Article

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

Authors:
  • Harpo SpA
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Abstract

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.

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... However, studies on plant performance and trait-based selection on EGRs are still scarce (Du et al., 2019a). Plant selection for EGRs should be ideally based on physiological traits related to drought and heat resistance, including water use strategies (iso-anisohydry; Raimondo et al., 2015), safety/ efficiency trade-offs, leaf water potential at turgor loss point (Ψ tlp , Du et al., 2019b), vulnerability to embolism formation and vulnerability of shoots and roots to heat stress (Savi et al., 2016). However, selection based on water use strategies and climate of origin alone does not per se ensure higher drought survival in EGR systems (Du et al., 2019a), requiring specific physiological performance tests on EGRs. ...
... Heat is a major constraint for plant health and survival in Mediterranean EGRs (Savi et al., 2016). In our experiment, we have quantified the root electrolyte leakage in the range of values registered in the substrate during the experiment (Figure 4). ...
... In our experiment, we have quantified the root electrolyte leakage in the range of values registered in the substrate during the experiment (Figure 4). Temperature effects on root plasma membrane integrity via electrolyte leakage tests have been previously examined only in perennial grasses (Zhang & Du, 2016), vegetable crops (Iglesias-Acosta et al., 2010) and some drought-adapted shrub species (Savi et al., 2016). Elevated temperatures can compromise plant growth and development and induce thermal stress in both plant shoots and roots, altering composition, fluidity and permeability of plasma membranes, as well as membrane protein activities with consequent leakage of ions and amino acids (e.g., Lindberg et al., 2005). ...
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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 (Ψs) have not been adequately investigated to date. An irrigation control unit based on Ψs thresholds for irrigation (MWS) was compared to a common irrigation timer maintaining Ψs ⁓ 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, turgor loss point) and root vulnerability to heat stress via electrolyte leakage was tested in four shrub species. Decreasing Ψs 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 °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 Ψs favoured the expansion of Hyperucum x moserianum. The results suggest that selecting proper Ψs 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.
... Thus, the selection of native species is already critical where the microclimatic conditions are extreme. It is very likely that if the climatic variations, the heat stress and the non-predictability of mid-to-long-term urban climate were considered, the greenery design would become more complex even in apparently milder climates, where drought and heat tolerant species could be also required (Savi et al. 2016). ...
... Further uncertainties in the expected service life would be introduced by heat stress. In addition, taking into account that high substrate temperature represents a stress factor affecting plant survival more than drought per se; it becomes even more difficult to estimate the expected service life in heat stress conditions (Savi et al. 2016). ...
... At the same time, the need to protect greenery from early deterioration due to heat stress and climatic variations seems to conflict with one of the main targets in green roof research of reducing substrate depth, to limit installation weight and costs (Cao et al. 2014). According to Savi et al. (2016) this might lead to even more extreme temperatures in the substrate and plants death during heat waves events or caused by UHI. ...
Article
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Urban greenery is a natural solution to cool cities and provide comfort, clean air and significant social, health and economic benefits. This paper aims to present the latest progress on the field of greenery urban mitigation techniques including aspects related to the theoretical and experimental assessment of the greenery cooling potential, the impact on urban vegetation on energy, health and comfort and the acquired knowledge on the best integration of the various types of greenery in the urban frame. Also to present the recent knowledge on the impact of climate change on the cooling performance of urban vegetation and investigate and analyse possible technological solutions to face the impact of high ambient temperatures.
... Further, due to the free-draining nature of green roof substrates, the onset of water-deficit can be very rapid (Cao et al., 2014;Farrell et al., 2013a). In hot and dry climates, plant mortality is triggered by drought and temperature stress on unirrigated green roofs (Savi et al., 2016;Schroll et al., 2011;Zhang et al., 2014), which has negative impacts on their benefits due to loss of plant cover, reducing evapotranspiration (Lundholm et al., 2010;Lundholm, 2015) and resulting in additional maintenance and costs. ...
... Physiological approaches, especially plant drought response and water use strategies are also being used to improve green roof plant selection beyond succulent species. Drought response strategies include minimum water potentials (ψ min ) and water potential at the turgor loss point (ψ tlp ) and plant water use strategies relate to evapotranspiration (ET) (Du et al., 2018;Farrell et al., 2013b;Raimondo et al., 2015;Savi et al., 2016). Plants with high water use have been hypothesised to optimize stormwater mitigation on green roofs as they facilitate substrate drying after rainfall (Farrell et al., 2013b). ...
... Contrary to our hypothesis, plant survival and health were not related to their turgor loss point (ψ tlp ), indicating that ψ tlp is not a useful measure to predict plant survival and health under green roof conditions. This is consistent with Savi et al. (2016) who showed that plant mortality of 11 Mediterranean shrub species was unrelated with their ψ tlp in shallow green roof modules (100 and 120 mm substrates). As ψ tlp is a leaf-level measure of drought tolerance, the lack of a relationship Fig. 5. Correlations between minimum water potentials (ψ min ), water potentials at turgor loss point (ψ tlp ), water use (evapotranspiration, ET) under well-watered conditions and survival and health for the 15 shrub species. ...
... The evaluation of native species with minimal water requirements under diverse growth conditions is considered a crucial stage in developing plants that are suitable for sustainable green roofs in semi-arid Mediterranean regions [9]. Apart from addressing the problem of limited water availability in such regions, biodiversity, including concern for pollinator reduction worldwide [11], and the preservation of the local character can be met via the use of native plants in landscaping and green roofs in particular [12,13]. ...
... However, although the S. fruticosa × S. ringens hybrid showed high plant mortality at the end of the hot and dry season under normal irrigation, similar to S. fruticosa, under sparse irrigation, in contrast to S. fruticosa, its plant mortality was among the lowest. Plant mortality in a green roof with low substrate depth (extensive type green roof) is affected by both drought and heat and is possibly affected to a larger extent by substrate temperature than drought per se [9]. The important role of temperature is further supported by a preliminary experiment (data not presented) we conducted the previous year (2020) with these five species of Salvia in the same green roof. ...
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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.
... Suitable native plant species for green roofs have been widely researched in Europe (Köhler, 2006;Madre et al., 2014) including the Mediterranean (Caneva et al., 2015;Raimondo et al., 2015;Savi et al., 2016;Van Mechelen et al., 2014) and North America (Butler et al., 2012;Dvorak and Volder, 2010;Monterusso et al., 2005;Sutton et al., 2012), yet there is a lag in the development and adoption of green roof technologies in Australia (Williams et al., 2021). Studies informing green roof plant selection are relatively few in Australia, with the majority conducted in temperate Southeast Australia (Farrell et al., 2013;Layt, 2011). ...
... The growth environment of green roofs is considered severe as a result of high exposure to solar radiation and wind and wide temperature fluctuations [2,3,12]. Therefore, the adverse conditions of the roof were probably responsible for the reduced plant growth, since aboveground plant biomass may be reduced under abiotic stress conditions [59][60][61]. Nevertheless, the difference in growth was not great, and plants grew satisfactorily on the roof (Figures 2 and 3). Although all plants sprouted in October 2016, after the first harvest period, they did not proceed in their development until March 2017, probably due to the low temperatures prevailing at this period. ...
Article
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The effect of substrate type and cultivation site in the urban fabric on growth, nutrient content and potentially toxic element (PTE) accumulation in tissues of the halophyte Crithmum maritimum was studied. Plantlets were cultivated for twelve months in containers with a green-roof infrastructure fitted and placed either on an urban second-floor roof or on ground level by the side of a moderate-traffic street. Two substrate types were used; one comprising grape marc compost, perlite and pumice (3:3:4, v/v) and one composed of grape marc compost, perlite, pumice and soil (3:3:2:2, v/v), with 10 cm depth. Plants grew well on both sites, although aboveground growth parameters and nutrient content in leaves were greater at street level. Both cultivation site and substrate type affected heavy-metal accumulation in plant tissues. Cu, Ni and Fe concentrations in leaves and Pb in roots were higher in street-level-grown plants compared to the roof-grown plants, and concentrations of Cu and Mn in leaves and Fe in both leaves and roots were lower in the soilless substrate compared to the soil-substrate, making the soilless type preferable in the interest of both safer produce for human consumption and lower construction weight in the case of green-roof cultivation.
... Study species and mean ± SEM for analysed physiological and morpho-anatomical traits (Table 2)and P50 of 11 out of 14 species are fromSavi et al. (2016b) Species ...
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Key message Easily measurable functional traits can be used as proxies in the selection of drought-tolerant saplings for reforestation in Mediterranean ecosystems. Abstract Heat and drought events—increasing both in frequency and severity—have led to forest decline, and are a serious threat for the Mediterranean biome. Whereas drought tolerance of adult trees of different Mediterranean species has been widely investigated, this is not the case for saplings and young trees. We analysed correlations and trade-offs among leaf (water potential at the turgor loss point, Ψtlp, modulus of elasticity, ε, osmotic potential at full turgor, π0, leaf capacitance, Cleaf_dw, leaf venation, VLA, leaf mass per area, LMA) and stem (wood capacitance and wood density, Cwood and Dwood, stem-specific conductivity and water potential inducing 50% loss of hydraulic conductance) functional traits of saplings for 14 woody species of the Mediterranean flora. The results support previously reported correlations among functional traits known to confer drought tolerance to plants. In particular, Ψtlp was positively correlated to π0, Cleaf_dw and VLA, while negatively correlated to ε and LMA. A highly significant correlation was highlighted between Cwood and Dwood. Overall, we observed surprisingly low symplastic and apoplastic resistance. We identify some easily measurable traits (π0 and LMA), which evidence seedlings’ ability to cope with drought, and which therefore could be used as proxies in the selection of drought-tolerant saplings for reforestation in Mediterranean areas.
