Plant performance on Mediterranean green roofs: interaction of species-specific hydraulic strategies and substrate water relations

Echium vulgare and Echium plantagineum: A Comparative Study to Evaluate Their Inclusion in Mediterranean Urban Green Roofs

Green roofs (GRs) are proposed to offset against numerous environmental and socio-economic concerns associated with climate change and urban sprawl. In Mediterranean urban areas, to protect and conserve biodiversity through GRs, the use of native plant species from arid environments and with shallow roots is generally recommended. In north Europe, Echium vulgare L. is widely used on GRs for its tolerance to abiotic stresses and its attractiveness for bees; unfortunately, since this species requires cold winters to induce flowering and warm wet summers for vegetative growths, its adaptability to Mediterranean GRs has been questioned. The current study is based on the hypothesis that Echium plantagineum L. can adapt better to the Mediterranean environment than E. vulgare and offer blooms to pollinators, thus providing the important urban ecosystem service (UES) of protecting entomofauna biodiversity. To compare the adaptability of E. plantagineum vs. E. vulgare, both Echium species were grown and studied on the extensive GR installed at ENEA Casaccia Research Center, in the north of Rome, Lazio, Italy. The comparative analysis of the GR performance of the two species was based on several plant-related traits, including seed morphology, rosette stage, inflorescence, flower and root-related traits, and their biological life cycle, most of them showing significant differences (for example, rosette area was 1.42-fold major in E. plantagineum than in E. vulgare). The information provided in this manuscript will be useful to update the herbarium records for conservation biology. A dramatic water stress was purposely applied in the GR before the end of the hot summer season, and while E. plantagineum faced with success the imposed dehydration (88.4% vegetation cover), E. vulgare did not (7.5% vegetation cover), presumably because of its biennial life cycle which did not allow it to complete seed maturation (only 46.9% percentage mature seeds in E. vulgare respect to 89.5% in E. plantagineum). In summary, as the main result, this work shows that in Mediterranean areas, the inclusion of E. plantagineum in seed mixes for flower meadow GRs could represent a valuable alternative to E. vulgare in temperate areas, providing a safeguard for pollinators and allowing water and energy saving.

Acclimatization of Mediterranean Native Sages (Salvia spp.) and Interspecific Hybrids in an Urban Green Roof under Regular and Reduced Irrigation

Native-to-Greece sage species, namely, Salvia fruticosa, S. officinalis, S. pomifera ssp. pomifera, S. ringens, S. tomentosa and interspecific hybrids, were evaluated for their acclimatization in an extensive Mediterranean green roof during summer under regular and reduced irrigation (every 2–3 days with substrate moisture 16–22% v/v and 4–5 days with substrate moisture 7–11% v/v, respectively). A substrate (grape-marc compost:perlite:pumice, 3:3:4, v/v) that was 10 cm deep was used. Regardless of the irrigation frequency, S. pomifera ssp. pomifera × S. ringens and S. officinalis × S. pomifera ssp. pomifera showed the highest survival of all hybrids and species, along with satisfactory growth, while S. fruticosa showed the lowest survival. Reduced irrigation resulted in the reduction of aboveground and root biomass, with no damage to the photosynthetic apparatus. S. fruticosa showed the highest (53%) aboveground biomass reduction and S. officinalis, S. officinalis × S. ringens and S. pomifera ssp. pomifera × S. ringens showed the lowest (28, 23 and 3%, respectively), while S. officinalis × S. pomifera ssp. pomifera and S. pomifera ssp. pomifera × S. ringens showed the lowest reduction in root biomass (13 and 16%, respectively). With a reservation for S. fruticosa, Greek Salvia spp. and their interspecific hybrids studied in the present work are recommended for sustainable exploitation in extensive green roofs in arid regions and generally in xeriscaping.

