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Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems
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Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems Nikolaos Ntoulas ∗ , Panayiotis A. Nektarios, Grigorios Kotopoulis, Paraskevas Ilia, Theodora Ttooulou Laboratory of Floriculture and Landscape Architecture, Dept. of Crop Science, Agricultural University of Athens, 75, Iera Odos, 118 55 Athens, Greece
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Article history: Received 31 May 2016 Received in revised form 18 January 2017 Accepted 4 March 2017 Available online xxx Keywords: Green turf cover Hybrid bermudagrass Leaf stomatal resistance Seashore paspalum Zoysiagrass
a b s t r a c t In an effort to increase the accessibility and functionality of shallow green roof systems, the ability of warm-season grasses to provide acceptable growth needs to be further investigated. In the current study, which was conducted during 2011 and 2012, three warm-season grasses (hybrid bermudagrass, Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy ‘MiniVerde’; seashore paspalum, Paspalum vaginatum Swartz ‘Platinum TE’ and zoysiagrass, Zoysia japonica Steud. ‘Zenith’) were established in outdoor lysimeters. The lysimeters were equipped with all necessary green roof layers placed below a coarse-textured substrate that comprised pumice, thermally treated attapulgite clay, peat, compost and zeolite. Half of the lysimeters had a substrate depth of 15 cm, while the other half had a substrate depth of 7.5 cm. Irrigation was applied at crop evapotranspiration (ETc ). Measurements included determination of substrate moisture content, green turf cover (GTC) and leaf stomatal resistance. Significant differences were observed in the values of GTC among the three turfgrass species and the two substrate depths. Zoysiagrass exhibited the best adaptation at the lower depths of shallow green roof systems. At 15 cm substrate depth, zoysiagrass managed to sustain green coverage for the two study periods. In addition, it was the only turfgrass species that managed to perform well at the substrate depth of 7.5 cm. Seashore paspalum exhibited limited green cover at both substrate depths, while hybrid bermudagrass could provide acceptable green coverage only at 15 cm substrate depth. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Green roofing is an urban greening technique that provides several environmental benefits, such as amelioration of the urban heat-island effect (Takebayashi and Moriyama, 2007), building energy savings (Kotsiris et al., 2012), storm water management (Berndtsson, 2010), improvement of air quality (Yang et al., 2008), improvement of urban landscape aesthetics and provision of new flora and fauna habitats (MacIvor and Lundholm, 2011). However, the above mentioned benefits are expected to accrue only provided that green roofing is implemented on broad city surfaces (Akbari et al., 2001; Getter and Rowe, 2006). Based on the
Abbreviations: GTC, green turf cover; ETc , crop evapotranspiration; NDVI, normalized difference vegetation index; SMC, substrate moisture content; LSR, leaf stomatal resistance. ∗ Corresponding author. E-mail addresses: [email protected] (N. Ntoulas), [email protected] (P.A. Nektarios), [email protected] (G. Kotopoulis), [email protected] (P. Ilia), [email protected] (T. Ttooulou).
latter, it is of great interest to select and investigate a methodology that is suitable for the application of green roofs to existing city buildings through retrofitting. This is of immense importance, considering that, in most cases, city buildings are aged and constructed based on obsolete design criteria, thus, being able to bear only minimal additional loads. The green roof industry has termed constructions with minimal weight as “extensive” green roofs. In these green roof types, weight reduction has mainly resulted from very shallow substrate depths that may vary from 5 to 20 cm depth (FLL, 2008). Due to shallow substrate depths and lack of irrigation, extensive green roofs demand specific plant species for their planting that are mainly succulents. The lack of plant variability in conjunction with the minimal wear tolerance of succulent plants to traffic has led extensive green roofs to be considered as “environmental” building structures rather than as a functional open green space that is accessible by ˜ the building residents. The report of Fernandez-Canero et al. (2013) demonstrates that local residents clearly preferred more aesthetically pleasing and accessible green roofs with variable plant types, such as groundcovers, shrubs and trees. In contrast, extensive green
http://dx.doi.org/10.1016/j.ufug.2017.03.005 1618-8667/© 2017 Elsevier GmbH. All rights reserved.
Please cite this article in press as: Ntoulas, N., et al., Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems. Urban Forestry & Urban Greening (2017), http://dx.doi.org/10.1016/j.ufug.2017.03.005
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roofs planted with Sedum succulents received low ratings in the public perception and preferences measurement. The preceding problems, concerning aesthetics and accessibility of extensive green roofs, become more severe in countries where governmental incentives are absent as well as in longitudes that are characterized by droughty summer periods such as in Mediterranean countries. As a result, Kotsiris et al. (2013) proposed the “adaptive green roofs” approach that focuses on light-weight green roof systems combined with improved aesthetics, accessibility and functionality. In adaptive green roofs, a shallow substrate depth provides a light structure that is compensated for by prudent irrigation applications along with the use of drought tolerant native species and turfgrasses (Ntoulas et al., 2013a). Turfgrasses have a unique ability to fulfil aesthetic, functional and recreation requirements, which are demanded of urban plants (Beard and Green, 1994). However, they have rarely been evaluated in relation to extensive green roofs due to their increased water demands compared to succulents or other xerophytic plants. Ntoulas et al. (2013a) evaluated the establishment and growth of Zoysia matrella (L.) Merr. on adaptive green roof systems. They reported higher green turf cover (GTC) and normalized difference vegetation index (NDVI) values when the substrate depth was 15 cm compared to a shallower substrate depth of 7.5 cm, during both the establishment and the water deficit periods. Ntoulas et al. (2013b) evaluated Z. matrella performance in two different green roof substrate types and depths (7.5 and 15 cm) and under two different irrigation regimes (3 mm or 6 mm every 3 days). They reported that GTC and NDVI values were mostly affected by substrate depth, moderately by irrigation regime and to a lesser extent by substrate type. Ntoulas and Nektarios (2015) reported that Paspalum vaginatum Swartz provided better green cover and demanded less water to retain its visual quality at a 15 cm substrate depth compared to a 7.5 cm depth. They also reported that at the 7.5 cm substrate depth P. vaginatum growth is possible if water inputs increase by 40% compared to the 15 cm substrate depth. Nektarios et al. (2014) reported that Festuca arundinacea L. can grow in a reduced substrate depth of 7.5 cm without being stressed compared to a substrate depth of 15 cm, when irrigation is provided at 85% of evapotranspiration. The variable response of different turfgrass species, growing on shallow green roof systems, indicated the need to perform a comparable study. The aim of the present study is to investigate the response of three warm-season grasses at different substrate depths of a shallow green roof system.
