Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs

Abstract

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 200 mm) 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 150 mm 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.

Appendix

Germination ecology

Materials and methods

Seed germination test

Seeds were disinfected with sodium hypochlorite (0.5% v/v) for 10 min before use, air dried, and imbibed on a single sheet of moistened filter paper (Whatman n. 1) and placed (50 seeds each) in 12 cm Petri dishes. Seeds were incubated in climate cabinets in different conditions of continuous white light (or darkness) and constant temperature (20°C). White light was obtained by using fluorescent tubes (Philips THL 20 W/33) at the intensity of roughly 50 µmol m⁻² s⁻¹. A spectroradiometer (model 1,800, Licor Inc., Lincoln, NE, USA) was used for the control of irradiance. In some cases seed treatments were necessary to reach a sufficient dormancy-breaking: 1) soaking with concentrated sodium hypochlorite (10 min followed by seed washing); 2) seed soaking with 200 ppm of gibberellic acid (GA₃); 3) seed extraction from the indehiscent fruits (Table 4). Germinated seeds were counted daily at cotyledon appearance. Germination counts were stopped when final germination percentages were reached. Mean germination time (MGT) was calculated as:

Table 4 Botanical and biological information and germination ecology of the 20 selected species Life forms: He= hemycryptophyte; Ch= Chamaephyte; Np= Nanophanerophyte. Means followed by one or two asterisks differ statistically for p < 0.05 and p < 0.01 respectively

$$ {\hbox{MGT}} = \Sigma \left( {{\hbox{n}} \times {\hbox{g}}} \right){\hbox{/N}} $$

Where n is the number of germinated seeds per day, g is the number of days after seeding, and N is the total number of germinated seeds.

Result and discussion

Seed germination

High germination rates (>90%) were observed only in 4 species: Dianthus cartusianorum, Verbascum thapsus, Satureja montana and Lavandula stoechas (Table 4), but in the case of the latter species germination occurred after a treatment with GA₃ which was necessary to remove the strong dormancy initially observed in non treated seeds Pre-germination treatments were necessary also for Critmum maritimum (extraction of the seed from the fruit), Euphorbia pythiusa, Othanthus maritimus (soaking with sodium hypochlorite), Glaucium flavum and Scrophularia canina (soaking with GA₃). Total dormancy was observed in the case of Sedum rupestre, even though various kinds of pre-germination treatments were performed, and the species was therefore successfully propagated by cuttings. The other species had germination rates between 22 and 70%, which can be considered acceptable considering that most wild plants have some kind of physiological, phyisical and/or morphological dormancy mechanism. Significantly different (p < 0.05 or p < 0.01) light requirements were observed in seven species (Crithmum maritimus, Dianthis cartusianorum, Euphorbia pythusa, Helichrysum spp., Glaucium flavum and Leontodon tuberosus), probably mediated by phytochrome, a typical chrome-protein pigment which, activated by light, elicits germination (see Shinomura 1997). The other species were indifferent to light, thereby underlining lower requirements in terms of space (underground depth of seed) and time (periods of the year with more or less light). Average germination time ranged between 1 week of the three Helychrisum species and 1 month of Othanthus maritimus. On average, most species had incubation times which ranged between 10 and 20 days. The only species in which light affected speed of germination was Dianthus cartusianorum, which germinated in a significantly slower longer time in darkness conditions.

Conclusion

Dormancy of the seeds tends to hinder their propagation, although some treatments are available to partially remove the dormancy, resulting in sufficient germination rates. Vegetative propagation is a possible alternative but was necessary only for the species Sedum rupestre which had an apparently unbreakable dormancy.