P.A. Nektariosa and N. Ntoulas
Laboratory of Floriculture and Landscape Architecture, Department of Crop Science, Agricultural University of Athens, Iera Odos 75, 118 55 Athens, Greece
Abstract
It is widely accepted that green roofs provide multiple benefits to the urban environment and the well-being of the cities’ residents. Despite their acknowledged contribution to the contemporary cityscape and environment, their broad implication in arid and semi-arid regions is extremely limited compared to northern countries.
Thus, it is of great interest and importance to identify the hurdles that delay the broad implication of green roofs in challenging climatic zones. These hurdles can be categorized either as governmental or technical or psychological even though they all intermingle and interact. More specifically, in the arid and semi-arid climatic zones, such as those of the Mediterranean basin, the governmental incentives are absent forcing the private sector and the homeowners to take the initiative. In those cases, investing in green roofing can only be justified if green roofs are usable and aesthetically pleasing since the preference of the public has been documented towards the intensive green roof types. However, taking into account that the majority of existing buildings are old and can bare minimal additional loading, it is obvious that green roof substrate depth must be minimal. All the above parameters are contradicting and have led us to the introduction of a new green roof category, namely the adaptive type. Adaptive green roofs have minimal substrate depth ranging from 5 to 15 cm but utilize a broad palette of plant species such as groundcovers, turfgrasses, medicinal and aromatic plants coppices and small shrubs. All the above are feasible due to minimal and prudent irrigation inputs. The current presentation provides a thorough review of the plant species evaluated in adaptive green roof systems concerning their response to different substrate types and depths, their tolerance to either deficit irrigation or drought and their irrigation requirements.
Keywords: adaptive green roof systems, Mediterranean zone, substrate depth, substrate type, native plant species, deficient irrigation, drought tolerance
INTRODUCTION
Green roofs formulaic classification separates them into extensive, simple intensive and intensive. Such a classification is based on substrate depth, the potential plant material that can be utilized and the intensity of maintenance procedures.
Intensive green roofs are equipped with deep substrates and can host a broad variety of plant forms such as trees, shrubs and groundcovers creating an aesthetically pleasing urban landscape. However, their maintenance intensity and the input of resources such as irrigation and fertilization are demanding. In contrast, extensive green roofs aim mainly to provide environmental benefits with minimal construction cost and maintenance requirements. They are characterized by shallow substrate depths and are usually planted with succulent plants mainly of Sedum and Delosperma species, which are characterized by extreme drought stress tolerance in conjunction with shallow root systems. Even though extensive green roofs seem to have a satisfactory market share in the northern climates, their application in the arid and semi‐arid Mediterranean climates is hindered due to the extreme summer temperatures and drought conditions along with the minimal adaptability of northern succulent species to such climatic conditions (Kokkinou, 2015). In addition, public conception is not in favor of green roof systems that are mainly constructed for environmental purposes in expense of functionality and aesthetically pleasing designs (Fernandez‐Cañero et al., 2013). Furthermore, Soulis et al. (2017) have proved that extensive green roofs planted with succulent plants present much less capacity to intercept rainfall compared to xerophytic plants.
However, the use of extensive green roofs seems to be the only feasible solution for most cities’ centers that lack urban green spaces due to the fact that most of the buildings are aged and have been constructed with obsolete design parameters and thus, they can withstand only minimal additional weight loads on their framework (Papafotiou et al., 2016). In addition, in the absence of governmental incentives building owners are reluctant to invest on green roofs solely for their environmental contribution rather than creating accessible green places on top of the buildings for socialization, utilization and aesthetical pleasure.
Therefore, the need to develop a flexible adaptive green roof system for new building constructions or for retrofitting old ones is a necessity for re‐introducing the lost flora in the city centers. The adaptive green roof systems were introduced by Kotsiris et al. in 2012 and since then several plant species were evaluated by the authors. The adaptive green roof concept utilizes shallow substrate depths (5‐15 cm) that are normally utilized by extensive green roofs. Substrate types are designed to retain increased but not excessive amounts of water with adequate porosity. Preferably, native but also foreign plant species are utilized in conjunction with prudent water applications. Thus, the aim of the present study is to collectively report research results produced so far in the concept of adaptive green roof systems.
MATERIALS AND METHODS
The review will report the response of several plant species that have been tested by the authors at the Agricultural University of Athens for their use on adaptive green roof systems. Several substrate types and depths have been evaluated over the course of 7 years
(2010‐2016) and the tolerance of various plant species to deficient irrigation or drought conditions. The studies are separated on those that have been performed on actual roof tops and those performed on ground level. In all cases, all necessary extensive green roof layering has been applied (protection mat, drainage board and geotextile) below the growth substrate. All reported results concern 2‐year studies and thus the presented data refers to the final conclusions of each study.
