Green roofs have been established as an environmentally correct action for the improvement of the environment and the quality of life in contemporary cities as a result of their proven environmental and social benefits. Despite their numerous benefits, green roof construction is progressing slowly in countries that do not provide incentives such as tax deductions or grants. To promote green roof establishment in the semiarid Mediterranean regions, solutions must be given, especially for the aged buildings that occupy the majority of the city cores and have limited additional load-bearing capacity on their frames. In such areas, extensive or semi-extensive green roofs seem to be the only applicable solution as a result of their low weight and reduced construction and maintenance costs. Extensive or semi-extensive green roofs can improve the aesthetics and, under certain conditions, the microclimate (Dunnett and Kingsbury, 2010; Getter and Rowe, 2006; Spronken-Smith and Oke, 1998; Takebayashi and Moriyama, 2009); thus, it is very important to investigate the potential of establishing such roofs by answering fundamental questions: 1) what substrate should be used, and at what depth, to minimize the load weight and yet provide sustainable plant growth; 2) which plants can provide adequate and sustainable growth on the particular green roof substrates and depths with minimal resources inputs; and 3) how much irrigation would be needed under the harsh semiarid climatic conditions of the Mediterranean region to sustain plant growth?
Green roof weight depends primarily on the substrate and secondly on the weight of the plant material (Scrivens, 1990). To create a system with minimal load, the weight of the substrate must be drastically reduced, whereas in extensive green roofs, the plant weight is negligible. This weight reduction could be accomplished both by the use of lightweight materials and by the reduction of substrate depth. However, apart from the weight factor, green roof substrates must fulfill several criteria: maintain adequate moisture for plant growth, facilitate the quick removal of excess water, provide support and anchoring of the plants, provide nutrients, and possess a pH and electrical conductivity (EC) appropriate for plant growth. Several researchers have evaluated different lightweight materials as green roof substrates (Beattie and Berghage, 2004, Nektarios et al., 2003; Rowe et al., 2006; Thuring et al., 2010). Beattie and Berghage (2004) indicated that a substrate must consist primarily of inorganic materials, whereas large quantities of composted or other organic substances should be avoided as a result of the substrate subsidence that results from their decomposition. Rowe et al. (2006) evaluated the use of heat-expanded shale in the establishment, growth, and survival of Sedum and native wild plants. The researchers evaluated 60%, 70%, 80%, 90%, and 100% of heat-expanded shale participation in the substrate mix in conjunction with varying proportions of sand, peat, and compost. They concluded that the substrates with higher shale contents resulted in slightly reduced plant growth and lower visual quality characteristics, irrespective of the plant species studied, but moderately high quantities of heat-expanded shale (80%) did not have any negative effects on plant growth and reduced the load weight of the structure. Thuring et al. (2010) evaluated the effects of expanded clay and shale amended with spent mushroom compost on the growth and dry weight of five succulent and herbaceous plants. Despite some individual plant responses, they found that plants grew better in expanded clay compared with expanded shale as a result of its better moisture and nutrient-holding capacity. This was especially profound during the drought stress periods.
Substrate depth is the second constituent that has a direct effect on green roof weight, construction cost, and plant survival and growth. The influence of varying extensive green roof substrate depth on plant growth has been investigated by several researchers to determine the smallest depth in which plants could grow in an effort to minimize the load to the building structure and the construction cost. The results are consistent because plant growth and survival improve as the depth of the substrate increases (Boivin et al., 2001; Dunnett et al., 2007; Durhman et al., 2007; Getter and Rowe, 2006; Thuring et al., 2010; VanWoert et al., 2005). However, in each case and ecoregion, it is of interest to determine the minimal substrate depth that achieves sufficient sustainability and visual quality of the green roof.