When Water Availability Is Low, Two Mediterranean Salvia Species Rely on Root Hydraulics

Increase in severity and frequency of drought events is altering plant community composition, exposing biomes to a higher risk of biodiversity losses. This is exacerbated in the most fragile areas as Mediterranean biome. Thus, identifying plant traits for forecasting species with a high risk of drought-driven mortality is particularly urgent. In the present study, we investigated the drought resistance strategy of two Mediterranean native species: Salvia ceratophylloides Ard. (Sc) and Salvia officinalis L. (So) by considering the impact of drought-driven water content decline on plant hydraulics. Well-watered samples of Sc displayed higher leaf and stemsaturated water content and lower shoot biomass than So samples, but similar root biomass. In response to drought, Sc showed a conservative water use strategy, as the prompt stomatal closure and leaves shedding suggested. A drought-tolerant mechanism was confirmed in So samples. Nevertheless, Sc and So showed similar drought-driven plant hydraulic conductance (Kplant) recover ability. Root hydraulic traits played a key role to reach this goal. Relative water content as well as loss of cell rehydration capability and membrane damages, especially of stem and root, were good proxies of drought-driven Kplant decline.

Too dry to survive: Leaf hydraulic failure in two Salvia species can be predicted on the basis of water content

Global warming is exposing plants to increased risks of drought-driven mortality. Recent advances suggest that hydraulic failure is a key process leading to plant death, and the identification of simple and reliable proxies of species-specific risk of irreversible hydraulic damage is urgently required. We assessed the predictive power of leaf water content and shrinkage for monitoring leaf hydraulic failure in two Mediterranean native species, Salvia ceratophylloides (Sc) and S. officinalis (So). The study species showed significant differences in relative water content (RWC) thresholds inducing loss of rehydration capacity, as well as leaf hydraulic conductance (K_L) impairment. Sc turned out to be more resistant to drought than So. However, Sc and So showed different leaf saturated water content values, so that different RWC values actually corresponded to similar absolute leaf water content. Our findings suggest that absolute leaf water content and leaf water potential, but not RWC, are reliable parameters for predicting the risk of leaf hydraulic impairment of two Salvia species, and their potential risk of irreversible damage under severe drought. Moreover, the lack of any K_L decline until the turgor loss point in Sc, coupled to consistent leaf shrinkage, rejects the hypothesis to use leaf shrinkage as a proxy to predict K_L vulnerability, at least in species with high leaf capacitance. Robust linear correlations between K_L decline and electrolyte leakage measurements suggested a role of membrane damage in driving leaf hydraulic collapse.

Hybridization Assays in Strawberry Tree toward the Identification of Plants Displaying Increased Drought Tolerance

Arbutus unedo L. is a small Ericaceae tree with a circum-Mediterranean distribution. It has a huge ecological impact on southern Europe forests and a great economic importance as a source of phytochemicals with bioactive properties and for fruit production. On the foreseen climate change context, breeding toward drought tolerance is necessary in order to ameliorate plant performance. Therefore, the aim of this work was to study the reproduction mechanisms of the strawberry tree, obtain new genetic combinations by hybridization, and select genotypes more tolerant to drought stress. A morphological analysis of flowers and pollen was carried out, and controlled pollinations were performed both in vitro and ex vitro. The very first approach on strawberry tree breeding by means of hybridization is also presented. Several physiological parameters were evaluated on 26 genotypes submitted to a water-deficit regime. Plant behavior under drought greatly varied among genotypes, which showed high phenotype plasticity. Three genotypes that were able to cope with water restriction without compromising net CO2 assimilation were identified as highly tolerant to drought stress. The results obtained elucidate the reproduction mechanisms of the strawberry tree and open the way for a long-term breeding program based on the selection of drought-tolerant plants.