... Configuration varies by region and climatic zone, but most green roofs typically have a shallow substrate (b100 mm) with a high hydraulic conductivity to promote drainage, as well as low organic matter content for long-term stability and to prevent the spread of fire (FLL, 2008). Consequently, these substrates have low water availability, such that in hot and dry climates green roofs need to be planted with drought-resistant species which can also survive intense heat and wind exposure (Farrell et al., 2012;Nagase and Dunnett, 2010;Rayner et al., 2016;Savi et al., 2016). Green roof water storage capacity can be manipulated by increasing the depth and water holding capacity of the substrate (Cao et al., 2014;Farrell et al., 2013a;Feitosa and Wilkinson, 2016) any by installing water-retention layers or reservoirs below the substrate (Savi et al., 2013;Simmons et al., 2008). ...
Article
Green roofs are increasingly being used among the suite of tools designed to reduce the volume of surface water runoff generated by cities. Plants provide the primary mechanism for restoring the rainfall retention capacity of green roofs, but selecting plants with high water use is likely to increase drought stress. Using empirically-derived plant physiological parameters, we used a water balance model to assess the trade-off between rainfall retention and plant drought stress under a 30-year climate scenario. We compared high and low water users with either drought avoidance or drought tolerance strategies. Green roofs with low water-using, drought-avoiding species achieved high rainfall retention (66-81%) without experiencing significant drought stress. Roofs planted with other strategies showed high retention (72-90%), but they also experienced >50days of drought stress per year. However, not all species with the same strategy behaved similarly, therefore selecting plants based on water use and drought strategy alone does not guarantee survival in shallow substrates where drought stress can develop quickly. Despite this, it is more likely that green roofs will achieve high rainfall retention with minimal supplementary irrigation if planted with low water users with drought avoidance strategies.
... Inconsistent results between green roof and lab germination suc- bicolor and S. tridentata in the hottest parts of the same green roof and other recent work suggests the effects of high temperature may be more important in promoting mortality than drought (Savi et al., 2016). Our results are consistent with these in that the featureless shallow microsites had the lowest seedling densities, the highest temperatures but not the lowest moisture contents. ...
Article
Urban areas benefit from the ecosystem services provided by low input green roofs. However, limited substrate depth on these green roofs creates challenging conditions for plant establishment and survival, leading to industry reliance on non-native succulents. Through a green roof and glasshouse study, we assessed the impact of simple design modifications to the green roof surface, including redistribution of substrate and addition of logs and pebble piles, on both substrate temperature and moisture content. We added seeds of 26 native species and quantified seedling density, species richness and composition over a single growing season. Overall effects of microsite heterogeneity on species diversity were assessed using species accumulation curves. The modifications altered substrate temperature and moisture. Deep substrate (10–12 cm) and the presence of surface features reduced the temperature by 14.6°C and, while surface features had mixed effects on substrate moisture on the green roof, pebble piles slowed moisture loss during a 6-week drought in the glasshouse. Following drought conditions, seedling density and species richness were greatest, relative to seeded controls, where substrate was deep on the green roof and where pebbles were present in glasshouse modules, despite high mortality overall. Design modifications did not result in differentiation of seedling communities among different microsite types. Species accumulation curves showed no difference in species richness between aggregates of modified vs. unaltered microsites. Synthesis and applications. Redistribution of green roof substrate and the addition of logs and pebble piles altered microsite conditions and created habitat heterogeneity on a green roof. These design modifications represent a minimalist strategy to ameliorate growing conditions, improve seedling survival and decrease species loss on shallow substrate green roofs. © 2017 The Authors. Journal of Applied Ecology
... The system can be applied manually and each module can be easily removed for maintenance purposes. The materials and plants used in the system aim to minimize the irrigation needs (Savi et al., 2016;, to improve buildings thermal behaviour (Manso and Castro-Gomes, 2016) and their acoustic conditions (Manso et al., 2017). ...
Article
The construction and use of buildings represent about half of the extracted materials and energy consumption, and around one third of the water consumption and waste produced in the European Union. Therefore it is becoming more important to use sustainable materials that reduce the environmental impacts of construction, by conserving and using resources more efficiently. Green walls can be used as a sustainable strategy to reduce the environmental impact of buildings. The aim of this study is to evaluate the environmental impact of a new modular system for green roofs and green walls (Geogreen) which uses waste and sustainable materials in its composition. A life cycle analysis (LCA) is used to evaluate the long term environmental benefits of this system. The life cycle analysis (LCA) is carried according to ISO 14040/44 using GaBi software and CML 2001 impact category indicators. The adopted functional unit is the square meter of each material required to assemble the Geogreen system. This study also compares the environmental performance of the Geogreen system with other living wall systems and other cladding materials using data from the literature. This LCA study of the Geogreen system became relevant to identify a curing process with a major impact on GWP due to the energy consumed in this process. A change on this process allowed reducing 74% of the overall GWP. After this change it can be noticed that the Geogreen System presents one of the lowest environmental burden when compared to other construction systems.
... One solution may be to use a coarse substrate on top to reduce moisture around shoots and a layered structure with a finer substrate deeper in the profile that is actually able to retain some water. Layered configurations may also be of wider interest, as high substrate temperature is a considerable problem for roof vegetation under dry Mediterranean conditions (Savi et al. 2016), but can be manipulated by substrate depths (Reyes et al. 2016) and to some extent by substrate composition (Sandoval et al. 2017). Further, roots are less frost-tolerant than shoots and hence benefit from substrates which they can forage into depths which are better frost insulated (Boivin et al. 2001) and layered structures may better handle both water amounts and contaminants (Wang et al. 2017). ...
Article
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Background and aimsLayered profiles of designed soils may provide long-term benefits for green roofs, provided the vegetation can exploit resources in the different layers. We aimed to quantify Sedum root foraging for water and nutrients in designed soils of different texture and layering. Methods In a controlled pot experiment we quantified the root foraging ability of the species Sedum album (L.) and S. rupestre (L.) in response to substrate structure (fine, coarse, layered or mixed), vertical fertiliser placement (top or bottom half of pot) and watering (5, 10 or 20 mm week−1). ResultsWater availability was the main driver of plant growth, followed by substrate structure, while fertiliser placement only had marginal effects on plant growth. Root foraging ability was low to moderate, as also reflected in the low proportion of biomass allocated to roots (5–13%). Increased watering reduced the proportion of root length and root biomass in deeper layers. Conclusions Both S. album and S. rupestre had a low ability to exploit water and nutrients by precise root foraging in substrates of different texture and layering. Allocation of biomass to roots was low and showed limited flexibility even under water-deficient conditions.
... While several studies have evaluated the drought tolerance of shrubs from dry and semidry habitats (Farrell et al. 2013, Papafotiou et al. 2013, Raimondo et al. 2015, Savi et al. 2016, there are no studies that have assessed shrubs based on physiological and climate of origin approaches. For stormwater retention, the ideal shrub species for green roofs would have high evapotranspiration under well-watered conditions, but will also tolerate water-deficit conditions as demonstrated by a more negative midday water potential (Farrell et al. 2013) (Fig. 1). ...
Article
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The ideal species for green or vegetated roofs should have high water use after rainfall to maximise 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 climate of origins, 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 (ΨMD) 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 ΨMD) under WD. Although some species which had high water use under WW conditions could avoid drought stress (high ΨMD) used. Water use was strongly related to plant biomass, total leaf area and leaf traits (SLA and LAR). Conversely, drought response (ΨMD) was not related to morphological traits. Species’ climate of origin was not related to drought response or water use. Drought avoiding shrubs (high ΨMD) 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. This article is protected by copyright. All rights reserved.
... Samples were then subjected to three freezing-thawing cycles (1 min in liquid nitrogen followed by 30 min at laboratory temperature), shaken for 5 min, and the final electrical conductivity was measured (C f ) on another 10 ll aliquot. REL was finally calculated as (C i / C f ) 9 100 (Savi et al., 2016). ...
Article
Synchrotron X‐ray computed micro‐tomography (microCT) has emerged as a promising noninvasive technique for in vivo monitoring of xylem function, including embolism build‐up under drought and hydraulic recovery following re‐irrigation. Yet, the possible harmful effects of ionizing radiation on plant tissues have never been quantified. We specifically investigated the eventual damage suffered by stem living cells of three different species exposed to repeated microCT scans. Stem samples exposed to one, two or three scans were used to measure cell membrane and RNA integrity, and compared to controls never exposed to X‐rays. Samples exposed to microCT scans suffered serious alterations to cell membranes, as revealed by marked increase in relative electrolyte leakage, and also underwent severe damage to RNA integrity. The negative effects of X‐rays were apparent in all species tested, but the magnitude of damage and the minimum number of scans inducing negative effects were species‐specific. Our data show that multiple microCT scans lead to disruption of fundamental cellular functions and processes. Hence, microCT investigation of phenomena that depend on physiological activity of living cells may produce erroneous results and lead to incorrect conclusions.
