Research articlePerformance evaluation of five Mediterranean species to optimize ecosystem services of green roofs under water-limited conditions
Introduction
The rapid increase of urban areas is considered one of the drivers of global climate change, with an important negative environmental impact. A lack of green infrastructures and an increase in impervious surfaces usually characterize urban areas, and as a consequence, cities are affected by the heat island effect, environmental pollution and altered hydrology that adversely affect quality of life and human health (Carter and Rasmussen, 2005, Jarrett et al., 2007). Green infrastructures can attenuate those negative impacts since they are able to provide a wide range of supporting, cultural and ecosystem-regulating services (Bendell et al., 2010, Hien et al., 2003, Peng and Jim, 2013). Among the services, the implementation of extensive green roofs has been generally recognized as a practice that provides several ecosystem services in urban areas, including atmospheric and acoustic pollution reduction (Veisten et al., 2012, Yang et al., 2008), stormwater management (Fioretti et al., 2010, Mentens et al., 2006, Palla et al., 2008, Schroll et al., 2011, Wolf and Lundholm, 2008), thermal regulation (Butler and Orians, 2011, Chen et al., 2009, Getter et al., 2011, Hien et al., 2003, Lundholm et al., 2010, Peng and Jim, 2013, Santamouris et al., 2007, Sharma et al., 2016) carbon sequestration (Getter et al., 2009, Whittinghill et al., 2014), biodiversity improvement (Kadas, 2006, Dvorak and Volder, 2013, Oberndorfer et al., 2007) (Clemants et al., 2006, Dvorak and Volder, 2013, Oberndorfer et al., 2007) and esthetic and social benefits (Fernandez-Cañero et al., 2013, Jungels et al., 2013).
Extensive green roof infrastructure was quickly embraced in European and North American cities during the second half of the 20th Century due to the synergy between the abovementioned ecosystem and social services (Oberndorfer et al., 2007). However, in most semi-arid and Mediterranean-climate areas long summers and water scarcity limit green roofs adoption. In the sense used here, the Mediterranean climate is characterized by hot, dry summers and cool winters, a combination that limits perennial plant performance of C4 and CAM drought-adapted species due to relatively low winter temperatures (Toscano et al., 2015), and of C3 species due to summer drought (Bevilacqua et al., 2016, Durhman et al., 2007, Price, 2013). Moreover, plant selection is also constrained by the invasive ability of many ornamental and exotic species (Gritti et al., 2006), suggesting the importance of evaluating the performance of native species and focus in plant selection, since plant species largely determine the functionality and efficiency of extensive green roofs (Lundholm et al., 2010, Vijayaraghavan, 2016).
In a global scenario in which water resources are scarce, sustainable water management appears to be one of the most challenging issues in the near future. Direct human consumption, industry and irrigation of both agricultural fields and urban green areas compete for water resources, significantly limiting water availability for urban green areas, since this is usually considered a low-priority use. This situation is even more dramatic in semi-arid and Mediterranean-climate areas (Van Mechelen et al., 2015), suggesting the relevance of providing accurate data regarding the water requirements of different species to be used in urban green areas (García-Navarro et al., 2004). Sedum species are commonly used in green roof implementation due to the drought tolerance and high ground cover ability under water-limited conditions conferred by their leaf morphology, photosynthetic metabolism and high water-use efficiency (Blanusa et al., 2013, Monterusso et al., 2005).