Substrates were customarily designed and mixed having as their main constituent pumice (Pum). Other inorganic materials that were tested as substrate mixture components were perlite, zeolite, attapulgite clay, sandy loam soil. Sphagnum peat moss, grape mark compost, garden waste and manure compost and olive mill waste compost were tested as organic amendments (Ntoulas et al., 2015). In Table 1 the composition of various tested substrates is reported along with their dry or saturated weight load.
Table 1. Dry and saturated weight load of various adaptive green roof substrates
Pum: Pumice, Per: Perlite, GWC: Garden waste and manure compost, Z: Zeolite, AC: Attapulgite clay, P: Peat
Substrate depth was 7.5 cm for the shallow ones and 15 cm for the deeper ones. The maximum depth of 15 cm was dictated from the need to restrain the weight load close to maximum allowance for additional building loading (200 kg m‐2). The 7.5 cm was selected as
a minimum depth to be tested for the selected species based on literature review and empirical observations.
Plant selection was mainly performed from the native and endemic flora, while limited exotic plants that seemed promising for use on green roofs were also tested. Due to the fact that public opinion shows a preference on green roofs that are functional and aesthetically pleasing (Fernandez‐Cañero et al., 2013), several turfgrass species were also evaluated apart from the native plants.
RESULTS AND DISCUSSION
The analysis of the 7‐year period results indicated that the inorganic portion of the tested substrates seemed to have the least impact on plant growth and sustainability. In contrast, the organic source as well as participation percentage in the substrate mixture significantly influenced plant response. In general, during water profusion, composts having either increased nitrogen or participating in increased proportions in the substrate (higher than 15% v/v) resulted in increased plant growth. In contrast, the same organic sources were proved to be more stressful to the plants during reduced irrigation regimes or drought. The additional stress resulted from the lush growth of the plants that created higher leaf area and increased evapotranspiration.
Apart from the organic amendments, substrate depth was in most cases the most influential factor in conjunction with the irrigation regime (Table 2). Most of the tested native species preferred the deeper substrates and performed better under higher irrigation
regimes. However, differences were profound between plant response during deficit irrigation regimes or drought. Among the most drought tolerant plants were the succulent species Sedum sediforme and Crithmum maritimum. Similar drought tolerance was exhibited by Dianthus fruticosus ssp. fruticosus that is suspected to have a facultative CAM metabolism based on the fact that it managed to grow during subsequent years without any irrigation. From the xerophytes Ballota acetabulosa, Helichrysum orientale and Rosmarinus officinalis exhibited very good drought tolerance, while Salvia fruticose and Melissa officinalis were the less tolerant species. Plant species tolerance either on deficit irrigation or drought are listed in Table 2.
From the tested turfgrass species Zoysia matrella and Zoysia japonica were listed as the most drought tolerant (Table 3). Their slow sward growth in conjunction with their shallow root system enables these warm‐season species to be considered as top candidates for turfgrass establishment on shallow green roof systems. In contrast, other drought tolerant turfgrass species such as bermudagrass, depend on drought avoidance mechanisms that rely on root system redistribution by increasing root length to access deeper water reservoirs (Ntoulas et al., 2017). These types of turfgrasses exhibited reduced drought tolerance due to the shallow green roof substrate depths that did not permit root redistribution.
CONCLUSIONS
The use of the adaptive approach has multiple advantages that include the reduction of the load exerted on the buildings’ framework by reducing substrate depth to less than 15 cm and by significantly increasing the palette of the potential plant species through prudent or minimal irrigation. These facts facilitate landscape architects and horticulturist to design accessible, functional and aesthetically pleasing green roofs even on aged building’s rooftops. In addition, our research provides valuable information about the irrigation inputs that are necessary to provide sustainable growth or minimum acceptable visual quality of green roofs’ plant material.
Table 2. Study parameters concerning succulent and xerophytic plants and minimum recommended substrate depths and irrigation regimes during water stress periods for achieving sustainable growth on adaptive green roof systems
1 Pum: Pumice, Per: Perlite, GWC: Garden waste and manure compost, Z: Zeolite, S: Sandy loam soil, AC: Attapulgite clay, P: Peat
2 Values indicate the tested substrate depths and irrigation regimes during water stress periods. The minimum substrate depth and irrigation regime that can sustain growth are indicated with bold.
Table 3. Turfgrass species irrigation demands and drought tolerance when grown on adaptive green roof systems having different substrate types and depths
1 S: Sandy loam soil, Pum: Pumice, Per: Perlite, P: Peat, Z: Zeolite, GWC: Garden waste and manure compost, AC: Attapulgite clay, GMC: Grape marc compost
2 Values indicate the tested substrate depths and irrigation regimes. The minimum substrate depth and irrigation regime that can sustain growth are indicated with bold.