... In regions with Mediterranean conditions, green roof plants must endure difficult environmental conditions to survive, therefore tolerance to drought can be considered one of the essential selection traits [11,31]. Thus, the type of plants chosen is quite determinant of the success of the green roof. ...
Article
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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.
... High water requirements of green roofs are a limiting factor for their development in water-scarce areas. To reduce water needs the use of drought and heat tolerant species are suggested (Savi et al., 2016). Another solution could be the use of alternative water resources such as greywater (Mahmoudi et al., 2021). ...
Article
This work focuses on the use of green roof as a modified shallow vertical flow constructed wetland for greywater treatment in buildings. Different design parameters such as substrate (perlite or vermiculite), substrate depth (15 cm or 25 cm), and plant species (Geranium zonale, Polygala myrtifolia or Atriplex halimus) were tested to determine optimum selection. In addition, the application of a 40% recirculation rate was applied during last month of the experiment to quantify the efficiency of pollutants removal. The experiment was conducted for a period of 12 months under typical Mediterranean climatic conditions in Lesvos island, Greece. Results showed that green roofs planted with Atriplex halimus and filled with 20 cm of vermiculite had the best COD (91%), BOD (91%), TSS (93%) and turbidity (93%) average removal efficiencies. In contrast, significant lower removals were observed when the substrate depth was decreased to 10 cm (60–75%). Green roof vegetation had significant impact on TN removal as the average TN concentration decreased from 6.5 ± 1.8 mg/L in the effluent of unplanted systems to 4.9 ± 2.7 mg/L in the effluent of green roofs planted with Atriplex halimus. The recirculation of a portion of the effluent in the influent had as a result a significant improvement of turbidity, organic matter and (especially) nitrogen removal. For example, BOD removal in green roofs planted with Atriplex halimus and filled with 20 cm of perlite increased from 76% to 92%, while TN removal in green roofs planted with the same plants and filled with 20 cm of vermiculite increased from 56% to 87%. Overall, the operation of green roofs as modified vertical unsaturated constructed wetlands seems a sustainable nature-based solution for greywater treatment and reuse in urban areas.
... Species were sampled in natural habitats in the Karst region (NE Italy), or in the Botanical Garden of University of Trieste. Additional data were obtained from previous studies performed in our laboratory (see Table S1 , Nardini et al., 2012;Savi et al., 2016a;Savi et al., 2016b;Savi et al., 2016c;Savi et al., 2017). ...
Article
Drought tolerance shapes the distribution of plant species, and it is mainly determined by the osmotic potential at full turgor (π0) and the water potential at turgor loss point (Ψtlp). We provide a simplified framework for π0 and Ψtlp measurements based on osmometer determination of π0 (π0_osm). Specifically, we ran regression models to i) improve the predictive power of the estimation of π0 from π0_osm and morpho-anatomical traits; ii) obtain the most accurate model to predict Ψtlp on the basis of the global relationship between π0 and Ψtlp. The inclusion of the leaf dry matter content (LDMC), an easy-to-measure trait, in the regression model improved the predictive power of the estimation of π0 from π0_osm. When π0_osm was used as a simple predictor of Ψtlp, discrepancies arose in comparison with global relationship between π0 and Ψtlp. Ψtlp values calculated as a function of the π0 derived from π0_osm and LDMC (π0_fit) were consistent with the global relationship between π0 and Ψtlp. The simplified framework provided here could encourage the inclusion of mechanistically sound drought tolerance traits in ecological studies.
... Whilst this strategy can be achieved by some plant functional types, woody plants exhibit a trade-off between drought response and water use (Du et al. 2018): drought resistant shrubs tend to have a low water use, whereas shrubs that have a high transpiration rates are not drought tolerant. Consequently, to select shrubs or other woody plants for future green roofs under water-deficit it will be more important to select species with high drought resistance and conservative water use Savi et al. 2016). ...
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Aims Green roofs are important novel urban ecosystems, but their shallow substrates can create plant water deficits in dry climates. Physiological approaches can improve green roof plant selection, and shrubs with high drought tolerance and conservative water use under water-deficit should perform well. The water potential at turgor loss point (Ψtlp) has been used to predict drought resistance. Therefore we aimed to determine whether Ψtlp could be used as a screening tool to assess drought resistance for green roof plant selection. Methods We evaluated 20 shrub species, originating from ecosystems varying in water availability, quantified by heat moisture index (HMI) and mean annual precipitation. We conducted a water-deficit experiment to measure Ψtlp, the degree of iso-anisohydry (△ΨMD) and water use (ET) in response to drought. Results Shrubs with lower Ψtlp were more anisohydric (greater △ΨMD) and had a more conservative water use (lower ET). However, Ψtlp, ΔΨMD and ET were not related to HMI. Conclusions These results suggest that Ψtlp could be used to select shrubs for green roofs, as species with lower Ψtlp tended to be more drought tolerant, more anisohydric and used less water under water-deficit. However, species with higher Ψtlp could also potentially survive through drought avoidance.
... Leaf water potential isotherms (PV-curves) were measured in order to evaluate the symplastic drought tolerance and its variations during acclimation to water deficit (Turner, 1988;Alsina et al., 2007;Savi et al., 2016). PV-curves mere measured at the beginning of June (high water availability), and at the peak of summer drought (July). ...
Article
Environmental sustainability of viticulture is negatively affected by prolonged droughts. In limestone dominated regions, there is limited knowledge on grapevine water status and on methods for accurate evaluation of actual water demand, necessary to appropriately manage irrigation. During a dry vintage, we monitored plant and soil water relations in old and young vines of Istrian Malvasia on Karst red soil. The vineyard with young vines was additionally subdivided into two areas, based on their soil type, 1) karst silty-clay loam, and 2) mixture of crushed rocks and karst silty-clay loam (stony soil). Seasonal changes in exploited water resources were estimated via analysis of oxygen isotope composition (δ18O) of rainfall, deep soil water, and xylem sap. We hypothesized that plants are able to thrive during drought thanks to the water stored in deep soil layers, while they rely less on superficial soil horizons. Our results show that vines growing on karstic substrates have deep roots securing the use of stable water sources during summer, with consequent favourable plant water status. In fact, both young and mature vines approached the threshold of severe water stress, but never surpassed it, as midday leaf water potentials were >-1.3MPa in all study sites. Vines roots showed flexible water uptake, i.e. the ability to absorb water from deep or shallow soil horizons during drought and after late-summer thunderstorms, which was particularly evident in vines growing on the stony soil. In fact, precipitations of 20mm were enough for plant water status recovery, due to fast infiltration. On the other hand, at least 50mm of rainfall were necessary to induce water status recovery in more compact soil (karst silty-clay loam). Our findings provide new knowledge on the rooting depth and water needs of vines growing on shallow soils overlying fractured limestone bedrock.
... Furthermore, green roofs, and particularly extensive green roof systems, are characterized by the additive effect of both water deficits (water stress) and high air and substrate temperatures (heat stress) [51,52]. During warm periods, the relationship between air temperature and water in a substrate of an extensive green roof strongly influences plant growth. ...
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In green roofs, the use of plant species that withstand dry arid environmental conditions and have reduced water requirements is recommended. The current study presents the effect of irrigation amount on the growth of four different species of lavender; Lavandula angustifolia, Lavandula dentata var. candicans, Lavandula dentata var. dentata, and Lavandula stoechas established on an extensive green roof system and used in urban agriculture. Two irrigation treatments (high and low) determined by the substrate hydraulic properties were applied. Plant growth studied at regular intervals included measurements of plant height, shoot canopy diameter, plant growth index, shoot dry weight and stomatal conductance. The results were consistent and showed that low irrigation reduced plant growth. With the exception of L. stoechas, the appearance of plants watered with the low irrigation treatment was satisfactory, and their use under low water amount irrigation is supported. Interspecies differences among lavender species were present in both irrigation treatments. Overall, L. dentata var. candicans showed the greatest growth, followed in descending order by L. dentata var. dentata and L. angustifolia. In parallel, for stomatal conductance, L. dentata var. candicans showed the lowest value, similar to L. dentata var. dentata, and L. angustifolia the largest. Differences in plant characteristics and size among the latter three species can be considered in the design of extensive green roof systems. The use of substrate hydraulic properties was shown to be important for irrigation management on extensive green roof systems.
... High root zone temperatures can also have detrimental effects on plant physiology, growth and biomass (Liu and Huang, 2005). Savi et al. (2016) also reported a significant positive correlation between root vulnerability to heat stress and mortality in hot and dry substrates. However, a pot-based study of Australian native species used on green roofs which exposed them to successive simulated heat waves, with irrigation, and under moderate irradiance, found only short-term effects on photosynthesis, few changes in plant biomass, and were able to acclimate to heat stress . ...