2.2. Green roof construction within the lysimeters The lysimeters had 30 cm internal diameter. Within each lysimeter a complete layered simulation of an extensive green roof system was constructed. The bottom of the lysimeter was covered with a protection mat that was a synthetic cloth made of non-rotting synthetic polyester fibers having 3 mm thickness and a dry weight of 0.32 kg m−2 . The mat also had the capacity to retain 3 L m−2 of water (TSM32, Zinco, Egreen, Athens, 10672, Greece). A drainage board layer was placed on top of the protection cloth. The drainage layer was made of recycled polyethylene with 25 mm height and 1.5 kg m−2 weight (FD25, Zinco, Egreen, Athens, 10672, Greece). It was equipped with water retaining troughs having a water holding capacity of 3 L m−2 and openings for improving subsurface aeration. The drainage layer was covered with a non-woven geotextile (SF, Zinco, Egreen, Athens, 10672, Greece) that was made of thermally strengthened polypropylene having 600 m thickness, a mass of 100 g m−2 , apparent opening size of D90 = 95 m and water flow rate of 0.07 m s−1 . The filter sheet was used to prevent fine particle migration from the substrate towards the drainage layer, thus ensuring that the drainage layer would not clog and would function effectively. The lysimeters were filled with a specialized green roof substrate that comprised 40% pumice, 40% thermally treated attapulgite clay, 8% peat, 7% compost and 5% clinoptilolite zeolite by volume. Pumice (LAVA, Mineral & Quarry A.D., Athens, 14123, Greece) had a particle distribution of 0.05–8 mm, thermally treated attapulgite clay 1–10 mm (GeoHellas SA, Athens, 17564, Greece) and zeolite 0.8-2.5 mm (S&B Industrial Minerals A.D., Athens, 14564, Greece). The organic portion of the substrate contained sphagnum peat, with a corrected pH of 5.5 and an organic matter of 90% (w/w), and compost that comprised straw, sawdust, yard waste (clippings and wood chips) as well as dairy cow, horse and chicken manure. The mechanical analysis and the water potential curve of the substrate are presented in Fig. 1A and 1B, respectively. Half of the lysimeters had a substrate depth of 7.5 cm and the other half had a 15 cm depth. Light compression and leveling was applied to the substrates after their placement into the lysimeters. The zoysiagrass lysimeters were seeded on 13 June 2011 and the hybrid bermudagrass and seashore paspalum lysimeters were sodded – using washed sod – on 22 June 2011. After seeding and sodding, the lysimeters were placed under a mist system in order to promote seed germination and sod rooting. Then, the lysimeters were transferred onto outdoor benches that were equipped with a rain-out shelter. However, no rainfall occurred during the first study year and only four minor rain events occurred during the second study year (Fig. 2).
2. Materials and methods 2.3. Turfgrass maintenance and irrigation 2.1. Experimental setup and hypothesis The outdoor study was conducted at the experimental field of the Laboratory of Floriculture and Landscape Architecture, Agricultural University of Athens, Greece. The initial study was performed from 3 Aug. until 10 Sep. 2011 and was replicated from 15 May until 19 July 2012. Treatments included: a) three warm season turfgrasses (hybrid bermudagrass, Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy ‘MiniVerde’; seashore paspalum, Paspalum vaginatum Swartz ‘Platinum TE’ and zoysiagrass, Zoysia japonica Steud. ‘Zenith’) and b) two green roof substrate depths (7.5 cm or 15 cm). Each treatment was replicated three times, totaling 18 lysimeters (3species x 2sub.depths × 3replications = 18 lysimeters). The hypotheses were to investigate whether the three warm season turfgrass species exhibit different green coverage when grown under green roof conditions and whether it improves as substrate depth increases.
Turfgrass sward was mowed at a height of 5 cm once a week with a handheld electric shear mower, and clippings were removed. Fertilization was applied once before the initiation of each study (6 July 2011 and 28 March 2012) with Floranid Permanent 16-715 (+2Mg, +7S + 0.5 Fe, Compo Hellas SA), at a rate of 10 g m−2 of fertilizer. At the initiation of each study, all lysimeters were irrigated close to saturation in order to produce uniform substrate moisture conditions. From then on, irrigation was performed on a daily basis using irrigation amounts at crop evapotranspiration (ETc ). Crop evapotranspiration was calculated based on the daily evaporation of a Class-A pan according to: ETc = Epan x Kp x Kc , where Epan is the evaporation of a Class-A pan, Kp is the pan coefficient used to convert pan evaporation to reference evapotranspiration, and Kc is the crop coefficient. Based on the weather data during the experimental periods, a Kp value of 0.65 (Doorenbos and Pruitt, 1977) and a Kc
Please cite this article in press as: Ntoulas, N., et al., Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems. Urban Forestry & Urban Greening (2017), http://dx.doi.org/10.1016/j.ufug.2017.03.005
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Fig. 1. (A) Particle size distribution of substrate that comprised 40% pumice, 40% thermally treated attapulgite clay, 8% peat, 7% compost and 5% clinoptilolite zeolite by volume. The grey area represents particle size distribution specified by FLL (2008) guidelines. (B) Water potential curve of the substrate used in the study. Values are the means of three replications (±SE).
value of 0.65 were utilized. In both years, recuperation from water stress period was achieved by daily water applications at 100% of A-pan evaporation. 2.4. Measurements The in situ moisture content of the substrate was determined daily, just before the irrigation of the sward, using a dielectric moisture sensor (FieldScout TDR 300 Soil Moisture Meter, Spectrum Technologies, IL, USA). Measurements of substrate profiles at different depths were achieved by interchanging the sensor rods. The rod length of the sensors was 7.5 cm and 12 cm for the lysimeters with substrate heights of 7.5 cm and 15 cm, respectively. Each rod length sensor was calibrated for the specific substrate according to the methodology described by Kargas et al. (2013). To evaluate the effects of substrate depth on each of the three turfgrass species, green turf cover (GTC) was determined using digital image analysis. During the study periods, GTC measurements were performed approximately every four days, before each irrigation event. Images were acquired using a Canon IXUS 100 IS (Canon Europe Ltd., UK) digital camera mounted on top of a sealed circular box (30 cm in diameter × 45 cm in height), equipped with a circular fluorescent lamp (1200 lumens) that secured consistent lighting conditions throughout the image acquisition process. In the course
of both studies, the camera settings remained constant and were the following: an aperture of F3.2, a focal length of 33 mm, white balance set to fluorescent and ISO sensitivity set to 200. The SigmaScan Pro, version 5.0, digital image analysis software (SigmaScan Pro, Systat Software Inc., Chicago, IL, USA) was employed to determine GTC percentage (%) according to Richardson et al. (2001). The images were in JPEG format and their size was 1600 × 1200 pixels. Each image was cropped using the Adobe PhotoShop CS3 software (Adobe Systems Inc., USA) in order to remove any unwanted image portion. Green leaves were selectively identified in the images by setting the hue range from 47 to 107 and saturation from 0 to 100. In each image, the GTC percentage was determined by dividing the number of green pixels by the total pixel count of each image. During the first study year (2011), the three turfgrass species were stressed until GTC reached values below 50%. In the second year (2012), the GTC stress limit was reduced to 20%. Leaf stomatal resistance (LSR) was determined using an AP4 diffusion porometer (Delta-T Devices, UK) before irrigation events, that is, at noon on cloudless days. Measurements were made on the abaxial side of young fully expanded leaves.
2.5. Meteorological data The ambient maximum, minimum and average temperatures as well as precipitation were recorded by the weather station of the National Observatory of Athens at the Gazi region, which is located 885 m from the experimental site (Fig. 2).