Green roofs in the tropics: design considerations and vegetation dynamics

Green roofs (GR) have been proposed as a possible solution for urban stressors that, integrated with other remediation and mitigation actions, can lead the way to a more sustainable society. Even when some aspects of green roof design are well established and known (i.e. depth arrangements, materials, structural components, etc.) there is a need for further development on ecological attributes. This study is a descriptive analysis of suitable plant species for their possible incorporation in green roof designs with tropical climate conditions. Green roof research has been mostly led by temperate climate countries and has neglected to address tropical areas; this study aims to move research towards this knowledge gap. The evaluation of the vegetation dynamics in these novel ecosystems was done through a case study in the renovated facilities of the International Institute of Tropical Forestry in Río Piedras, Puerto Rico, which incorporated a set of green roofs in their infrastructure. We also sampled an older green roof built in the Social Sciences Faculty at the University of Puerto Rico at Río Piedras. A three-dimensional approach, the Point-Intercept Method, was taken in the vegetation surveys to capture as much as possible the green infrastructure of the roofs. Most of the originally planted species did not appear in these surveys. On the contrary, mainly new species dominated the areas. Along with the findings of these surveys and those in other tropical countries, a list of suitable species for green roofs in Puerto Rico is suggested, and some general recommendations are made for the better management of green roofs in tropical zones.

Is plant survival on green roofs related to their drought response, water use or climate of origin?

Green roofs are novel urban ecosystems with shallow substrate depths and low water availability. Hence, it is critical to select green roof plants that can survive water-deficits, particularly in climates with hot and dry summers. Shrubs are perennial plants which can be drought resistant and may be suitable for green roofs. However, studies about survival and health of shrubs are limited. The aim of this study was to determine whether plant climate of origin aridity, drought response and water use strategies could be used to select shrubs which can survive on green roofs that experience water-deficit. We selected 15 shrub species from a range of climates (dry, mesic and wet) and planted them together in 20 replicate green roof modules with 130 mm deep substrate. We monitored substrate water contents, plant minimum water potentials (ψ_min), health (visual score), percentage survival and related survival with their turgor loss point (ψ_tlp) and water use strategies (evapotranspiration rates in a related glasshouse experiment). We also determined whether plants could recover after dry periods by rewatering after the summer. Mean gravimetric soil water content decreased to approximately 5% after summer drought, which resulted in mortality. Overall, survival ranged between 10% and 100% for the 15 species. However, survival was not related to their ψ_tlp or water use strategies. While shrubs from more arid climates had lower ψ_min in response to dry substrates, this did not result in greater survival and health. Following rewatering, only four shrub species resprouted. Hence, as plant drought response, water use strategy and climate of origin were not strongly related to survival, we suggest survival on green roofs is likely to be determined by a combination of physiological traits. Emergency irrigation for shrubs growing on green roofs in hot and dry climates is recommended during summer to keep them alive.

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

In order to consider green roofs as an environmentally friendly technology, the selection of efficient and sustainable components is extremely important. Previous review papers have mainly focused on the performance and advantages of green roofs. The objective of this paper is to examine the primary layers: The waterproof and anti-root membranes; the protection, filter, and drainage layers; the substrate; and the vegetation. First, the history, modern applications, benefits and classification are analyzed in order to present a well-defined state of the art of this technology. Then, the roles, requirements, characteristics, and materials are assessed for each green roof layers. This technology was compared to a conventional roof technology, Mediterranean climate conditions and their influence on green roof design were assessed, also comparing them with Tropical area and focusing on irrigation systems, examples about the commercial materials and products available in the market were provided and innovative materials coming from recycled sources were analyzed. Future research should evaluate new materials for green roof technologies, in order to enhance their performance and increase their sustainability. The information provided in this review paper will be useful to develop Mediterranean green roof guidelines for selecting suitable components and materials during the design and installation phases.