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Australian cities have been slow to implement green roofs. This is because there are many potential barriers to their widely acceptance as a nature based solution that can make cities more liveable and help them adapt to, and mitigate, climate change. Due to significant differences in rainfall, temperature, available substrates and suitable vegetation relying on northern hemisphere research and experience is problematic as many of the environmental and economic benefits of green roofs are location specific. This paper aims to 1. synthesise a decade of Australian green roof research that has sought to overcome these barriers, 2. assess the current status of the Australian green roof industry and remaining knowledge gaps, and 3. provide a roadmap for future progress developed in multidisciplinary industry workshops. Many of these insights will be applicable to areas with similar seasonally hot and dry climates or emerging green roof markets. We identified that significant progress has been made in addressing the barriers to green roofs in Australia. Research has focused on developing green roofs for local conditions and quantifying their benefits. Substrate research has investigated the suitability of locally available materials with a focus on how water retention additives and organic waste materials can increase plant available water and therefore survival. By taking a plant physiology approach Australian researchers have gained a strong functional understanding of suitable green roof plants and the benefits they provide, considerably expanding the available palette beyond the succulents commonly used internationally. Research has quantified green roofs’ stormwater retention and building insulation and energy benefits and provided evidence that they benefit well-being and performance, important for employee productivity.
... However, plant survival on green roofs is challenging as green roofs often have shallow substrates (< 20 cm depth) to reduce weight-loading on buildings, which reduces plant water availability (Vijayaraghavan, 2016;Shafique et al., 2018). This is exacerbated in hot and dry climates where plants can experience long periods without water in summer, resulting in mortality of many species (Rayner et al., 2016;Savi et al., 2016). Plant survival on green roofs can be improved by increasing water availability through irrigation, improved substrate water retention properties, greater substrate depths or by selecting plants that have greater water availability through succulence (Farrell et al., 2012;Razzaghmanesh et al., 2014b). ...
Article
The many ecosystem services that green roofs can provide rely on good plant coverage and plant survival, which is challenging in hot and dry climates. While true succulents like Sedum spp. have been shown to survive well on green roofs, there are limited studies relating individual traits or trait combinations to survival in other life-forms. Succulence is a rarely studied trait that describes plant water storage in leaves, stems and roots, regardless of life-form. This means that succulence can occur in plants which are not considered to be true succulents like Sedum or Crassula species. Improving water availability through succulence may improve survival on unirrigated green roofs, but succulence, as a trait, has rarely been investigated in plants which are not true succulents. We investigated whether succulence or trait combinations can relate well with survival and could be used to improve plant selection for different substrate depths on hot and dry green roofs. We conducted two experiments with the same 11 Mediterranean species (five herbs, three sub-shrubs and three shrubs); (1) a pot experiment to determine traits under well-watered conditions including: succulence, leaf lethal temperature, water use, root:shoot ratio and leaf area. These individual traits and the combinations of all these traits were used in the analyses of this experiment; and (2) a green-roof module experiment to determine survival in four substrate depths (10, 15, 20, and 25 cm). Survival was not related to succulence, indicating that increased internal water storage in non-succulent plants does not per se lead to greater survival in extreme conditions. Survival was also not related to individual traits relating to water use and leaf heat tolerance or trait combinations. Nevertheless, plants in the same functional groups had similar survival, which suggests plants with similar trait combinations can have similar survival on green roofs.
... This suggests that in hot climate root system, especially on top of the substrate, is challenged by high temperature. Selecting plants with high tolerance to substrate temperature fluctuations (Savi et al., 2016; Table 1), defining growing medium components and depth with the aim to reduce thermal conductivity and heat capacity in green roofs in extreme environments seems necessary. Research evidence has observed that deeper substrate better reduces temperature fluctuations and results in better plant coverage (see Getter et al., 2009 in Table 1). ...
Article
Growing media (substrate) is a fundamental part of a green roof, providing water, nutrients and support to plants. However, little research has reviewed how it affects plant performances in different climatic regions. This study aims to analyse published research on green roof growing medium across world’s climate zones. Findings are structured according to Köppen–Geiger climate classification, aiming to investigate the prevalence of research conducted in different climate zones. Results from full-scale studies and laboratory or greenhouse experiments were reviewed. The later were included as they provide systematic knowledge on the effect of individual factors on system performances although cannot provide climate specific information. Studies discussed effects of major substrate components and depths on plant survival and establishment using standard test procedures. Results showed that most research in the subject were in temperate (group C climate classification), continental (group D) and dry climates (group B), respectively. Considerable number of investigations was conducted in controlled laboratory or greenhouse environments. Based on the results, future green roof research and guidelines should consider climate specifications of the region in designing growing medium, depths and attribute of green roof substrates in order to ensure enhanced plant performance. Especially, for more fragile but less investigated dry climate, considerations should be made to tackle heat fluctuations and drought stress by enhancing water holding capacity and thermal isolation of the substrate. To move forward, sustainable building solutions as a part of future urban forms, climate-adaptive green roof systems should be included into future research, practice and guidelines.
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The multifunctionality and delivery of ecosystem services from green roofs is improved by biological diversity of the roof vegetation. However, the frequency and intensity of drought episodes on extensive green roofs may limit the use of non-succulent species and the potential functional and phylogenetic diversity of the vegetation. Wind accelerates water use by plants and desiccation of the green roof substrate, and may be a key factor in selection of non-succulent plant species for green roofs. In this study, we tested wind interactions with green roof substrate composition and the effects on plant and substrate water balance, overall plant performance, and wilting and survival of three non-succulent species (Plantago maritima L., Hieracium pilosella L., and Festuca rubra L.) under realistic prolonged water deficit conditions. We found that, regardless of species or substrate tested, wind accelerated drought response. Drought-stressed plants exposed to wind wilted and died earlier, mostly due to more rapid desiccation of the growth substrate (critical substrate moisture content was 6-8%). The moderate wind levels applied did not affect plant performance when not combined with drought. Species with contrasting growth forms showed similar responses to treatments, but there were some species-specific responses. This highlights the importance of including wind to increase realism when evaluating drought exposure in non-succulent green roof vegetation.
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Roof gardens, which perform many useful functions for humans, are one of the most modern ways to create a harmonious and aesthetic environment in cities. Therefore, the study of the list of woody plants that successfully overwinter and tolerate critical summer temperatures on the roofs with minimal tending, is an important task for green space workers. The taxonomic composition of the dendroflora of 24 flat of extensive type in the city of Lviv at heights from 5 to 40 m was studied and its system-structural analysis was performed. It was found that the dendroflora of the examined objects was composed of 167 taxonomic units of woody plants (of which 109 species, 76 cultivars, 10 hybrids). The family Rosaceae Juss is numerically superior (13 genera (21.1% of the total number of genera), 32 species and hybrids (26.9%); by the number of cultivars, the Cupressaceae family dominates - 32 cultivars (42.7%). The genus Juniperus dominates in the genus spectrum in terms of the number of species, hybrids and cultivars - 10 species (8.8% of the total number), 13 cultivars (17.1%). For greening the roofs, 93 species and cultivars of deciduous plants, 74 evergreen phanerophytes (28 mesophanerophytes, 69 microfanerophytes, 60 nanophanerophytes and 10 woody lianas) were used. According to the results of the chorological analysis, it was found that species from the East Asian region and North America predominate on the roofs, indigenous species are practically not used. The plant compositions on the roofs are dominated by species regionalized for USDA-frost resistance zones 4 and 5, while the territory of Lviv lies within zone 6. Our 4-year observations proved the success of the introduction on roofs of species of a higher USDA zone than that defined for Lviv. Further study of the dendroflora of green roofs is very important, taking into account several points: the system-structural patterns of species selection for greenery have not been fully clarified; methods of specific introduction of plants on roofs and the possibility of their acclimatization, which is different from terrestrial objects, have not been tested; the possibilities of introducing a list of plants that are not found in garden and park compositions of the city have not been fully studied. Compositional solutions of green roofs are determined by various ways of planting plants - directly into the substrate, in beds or containers raised above the surface. Such studies highlight the peculiarities of the taxonomic composition and structure of the dendroflora of green roofs, which will make it possible to significantly adjust the agronomic techniques of plant maintenance and identify the most promising group of woody plants for greening roofs.
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Green roofs are becoming increasingly prevalent in the United States due to their economical and environmental benefits as compared with conventional roofs. Plant selection for green roofs in the variable climate of the southeastern United States has not been well evaluated. Shallow substrates on green roofs provide less moderation of temperature and soil moisture than deeper soils in traditional landscapes, necessitating empirical evaluation in green roof environments to make informed recommendations for green roof plant selection. Nineteen species and cultivars, including succulents, grasses, and forbs, were evaluated under seasonal irrigated and non-irrigated conditions in experimental green roofs. Plants were planted on 26 Oct. 2009 and each evaluated for survival and increase in two-dimensional coverage of the substrate during establishment, after overwintering, and after the first growing season. The winter 2009-10 was colder than normal, and some plants, such as ice plants (Delosperma spp.), considered to be cold-hardy in this climate did not survive through the winter. Irrigation influenced survival for the summer period and only succulent plants like stonecrops (Sedum spp.) survived without irrigation. Irrigated experimental green roofs had significantly lower summer substrate temperatures (up to 20 F lower) and plants survived in irrigated conditions. Plants that survived both winter and summer under irrigated conditions include pussytoes (Antennaria plantaginifolia), mouse-ear tickseed (Coreopsis auriculata), eastern bottlebrush grass (Elymus hystrix), glade cleft phlox (Phlox bifida stellaria), and eggleston's violet (Viola egglestonii). Irrigation is recommended on extensive green roofs to increase the palette for plant selection by protecting against plant mortality due to drought and extreme soil temperatures.
