Substrate Composition and Depth Affect Soil Moisture Behavior and Plant-Soil Relationship on Mediterranean Extensive Green Roofs

Abstract

Abstract:The Mediterranean basin is extremely vulnerable to climate change, and one of the areas most impacted by human water demand. Yet the green roofs increasingly created both for aesthetic reasons and to limit pollution and urban runoff are themselves very water-demanding. Successful green roof installation depends on the establishment of the vegetation, and the substrate is the key element: it conserves water, and provides the nutrients and physical support indispensable for plant growth. Since typical Mediterranean plant communities require no maintenance, this study seeks to develop techniques for creating maintenance-and watering-free horizontal green roofs for public or private buildings in a Mediterranean context. The innovative aspect of this study lies in creating two soil mixes, fine elements (clay and silt) and coarse elements (pebbles of all sizes), in two different thicknesses, to assess vegetation development. Monitoring of substrate moisture was carried out and coupled with local rainfall measurements during summer and autumn. As expected, substrate moisture is mainly influenced by substrate depth (the deeper, the moister) and composition (the finer the particles (clays and silts), the higher the moisture content). Vegetation cover impacts moisture to a lesser extent but is itself affected by the composition and depth of the substrates. These results are subsequently discussed with relation to the issue of sustainable green roofs in Mediterranean climates. Considering applications of our results, for an optimal colonization of a Mediterranean vegetation, a substrate thickness of 15 cm composed mainly of fine elements (75% clay-silt and 25% pebble-sand) would be recommended in green roofs.

Effect of Green Roof Configuration and Hydrological Variables on Runoff Water Quantity and Quality

“…The results of the t-test indicate no statistically significant differences between the two groups (p-value = 0.548), obtaining similar results as in some previous studies [38,42,43], which have stated that vegetation does not have a clear effect on water retention efficiency for GRs. The previous result contrasts with that of Chenot et al [17] who concluded that vegetated coverage does have an influence on water retention even though substrate composition and depth have a greater effect. This disagreement may be the result of not considering the hydrological conditions.…”

Green Roof Design: State of the Art on Technology and Materials

“…Chenot et al [88] assessed the role of substrate thickness and composition in maintaining the moisture necessary for good vegetation cover. The authors mixed fine elements (clays and silts) with coarse elements (pebbles of all sizes) with the aim of allowing typical pioneer Mediterranean vegetation communities to be maintained without human intervention (no watering, mechanical or chemical weeding).…”

Improving Extensive Green Roofs for Endangered Ground-Nesting Birds

“…For example, 3-5 years after planting, a roof subject to average Swiss rainfall conditions with a ≥10-cmthick substrate is likely to support a meadow-like plant community (Nagase and Dunnett 2013). Also, the variability of substrate thickness, particle size and soil type and the percentage of organic matter may strongly influence plant diversity (Chenot et al 2017;Dunnett et al 2008).…”