Green Roof Design Techniques to Improve Water Use under Mediterranean Conditions

Green roof typology can vary depending on buildings structure, climate conditions, substrate, and plants used. In regions with hot and dry summers, such as the Mediterranean region, irrigation plays an essential role, as the highest temperatures occur during the driest period of the year. Irrigation might reduce the heat island effect and improve the cooling of buildings during this period, however, the added cost of maintenance operations and additional energy consumption could outrun the benefits provided by the project. Moreover, in situations where water is scarce or primarily channelled to other uses (e.g., domestic, agriculture or industry) during drought occurrence, it is advisable to implement green roof projects with the lowest use of water possible. The objective of the present work is to investigate solutions to optimize water use in green roofs under Mediterranean conditions, such as those of southern Europe. Two case studies are presented for Portugal, and potential techniques to reduce irrigation requirements in green roofs were tested. These addressed the use of native plant species, including the extreme type of a non-irrigated green roof (Biocrust roof) and techniques for plant installation. Plant drought tolerance was found to be an advantage in green roofs under these climatic conditions and, for the species studied, aesthetic value could be maintained when irrigation decreased.

Mediterranean Green Roof Simulation in Caldes de Montbui (Barcelona): Thermal and Hydrological Performance Test of Frankenia laevis L., Dymondia margaretae Compton and Iris lutescens Lam

Green roofs provide a number of environmental advantages like increasing urban biodiversity, reducing pollution, easing burdens on drainage systems, and lowering energy costs thanks to thermal insulation. Frankenia laevis, Dymondia margaretae and Iris lutescens were tested in a green roof installation. For all three species, we assessed two minimal irrigation treatments and one rain-fed treatment to resemble Mediterranean climate conditions analyzing the thermal and hydrological performance of all three species and their substrates through an evaluation of green cover, mortality, and biomass. The most influential factors registered for all three species are the relationship between air and water in the substrate and the interaction between green cover and substrate, respectively, for summer and winter seasons. In particular, D. margaretae preserved more water in its substrate than the other species both in summer and winter and after each rainfall event. F. laevis registered the highest level of variation in terms of substrate water content and of rainwater retention. I. lutescens achieved low hydrological performance, a limited amount of green cover, and slow growth. Our results suggest the absolute need of additional irrigation, managed in accordance with specific functional objectives, for all three species analyzed under Mediterranean conditions and different water regime.

Relationships between plant drought response, traits, and climate of origin for green roof plant selection

The ideal species for green or vegetated roofs should have high water use after rainfall to maximize stormwater retention but also survive periods with low water availability in dry substrates. Shrubs have great potential for green roofs because they have higher rates of water use, and many species are also drought tolerant. However, not all shrub species will be suitable and there may be a trade-off between water use and drought tolerance. We conducted a glasshouse experiment to determine the possible trade-offs between shrub water use for stormwater management and their response to drought conditions. We selected 20 shrubs from a wide range of climates of origin, represented by heat moisture index (HMI) and mean annual precipitation (MAP). Under well-watered (WW) and water-deficit (WD) conditions, we assessed morphological responses to water availability; evapotranspiration rate (ET) and midday water potential (Ψ_MD ) were used to evaluate species water use and drought response. In response to WD, all 20 shrubs adjusted their morphology and physiology. However, there were no species that simultaneously achieved high rates of water use (high ET) under WW and high drought tolerance (low Ψ_MD ) under WD conditions. Although some species which had high water use under WW conditions could avoid drought stress (high Ψ_MD ). Water use was strongly related to plant biomass, total leaf area, and leaf traits (specific leaf area [SLA] and leaf area ratio [LAR]). Conversely, drought response (Ψ_MD ) was not related to morphological traits. Species' climate of origin was not related to drought response or water use. Drought-avoiding shrubs (high Ψ_MD ) could optimize rainfall reduction on green roofs. Water use was related to biomass, leaf area, and leaf traits; thus, these traits could be used to assist the selection of shrubs for stormwater mitigation on green roofs. The natural distribution of species was not related to their water use or drought response, which suggests that shrubs from less arid climates may be suitable for use on green roofs. Selecting species based on traits and not climate of origin could both improve green roof performance and biodiversity outcomes by expanding the current plant palette.