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Recent studies have highlighted the ecological, economical 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 plant hydraulic conductance were measured at different time intervals following the last irrigation. The substrate type significantly affected water status. A. 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 revealed 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. Published by Oxford University Press on behalf of the Annals of Botany Company.
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Xylem cavitation resistance has profound implications for plant physiology and ecology. This process is characterized by a 'vulnerability curve' (VC) showing the variation of the percentage of cavitation as a function of xylem pressure potential. The shape of this VC varies from 'sigmoidal' to 'exponential'. This review provides a panorama of the techniques that have been used to generate such a curve. The techniques differ by (i) the way cavitation is induced (e.g. bench dehydration, centrifugation, or air injection), and (ii) the way cavitation is measured (e.g. percentage loss of conductivity (PLC) or acoustic emission), and a nomenclature is proposed based on these two methods. A survey of the literature of more than 1200 VCs was used to draw statistics on the usage of these methods and on their reliability and validity. Four methods accounted for more than 96% of all curves produced so far: bench dehydration-PLC, centrifugation-PLC, pressure sleeve-PLC, and Cavitron. How the shape of VCs varies across techniques and species xylem anatomy was also analysed. Strikingly, it was found that the vast majority of curves obtained with the reference bench dehydration-PLC method are 'sigmoidal'. 'Exponential' curves were more typical of the three other methods and were remarkably frequent for species having large xylem conduits (ring-porous), leading to a substantial overestimation of the vulnerability of cavitation for this functional group. We suspect that 'exponential' curves may reflect an open-vessel artefact and call for more precautions with the usage of the pressure sleeve and centrifugation techniques.
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The pressure-bomb technique as developed by Scholander and colleagues is reviewed. A theoretical analysis of the equilibrium water-relations of individual cells of a twig is derived taking due account of the fact that each cell has a unique solute concentration, fluid volume, shape, and unique mechanical constraint by virtue of its cell-wall structure and attachment to nearest neighbours. These equations combine to give a complete description of the whole twig in response to mechanical (air pressure) stress. Our theoretical analysis suggests that the ‘pressure-volume curve’ can be related quantitatively to meaningful bulk parameters of water relations: viz. the total osmolar content of the symplast Ns, the original volume of the symplast Vo, the volume expressed from the symplast Ve, the gas-pressure of the bomb P, and the volume-averaged turgor pressure (the sum of the products of the relative volume and turgor pressure of each cell). An empirical relation for the volume-averaged turgor pressure of twigs is found which fits all species examined; it also fits the turgor pressure relation for single (Nitella) cells.
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Extensive green roof habitats are characterized by shallow substrates and extreme soil-moisture conditions. This set of characteristics, or "habitat template," has natural analogs in rock barren ecosystems such as cliffs, scree slopes, and limestone pavements. Typical plants used in green roof initiatives often have their origins in rocky habitats, as do a host of other common urban species. This paper examines the implications of using natural ecosystems as templates for green roof design. While green roof plant selection has targeted drought-tolerant species, the incorporation of other features of rocky habitats may improve green roof functions.
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Background and aims Green roofs are often installed to reduce urban stormwater runoff. To optimally achieve this, green roof plants need to use water when available, but reduce transpiration when limited to ensure survival. Succulent species commonly planted on green roofs do not achieve this. Water availability on green roofs is analogous to natural shallow-soil habitats including rock outcrops. We aimed to determine whether granite outcrop species could improve green roof performance by evaluating water use strategies under contrasting water availability. Methods Physiological and morphological responses of 12 granite outcrop species with different life-forms (monocots, herbs and shrubs) and a common green roof succulent were compared in well watered (WW) and water deficit (WD) treatments. Key results Granite outcrop species showed a variety of water-use strategies. Unlike the green roof succulent all of the granite outcrop species showed plasticity in water use. Monocot and herb species showed high water use under WW but also high water status under WD. This was achieved by large reductions in transpiration under WD. Maintenance of water status was also related to high root mass fraction. Conclusions By developing a conceptual model using physiological traits we were able to select species suitable for green roofs. The ideal species for green roofs were high water users which were also drought tolerant.
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Shifts in rainfall patterns and increasing temperatures associated with climate change are likely to cause widespread forest decline in regions where droughts are predicted to increase in duration and severity. One primary cause of productivity loss and plant mortality during drought is hydraulic failure. Drought stress creates trapped gas emboli in the water transport system, which reduces the ability of plants to supply water to leaves for photosynthetic gas exchange and can ultimately result in desiccation and mortality. At present we lack a clear picture of how thresholds to hydraulic failure vary across a broad range of species and environments, despite many individual experiments. Here we draw together published and unpublished data on the vulnerability of the transport system to drought-induced embolism for a large number of woody species, with a view to examining the likely consequences of climate change for forest biomes. We show that 70% of 226 forest species from 81 sites worldwide operate with narrow (<1 megapascal) hydraulic safety margins against injurious levels of drought stress and therefore potentially face long-term reductions in productivity and survival if temperature and aridity increase as predicted for many regions across the globe. Safety margins are largely independent of mean annual precipitation, showing that there is global convergence in the vulnerability of forests to drought, with all forest biomes equally vulnerable to hydraulic failure regardless of their current rainfall environment. These findings provide insight into why drought-induced forest decline is occurring not only in arid regions but also in wet forests not normally considered at drought risk.
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Unlabelled: • Premise of the study: Vessels are the chief conduit for long-distance water transport in the majority of flowering plants. Vessel length is a key trait that determines plant hydraulic efficiency and safety, yet relatively little is known about this xylem feature. • Methods: We used previously published studies to generate a new global data set of vessel length in woody plants. These data were used to examine how evolutionary history, plant habit, environment, and growth ring porosity influenced vessel length. We also examined the relationship between mean vessel length and mean vessel diameter and maximum vessel length. • Key results: Data on mean vessel length were available for stems of 130 species and on maximum vessel length for stems of 91 species. A phylogenetic analysis indicated that vessel length did not exhibit significant phylogenetic signal. Liana species had longer vessel lengths than in tree or shrub species. Vessel diameter was not predictive of mean vessel length, but maximum vessel length strongly predicted mean vessel length. Vessel length did not vary between species that differed in growth ring porosity. • Conclusions: Many traits often assumed to be linked to vessel length, including growth ring porosity and vessel diameter, are not associated with vessel length when compared interspecifically. Sampling for vessel length has been nonrandom, e.g., there are virtually no data available for roots, and sampling for environment has been confounded with sampling for habit. Increased knowledge of vessel length is key to understanding the structure and function of the plant hydraulic pathway.
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Several techniques have been developed to quantify the degree of embolism of the xylem using hydraulic conductance. Although there have been several improvements to these techniques, their reliability is still questionable and many technical pitfalls persist. We are proposing here a manometric approach to improve the accuracy of xylem cavitation measurement by the original air-injection technique which uses twigs exposed to pressurized air to cause cavitation. The measured parameter is air bubble production (P b) caused by xylem cavitation in birch (Betula pendula Roth) twigs from which the percent increase in bubble production is calculated to quantify xylem cavitation. Data produced by three different methods (bench-drying, air-injection, and manometric approach) are compared. Xylem vulnerability curves (VCs) constructed by the reference and reliable bench-drying technique and the manometric approach show similar sigmoid “S” shape, but a small anomaly appeared in the VC constructed by the original air-injection technique. The xylem pressure inducing 50% of embolism (P 50) was the same with the three techniques. Furthermore, there was a strong positive correlation between the estimators of xylem cavitation measured by the three different methods. For its reliability, precision and ease we recommend the manometric technique as an improved version of the original hydraulic air-injection method.
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The growing desire to make the urban environment more sustainable from an ecological point of view has stimulated research on the architectural and agronomic aspects of green roofs. The practical realisation of green roofs, is however limited by economic and ecological issues. More specifically, water availability is the most limiting factor, and is likely to be ever more so in the future in the light of climate change. For this reason, we evaluated the agronomic performance of several xerophytes in a simulated dry green roof. Seeds of 20 species were collected in typically dry habitats (abandoned quarries, rocky soils, dunes, etc.) and studied in the laboratory for germination ecology. In cases of strong dormancy, methods were tested to stimulate germination and their germination ecology was studied. The resulting seedlings were transplanted in spring 2008 in two green roof types that differ in substrate depth (150 and 200mm) made up of lapil, pumice, zeolites and peat, resting on a drainage layer of hydroperlite. Temperature and humidity in the substrate and drainage layer were measured during the whole test period. Survival of the seedlings in both substrate depths was almost 100%, favoured by a rainy spring. Most of the tested species showed an excellent performance during the hot and dry summer months in terms of survival rates, growth, and vegetation cover dynamics, notwithstanding the difficult ecological conditions (temperatures around 50°C; hydric potential Ψ -15 bars). Furthermore, most of the species had a long flowering stage in the first year of growth. Plants in the green roof with the deeper substrate depth produced, for most of the tested taxa, a significantly higher vegetation cover and growth compared to when they were placed in the 150mm substrate. The results of this study show that some Mediterranean xerophytes have biological characteristics suitable for their use in dry green roofs, although an irrigation system for emergency use seems advisable. To conclude, further research should focus on long term evaluation of green roof vegetation in terms of plant survival and flowering dynamics. KeywordsGreen roof-Substrate depth-Mediterranean flora-Dry tolerance-Biodiversity
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Fifteen herbaceous perennial grass and herb species were planted into experimental green roofs in spring 2001. The species differed widely in their origins, heights, flowering times, life spans and growth forms but all were typical of dry and nutrient-stressed habitats. Three individuals of each species of a standardised size were randomly assigned to a planting grid at 20cm apart in each experimental replicate plot at substrate depth of either 100 or 200mm. Each treatment was replicated three times. During each growing season, the mean height and spread of each individual was recorded, together with flowering performance and % vegetation cover. In addition the numbers and % covers of all spontaneous colonised species were recorded. Greatest survival, diversity, size and flowering performance of planted species occurred at 200mm depth. Bare ground and moss cover was greatest at 100mm, as was diversity of colonising species. Differences between the early years and the final years of the experiment indicate the need for long-term monitoring of green roofs in addition to short-term experiments.