Please cite this article in press as: Ntoulas, N., et al., Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems. Urban Forestry & Urban Greening (2017), http://dx.doi.org/10.1016/j.ufug.2017.03.005
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Date Fig. 3. Effects of plant species (hybrid bermudagrass, Cynodon dactylon x C. transvaalensis ‘MiniVerde’; seashore paspalum, Paspalum vaginatum ‘Platinum TE’; zoysiagrass, Zoysia japonica ‘Zenith’) and substrate depth (Shallow, 7.5 cm or Deep, 15 cm) on green turf cover (%) during the two study years (2011 and 2012). Values are the means of 3 replications. Bars represents Fisher’s least significant difference (LSD) at P < 0.05.
3. Results and discussion In 2011, the study took place in August while in 2012 it was conducted during late May and June. Due to the replication of the study at different times of the year, ambient temperatures differed between the two study years (Fig. 2). In 2011, ambient average air temperature fluctuated from 26.4 ◦ C to 31.0 ◦ C, while in 2012 it fluctuated from 14.6 ◦ C to 34.1 ◦ C. However, soon after the initiation of the 2012 study period, the mean air temperature rose and fluctuated from 26.4 ◦ C to 34.1 ◦ C after 9 June 2012. This was very similar to air temperatures observed during the 2011 study period. No rainfall events occurred during the 2011 study period, while 4 minor rainfall events were recorded in 2012 (Fig. 2). In all four instances, the lysimeters were covered during rainfall events. 3.1. Green turf cover In the course of both years, green turf cover remained stable for a week after the initiation of the study. From then on, a significant GTC reduction was recorded that depended on the treatments. More specifically, in 2011 the most abrupt reduction occurred at shallow depth substrates that were covered with seashore paspalum and hybrid bermudagrass (Fig. 3). Based on the curve slopes,
seashore paspalum and hybrid bermudagrass growing in the 15 cm substrate depth exhibited a similar GTC reduction rate compared to the 7.5 cm substrate depth. However, in the case of the 15 cm substrate depth, the initiation of the reduction was delayed by 3 days. Zoysiagrass growing in the 7.5 cm substrate depth followed the same GTC reduction patterns as those of seashore paspalum and hybrid bermudagrass growing in the 15 cm substrate depth. Zoysiagrass growing in the deeper substrates exhibited a similar reduction to the other two turfgrass species until 21 Aug. 2011. However, after that date, it was clearly separated from the other treatments, since GTC was not reduced any further and remained stable until after 3 Sep. 2011 when it actually increased. Similar results were obtained from the second study year (2012), when the GTC limit for the stress period was reduced to 20%. In this case, the only significant difference, compared to 2011, was seashore paspalum that exhibited a fast GTC reduction regardless of substrate depth. Based on the obtained results, it was concluded that zoysiagrass exhibited the best response in retaining GTC on shallow green roof systems. In contrast, at shallow substrate depths seashore paspalum and hybrid bermudagrass did not undergo the same adaptation as zoysiagrass. These observed differences among the three turfgrass species might rest on the different mechanisms responsible for overcoming water limiting conditions and drought stresses. Huang et al. (1997b) compared four seashore paspalums to common bermudagrass and zoysiagrass along their capacity to alter morphological and physiological traits in relation to drought resistance. They reported that the tested grasses exhibited different responses after being imposed to drought stress. Some seashore paspalum varieties extended their root system into greater depths, indicating that a drought avoidance mechanism was triggered. In contrast, zoysiagrass did not extend its root length, indicating that its drought resistance was not based on retrieving and utilizing water from greater soil depths. In the same experiment, Huang et al. (1997a) reported that shoot dry weight was significantly lower for zoysiagrass than for the other tested grasses, thus minimizing its evapotranspiration demands. In addition, water-stressed zoysiagrass responded in a very positive way with regard to chlorophyll content, canopy temperature and relative water content compared to the well-watered control. All the above indicate that the drought resistance mechanisms of zoysiagrass are well-suited to green roof systems, where substrate depth has been shown to be the most limiting factor (Ntoulas et al., 2013a,b). It is obvious that the drought avoidance mechanisms that rely on root system redistribution by increasing root length in order to access deeper water reservoirs are negated at shallow green roof substrate depths. Consequently, zoysiagrass, which does not rely on the latter mechanism, appears to possess a significant advantage for usage in shallow substrates. This is further supported by Huang (1999) who concluded that zoysiagrass has extensive roots in the surface and mainly utilizes surface water. Moreover, Qian and Fry (1997) reported that zoysiagrass had a shallow root system but exhibited intermediate evapotranspiration rate and a high level of osmotic adjustment. In addition, Carmo-Silva et al. (2009) reported increased amino acid production (proline, phenyladenine, valine, isoleucine and leucine) of water-stressed zoysiagrass compared to bermudagrass and Paspalum dilatatum Poir., which was probably related to enhanced secondary metabolism. All the above demonstrate the differences between zoysiagrass and seashore paspalum and bermudagrass concerning their drought resistance mechanism. 3.2. Substrate moisture content The substrate moisture content (SMC) displayed a reverse pattern to that of GTC. In 2011, at the initiation of the study, an increased SMC was observed for seashore paspalum growing in
Please cite this article in press as: Ntoulas, N., et al., Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems. Urban Forestry & Urban Greening (2017), http://dx.doi.org/10.1016/j.ufug.2017.03.005
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Fig. 4. Effects of plant species (hybrid bermudagrass, Cynodon dactylon x C. transvaalensis ‘MiniVerde’; seashore paspalum, Paspalum vaginatum ‘Platinum TE’; zoysiagrass, Zoysia japonica ‘Zenith’) and substrate depth (Shallow, 7.5 cm or Deep, 15 cm) on substrate moisture content (% v/v) during the two study years (2011 and 2012). Values are the means of 3 replications. Bars represents Fisher’s least significant difference (LSD) at P < 0.05.
both 7.5 cm and 15 cm substrate depths (Fig. 4). SMC for seashore paspalum at 15 cm remained high for the whole duration of the study, while after 28 Aug. zoysiagrass at shallow substrate depths exhibited higher SMC. In contrast, zoysiagrass and hybrid bermudagrass growing in deeper substrate depths showed the least SMC. However, it must be emphasized that all treatments reached a very low SMC of less than 10% v/v within 5–10 days after the initiation of the study. A similar response of SMC between treatments was observed in the 2012 study year. 3.3. Leaf stomatal resistance Leaf stomatal resistance of the three species fluctuated according to the monitored air temperature (Figs. 2 and 5). The response of the turfgrass plants was immediate and as soon as ambient temperature was reduced, LSR followed the same reduction. Similarly, whenever air temperature was increased, LSR was also raised. Leaf stomatal resistance provided interesting results that were in agreement with GTC. During the first study year, LSR increased sharply after 13 days (16 Aug. 2011) for seashore paspalum and hybrid bermudagrass growing in the shallow substrate depth of 7.5 cm (Fig. 5). A similar response was observed for seashore paspalum growing in the deeper substrate of 15 cm on the next sampling date (18 Aug. 2011). This coincides with the fast GTC
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Fig. 5. Effects of plant species (hybrid bermudagrass, Cynodon dactylon x C. transvaalensis ‘MiniVerde’; seashore paspalum, Paspalum vaginatum ‘Platinum TE’; zoysiagrass, Zoysia japonica ‘Zenith’) and substrate depth (Shallow, 7.5 cm or Deep, 15 cm) on leaf stomatal resistance (s cm−1 ) during the two study years (2011 and 2012). Values are the means of 3 replications. Bars represents Fisher’s least significant difference (LSD) at P < 0.05.