Performance evaluation of five Mediterranean species to optimize ecosystem services of green roofs under water-limited conditions

Rapid urban growth in Mediterranean cities has become a serious environmental concern. Due to this expansion, which covers adjacent horizontal ground, a critical deficit of green areas has been increasing. Moreover, irrigation is considered an important issue since water is one of the most limiting natural resources all over the world. The main objective of this study was to perform a long-term experiment to assess five Mediterranean species for extensive green roof implementation in Mediterranean-climate conditions. Brachypodium phoenicoides, Crithmum maritimum, Limonium virgatum, Sedum sediforme and Sporobolus pungens were grown in experimental modules under well-watered and water-limited conditions (irrigation at 50% and 25% ET₀, respectively). Plant growth and cover, relative appearance, color evolution and water use were determined periodically for two years. Shoot and root biomass were quantified at the end of the experimental period. The effects of the irrigation treatments and seasonal changes were assessed to identify the advantages and disadvantages of each species according to their environmental performance. All species survived and showed adequate esthetic performance and plant cover during the experiment. S. sediforme registered the lowest variation of relative appearance along the experiment, the highest biomass production and the lowest water consumption. Nevertheless, B. phoenicoides appeared to be an interesting alternative to S. sediforme, showing high esthetic performance and water consumption throughout the rainy season, suggesting a potential role of this species in stormwater regulation related with runoff reduction. S. pungens performed well in summer but presented poor esthetics during winter.

Growing substrates for aromatic plant species in green roofs and water runoff quality: pilot experiments in a Mediterranean climate

Green roof technology has evolved in recent years as a potential solution to promote vegetation in urban areas. Green roof studies for Mediterranean climates, where extended drought periods in summer contrast with cold and rainy periods in winter, are still scarce. The present research study assesses the use of substrates with different compositions for the growth of six aromatic plant species - Lavandula dentata, Pelargonium odoratissimum, Helichrysum italicum, Satureja montana, Thymus caespititius and T. pseudolanuginosus, during a 2-year period, and the monitoring of water runoff quality. Growing substrates encompassed expanded clay and granulated cork, in combination with organic matter and crushed eggshell. These combinations were adequate for the establishment of all aromatic plants, allowing their propagation in the extensive system located on the 5th storey. The substrate composed of 70% expanded clay and 30% organic matter was the most suitable, and crushed eggshell incorporation improved the initial plant establishment. Water runoff quality parameters - turbidity, pH, conductivity, NH₄⁺, NO₃^-, PO₄^3- and chemical oxygen demand - showed that it could be reused for non-potable uses in buildings. The present study shows that selected aromatic plant species could be successfully used in green roofs in a Mediterranean climate.

Drought versus heat: What's the major constraint on Mediterranean green roof plants?

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.

Leaf hydraulic vulnerability protects stem functionality under drought stress in Salvia officinalis

Functional coordination between leaf and stem hydraulics has been proposed as a key trait of drought-resistant plants. A balanced water transport efficiency and safety of different plant organs might be of particular importance for plant survival in the Mediterranean climate. We monitored seasonal changes of leaf and stem water relations of Salvia officinalis L. in order to highlight strategies adopted by this species to survive in harsh environmental conditions. During summer drought, the water potential dropped below the turgor loss point thus reducing water loss by transpiration, whereas the photosynthetic efficiency remained relatively high. Leaves lost their water transport efficiency earlier than stems, although in both plant organs P50 (water potential inducing 50% loss of hydraulic conductivity) indicated surprisingly high vulnerability when compared with other drought-tolerant species. The fast recovery of leaf turgor upon restoration of soil water availability suggests that the reduction of leaf hydraulic conductance is not only a consequence of vein embolism, but cell shrinkage and consequent increase of resistance in the extra-xylem pathway may play an important role. We conclude that the drought tolerance of S. officinalis arises at least partly as a consequence of vulnerability segmentation.