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Green roofs have the potential to retain stormwater on the roof surface and lower the thermal loading on buildings. Because of this, the greatest environmental benefits from green roofs might be achieved in subtropical climates characterized by high temperatures and intense rain events. There is, however, little research to support this. In a replicated study in Texas, we compared the performance of six different extensive green roof designs vegetated with native species, to non-reflective (black) roofs, and reflective (white) roofs. Preliminary hydrologic and thermal profile data indicated not only differences between green and non-vegetated roofs, but also among green roof designs. Maximum green roof temperatures were cooler than conventional roofs by 38°C at the roof membrane and 18°C inside air temperature, with little variation among green roofs. Maximum run-off retention was 88% and 44% for medium and large rain events but some green roof types showed very limited retention characteristics. These data demonstrate indicate that: 1. Green roofs can greatly affect the roof temperature profile—cooling surface layers and internal space on warm days. 2. Green roofs can retain significant amounts of rainfall, this is dependent on the size of the rain event and design and can fail if not designed correctly. We suggest that as green roofs vary so much in their design and performance, they must be designed according to specific goals rather than relying on assumed intrinsic attributes.
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The aim of this study was to extent the range of knowledge about water relations and stomatal responses to water stress to ten Mediterranean plants with different growth forms and leaf habits. Plants were subjected to different levels of water stress and a treatment of recovery. Stomatal attributes (stomatal density, StoD), stomatal conductance (g s), stomatal responsiveness to water stress (SR), leaf water relations (pre-dawn and midday leaf water potential and relative water content), soil to leaf apparent hydraulic conductance (K L) and bulk modulus of elasticity (ε) were determined. The observed wide range of water relations and stomatal characteristics was found to be partially depended on the growth form. Maximum g s was related to StoD and the stomatal area index (SAI), while g s evolution after water stress and recovery was highly correlated with K L. Relationships between SR to water deficit and other morphological leaf traits, such as StoD, LMA or ε, provided no general correlations when including all species. It is concluded that a high variability is present among Mediterranean plants reflecting a continuum of leaf water relations and stomatal behaviour in response to water stress.
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Greenhouse gas emissions have significantly altered global climate, and will continue to do so in the future. Increases in the frequency, duration, and/or severity of drought and heat stress associated with climate change could fundamentally alter the composition, structure, and biogeography of forests in many regions. Of particular concern are potential increases in tree mortality associated with climate-induced physiological stress and interactions with other climate-mediated processes such as insect outbreaks and wildfire. Despite this risk, existing projections of tree mortality are based on models that lack functionally realistic mortality mechanisms, and there has been no attempt to track observations of climate-driven tree mortality globally. Here we present the first global assessment of recent tree mortality attributed to drought and heat stress. Although episodic mortality occurs in the absence of climate change, studies compiled here suggest that at least some of the world's forested ecosystems already may be responding to climate change and raise concern that forests may become increasingly vulnerable to higher background tree mortality rates and die-off in response to future warming and drought, even in environments that are not normally considered water-limited. This further suggests risks to ecosystem services, including the loss of sequestered forest carbon and associated atmospheric feedbacks. Our review also identifies key information gaps and scientific uncertainties that currently hinder our ability to predict tree mortality in response to climate change and emphasizes the need for a globally coordinated observation system. Overall, our review reveals the potential for amplified tree mortality due to drought and heat in forests worldwide.
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Extensive past efforts have been taken toward understanding heat tolerance mechanisms of the aboveground organs. Root systems play critical roles in whole-plant adaptation to heat stress, but are less studied. This review discusses recent research results revealing some critical physiological and metabolic factors underlying root thermotolerance, with a focus on temperate perennial grass species. Comparative analysis of differential root responses to supraoptimal temperatures by a heat-adapted temperate C3 species, Agrostis scabra, which can survive high soil temperatures up to 45 °C in geothermal areas in Yellow Stone National Park, and a heat-sensitive cogeneric species, Agrostis stolonifera, suggested that efficient carbon and protein metabolism is critical for root thermotolerance. Superior root thermotolerance in a perennial grass was associated with a greater capacity to control respiratory costs through respiratory acclimation, lowering carbon investment in maintenance for protein turnover, and efficiently partitioning carbon into different metabolic pools and alternative respiration pathways. Proteomic analysis demonstrated that root thermotolerance was associated with an increased maintenance of stability and less degradation of proteins, particularly those important for metabolism and energy production. In addition, thermotolerant roots are better able to maintain growth and activity during heat stress by activating stress defence proteins such as those participating in antioxidant defence (i.e. superoxide dismutase, peroxidase, glutathione S-transferase) and chaperoning protection (i.e. heat shock protein).
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High temperature (HT) and water deficit (WD) are frequent environmental constraints restricting plant growth and productivity. These stresses often occur simultaneously in the field, but little is known about their combined impacts on plant growth, development and physiology. We evaluated the responses of 10 Arabidopsis thaliana natural accessions to prolonged elevated air temperature (30 °C) and soil WD applied separately or in combination. Plant growth was significantly reduced under both stresses and their combination was even more detrimental to plant performance. The effects of the two stresses were globally additive, but some traits responded specifically to one but not the other stress. Root allocation increased in response to WD, while reproductive allocation, hyponasty and specific leaf area increased under HT. All the traits that varied in response to combined stresses also responded to at least one of them. Tolerance to WD was higher in small-sized accessions under control temperature and HT and in accessions with high biomass allocation to root under control conditions. Accessions that originate from sites with higher temperature have less stomatal density and allocate less biomass to the roots when cultivated under HT. Independence and interaction between stresses as well as the relationships between traits and stress responses are discussed.
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Leaves are extraordinarily variable in form, longevity, venation architecture, and capacity for photosynthetic gas exchange. Much of this diversity is linked with water transport capacity. The pathways through the leaf constitute a substantial (>or=30%) part of the resistance to water flow through plants, and thus influence rates of transpiration and photosynthesis. Leaf hydraulic conductance (K(leaf)) varies more than 65-fold across species, reflecting differences in the anatomy of the petiole and the venation architecture, as well as pathways beyond the xylem through living tissues to sites of evaporation. K(leaf) is highly dynamic over a range of time scales, showing circadian and developmental trajectories, and responds rapidly, often reversibly, to changes in temperature, irradiance, and water supply. This review addresses how leaf structure and physiology influence K(leaf), and the mechanisms by which K(leaf) contributes to dynamic functional responses at the level of both individual leaves and the whole plant.
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Plant selection for extensive green roofs has largely been based on cool, temperate climate research. However, as green roof implementation in hotter and drier climates increases, there is a need to evaluate plant performance under these climatic conditions. Succulents have been shown to be successful in hot and dry green roofs, although survival differs between species and the role of leaf succulence in survival has not been fully explored. For non-succulent plants, habitats with conditions similar to green roofs (‘habitat templates’) have been used to select plants, although few studies have discussed the performance of these selections under green roof conditions. Therefore, we evaluated establishment of 32 plant species on an unirrigated extensive (125 mm deep) green roof in Melbourne, Australia over a 42 week period (from winter through summer into autumn). Plants were selected on the basis of life-form, succulence, appropriate habitat templates and/or successful use on green roofs internationally. Climatic conditions during the experiment were often extreme, with evaporation regularly exceeding rainfall and a hot and dry summer (mean maximum air temperature 35 °C and 80.6 mm total rainfall), leading to roof temperatures of 65 °C. After 42 weeks, only succulent plants remained alive and only three of the succulent species had 100% survival. Survival was positively related to the degree of leaf succulence (g H2O leaf area cm−1) making this a useful trait for plant selection for unirrigated green roofs in hot, dry climates. The failure of most species, despite being chosen from appropriate habitats, demonstrates the need to evaluate potential plants on green roofs under extreme climatic conditions. Supplementary irrigation may be essential to sustain non-succulent species during extreme weather in hot and dry climates.
