Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield
Water use strategies to improve green roof performance: Balancing survival and stormwater mitigation C. Farrell, C. Szota, N.S.G. Williams and S. Arndt The University of Melbourne, c.farrell@unimelb.edu.au Introduction Green roofs are increasingly being used to reduce stormwater runoff from cities. However, most roofs are planted with succulent species which do not effectively reduce runoff due to conservative water use strategies (Farrell et al. 2012). Clearly, plant selection needs to move beyond these species to include those which have both high water use after rain events as well as drought tolerance when water is limiting. As water availability on green roofs is analogous to natural shallow soil habitats we hypothesised that granite outcrop species could improve green roof performance. Granite outcrop species growing in shallow soil-filled depressions are exposed to seasonal and diurnal extremes of water availability (Hopper 2000). Species have adapted to drought stress through physiological control of water use or morphological features such as sclerophyllous leaves, and underground storage organs (geophytes and some monocots) (Hopper 2000). In our study, we aimed to determine whether granite outcrop plants could achieve both high water use and drought tolerance and therefore be suitable for green roofs.
Overview of Methodology We compared the physiological and morphological responses of 12 granite outcrop species of different life-forms (monocot, herb and shrub) and a commonly used green roof succulent (Sedum pachyphyllum) to contrasting water availability (well watered and water deficit treatments). This was a glasshouse experiment with plants in individual 4L pots containing scoria green roof substrate (see Farrell et al. 2012 for substrate properties). There were 5 replicates of each species in each watering treatment. Plants in the water deficit treatment received 20% of the water used by the well-watered plants (on a species basis) at each watering. Water use was determined by weighing pots before and after watering, with bare pots to determine loss through evaporation. Predawn water potential measurements were done at the end of the experiment to determine plant water status. Plants with predawn water potentials <1.5 MPa were considered to have maintained water status under water deficit conditions.
Key Findings As expected, all 12 granite outcrop species used more water than the green roof succulent under both well-watered and water deficit conditions. All granite outcrop species showed plasticity in water use but the most ideal species for green roofs were those which had high water use under well watered conditions and maintained water status under water deficit.
Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield Maintenance of water status was achieved by large reductions in transpiration under water deficit by these species. Using these physiological traits we developed a framework to select the most appropriate species for green roofs (Table 1). Based on our water use strategy criteria, the most ideal species were the monocots and herbs which showed high water use under well-watered conditions but also maintained water status under water deficit. Table 1 Framework for selecting species according to plant water use strategies and drought tolerance. Species categorised according to: water use strategies under well watered (WW) and water deficit (WD) conditions and water status (predawn water potential; ΨPD) under water deficit at the end of the experiment. Life-form indicated in parentheses with M = monocot; H = herb and S = shrub. Shading represents most desirable characteristics for green roof plant species. Characteristic
Category 1
Category 2
Category 3
Water use strategy when well-watered (WW)
High water users (>3 kg H2O pot-1) D. admixta (M) S. glauca (M) D. perfoliata (H) I. axillaris (H)
Moderate water users (1.5-3 kg H2O pot-1) A. milleflorum (M) L. longifolia (M) B. multifida (H) C. semipapposum (H) C. reflexa (S) C. tetragona (S)
Low water users (<1.5 kg H2O pot-1) G. alpina (S) H. obtusifolia (S)
High water users (>1 kg H2O pot-1) S. glauca (M) D. perfoliata (H) I. axillaris (H)
Moderate water users (0.5-1 kg H2O pot-1) A. milleflorum (M) D. admixta (M) L. longifolia (M) B. multifida (H) C. semipapposum (H) C. reflexa (S) C. tetragona (S)
Low water users (<0.5 kg H2O pot-1) G. alpina (S) H. obtusifolia (S)
High water status (ΨPD<-1.5 MPa) A. milleflorum (M) D. admixta (M) L. longifolia (M) S. glauca (M) I. axillaris (H)
Moderate water status (ΨPD -1.5 to -3.0 MPa) D. perfoliata (H) C. tetragona (S)
Low water status (ΨPD>-3.0 MPa) B. multifida (H) C. semipapposum (H) G. alpina (S) H. obtusifolia (S) C. reflexa (S)
Water use strategy under water deficit (WD)
Water status (ΨPD) under water deficit (WD)
Further Reading Farrell C, Mitchell R E, Szota C, Rayner J P and Williams N S G 2012 Green roofs for hot and dry climates: Interacting effects of plant water use, succulence and substrate. Ecol Eng 49, 270-276. Hopper S 2000 Floristics of Australian granitoid inselberg vegetation. In Inselbergs: Biotic diversity of isolated rock outcrops in tropical and temperate regions Eds. W Barthlott and S Porembski. pp 391407. Springer. Melbourne School of Land and Environment Green Infrastructure Research Group: http://www.landfood.unimelb.edu.au/green/