reduction, indicating that these treatments experienced increased stress (Fig. 3). Stomata closure is a physiological plant response in an effort to minimize water losses. However, if the exerted stress is prolonged, stomata closure negatively affects photosynthetic activity, which results in plant physiological deterioration. When plants have their stomata closed, transpiration is reduced and, thus, SMC is expected to reduce at a slower rate. This explains the increased SMC (Fig. 4) that seashore paspalum exhibited at the initiation of the 2011 study (3–16 Aug. 2011). Hybrid bermudagrass growing in the shallow substrate of 7.5 cm exhibited an increased LSR from 18 to 20 Aug. 2011, which was, however, lower than that of seashore paspalum. The lower LSR of hybrid bermudagrass, compared to that of seashore paspalum, indicated a slightly better drought resistance. When growing in the deeper substrate of 15 cm, this difference between seashore paspalum and hybrid bermudagrass became even greater. In deeper substrates, bermudagrass exhibited a moderate LSR until 20 Aug. 2011 that increased sharply on 6 Sep. 2011, thus, resulting in the lowest GTC value (Figs. 3 and 5). In contrast to the other two turfgrass species, zoysiagrass showed the least LSR at both substrate depths, during the 2011 study period, indicating that its anatomical and physiological status, as described above (Huang et al., 1997a,b; Qian and Fry, 1997), permitted an adequate physiological function at the shallow green roof depths. Similar results were obtained from the second study year (2012) for seashore paspalum that again showed the highest LSR until
Please cite this article in press as: Ntoulas, N., et al., Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems. Urban Forestry & Urban Greening (2017), http://dx.doi.org/10.1016/j.ufug.2017.03.005
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Table 1 Recovery time (days) to reach 90% green turf cover (GTC) for the three turfgrass species after reaching 50% GTC in 2011 and 20% GTC in 2012. (N/A refers to the first year of the treatments during which GTC did not fall below 50%). Turfgrass species
Substrate depth
Recovery time
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(days) Year 2011
Year 2012
Paspalum vaginatum
7.5 15
N/A 16
21 18
Zoysia japonica
7.5 15
N/A N/A
18 25
Cynodon dactylon x C. transvaalensis
7.5 15
N/A 12
28 36
Ethical approval The mention of a trade mark, proprietary product, or vendor does not imply endorsement by the authors, nor does it imply approval to the exclusion of other products that may also be suitable. Acknowledgments The authors would like to thank GeoHellas S.A. for providing attapulgite clay, L. Cambanis S.A. for providing compost, LAVA Mining & Quarrying S.A. for providing pumice, Compo Hellas SA for donating the fertilizer and Hellasod S.A. for providing turfgrass sod. References
1 June 2012 at both substrate depths. In contrast, zoysiagrass was, once more, the species that exhibited the least stress for the whole duration of the 2012 study. Hybrid bermudagrass had either a low (deeper substrates) or a moderate (swallow substrate) LSR until 1 June 2012. From then on and until 12 June 2012, hybrid bermudagrass at both substrate depths was the turfgrass species with the highest LSR along with seashore paspalum growing in the 7.5 cm substrate depth. The observed increase in LSR for hybrid bermudagrass 24 days after initiation (9 May to 1 June 2012) further substantiated the findings of the first study year, when a similar increase was observed 35 days after initiation (3 Aug. to 6 Sep. 2011, Fig. 5). This indicated that at shallow substrate profiles, hybrid bermudagrass can tolerate water stress better than seashore paspalum, although this difference is temporary. 3.4. Recovery period All treatments were able to recover in both study years (Table 1), indicating that GTC loss was caused by dormancy induction due to the reduced SMC. During the 2012 study period, when turfgrasses were stressed to a greater extent compared to 2011, the quickest recovery was achieved by seashore paspalum growing in the 15 cm substrate depth and zoysiagrass growing in the shallow depth of 7.5 cm. Recovery was slower for hybrid bermudagrass growing in both substrate depths and lasted 28 and 36 days for the 15 cm and the 7.5 cm substrate depths, respectively. 4. Conclusions The observed differences in GTC among the three turfgrass species highlighted the importance of investigating the ability of each turfgrass species to adapt to shallow green roof systems and provide acceptable green cover. Zoysiagrass seems to be one of the best options for providing an aesthetically pleasing as well as functional surface on roof tops. Its green cover was not hindered to the same extent as for the other two turfgrass species tested, which suggests a good adaptation to the growing conditions on swallow green roof systems based on anatomical and metabolic mechanisms. Hybrid bermudagrass needed at least 15 cm of substrate to provide acceptable green cover, while seashore paspalum exhibited greater difficulties even at the 15 cm substrate depth. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Akbari, H., Pomerantz, M., Taha, H., 2001. Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas. Solar energy 70 (3), 295–310. Beard, J.B., Green, R.L., 1994. The role of turfgrasses in environmental protection and their benefits to humans. J. Environ. Qual. 23 (3), 452–460. Berndtsson, J.C., 2010. Green roof performance towards management of runoff water quantity and quality: a review. Ecol. Eng. 36 (4), 351–360. Carmo-Silva, A.E., Francisco, A., Powers, S.J., Keys, A.J., Ascensão, L., Parry, M.A., Arrabac¸a, M.C., 2009. Grasses of different C4 subtypes reveal leaf traits related to drought tolerance in their natural habitats: changes in structure, water potential, and amino acid content. Am. J. Bot. 96 (7), 1222–1235. Doorenbos, J., Pruitt, W.O., 1977. Guidelines for Predicting Crop Water Requirements FAO Irrigation and Drainage Paper No. 24. Food and Agriculture Organization, Rome, Italy. FLL-Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V. 2008. Richtlinien für die Planung, Ausführung und Pflege von Dachbegrünungen. Richtlinien für Dachbegrünungen (Guideline for the planning, execution and upkeep of green-roof sites), Bonn, Germany. ˜ R., Emilsson, T., Fernandez-Barba, C., Machuca, M.