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Irrigation management in extensive green roofs (EGRs) is crucial in Mediterranean and semi-arid climates, as it should guarantee efficient water use while ensuring plant survival and vegetation cover. However, benefits of maintaining moderately low substrate water potential (psi(s)) have not been adequately investigated to date. An irrigation control unit based on psi(s) thresholds for irrigation (MediWater Safe [MWS]) was compared to a common irrigation timer maintaining psi(s) similar to 0 MPa (CTR) in shrub-vegetated Mediterranean EGR modules. The effect of the different irrigation regimes on substrate temperature, plant water relations (leaf conductance to water vapour, midday water potential and turgor loss point) and root vulnerability to heat stress via electrolyte leakage was tested in four shrub species. Decreasing psi(s) thresholds to -0.4 MPa reduced irrigation volumes by 68% in 3 summer months. However, the MWS unit neither influenced plant water status and vegetation cover nor induced physiological acclimation responses. Brief irrigation cycles imposed by MWS in the warmest hours reduced substrate surface temperature by 3 degrees C compared to CTR. Plant water status dynamics and root vulnerability to heat were species specific. Progressive stomatal closure and plant decline occurred only in Ceanothus thyrsiflorus and were associated to high root vulnerability to heat. Mortality occurred only in some Ceanothus plants in the CTR module, where higher psi(s) favoured the expansion of Hyperucum x moserianum. The results suggest that selecting proper psi(s) thresholds for irrigation could optimize EGR benefits, guaranteeing substantial water savings and proper plant establishment. Moreover, we claim root resistance to heat as a key parameter for plant selection in Mediterranean EGRs.

We investigated the percolation of water within a controlled environment in order to recover the EM properties and the internal structure of the materials, and to better understand the drainage processes involved. We implemented an analogue model that accurately replicates a green roof, from the construction materials to the geometry. We acquired several common offset GPR data sets on the entire surface of the model, both before and during different irrigation tests. We separately analyzed variations in the reflected amplitudes and in the EM velocities, the latter of which were calculated by combining the measured thickness of the model with the arrival times of the recorded reflections. The signal amplitudes obtained before and during irrigation are compared in order to estimate the attenuation factor at the different stages of the irrigation process. We use the attenuation factor and the EM velocities to estimate the relative permittivity of the topsoil mixture, from which we then calculate the water content, the porosity, and the relative electrical permittivity of the solid matrix by applying well known empirical formulas. The results were validated by comparing them with independent direct measurements.

Green roofs are gaining momentum in the arid and semi-arid regions due to their multiple benefits as compared with conventional roofs. One of the most critical steps in green roof installation is the selection of drought and heat tolerant species that can thrive under extreme microclimate conditions. We monitored the water status, growth and survival of 11 drought-adapted shrub species grown on shallow green roof modules (10 and 13cm deep substrate) and analyzed traits enabling plants to cope with drought (symplastic and apoplastic resistance) and heat stress (root membrane stability). The physiological traits conferring efficiency/safety to the water transport system under severe drought influenced plant water status and represent good predictors of both plant water use and growth rates over green roofs. Moreover, our data suggest that high substrate temperature represents a stress factor affecting plant survival to a larger extent than drought per se. In fact, the major cause influencing seedling survival on shallow substrates was the species-specific root resistance to heat, a single and easy measurable trait that should be integrated into the methodological framework for screening and selection of suitable shrub species for roof greening in the Mediterranean.

Climate features of the Mediterranean area make plant survival over green roofs challenging, thus calling for research work to improve water holding capacities of green roof systems. We assessed the effects of polymer hydrogel amendment on the water holding capacity of a green roof substrate, as well as on water status and growth of Salvia officinalis. Plants were grown in green roof experimental modules containing 8 cm or 12 cm deep substrate (control) or substrate mixed with hydrogel at two different concentrations: 0.3 or 0.6%. Hydrogel significantly increased the substrate's water content at saturation, as well as water available to vegetation. Plants grown in 8 cm deep substrate mixed with 0.6% of hydrogel showed the best performance in terms of water status and membrane integrity under drought stress, associated to the lowest above-ground biomass. Our results provide experimental evidence that polymer hydrogel amendments enhance water supply to vegetation at the establishment phase of a green roof. In particular, the water status of plants is most effectively improved when reduced substrate depths are used to limit the biomass accumulation during early growth stages. A significant loss of water holding capacity of substrate-hydrogel blends was observed after 5 months from establishment of the experimental modules. We suggest that cross-optimization of physical-chemical characteristics of hydrogels and green roof substrates is needed to improve long term effectiveness of polymer-hydrogel blends.

