The rapid development of contemporary cities has caused dramatic changes in urban landscape and climate. Increased population density, particularly in city centers, in conjunction with the sealing of major land portions through building and construction work, have resulted in the lack of urban open and green spaces (Ferguson, 1998). All the above have negatively affected the urban microclimate, including air and water quality, and have caused environmental deterioration, thus endangering public health, while also degrading life quality along with the comfort and wellbeing of the inhabitants.
Green roofs provide contemporary technical solutions that could increase urban green spaces and contribute to the amelioration of environmental problems. Roofs and recessed penthouses cover a large area of the built urban spaces, especially in city areas which are characterized by dense building networks. Several researchers have reported, or forecasted through modeling, that green roofs could decrease ambient temperature (cooling) during summer, increase relative humidity, reduce infrared, and diffuse radiation (Kumar and Kaushik, 2005; Simmons et al., 2008). Further advantages resulting from green roof implementation include oxygen production by photosynthesis (Getter et al., 2009), building energy savings for summer cooling (Kotsiris et al., 2012a), reduction of air pollutants (Czemiel Berndtsson, 2010; Rowe, 2011), regulation of stormwater runoff and minimization of flooding events (Czemiel Berndtsson, 2010; Fioretti et al., 2010; Oberndorfer et al., 2007; Simmons et al., 2008; Van Woert et al., 2005), and amelioration of urban heat island effect (Akbari et al., 2001; Getter and Rowe, 2006).
Thus, the general concept is the development of a green roof networking which can contribute to the improvement of the microclimate in multiple ways if green roofs are largely implemented in urban areas. Therefore, it is necessary to seek ways to construct them on top of existing buildings. Because of the minimum load-bearing capacity of most existing buildings, either extensive or adaptive green roofs (Nektarios et al., 2011, 2015; Ntoulas et al., 2012, 2013b) are appropriate for green roof implementation. Extensive green roofs are characterized by minimal substrate depths (2–15 cm) that result in loads between 20 to 120 kg·m−2. They are usually planted with succulent plants and require low or no maintenance (FLL, 2008). Adaptive green roof systems also use minimal substrate depth (5–15 cm), but are planted with various plant types such as aromatic and medicinal plants, turfgrasses, and groundcovers. In contrast with the extensive green roofs, the adaptive ones are accessible and require minimal irrigation inputs (Kotsiris et al., 2013; Ntoulas et al., 2012, 2013a, 2013b; Ntoulas and Nektarios, 2015). Both extensive and adaptive green roofs require the use of plant species that have adequate water-stress tolerance and are capable of growing in shallow substrate depths.
Plant species for either extensive or adaptive green roof systems should be preferably native and adapted to local environmental conditions. Dimopoulos et al. (2013) reported that the prolific flora of Greece consists of 5752 kinds of plant species (1278 endemic), of which many of them are herbs, aromatics, and pharmaceuticals that could be considered appropriate for sustainable growth on Mediterranean green roof systems in conjunction with minimal water inputs (Benvenuti and Bacci, 2010; Kotsiris et al., 2012b, 2013; Nektarios et al., 2011, 2015; Papafotiou et al., 2013; Paraskevopoulou et al., 2015; Tassoula et al., 2015).
The aim of the present study is to determine the irrigation threshold based on Epan for five medicinal and aromatic plant species. The findings of this research may be valuable in selecting the most appropriate plants for Mediterranean green roof systems and in predicting the minimal required irrigation inputs for sustainable growth.
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