Á.H., 2013. Fernandez-Canero, Green roof systems: a study of public attitudes and preferences in southern Spain. J. Environ. Manage. 128, 106–115. Getter, K.L., Rowe, D.B., 2006. The role of extensive green roofs in sustainable development. HortScience 41 (5), 1276–1285. Huang, B., Duncan, R.R., Carrow, R.N., 1997a. Drought-resistance mechanisms of seven warm-season turfgrasses under surface soil drying: I. Shoot response. Crop Sci. 37 (6), 1858–1863. Huang, B., Duncan, R.R., Carrow, R.N., 1997b. Drought-resistance mechanisms of seven warm-season turfgrasses under surface soil drying: II. Root aspects. Crop Sci. 37 (6), 1863–1869. Huang, B., 1999. Water relations and root activities of Buchloe dactyloides and Zoysia japonica in response to localized soil drying. Plant Soil 208 (2), 179–186. Kargas, G., Ntoulas, N., Nektarios, P.A., 2013. Moisture content measurements of green roof substrates using two dielectric sensors. HortTechnology 23 (2), 177–186. Kotsiris, G., Androutsopoulos, A., Polychroni, E., Nektarios, P.A., 2012. Dynamic U-value estimation and energy simulation for green roofs. Energy Build. 45, 240–249. Kotsiris, G., Nektarios, P.A., Ntoulas, N., Kargas, G., 2013. An adaptive approach to intensive green roofs in the Mediterranean climatic region. Urban For. Urban Green. 12 (3), 380–392. MacIvor, J.S., Lundholm, J., 2011. Insect species composition and diversity on intensive green roofs and adjacent level-ground habitats. Urban Ecosyst. 14 (2), 225–241. Nektarios, P.A., Ntoulas, N., Kotopoulis, G., Ttoulou, T., Ilia, P., 2014. Festuca arundinacea drought tolerance and evapotranspiration when grown on two extensive green roof substrate depths and under two irrigation regimes. Eur. J. Hortic. Sci. 79, 142–149. Ntoulas, N., Nektarios, P.A., 2015. Paspalum vaginatum drought tolerance and recovery in adaptive extensive green roof systems. Ecol. Eng. 82, 189–200. Ntoulas, N., Nektarios, P.A., Charalambous, E., Psaroulis, A., 2013a. Zoysia matrella cover rate and drought tolerance in adaptive extensive green roof systems. Urban For. Urban Green. 12 (4), 522–531. Ntoulas, N., Nektarios, P.A., Nydrioti, E., 2013b. Performance of Zoysia matrella ‘Zeon’in shallow green roof substrates under moisture deficit conditions. HortScience 48 (7), 929–937. Qian, Y., Fry, J.D., 1997. Water relations and drought tolerance of four turfgrasses. J. Am. Soc. Hortic. Sci. 122 (1), 129–133. Richardson, M.D., Karcher, D.E., Purcell, L.C., 2001. Quantifying turfgrass cover using digital image analysis. Crop Sci. 41 (6), 1884–1888. Takebayashi, H., Moriyama, M., 2007. Surface heat budget on green roof and high reflection roof for mitigation of urban heat island. Build. Environ. 42 (8), 2971–2979. Yang, J., Yu, Q., Gong, P., 2008. Quantifying air pollution removal by green roofs in Chicago. Atmos. Environ. 42 (31), 7266–7273.
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Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems Nikolaos Ntoulas ∗ , Panayiotis A. Nektarios, Grigorios Kotopoulis, Paraskevas Ilia, Theodora Ttooulou Laboratory of Floriculture and Landscape Architecture, Dept. of Crop Science, Agricultural University of Athens, 75, Iera Odos, 118 55 Athens, Greece
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Article history: Received 31 May 2016 Received in revised form 18 January 2017 Accepted 4 March 2017 Available online xxx Keywords: Green turf cover Hybrid bermudagrass Leaf stomatal resistance Seashore paspalum Zoysiagrass
a b s t r a c t In an effort to increase the accessibility and functionality of shallow green roof systems, the ability of warm-season grasses to provide acceptable growth needs to be further investigated. In the current study, which was conducted during 2011 and 2012, three warm-season grasses (hybrid bermudagrass, Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy ‘MiniVerde’; seashore paspalum, Paspalum vaginatum Swartz ‘Platinum TE’ and zoysiagrass, Zoysia japonica Steud. ‘Zenith’) were established in outdoor lysimeters. The lysimeters were equipped with all necessary green roof layers placed below a coarse-textured substrate that comprised pumice, thermally treated attapulgite clay, peat, compost and zeolite. Half of the lysimeters had a substrate depth of 15 cm, while the other half had a substrate depth of 7.5 cm. Irrigation was applied at crop evapotranspiration (ETc ). Measurements included determination of substrate moisture content, green turf cover (GTC) and leaf stomatal resistance. Significant differences were observed in the values of GTC among the three turfgrass species and the two substrate depths. Zoysiagrass exhibited the best adaptation at the lower depths of shallow green roof systems. At 15 cm substrate depth, zoysiagrass managed to sustain green coverage for the two study periods. In addition, it was the only turfgrass species that managed to perform well at the substrate depth of 7.5 cm. Seashore paspalum exhibited limited green cover at both substrate depths, while hybrid bermudagrass could provide acceptable green coverage only at 15 cm substrate depth. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Green roofing is an urban greening technique that provides several environmental benefits, such as amelioration of the urban heat-island effect (Takebayashi and Moriyama, 2007), building energy savings (Kotsiris et al., 2012), storm water management (Berndtsson, 2010), improvement of air quality (Yang et al., 2008), improvement of urban landscape aesthetics and provision of new flora and fauna habitats (MacIvor and Lundholm, 2011). However, the above mentioned benefits are expected to accrue only provided that green roofing is implemented on broad city surfaces (Akbari et al., 2001; Getter and Rowe, 2006). Based on the
Abbreviations: GTC, green turf cover; ETc , crop evapotranspiration; NDVI, normalized difference vegetation index; SMC, substrate moisture content; LSR, leaf stomatal resistance. ∗ Corresponding author. E-mail addresses: [email protected] (N. Ntoulas), [email protected] (P.A. Nektarios), [email protected] (G. Kotopoulis), [email protected] (P. Ilia), [email protected] (T. Ttooulou).