Literature cited
Fernandez‐Cañero, R., Emilsson, T., Fernandez‐Barba, C., and Herrera Machuca, M.A . (2013). Green roof systems: a study of public attitudes and preferences in southern Spain. J. Environ. Manage. 128, 106–115 https://doi.org/10.1016/j.jenvman.2013.04.052. PubMed
Kokkinou, I. (2015) Water stress effect on native and foreign plant species grown under extensive green roof conditions. MLA thesis (Greece: Agricultural Univ. of Athens).
Kokkinou, I., Ntoulas, N., Nektarios, P.A., and Varela, D. (2016). Response of native aromatic and medicinal plant species to water stress when grown on extensive green roof systems. HortScience 10, 1327–1333.
Kotsiris, G., Nektarios, P.A., Ntoulas, N., and Kargas, G. (2013). An adaptive approach to intensive green roofs in the Mediterranean climatic region. Urban For. Urban Green. 12 (3), 380–392 https://doi.org/10.1016/j.ufug.2013.05.001.
Nektarios, P.A., Amountzias, I., Kokkinou, I., and Ntoulas, N. (2011). Green roof substrate type and depth affect the growth of the native species Dianthus fruticosus under reduced irrigation regimens. HortScience 46 (8), 1208–1216.
Nektarios, P.A., Ntoulas, N., Kotopoulis, G., Ttoulou, T., and 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 (3), 142–149.
Nektarios, P.A., Ntoulas, N., Nydrioti, E., Kokkinou, I., Bali, E.‐M., and Amountzias, I. (2015). Drought stress response of Sedum sediforme grown in extensive green roof systems with different substrate types and depths. Sci. Hortic. (Amsterdam) 181, 52–61 https://doi.org/10.1016/j.scienta.2014.10.047.
Nektarios, P.A., Nydrioti, E., Kapsali, T., and Ntoulas, N. (2016a). Substrate type, depth and irrigation regime effects on Ebenus cretica growth in extensive green roof. Acta Hortic. 1108, 297–302
https://doi.org/10.17660/ActaHortic.2016.1108.39.
Nektarios, P.A., Nydrioti, E., Kapsali, T., and Ntoulas, N. (2016b). Crithmum maritimum growth in extensive green roof systems with different substrate type, depth and irrigation regime. Acta Hortic. 1108, 303–308 https://doi.org/10.17660/ActaHortic.2016.1108.40.
Ntoulas, N., and Nektarios, P.A. (2015). Paspalum vaginatum drought tolerance and recovery in adaptive extensive green roof systems. Ecol. Eng. 82, 189–200 https://doi.org/10.1016/j.ecoleng.2015.04.091.
Ntoulas, N., and Nektarios, P.A. (2017). Paspalum vaginatum NDVI when grown on shallow green roof systems and under moisture deficit conditions. Crop Sci. 57 (Suppl1), 147–160
https://doi.org/10.2135/cropsci2016.05.0381.
Ntoulas, N., Nektarios, P.A., Spaneas, K., and Kadoglou, N. (2012). Semi‐extensive green roof substrate type and depth effects on Zoysia matrella ‘Zeon’ growth and drought tolerance under different irrigation regimes. Acta Agriculturae Scandinavica, Section B‐Soil & Plant Sci. 62 (sup1), 165–173 http://dx.doi.org/10.1080/ 09064710.
2012.681391.
Ntoulas, N., Nektarios, P.A., Charalambous, E., and Psaroulis, A. (2013a). Zoysia matrella cover rate and drought tolerance in adaptive extensive green roof systems. Urban For. Urban Green. 12 (4), 522–531 https://doi.org/10.1016/j.ufug.2013.07.006.
Ntoulas, N., Nektarios, P.A., and Nydrioti, E. (2013b). Performance of Zoysia matrella ‘Zeon’ in shallow green roof substrates under moisture deficit conditions. HortScience 48 (7), 929–937.
Ntoulas, N., Nektarios, P.A., Kapsali, T.E., Kaltsidi, M.P., Han, L., and Yin, S. (2015). Determination of the physical, chemical, and hydraulic characteristics of locally available materials for formulating extensive green roof substrates. Horttechnology 25 (6), 774–784.
Ntoulas, N., Nektarios, P.A., Kotopoulis, G., Ilia, P., and Ttooulou, T. (2017). Quality assessment of three warmseason turfgrasses growing in different substrate depths on shallow green roof systems. Urban For. Urban Green. 26, 163–168 https://doi.org/10.1016/j.ufug.2017.03.005.
Papafotiou, M., Tassoula, L., Liakopoulos, G., and Kargas, G. (2016). Effect of substrate type and irrigation frequency on growth of Mediterranean xerophytes on green roofs. Acta Hortic. 1108, 309–316 https://doi.org/10.17660/ActaHortic.2016.1108.41.
Soulis, K.X., Ntoulas, N., Nektarios, P.A., and Kargas, G. (2017). Runoff reduction from extensive green roofs having different substrate depth and plant cover. Ecol. Eng. 102, 80–89 https://doi.org/10.1016/j.ecoleng.2017.01.031.