In addition to water limitation during summer, oversaturation of urban drainage systems during the rainy season has been identified as a problem in many Mediterranean cities. The reduction and delay of runoff are considered potential green roof services (Fioretti et al., 2010, Mentens et al., 2006, Palla et al., 2008, Jefferies and Doménech, 2015). However, that service would be more efficiently performed if species with high water consumption abilities were used in green roof implementation suggesting a trade-off among vegetation functional goals (Benvenuti and Bacci, 2010, Blanusa et al., 2013, MacIvor and Lundholm, 2011, MacIvor et al., 2013, MacIvor and Lundholm, 2011, Eksi et al., 2017, Schweitzer and Erell, 2014), since those species that are able to consume large amounts of water usually show low performance under water-limited conditions (Durhman et al., 2007). Nevertheless, some herbs, grasses and geophytes, usually high water consumers, have reported high survival rates and optimal recovery after a drought period. (Van Mechelen et al., 2015, Szota et al., 2017). Those species have shown a number of responses to water stress such as root growth increase, leaf size reduction, high stomatal response and isohydric behavior (Farrell et al., 2013, MacIvor et al., 2013, MacIvor and Lundholm, 2011, Nagase and Dunnett, 2010). However, in addition to plant species traits, the irrigation regime and the substrate characteristics would also determine to a great extent the ability of a green roof system to reduce stormwater runoff and delay peak flows (Fioretti et al., 2010, Mentens et al., 2006, Palla et al., 2008, Wolf and Lundholm, 2008).
The esthetic value is also recognized as an ecosystem benefit provided by green roofs (Bendell et al., 2010, Fernandez-Cañero et al., 2013), though it is rarely emphasized, and very little information is known about this benefit. Several studies have suggested that future research on plant selection should consider and better quantify plant architecture and form, flowering length, color and greening variation during the year and biomass production, and how water supply and other management techniques affect vegetation and green roof performance (Benvenuti and Bacci, 2010, Dunnett et al., 2008, Durhman et al., 2007, Emilsson, 2008, Fernandez-Cañero et al., 2013, Lundholm et al., 2010, MacIvor et al., 2013, Monterusso et al., 2005, Nagase and Dunnett, 2010).
In this context, identifying species able to withstand drought with an optimal appearance and growth but also capable of achieving high water consumption during the rainy season in order to enhance runoff reduction would greatly improve green roof functionality in Mediterranean-climate areas. The present experimental and long-term study aims to study the performance of 5 Mediterranean species in experimental extensive green roofs under two different water regimes. The following specific objectives are addressed: i) to study the ability of 5 Mediterranean species to cope with water limited condition by determining plant cover, biomass accumulation, relative appearance and coloration; and ii) to determine the water use of those species in order to evaluate their potential uses in stormwater management and estimate their water-use efficiency under extensive green roof conditions.
Section snippets
Plant material and experimental design
The experiment was performed at the University of the Balearic Islands, Spain, (West Mediterranean Basin, 39°38′ N, 2°38′ E) at 80 m a.s.l., where the mean annual rainfall is 427 mm and the annual mean temperature is 17 °C (AEMET, 2011).
Five Mediterranean species, Asteriscus maritimus L. Less., Brachypodium phenicoides (L.) Roem. Et schultes, Crithmum maritmun L., Limonium virgatum (Willd.) Fourr, Sedum sediforme (jacq.) Pau and Sporobolus pungens (schreber) kunth (Table 1) were considered in
Results
Meteorological (temperature and precipitation) data during the experimental period were recorded daily, and the averages are shown in Fig. 1. The study period showed typical Mediterranean conditions with dry and hot summers, cool and wet winters, and a highly irregular precipitation pattern (Fig. 1).
Discussion
In this work, we assessed the performance of five Mediterranean species under experimental extensive green roof conditions throughout two years. We focused on the contribution of each species to three common ecosystem and social services – esthetic interest, stormwater management and carbon sequestration – as well as on the effect of water availability on plant performance. Sedum sediforme, a species commonly used in extensive green roof implementation and native to the Mediterranean Basin,
Conclusions
In conclusion, the present study shows that in addition to S. sediforme, the species that altogether performed the best, there are a number of Mediterranean species that can be considered in the implementation of extensive green roofs. Moreover, succulent CAM or CAM-facultative species have evidenced a good adaptation to extensive green roof conditions under semi-arid and Mediterranean conditions. However, this work shows that there are other plant adaptations to drought that can provide a good
Acknowledgments
This work is part of the project INNPACTO (IPT 2011-1017-310000), funded by the Spanish Government. First author was granted by the Agencia Española de Cooperación Internacional para el desarrollo (AECID).
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