latter, it is of great interest to select and investigate a methodology that is suitable for the application of green roofs to existing city buildings through retrofitting. This is of immense importance, considering that, in most cases, city buildings are aged and constructed based on obsolete design criteria, thus, being able to bear only minimal additional loads. The green roof industry has termed constructions with minimal weight as “extensive” green roofs. In these green roof types, weight reduction has mainly resulted from very shallow substrate depths that may vary from 5 to 20 cm depth (FLL, 2008). Due to shallow substrate depths and lack of irrigation, extensive green roofs demand specific plant species for their planting that are mainly succulents. The lack of plant variability in conjunction with the minimal wear tolerance of succulent plants to traffic has led extensive green roofs to be considered as “environmental” building structures rather than as a functional open green space that is accessible by ˜ the building residents. The report of Fernandez-Canero et al. (2013) demonstrates that local residents clearly preferred more aesthetically pleasing and accessible green roofs with variable plant types, such as groundcovers, shrubs and trees. In contrast, extensive green
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Please cite this article in press as: Ntoulas, N., et al., Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems. Urban Forestry & Urban Greening (2017), http://dx.doi.org/10.1016/j.ufug.2017.03.005
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roofs planted with Sedum succulents received low ratings in the public perception and preferences measurement. The preceding problems, concerning aesthetics and accessibility of extensive green roofs, become more severe in countries where governmental incentives are absent as well as in longitudes that are characterized by droughty summer periods such as in Mediterranean countries. As a result, Kotsiris et al. (2013) proposed the “adaptive green roofs” approach that focuses on light-weight green roof systems combined with improved aesthetics, accessibility and functionality. In adaptive green roofs, a shallow substrate depth provides a light structure that is compensated for by prudent irrigation applications along with the use of drought tolerant native species and turfgrasses (Ntoulas et al., 2013a). Turfgrasses have a unique ability to fulfil aesthetic, functional and recreation requirements, which are demanded of urban plants (Beard and Green, 1994). However, they have rarely been evaluated in relation to extensive green roofs due to their increased water demands compared to succulents or other xerophytic plants. Ntoulas et al. (2013a) evaluated the establishment and growth of Zoysia matrella (L.) Merr. on adaptive green roof systems. They reported higher green turf cover (GTC) and normalized difference vegetation index (NDVI) values when the substrate depth was 15 cm compared to a shallower substrate depth of 7.5 cm, during both the establishment and the water deficit periods. Ntoulas et al. (2013b) evaluated Z. matrella performance in two different green roof substrate types and depths (7.5 and 15 cm) and under two different irrigation regimes (3 mm or 6 mm every 3 days). They reported that GTC and NDVI values were mostly affected by substrate depth, moderately by irrigation regime and to a lesser extent by substrate type. Ntoulas and Nektarios (2015) reported that Paspalum vaginatum Swartz provided better green cover and demanded less water to retain its visual quality at a 15 cm substrate depth compared to a 7.5 cm depth. They also reported that at the 7.5 cm substrate depth P. vaginatum growth is possible if water inputs increase by 40% compared to the 15 cm substrate depth. Nektarios et al. (2014) reported that Festuca arundinacea L. can grow in a reduced substrate depth of 7.5 cm without being stressed compared to a substrate depth of 15 cm, when irrigation is provided at 85% of evapotranspiration. The variable response of different turfgrass species, growing on shallow green roof systems, indicated the need to perform a comparable study. The aim of the present study is to investigate the response of three warm-season grasses at different substrate depths of a shallow green roof system.
2.2. Green roof construction within the lysimeters The lysimeters had 30 cm internal diameter. Within each lysimeter a complete layered simulation of an extensive green roof system was constructed. The bottom of the lysimeter was covered with a protection mat that was a synthetic cloth made of non-rotting synthetic polyester fibers having 3 mm thickness and a dry weight of 0.32 kg m−2 . The mat also had the capacity to retain 3 L m−2 of water (TSM32, Zinco, Egreen, Athens, 10672, Greece). A drainage board layer was placed on top of the protection cloth. The drainage layer was made of recycled polyethylene with 25 mm height and 1.5 kg m−2 weight (FD25, Zinco, Egreen, Athens, 10672, Greece). It was equipped with water retaining troughs having a water holding capacity of 3 L m−2 and openings for improving subsurface aeration. The drainage layer was covered with a non-woven geotextile (SF, Zinco, Egreen, Athens, 10672, Greece) that was made of thermally strengthened polypropylene having 600 m thickness, a mass of 100 g m−2 , apparent opening size of D90 = 95 m and water flow rate of 0.07 m s−1 . The filter sheet was used to prevent fine particle migration from the substrate towards the drainage layer, thus ensuring that the drainage layer would not clog and would function effectively. The lysimeters were filled with a specialized green roof substrate that comprised 40% pumice, 40% thermally treated attapulgite clay, 8% peat, 7% compost and 5% clinoptilolite zeolite by volume. Pumice (LAVA, Mineral & Quarry A.D., Athens, 14123, Greece) had a particle distribution of 0.05–8 mm, thermally treated attapulgite clay 1–10 mm (GeoHellas SA, Athens, 17564, Greece) and zeolite 0.8-2.5 mm (S&B Industrial Minerals A.D., Athens, 14564, Greece). The organic portion of the substrate contained sphagnum peat, with a corrected pH of 5.5 and an organic matter of 90% (w/w), and compost that comprised straw, sawdust, yard waste (clippings and wood chips) as well as dairy cow, horse and chicken manure. The mechanical analysis and the water potential curve of the substrate are presented in Fig. 1A and 1B, respectively. Half of the lysimeters had a substrate depth of 7.5 cm and the other half had a 15 cm depth. Light compression and leveling was applied to the substrates after their placement into the lysimeters. The zoysiagrass lysimeters were seeded on 13 June 2011 and the hybrid bermudagrass and seashore paspalum lysimeters were sodded – using washed sod – on 22 June 2011. After seeding and sodding, the lysimeters were placed under a mist system in order to promote seed germination and sod rooting. Then, the lysimeters were transferred onto outdoor benches that were equipped with a rain-out shelter. However, no rainfall occurred during the first study year and only four minor rain events occurred during the second study year (Fig. 2).
2. Materials and methods 2.3. Turfgrass maintenance and irrigation 2.1. Experimental setup and hypothesis The outdoor study was conducted at the experimental field of the Laboratory of Floriculture and Landscape Architecture, Agricultural University of Athens, Greece. The initial study was performed from 3 Aug. until 10 Sep. 2011 and was replicated from 15 May until 19 July 2012. Treatments included: a) three warm season turfgrasses (hybrid bermudagrass, Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy ‘MiniVerde’; seashore paspalum, Paspalum vaginatum Swartz ‘Platinum TE’ and zoysiagrass, Zoysia japonica Steud. ‘Zenith’) and b) two green roof substrate depths (7.5 cm or 15 cm). Each treatment was replicated three times, totaling 18 lysimeters (3species x 2sub.depths × 3replications = 18 lysimeters). The hypotheses were to investigate whether the three warm season turfgrass species exhibit different green coverage when grown under green roof conditions and whether it improves as substrate depth increases.
Turfgrass sward was mowed at a height of 5 cm once a week with a handheld electric shear mower, and clippings were removed. Fertilization was applied once before the initiation of each study (6 July 2011 and 28 March 2012) with Floranid Permanent 16-715 (+2Mg, +7S + 0.5 Fe, Compo Hellas SA), at a rate of 10 g m−2 of fertilizer. At the initiation of each study, all lysimeters were irrigated close to saturation in order to produce uniform substrate moisture conditions. From then on, irrigation was performed on a daily basis using irrigation amounts at crop evapotranspiration (ETc ). Crop evapotranspiration was calculated based on the daily evaporation of a Class-A pan according to: ETc = Epan x Kp x Kc , where Epan is the evaporation of a Class-A pan, Kp is the pan coefficient used to convert pan evaporation to reference evapotranspiration, and Kc is the crop coefficient. Based on the weather data during the experimental periods, a Kp value of 0.65 (Doorenbos and Pruitt, 1977) and a Kc
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Fig. 1. (A) Particle size distribution of substrate that comprised 40% pumice, 40% thermally treated attapulgite clay, 8% peat, 7% compost and 5% clinoptilolite zeolite by volume. The grey area represents particle size distribution specified by FLL (2008) guidelines. (B) Water potential curve of the substrate used in the study. Values are the means of three replications (±SE).
