Green roofs are becoming increasingly popular for their environmental, economic, and social benefits. Depending on substrate depth, grasses, herbaceous plants, or even woody shrubs and trees can be planted. Using native and functionally diverse species could increase biodiversity and benefit several green roof functions. However, due to the extreme conditions on the roof, suitable species need to be identified.
This study aimed to evaluate whether four Alpine dwarf shrubs (Arctostaphylos uva-ursi, Vaccinium myrtillus, V. vitis-idaea, Calluna vulgaris) are suitable and physiologically adapted to the harsh conditions of green roofs. We assessed this in two experimental green roof plots with 10 and 30 cm substrate to reflect the conditions of an extensive and an intensive green roof, respectively. Various climatic parameters (air temperature, relative humidity, solar radiation, soil temperature, and soil water potential) were monitored throughout the experiment. Seasonal variations in midday leaf water potential (Ψmidday, MPa), leaf temperature, gas exchange, fluorescence, and daily variations in these parameters during a humid and a dry period were recorded.
Leaf water potentials showed a distinct decrease during the dry period compared to the more humid period. Accordingly, photosynthetic rate and stomatal conductance decreased under dry conditions. Mean Ψmidday was more negative, and mean midday leaf temperature was higher for shrubs in the 10 cm plot, whereas stomatal conductance and photosynthetic rate were higher for shrubs in the 30 cm plot. Vaccinium spp. and C. vulgaris showed very low transpiration and photosynthetic rates compared to A. uva-ursi, and severe defoliation and partial dieback after the dry period.
Overall, dwarf shrubs performed well in the 30 cm plot, with all species surviving the dry period, while only A. uva-ursi did not show drought-induced damage in the 10 cm plot. Therefore, different substrate depths on green roofs may create different ecological niches to support various species. The use of appropriate Alpine dwarf shrubs may help to improve species richness on green roofs.