Abstract
Recent studies have highlighted the ecological, economic and social benefits assured by green roof technology to urban areas. However, green roofs are very hostile environments for plant growth because of shallow substrate depths, high temperatures and irradiance and wind exposure. This study provides experimental evidence for the importance of accurate selection of plant species and substrates for implementing green roofs in hot and arid regions, like the Mediterranean area. Experiments were performed on two shrub species (Arbutus unedo L. and Salvia officinalis L.) grown in green roof experimental modules with two substrates slightly differing in their water retention properties, as derived from moisture release curves. Physiological measurements were performed on both well-watered and drought-stressed plants. Gas exchange, leaf and xylem water potential and also plant hydraulic conductance were measured at different time intervals following the last irrigation. The substrate type significantly affected water status. Arbutus unedo and S. officinalis showed different hydraulic responses to drought stress, with the former species being substantially isohydric and the latter one anisohydric. Both A. unedo and S. officinalis were found to be suitable species for green roofs in the Mediterranean area. However, our data suggest that appropriate choice of substrate is key to the success of green roof installations in arid environments, especially if anisohydric species are employed.
Highlights
Green roofs are engineered ecosystems designed to favour plant establishment on manufactured layers installed over rooftops, and typically comprise lightweight mineral substrate, drainage and moisture retention layers and a root-resistant waterproofing barrier (VanWoert et al 2005; Berndtsson 2010)
Data recorded in the present study suggest that arbutus plants could overcome intense drought conditions and
The higher water use of the latter species might imply the need of additional irrigation to prevent foliage damage and/or desiccation under prolonged drought
Summary
Green roofs are engineered ecosystems designed to favour plant establishment on manufactured layers installed over rooftops, and typically comprise lightweight mineral substrate, drainage and moisture retention layers and a root-resistant waterproofing barrier (VanWoert et al 2005; Berndtsson 2010). This, in turn, limits the number of species that can thrive over green roofs, especially in hot and arid regions like Mediterranean countries (Fioretti et al 2010; Nardini et al 2012b), where drought, high irradiance and temperatures are common stress factors even for natural vegetation (Sanchez-Gomez et al 2006; David et al 2007; Nardini et al 2014) Under these environmental conditions, the plants’ growth over green roofs is challenging and requires specific technological and ecophysiological strategies to improve plant survival (Dvorak and Volder 2013)
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