value of 0.65 were utilized. In both years, recuperation from water stress period was achieved by daily water applications at 100% of A-pan evaporation. 2.4. Measurements The in situ moisture content of the substrate was determined daily, just before the irrigation of the sward, using a dielectric moisture sensor (FieldScout TDR 300 Soil Moisture Meter, Spectrum Technologies, IL, USA). Measurements of substrate profiles at different depths were achieved by interchanging the sensor rods. The rod length of the sensors was 7.5 cm and 12 cm for the lysimeters with substrate heights of 7.5 cm and 15 cm, respectively. Each rod length sensor was calibrated for the specific substrate according to the methodology described by Kargas et al. (2013). To evaluate the effects of substrate depth on each of the three turfgrass species, green turf cover (GTC) was determined using digital image analysis. During the study periods, GTC measurements were performed approximately every four days, before each irrigation event. Images were acquired using a Canon IXUS 100 IS (Canon Europe Ltd., UK) digital camera mounted on top of a sealed circular box (30 cm in diameter × 45 cm in height), equipped with a circular fluorescent lamp (1200 lumens) that secured consistent lighting conditions throughout the image acquisition process. In the course
of both studies, the camera settings remained constant and were the following: an aperture of F3.2, a focal length of 33 mm, white balance set to fluorescent and ISO sensitivity set to 200. The SigmaScan Pro, version 5.0, digital image analysis software (SigmaScan Pro, Systat Software Inc., Chicago, IL, USA) was employed to determine GTC percentage (%) according to Richardson et al. (2001). The images were in JPEG format and their size was 1600 × 1200 pixels. Each image was cropped using the Adobe PhotoShop CS3 software (Adobe Systems Inc., USA) in order to remove any unwanted image portion. Green leaves were selectively identified in the images by setting the hue range from 47 to 107 and saturation from 0 to 100. In each image, the GTC percentage was determined by dividing the number of green pixels by the total pixel count of each image. During the first study year (2011), the three turfgrass species were stressed until GTC reached values below 50%. In the second year (2012), the GTC stress limit was reduced to 20%. Leaf stomatal resistance (LSR) was determined using an AP4 diffusion porometer (Delta-T Devices, UK) before irrigation events, that is, at noon on cloudless days. Measurements were made on the abaxial side of young fully expanded leaves.
2.5. Meteorological data The ambient maximum, minimum and average temperatures as well as precipitation were recorded by the weather station of the National Observatory of Athens at the Gazi region, which is located 885 m from the experimental site (Fig. 2).
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Date Fig. 3. Effects of plant species (hybrid bermudagrass, Cynodon dactylon x C. transvaalensis ‘MiniVerde’; seashore paspalum, Paspalum vaginatum ‘Platinum TE’; zoysiagrass, Zoysia japonica ‘Zenith’) and substrate depth (Shallow, 7.5 cm or Deep, 15 cm) on green turf cover (%) during the two study years (2011 and 2012). Values are the means of 3 replications. Bars represents Fisher’s least significant difference (LSD) at P < 0.05.
3. Results and discussion In 2011, the study took place in August while in 2012 it was conducted during late May and June. Due to the replication of the study at different times of the year, ambient temperatures differed between the two study years (Fig. 2). In 2011, ambient average air temperature fluctuated from 26.4 ◦ C to 31.0 ◦ C, while in 2012 it fluctuated from 14.6 ◦ C to 34.1 ◦ C. However, soon after the initiation of the 2012 study period, the mean air temperature rose and fluctuated from 26.4 ◦ C to 34.1 ◦ C after 9 June 2012. This was very similar to air temperatures observed during the 2011 study period. No rainfall events occurred during the 2011 study period, while 4 minor rainfall events were recorded in 2012 (Fig. 2). In all four instances, the lysimeters were covered during rainfall events. 3.1. Green turf cover In the course of both years, green turf cover remained stable for a week after the initiation of the study. From then on, a significant GTC reduction was recorded that depended on the treatments. More specifically, in 2011 the most abrupt reduction occurred at shallow depth substrates that were covered with seashore paspalum and hybrid bermudagrass (Fig. 3). Based on the curve slopes,
seashore paspalum and hybrid bermudagrass growing in the 15 cm substrate depth exhibited a similar GTC reduction rate compared to the 7.5 cm substrate depth. However, in the case of the 15 cm substrate depth, the initiation of the reduction was delayed by 3 days. Zoysiagrass growing in the 7.5 cm substrate depth followed the same GTC reduction patterns as those of seashore paspalum and hybrid bermudagrass growing in the 15 cm substrate depth. Zoysiagrass growing in the deeper substrates exhibited a similar reduction to the other two turfgrass species until 21 Aug. 2011. However, after that date, it was clearly separated from the other treatments, since GTC was not reduced any further and remained stable until after 3 Sep. 2011 when it actually increased. Similar results were obtained from the second study year (2012), when the GTC limit for the stress period was reduced to 20%. In this case, the only significant difference, compared to 2011, was seashore paspalum that exhibited a fast GTC reduction regardless of substrate depth. Based on the obtained results, it was concluded that zoysiagrass exhibited the best response in retaining GTC on shallow green roof systems. In contrast, at shallow substrate depths seashore paspalum and hybrid bermudagrass did not undergo the same adaptation as zoysiagrass. These observed differences among the three turfgrass species might rest on the different mechanisms responsible for overcoming water limiting conditions and drought stresses. Huang et al. (1997b) compared four seashore paspalums to common bermudagrass and zoysiagrass along their capacity to alter morphological and physiological traits in relation to drought resistance. They reported that the tested grasses exhibited different responses after being imposed to drought stress. Some seashore paspalum varieties extended their root system into greater depths, indicating that a drought avoidance mechanism was triggered. In contrast, zoysiagrass did not extend its root length, indicating that its drought resistance was not based on retrieving and utilizing water from greater soil depths. In the same experiment, Huang et al. (1997a) reported that shoot dry weight was significantly lower for zoysiagrass than for the other tested grasses, thus minimizing its evapotranspiration demands. In addition, water-stressed zoysiagrass responded in a very positive way with regard to chlorophyll content, canopy temperature and relative water content compared to the well-watered control. All the above indicate that the drought resistance mechanisms of zoysiagrass are well-suited to green roof systems, where substrate depth has been shown to be the most limiting factor (Ntoulas et al., 2013a,b). It is obvious that the drought avoidance mechanisms that rely on root system redistribution by increasing root length in order to access deeper water reservoirs are negated at shallow green roof substrate depths. Consequently, zoysiagrass, which does not rely on the latter mechanism, appears to possess a significant advantage for usage in shallow substrates. This is further supported by Huang (1999) who concluded that zoysiagrass has extensive roots in the surface and mainly utilizes surface water. Moreover, Qian and Fry (1997) reported that zoysiagrass had a shallow root system but exhibited intermediate evapotranspiration rate and a high level of osmotic adjustment. In addition, Carmo-Silva et al. (2009) reported increased amino acid production (proline, phenyladenine, valine, isoleucine and leucine) of water-stressed zoysiagrass compared to bermudagrass and Paspalum dilatatum Poir., which was probably related to enhanced secondary metabolism. All the above demonstrate the differences between zoysiagrass and seashore paspalum and bermudagrass concerning their drought resistance mechanism. 3.2. Substrate moisture content The substrate moisture content (SMC) displayed a reverse pattern to that of GTC. In 2011, at the initiation of the study, an increased SMC was observed for seashore paspalum growing in
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Fig. 4. Effects of plant species (hybrid bermudagrass, Cynodon dactylon x C. transvaalensis ‘MiniVerde’; seashore paspalum, Paspalum vaginatum ‘Platinum TE’; zoysiagrass, Zoysia japonica ‘Zenith’) and substrate depth (Shallow, 7.5 cm or Deep, 15 cm) on substrate moisture content (% v/v) during the two study years (2011 and 2012). Values are the means of 3 replications. Bars represents Fisher’s least significant difference (LSD) at P < 0.05.