The influence of different green roof layering types on the amount of water available to plants was investigated in the specific climatic context of the Mediterranean region. Water status, productivity and survival rate of Salvia officinalis L. plants growing in experimental green roof modules were monitored between early spring and late summer. Experimental data showed that: (a) substrate and water retention layer retained respectively 34% and 90% in volume of water potentially available to plants; (b) water retention layer had a positive effect on plant water status and survival; (c) the design of the overall green roof system, and in particular the characteristics of the drainage layer, influenced the amount of water transferred between different green roof elements, thus significantly influencing the amount of water available to plants. In particular, significant amounts of water were shown to be transferred from the retention layer into the cavities of the plastic drainage layer in response to temperature fluctuations, leading to day/night cycles of water evaporation/condensation. Targeted modifications of the geometrical and technical features of drainage elements were shown to be potentially useful to improve plant survival during intense and/or prolonged drought events.

Mediterranean native shrubs can be very useful for green roofs in hot and arid regions. Our data show that both Arbutus unedo L. and Salvia officinalis L. could be successfully utilized, although the choice of species should be based on the drought-resistant strategy relative to the desired technical performance of the green roof. Moreover, substrate selection was found to have a crucial role in the success of green roof installations in the Mediterranean area., 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.

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.

The relative contribution of substrate depth and vegetation type on temperature mitigation and stormwater runoff reduction was studied in an experimental green roof in North eastern Italy. Two substrate depths (120 and 200 mm) and two vegetation types (herbaceous plants and shrubs, respectively) were used, and compared to control modules with similar substrate depths but left bare of vegetation. Experimental observations showed that: a) green roofs substantially reduce thermal load over the rooftop, with significant effects of substrate depth and no apparent impact of vegetation type; b) thermal effects are strongly influenced by substrate water content; c) green roofs strongly reduce water runoff with significant substrate x vegetation effects. Our data suggest that green roof design addressed to optimization of the thermal functions should take into account adequate planning of substrate depth. Moreover, our data show that vegetated modules out-competed medium-only ones in terms of runoff reduction capacity, in accordance with some previous studies. Both shrub-vegetated and herbaceous modules intercepted and stored more than 90% rainfall during intense precipitation events, with no significant difference between the two vegetation types despite different substrate depths.

The circadian regulation of leaf hydraulic conductance (Kleaf) was investigated in Helianthus annuus L. (sunflower). Kleaf was measured with an high pressure flow meter during the light and dark period from plants growing at a photoperiod of 12 h. Kleaf was 4.0 e-4 kg s -1 m-2 MPa-1 during the light period (LL) and 30-40% less during the dark period (DL). When photoperiod was inverted and leaves were measured for Kleaf at their subjective light or dark periods, Kleaf adjusted to the new conditions requiring 48 h for increasing from dark to light values and 4 d for the opposite transition. Plants put in continuous dark showed Kleaf oscillating from light to dark values in phase with their subjective photoperiod indicating that K leaf changes were induced by the circadian clock. Several cuts through the minor veins reduced leaf hydraulic resistance (Rleaf) of both LL and DL to the same value (1.0 e + 3 MPa m2 s kg-1) that equalled the vascular resistance (Rv). The contribution of the non-vascular leaf resistance (Rnv) to Rleaf was of 71.9% in DL and of 58.4% in LL. The dominant Rnv was shown to be reversibly modulated by mercurials, suggesting that aquaporins play a role in diurnal changes of Kleaf.