In extensive green roofs, irrigation may compensate for some substrate depth reduction (VanWoert et al., 2005). Although the German green roof specifications (FLL, 2002) have been formulated for northern climates and provide minimal information about green roof irrigation, it has been acknowledged by several researchers that if extensive green roofs are to be installed in semiarid climates, irrigation might be a necessity, especially during the first 1 or 2 years of establishment and during severe drought periods in the summer (Getter and Rowe, 2006; Latocha and Batorska, 2007; Williams et al., 2010; Wolf and Lundholm, 2008). Thuring et al. (2010) found that apart from the irrigation amount, the timing of drought imposition (soon after establishment or later) had also a significant impact on green roof plant establishment, because early drought exhibited adverse effects on plant growth. An additional issue that needs to be addressed concerning the debate about irrigation application on extensive green roofs is that, if they are expected to contribute to the mitigation of urban heat island effect, the green roof plants must transpire as a result of irrigation (Dunnett and Kingsbury, 2010; Spronken-Smith and Oke, 1998).
The final success of an extensive green roof is evaluated by the survival and sustainable growth of the plants. Therefore, the choice of suitable plant material that can withstand the stresses imposed on extensive green roofs under minimal management is of paramount importance. The selection of extensive green roof plant species has been the goal of several research studies with the most successful being the succulent plants, particularly those from the genus Sedum (Durhman et al., 2007; Getter and Rowe, 2008, 2009; Monterusso et al., 2005; Nagase and Dunnett, 2010; Snodgrass, 2005; VanWoert et al., 2005; Wolf and Lundholm, 2008). As a result of the demanding and harsh conditions that the plant material is confronted with on a green roof, the plant species are limited and must comply with several criteria (Dunnett and Kingsbury, 2010); thus, both native and non-native species are usually used. However, native plants are preferred for urban green roofs as a result of their adaptation to local climatic conditions, their known growth pattern in the specific climatic region that permits the selection of non-invasive species, and their contribution to replace the lost flora in the urban environment (Bousselot et al., 2011; Dunnett and Kingsbury, 2010; Nagase and Dunnett, 2010). Native plants are also able to provide a familiar habitat for the local fauna of each region, including the attraction of pollinating insects, thus increasing the biodiversity on green roofs and enriching the existing roof vegetation with even more species of the native flora (Nagase and Dunnett, 2010).
However, the variety of native species available for selection is significantly reduced when the harsh environmental conditions and shallow substrate depth of the extensive green roofs are taken into consideration. Monterusso et al. (2005), after screening and examining the suitability of 18 native species for potential use on extensive green roofs in Michigan, concluded that only four species were able to survive on the non-irrigated extensive green roof. Consequently, the establishment of native species in extensive green roofs is appropriate under certain conditions of irrigation, substrate depth, and nutrient levels.
Based on all of these factors, it is obvious that research must be undertaken for each region to select primarily native and secondly non-native plant species and evaluate their growth performance under the particular parameters of an extensive green roof. Dianthus fruticosus is a native plant species of the Mediterranean region; it has a bushy appearance and reaches a height of 50 cm. Dianthus fruticosus sub. fruticosus is an endangered species found in the Cycladic Islands that grows in rock crevices at 0 to 400 m altitude. It has spiral stems with narrow fleshy leaves and pink flowers; it exhibits prolonged flowering from June to August. It withstands temperature extremes and drought well, whereas its strong fibrous root system provides adequate anchorage to resist wind drifts. Based on these characteristics, D. fruticosus sub. fruticosus seems to be a promising native plant able to withstand the stresses of extensive green roofs in the Mediterranean semiarid region.
Therefore, the aims of the study were: 1) to determine the effects of substrate type and depth on growth response of D. fruticosus sub. fruticosus; 2) to evaluate the capacity of D. fruticosus sub. fruticosus to survive throughout the summer under the imposition of deficient irrigation; and 3) to determine the effects of substrate type and depth and irrigation regimens on D. fruticosus sub. fruticosus growth and physiological status during and after summer water stress conditions.
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