both 7.5 cm and 15 cm substrate depths (Fig. 4). SMC for seashore paspalum at 15 cm remained high for the whole duration of the study, while after 28 Aug. zoysiagrass at shallow substrate depths exhibited higher SMC. In contrast, zoysiagrass and hybrid bermudagrass growing in deeper substrate depths showed the least SMC. However, it must be emphasized that all treatments reached a very low SMC of less than 10% v/v within 5–10 days after the initiation of the study. A similar response of SMC between treatments was observed in the 2012 study year. 3.3. Leaf stomatal resistance Leaf stomatal resistance of the three species fluctuated according to the monitored air temperature (Figs. 2 and 5). The response of the turfgrass plants was immediate and as soon as ambient temperature was reduced, LSR followed the same reduction. Similarly, whenever air temperature was increased, LSR was also raised. Leaf stomatal resistance provided interesting results that were in agreement with GTC. During the first study year, LSR increased sharply after 13 days (16 Aug. 2011) for seashore paspalum and hybrid bermudagrass growing in the shallow substrate depth of 7.5 cm (Fig. 5). A similar response was observed for seashore paspalum growing in the deeper substrate of 15 cm on the next sampling date (18 Aug. 2011). This coincides with the fast GTC
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Fig. 5. Effects of plant species (hybrid bermudagrass, Cynodon dactylon x C. transvaalensis ‘MiniVerde’; seashore paspalum, Paspalum vaginatum ‘Platinum TE’; zoysiagrass, Zoysia japonica ‘Zenith’) and substrate depth (Shallow, 7.5 cm or Deep, 15 cm) on leaf stomatal resistance (s cm−1 ) during the two study years (2011 and 2012). Values are the means of 3 replications. Bars represents Fisher’s least significant difference (LSD) at P < 0.05.
reduction, indicating that these treatments experienced increased stress (Fig. 3). Stomata closure is a physiological plant response in an effort to minimize water losses. However, if the exerted stress is prolonged, stomata closure negatively affects photosynthetic activity, which results in plant physiological deterioration. When plants have their stomata closed, transpiration is reduced and, thus, SMC is expected to reduce at a slower rate. This explains the increased SMC (Fig. 4) that seashore paspalum exhibited at the initiation of the 2011 study (3–16 Aug. 2011). Hybrid bermudagrass growing in the shallow substrate of 7.5 cm exhibited an increased LSR from 18 to 20 Aug. 2011, which was, however, lower than that of seashore paspalum. The lower LSR of hybrid bermudagrass, compared to that of seashore paspalum, indicated a slightly better drought resistance. When growing in the deeper substrate of 15 cm, this difference between seashore paspalum and hybrid bermudagrass became even greater. In deeper substrates, bermudagrass exhibited a moderate LSR until 20 Aug. 2011 that increased sharply on 6 Sep. 2011, thus, resulting in the lowest GTC value (Figs. 3 and 5). In contrast to the other two turfgrass species, zoysiagrass showed the least LSR at both substrate depths, during the 2011 study period, indicating that its anatomical and physiological status, as described above (Huang et al., 1997a,b; Qian and Fry, 1997), permitted an adequate physiological function at the shallow green roof depths. Similar results were obtained from the second study year (2012) for seashore paspalum that again showed the highest LSR until
Please cite this article in press as: Ntoulas, N., et al., Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems. Urban Forestry & Urban Greening (2017), http://dx.doi.org/10.1016/j.ufug.2017.03.005
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Table 1 Recovery time (days) to reach 90% green turf cover (GTC) for the three turfgrass species after reaching 50% GTC in 2011 and 20% GTC in 2012. (N/A refers to the first year of the treatments during which GTC did not fall below 50%). Turfgrass species
Substrate depth
Recovery time
(cm)
(days) Year 2011
Year 2012
Paspalum vaginatum
7.5 15
N/A 16
21 18
Zoysia japonica
7.5 15
N/A N/A
18 25
Cynodon dactylon x C. transvaalensis
7.5 15
N/A 12
28 36
Ethical approval The mention of a trade mark, proprietary product, or vendor does not imply endorsement by the authors, nor does it imply approval to the exclusion of other products that may also be suitable. Acknowledgments The authors would like to thank GeoHellas S.A. for providing attapulgite clay, L. Cambanis S.A. for providing compost, LAVA Mining & Quarrying S.A. for providing pumice, Compo Hellas SA for donating the fertilizer and Hellasod S.A. for providing turfgrass sod. References
1 June 2012 at both substrate depths. In contrast, zoysiagrass was, once more, the species that exhibited the least stress for the whole duration of the 2012 study. Hybrid bermudagrass had either a low (deeper substrates) or a moderate (swallow substrate) LSR until 1 June 2012. From then on and until 12 June 2012, hybrid bermudagrass at both substrate depths was the turfgrass species with the highest LSR along with seashore paspalum growing in the 7.5 cm substrate depth. The observed increase in LSR for hybrid bermudagrass 24 days after initiation (9 May to 1 June 2012) further substantiated the findings of the first study year, when a similar increase was observed 35 days after initiation (3 Aug. to 6 Sep. 2011, Fig. 5). This indicated that at shallow substrate profiles, hybrid bermudagrass can tolerate water stress better than seashore paspalum, although this difference is temporary. 3.4. Recovery period All treatments were able to recover in both study years (Table 1), indicating that GTC loss was caused by dormancy induction due to the reduced SMC. During the 2012 study period, when turfgrasses were stressed to a greater extent compared to 2011, the quickest recovery was achieved by seashore paspalum growing in the 15 cm substrate depth and zoysiagrass growing in the shallow depth of 7.5 cm. Recovery was slower for hybrid bermudagrass growing in both substrate depths and lasted 28 and 36 days for the 15 cm and the 7.5 cm substrate depths, respectively. 4. Conclusions The observed differences in GTC among the three turfgrass species highlighted the importance of investigating the ability of each turfgrass species to adapt to shallow green roof systems and provide acceptable green cover. Zoysiagrass seems to be one of the best options for providing an aesthetically pleasing as well as functional surface on roof tops. Its green cover was not hindered to the same extent as for the other two turfgrass species tested, which suggests a good adaptation to the growing conditions on swallow green roof systems based on anatomical and metabolic mechanisms. Hybrid bermudagrass needed at least 15 cm of substrate to provide acceptable green cover, while seashore paspalum exhibited greater difficulties even at the 15 cm substrate depth. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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Please cite this article in press as: Ntoulas, N., et al., Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems. Urban Forestry & Urban Greening (2017), http://dx.doi.org/10.1016/j.ufug.2017.03.005