Does shallow substrate improve water status of plants growing on green roofs? Testing the paradox in two sub-Mediterranean shrubs

• Green roof technology is under-represented in warm sub-Mediterranean areas.
• Substrate depth reduction is mandatory in order to limit installation weight.
• Water status of drought-adapted shrubs was monitored in 10 or 13 cm deep substrate.
• Reduced substrate depth translates into less severe water stress suffered by plants.
• Rainfalls lead to faster water availability recovery if shallow substrates are used.

Green roofs are artificial ecosystems providing ecological, economic, and social benefits to urban areas. Recently, the interest in roof greening has increased even in Mediterranean and sub-Mediterranean areas, despite the climatic features and reduced substrate depth expose plants to extreme stress. To limit installation weight and costs, recent green roof research aims to reduce substrate depth, which apparently contrasts with the need to maximize the amount of water available to vegetation. We monitored water status, growth, and evapotranspiration of drought-adapted shrubs (Cotinus coggygria, Prunus mahaleb) growing in experimental green roof modules filled with 10 or 13 cm deep substrate. Experimental data showed that: (a) reduced substrate depth translated into less severe water stress experienced by plants; (b) shallower substrate indirectly promoted lower water consumption by vegetation as a likely consequence of reduced plant biomass; (c) both large and small rainfalls induced better recovery of water content of substrate, drainage, and water retention layers when shallow substrate was used. Evidence was provided for the possibility to install extensive green roofs vegetated with stress-tolerant shrubs in sub-Mediterranean areas using 10 cm deep substrate. Green roofs based on the combination of shallow substrate and drought-tolerant plants may be an optimal solution for solving urban ecological issues.