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From its beginnings in Germany in the twentieth century, a thriving extensive green roof industry has become established in many countries in temperate climates. Based upon the success of the industry, and with an expectation that this technology will be adopted in other climates, this review of the ecological research of extensive green roofs aims to evaluate the application of this knowledge. The modern extensive green roof is the product of research in the 1970s by German green roof pioneers; the selection of suitable species from analogue habitats led to green roof vegetation dominated by drought tolerant taxa. The commercial success of extensive green roof systems can be attributed to engineering and horticultural research, to policy mechanisms in some places, and to a market that encourages innovation, and the origins in ecological design are now easily overlooked. Some of the work reviewed here, including the classification of spontaneous roof vegetation into plant communities, is not widely known due to its confinement to the German literature. By re-visiting the history of the extensive green roof and reviewing the ecological research that has contributed to our understanding of it, the intention is, for this paper, to inform those considering green roofs in other climatic regions, to apply an ecologically informed approach in using local knowledge for developing installations that are suited to the bioregion in which they occur. Finally the paper considers some future directions for research and practice.
Article
Appropriate vegetation is critical for green roofs to perform their social and ecological functions. This study was conducted on a third-story rooftop to investigate the effect of substrate depth on the establishment of plants for potential use on unirrigated extensive green roofs in Beijing, China. Eighteen non-succulent herbaceous perennial species were tested on substrates of three different depths (10 cm, 15 cm and 20 cm) from October 2012 to November 2013. Measurements included plant survival, growth index (GI), visual appearance, coverage and substrate moisture content. It was concluded that the major cause of plant mortality in unirrigated green roofs in Beijing was not summer water deficit, but the water deficit that occurs during Beijing's natural dry spring. Deeper substrate promoted higher survival during this water stressed period. Most species exhibited the greatest GI and visual rating at the depth of 20 cm. Of the 18 species tested, three geophytes were the most drought-tolerant. Only Allium senescens was found to be suitable for use in unirrigated green roofs with substrates no deeper than 10 cm. Iris 'Music Box' and Hemerocallis 'Stella de Oro' are also suitable for unirrigated green roofs if planted at substrate depths of at least 15 cm and a depth of 20 cm facilitated survival of the greatest number species. Supplemental irrigation is highly recommended for extensive green roofs with non-succulent herbaceous perennials in Beijing and other similar climate regions with the same temporal distribution of precipitation. Nepeta cataria and Saponaria ocymo ides are also potential species for unirrigated extensive green roofs because of their capacity for self seeding.
Article
The need of planning more sustainable cities leads to a wider use of extensive green roofs (EGRs) as they provide significant advantages to the urban environment (e.g., energy conservation and increase of biodiversity). In Central and North Europe, as well as in North America and Asia, EGRs are generally included in new building designs, whereas they are still uncommon in Mediterranean countries. The adaptations of many Mediterranean plants to drought stress and their floristic diversity constitute, however, positive elements in finding solutions for them. This research proposes a methodological approach to select wild species for EGRs based on ecological characteristics (using natural ecosystems as templates for green roof design). An extensive bibliographic search on plants proposed for EGRs in Mediterranean countries has led to the creation of a wide database. Other plants were selected considering their synecological, structural, and autoecological characteristics. All the data were integrated in a comprehensive database of 138 taxa potentially suitable for setting EGRs according to their syntaxonomical classification and their ecological behavior (fitting both the Mediterranean and EGR environmental conditions). The selected taxa could enlarge the pool of species for EGRs in Mediterranean cities, increasing urban biodiversity.
Article
Many studies worldwide have investigated the potential benefits achievable by transforming brown roofs of buildings to green roofs. However, little literature examined the runoff quality/sorption ability of green roofs. As the green roof substrate is the main component to alter the quality of runoff, this investigation raises the possibility of using a mixture of low-cost inorganic materials to develop a green roof substrate. The tested materials include exfoliated vermiculite, expanded perlite, crushed brick and sand along with organic component (coco-peat). Detailed physical and chemical analyses revealed that each of these materials possesses different characteristics and hence a mix of these materials was desirable to develop an optimal green roof substrate. Using factorial design, 18 different substrate mixes were prepared and detailed examination indicated that mix-12 exhibited desirable characteristics of green roof substrate with low bulk density (431 kg/m(3)), high water holding capacity (39.4%), air filled porosity (19.5%), and hydraulic conductivity (4570 mm/h). The substrate mix also provided maximum support to Portulaca grandiflora (380% total biomass increment) over one month of growth. To explore the leaching characteristics and sorption capacity of developed green roof substrate, a down-flow packed column arrangement was employed. High conductivity and total dissolved solids along with light metal ions (Na, K, Ca and Mg) were observed in the leachates during initial stages of column operation; however the concentration of ions ceased during the final stages of operation (600 min). Experiments with metal-spiked deionized water revealed that green roof substrate possess high sorption capacity towards various heavy metal ions (Al, Fe, Cr, Cu, Ni, Pb, Zn and Cd). Thus the developed growth substrate possesses desirable characteristics for green roofs along with high sorption capacity.
Article
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.
Article
Mediterranean-type biomes characterized by warm summers with a distinct drought period lasting from 2 up to 10 months occur in several world regions including the Mediterranean basin, S-California, Chile, S-Africa and SW-Australia. All these areas are covered by a peculiar and hyper-diverse vegetation dominated by evergreen trees and shrubs with small and coriaceous leaves. Drought adaptation of Mediterranean plants relies on different mechanisms including deep rooting patterns, avoidance or resistance of cavitation-induced embolism, compensation or repair of embolism-induced hydraulic damage. The complementarity and/or co-occurrence of these physiological traits in different species inhabiting Mediterranean biomes is probably the basis for high plant biodiversity in these fascinating habitats. Ongoing climate changes, leading to enhanced frequency and intensity of drought episodes in Mediterranean biomes, represent a major threat to future conservation of these fragile ecosystems, especially if future harsher climate conditions will overcome the drought resistance limits of Mediterranean plants. Current knowledge about drought resistance mechanisms as well as about processes leading to decline and death of woody plants under extreme climatic conditions is revised and directions for future research are suggested.
Article
Green roofs as one of the components of water-sensitive urban design have become widely used in recent years. This paper describes performance monitoring of four prototype-scale experimental green roofs in a northern suburb of Adelaide, South Australia, undertaken over a 1-year period. Four species of indigenous Australian ground cover and grass species comprising Carpobrotus rossii, Lomandra longifolia 'Tanika,' Dianella caerula 'Breeze' and Myoporum parvifolium were planted in extensive and intensive green roof configurations using two different growing media. The first medium consisted of crushed brick, scoria, coir fibre and composted organics while the second comprised scoria, composted pine bark and hydro-cell flakes. Plant growth indices including vertical and horizontal growth rate, leaf succulence, shoot and root biomasses, water use efficiency and irrigation regimes were studied during a 12-month period. The results showed that the succulent species, C. rossii, can best tolerate the hot, dry summer conditions of South Australia, and this species showed a 100% survival rate and had the maximum horizontal growth rate, leaf succulence, shoot biomass and water use efficiency. All of the plants in the intensive green roofs with the crushed brick mix media survived during the term of this study. It was shown that stormwater can be used as a source of irrigation water for green roofs during 8months of the year in Adelaide. However, supplementary irrigation is required for some of the plants over a full annual cycle.
Article
Green roofs have been proposed for sustainable buildings in many countries with different climatic conditions. A state-of-the-art review of green roofs emphasizing current implementations, technologies, and benefits is presented in this paper. Technical and construction aspects of green roofs are used to classify different systems. Environmental benefits are then discussed mainly by examining measured performances. By reviewing the benefits related to the reduction of building energy consumption, mitigation of urban heat island effect, improvement of air pollution, water management, increase of sound insulation, and ecological preservation, this paper shows how green roofs may contribute to more sustainable buildings and cities. However, an efficient integration of green roofs needs to take into account both the specific climatic conditions and the characteristics of the buildings. Economic considerations related to the life-cycle cost of green roofs are presented together with policies promoting green roofs worldwide. Findings indicate the undeniable environmental benefits of green roofs and their economic feasibility. Likewise, new policies for promoting green roofs show the necessity for incentivizing programs. Future research lines are recommended and the necessity of cross-disciplinary studies is stressed.
Article
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.
Article
Plant water status and hydraulics were measured in six woody angiosperms growing in a karstic woodland, during an extreme summer drought. Our aim was to take advantage of an unusual climatic event to identify key traits related to species-specific drought damage. The damage suffered by different species was assessed in terms of percentage of individuals showing extensive crown desiccation. Stem water potential (Ψstem ) and percent loss of hydraulic conductivity (PLC) were measured in healthy and desiccated individuals. Vulnerability to cavitation was assessed in terms of stem water potential inducing 50% PLC (Ψ50 ). Stem density (ρstem ) was also measured. Species-specific percentage of desiccated individuals was correlated to Ψ50 and ρstem . Crown desiccation was more widespread in species with less negative Ψ50 and lower ρstem . Desiccated individuals had lower Ψstem and higher PLC than healthy ones, suggesting that hydraulic failure was an important mechanism driving shoot dieback. Drought-vulnerable species showed lower safety margins (Ψstem - Ψ50 ) than resistant ones. The Ψ50 , safety margins and ρstem values emerge as convenient traits to be used for tentative predictions of differential species-specific impact of extreme drought events on a local scale. The possibility that carbohydrate depletion was also involved in induction of desiccation symptoms is discussed.
