (Chiquet et al., 2013), and thus contribute to a diverse urban green infrastructure (Collins et al., 2017). Although there is a growing body of empirical evidence on native species that can thrive on green walls (Collins et al., 2017; Fernández-Cañero et al., 2018), the plant selection in practice is still limited and chosen without regard to biodiversity (Riley, 2017). These aspects led the idea of this paper, which is to identify native species that can thrive in the vertical position and are tolerant to the amplitudes in precipitation.
In order to withstand harsh conditions that involve angled rooting and restricted growth space (Jørgensen et al., 2014a, 2014b) the plants need to possess traits necessary to survive extended periods without water. Plants use several drought-survival mechanisms, including short-term physiological responses such as stomatal closure (Tradieu, 2013; Martinez-Vilalta and Garcia-Forner, 2017) or long-term plant growth adaptations such as decreases in shoot and leaf growth (Chenu et al., 2008), changes in leaf shape, orientation and visual appearance (flowering, wilting or leaf burn) (Tradieu, 2013). Water storage in roots also plays a role in plant recovery during drought periods: traits such as succulence of roots, leaves or stems may indicate better drought resistance (Tradieu, 2013; Jørgensen, 2014; Farrell et al., 2017). Nagase and Dunnett’s (2011) drought test of a variety of species from similar habitats on shallow green roof substrates shows that species survival might depend more on a plant’s morphology and size. This work builds on their (Nagase and Dunnett, 2010) observation that shallow-taproot species such as Armeria can adapt to thinner substrate layers and be more drought-tolerant than deep taprooted species or forbs with shallow and spreading fibrous roots. Similar studies of prolonged drought effects on plant survival and recovery in living wall systems are needed (Medl et al., 2018), especially when looking at the development of emerging stormwater reuse technologies, where drought might become a critical factor when establishing well-functioning systems (Palermo and Turco, 2020).
Winter-hardy, herbaceous perennials from ruderal communities on roadsides and in dry meadows may represent a good species source, as this group has proved to have high transpiration and simultaneously good drought resilience on green roofs (Farrell et al., 2017). Only a limited number of studies have investigated the growth of perennials on living walls (Ghazalli et al., 2019). The conclusions are that only a subset of perennials can be placed on walls, and an even smaller subset conforms with expectations regarding visual and ornamental quality (Mårtensson et al., 2014; Ghazalli et al., 2019). Furthermore, the studies on water use of species on green walls are also lacking. It has been suggested that the growth medium properties have an influence on the plant water uptake (Perez Urrestarazu et al., 2014; Jørgensen et al., 2018). Yet, Prodanovic et al. (2019) tested five species to establish that the plant seasonal activity was a factor in varying transpiration, but also observed differences between plant species placed in the same substrate, which indicates that in similar conditions, plant properties impact the water requirements. None of those studies focused on establishing which plant traits are important regarding water use of species in living walls.
The objective of this study was to identify and test plant species suitable for growth on vertical rain fed green wall systems, under different water conditions. We compared responses of eight native herbaceous perennials from Scandinavian climate to water conditions that mimicked well-watered conditions after heavy rainfall (maximum water capacity) followed by drought periods of one, two, four and six weeks. The plants were tested under greenhouse conditions in a vertical, mineral wool-based setup. Our specific objectives were to (1) assess the water use of species; (2) examine drought responses of species; and (3) determine which functional plant traits are most relevant to plant choice for rain-fed living walls.
Section snippets
Plant material
Eight winter-hardy herbaceous perennials, all native to Denmark, with a size between 30–80 cm in mature form, and typical of the sun-exposed and dry habitat expected on a green wall, were selected to represent specific traits (Table 1): approximate size; root type; and leaf type and size. Plants were purchased as plug plants with plug size 4 cm by 8 cm from a plant nursery in southern Sweden (Pratensis). These specific plants will be referred to as ‘replicates’, in order to distinguish the
Mean water use
Mean ET rate for all species was 837 g H2O m-2 d-1. However, it varied by up to 30 % among species and regimes (Table 2). Significantly higher ET rates as compared to the overall mean were registered for L. vulgare (1099 g H2O m-2d-1), K. arvensis (923 g H2O m-2d-1) and H. perforatum (935,59 g H2O m-2d-1), while significantly lower rates were registered for A. maritima (698,17 g H2O m-2d-1) and S. granulata (713,52 g H2O m-2d-1). No clear pattern could be seen for K. arvensis. The remaining
Water use strategies
This paper presents insights into the ability of ruderal, herbaceous perennials to withstand living wall conditions and provide functions that support urban stormwater systems: a high evapotranspiration rate when water is abundant (to remove water quickly) and low water consumption in periods of water shortage. Even though the analysed species are found in similar habitats, e.g. roadsides, their water use strategies are rather different (Fig. 6).
The general trend is that species with higher
Conclusions
By analyzing water use strategies and responses to drought, we determined that it is possible to establish stormwater living walls that can thrive by relying on rainfall with a summer drought period of up to three weeks. The species recommended for their high evapotranspiration rates and high-water status during drought are Knautia arvensis and Geranium sanguineum. In terms of drought resilience, the results indicate that Armeria maritima, Campanula persicifolia and Saxifraga granulata can
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The study has been funded by Københavns Universitet and Ministry of Environment and Food of Denmark (program MUDP); Grant number: NST-404-00177.
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