Article
Abstract Previous studies suggest that high temperature stress on wheat (Triticum aestivum L.) involves root processes and acceleration of monocarpic senescence. Physiological changes in wheat roots and shoots were investigated to elucidate their relationship to injury from elevated temperatures after anthesis. Plants were grown under uniform conditions until 10 d after anthesis, when shoot/root regimes of 25°C/25°C, 25°C/35°C, 35°C/25°C and 35°C/35°C were imposed. Growth and senescence of shoots and grain were influenced more by root temperatures than by shoot temperatures. High root temperatures increased activities of protease and RNasc enzymes, and loss of chlorophyll, protein and RNA from shoots, whereas low root temperatures had opposite effects. High root temperatures appeared to induce shoot senescence directly. High shoot temperatures probably disrupted root processes, including export of cytokinins, and induced high leaf protease activity, senescence and cessation of grain development. The authors concluded that responses of wheat to high temperatures, whether of roots or shoots, are manifested as acceleration of senescence and may be mediated by roots during grain development.
Article
Pressure–volume (P–V) curves for leaves or terminal shoots summarize leaf-level responses to increasing water deficit. P–V curve traits and field-measured shoot xylem pressures were characterized across 62 species from four sites differing in rainfall and soil phosphorus. Within-species variation in the measured traits was small relative to differences among species and between environments. P–V curve traits tended to differ with site rainfall but not with soil phosphorus. Turgor loss points (TLPs) varied widely and averaged more negative in species from lower-rainfall sites. Differences between species in TLP were driven mainly by differences in solute potential, rather than by differences in cell wall elasticity. Among species at individual sites, species seemed to vary in leaf-response strategy reflected in TLP independently from water-uptake strategy reflected in predawn xylem pressures and in xylem pressure drop from predawn to midday.
Article
Two-year-old olive trees (Olea europaea L., cv. Coratina) were subjected to a 15-day period of water deficit, followed by 12days of rewatering. Water deficit caused decreases in predawn leaf water potential (Ψw), relative water content and osmotic potential at full turgor (Ψ π100) of leaves and roots, which were normally restored upon the subsequent rewatering. Extracts of leaves and roots of well-watered olive plants revealed that the most predominant sugars are mannitol and glucose, which account for more than 80% of non-structural carbohydrates and polyols. A marked increase in mannitol content occurred in tissues of water-stressed plants. During water deficit, the levels of glucose, sucrose and stachyose decreased in thin roots (with a diameter <1mm), whereas medium roots (diameter of 1–5mm) exhibited no differences. Inorganic cations largely contribute to Ψ π100 and remained stable during the period of water deficit, except for the level of Ca2+, which increased of 25% in water-stressed plants. The amount of malate increased in both leaves and roots during the dry period, whereas citrate and oxalate decreased. Thin roots seem to be more sensitive to water deficit and its consequent effects, while medium roots present more reactivity and a higher osmotic adjustment. The results support the hypothesis that the observed decreases in Ψw and active osmotic adjustment in leaves and roots of water-stressed olive plants may be physiological responses to tolerate water deficit.
Article
Green roofs are a passive cooling technique that stop incoming solar radiation from reaching the building structure below. Many studies have been conducted over the past 10 years to consider the potential building energy benefits of green roofs and shown that they can offer benefits in winter heating reduction as well as summer cooling.This paper reviews the current literature and highlights the situations in which the greatest building energy savings can be made. Older buildings with poor existing insulation are deemed to benefit most from a green roof as current building regulations require such high levels of insulation that green roofs are seen to hardly affect annual building energy consumption.As over half of the existing UK building stock was built before any roof insulation was required, it is older buildings that will benefit most from green roofs. The case for retrofitting existing buildings is therefore reviewed and it is found there is strong potential for green roof retrofit in the UK.
Article
Heat stress due to increased temperature is an agricultural problem in many areas in the world. Transitory or constantly high temperatures cause an array of morpho-anatomical, physiological and biochemical changes in plants, which affect plant growth and development and may lead to a drastic reduction in economic yield. The adverse effects of heat stress can be mitigated by developing crop plants with improved thermotolerance using various genetic approaches. For this purpose, however, a thorough understanding of physiological responses of plants to high temperature, mechanisms of heat tolerance and possible strategies for improving crop thermotolerance is imperative. Heat stress affects plant growth throughout its ontogeny, though heat-threshold level varies considerably at different developmental stages. For instance, during seed germination, high temperature may slow down or totally inhibit germination, depending on plant species and the intensity of the stress. At later stages, high temperature may adversely affect photosynthesis, respiration, water relations and membrane stability, and also modulate levels of hormones and primary and secondary metabolites. Furthermore, throughout plant ontogeny, enhanced expression of a variety of heat shock proteins, other stress-related proteins, and production of reactive oxygen species (ROS) constitute major plant responses to heat stress. In order to cope with heat stress, plants implement various mechanisms, including maintenance of membrane stability, scavenging of ROS, production of antioxidants, accumulation and adjustment of compatible solutes, induction of mitogen-activated protein kinase (MAPK) and calcium-dependent protein kinase (CDPK) cascades, and, most importantly, chaperone signaling and transcriptional activation. All these mechanisms, which are regulated at the molecular level, enable plants to thrive under heat stress. Based on a complete understanding of such mechanisms, potential genetic strategies to improve plant heat-stress tolerance include traditional and contemporary molecular breeding protocols and transgenic approaches. While there are a few examples of plants with improved heat tolerance through the use of traditional breeding protocols, the success of genetic transformation approach has been thus far limited. The latter is due to limited knowledge and availability of genes with known effects on plant heat-stress tolerance, though these may not be insurmountable in future. In addition to genetic approaches, crop heat tolerance can be enhanced by preconditioning of plants under different environmental stresses or exogenous application of osmoprotectants such as glycinebetaine and proline. Acquiring thermotolerance is an active process by which considerable amounts of plant resources are diverted to structural and functional maintenance to escape damages caused by heat stress. Although biochemical and molecular aspects of thermotolerance in plants are relatively well understood, further studies focused on phenotypic flexibility and assimilate partitioning under heat stress and factors modulating crop heat tolerance are imperative. Such studies combined with genetic approaches to identify and map genes (or QTLs) conferring thermotolerance will not only facilitate marker-assisted breeding for heat tolerance but also pave the way for cloning and characterization of underlying genetic factors which could be useful for engineering plants with improved heat tolerance.
Article
Ecology Letters (2012) 15: 393–405 Increasing drought is one of the most critical challenges facing species and ecosystems worldwide, and improved theory and practices are needed for quantification of species tolerances. Leaf water potential at turgor loss, or wilting (πtlp), is classically recognised as a major physiological determinant of plant water stress response. However, the cellular basis of πtlp and its importance for predicting ecological drought tolerance have been controversial. A meta-analysis of 317 species from 72 studies showed that πtlp was strongly correlated with water availability within and across biomes, indicating power for anticipating drought responses. We derived new equations giving both πtlp and relative water content at turgor loss point (RWCtlp) as explicit functions of osmotic potential at full turgor (πo) and bulk modulus of elasticity (ε). Sensitivity analyses and meta-analyses showed that πo is the major driver of πtlp. In contrast, ε plays no direct role in driving drought tolerance within or across species, but sclerophylly and elastic adjustments act to maintain RWCtlp, preventing cell dehydration, and additionally protect against nutrient, mechanical and herbivory stresses independent of drought tolerance. These findings clarify biogeographic trends and the underlying basis of drought tolerance parameters with applications in comparative assessments of species and ecosystems worldwide.
Article
Although itisnow generally recognized that solutes maypermeate someappreciable fraction ofthe volumeofcertain plantorgansbyfreediffusion (4,6,10,19),themagnitude andphysiological sig- nificance offreespace remain highly debatable. Free spacemeasurements havebeenquestioned morewith regard totheadequacy ofremoval ofsuperficial solu- tionthanforanyotherreason. Theretention ofa verythinfilm, especially onfineroots after blotting orcentrifuging mayfalsely magnifythefreespace value(20).To obviate thisdifficulty, theauthors havedeveloped atechnique fortagging theexternal mediumwithacolloid whichdoesnotpenetrate free space.India inkparticles (Higgins American India InkNo.4415, Waterproof Black, Higgins InkCom- pany,Brooklyn, N.Y.)arenotadsorbed toorre- pelled fromrootsurfaces underanyoftheexperi- mental conditions described. Thiswasconfirmed by microscopic examination ofrootsmountedinIndia inkfollowing a variety ofpretreatments including leaching indistilled waterand/orequilibration with NaClsolutions. Bymeasuring theamountofIndia inkcarried overonthesurface oftheroot, thevolume ofrootmediumsuperficially retained bytherootis determined. Thequantity oftestsolute present in thisvolume ofmediumissubtracted fromthetotal quantity oftestsolute carried overwiththerootto givetheamountofsolute inthefreespace oftheroot. Usingthistechnique, theapparent freespace values weremeasured forroots ofanumberofplant species forwhichdataareavailable intheliterature. Inthecourse ofthesepreliminary studies, variable freespacevalues werefoundforpeaandbeanroots depending onexperimental conditions. Thesecondi- tions (duration ofwashing ofroots indistilled water, natureoftestsolute, andduration ofequilibration withsolute) weretherefore examined critically so thatareasonable explanation oftheobserved effects could begiven. Theterm"apparent free space" (AFS)includes a built-in apology: Theuserconcedes thatAFS de- scribes ahypothetical partial volumewhichisatrue measure ofthefreespaceonlyifthetestsolute con-