Alpine dwarf shrubs on green roofs: species-specific physiological responses to substrate depth and drought intensity

Background and aims Green roofs can enhance urban biodiversity when local, functionally diverse species are used. However, extreme roof conditions require species selection based on plant eco-physiological characteristics. Methods This study evaluated the performance of four Alpine dwarf shrubs ( Arctostaphylos uva-ursi , Vaccinium myrtillus , V. vitis-idaea , Calluna vulgaris ) in green roof plots with 10 and 30 cm substrate under natural conditions (temperate climate) and compared it with the drought-tolerant shrub Salvia officinalis as a reference. Seasonal and daily variations in leaf temperature, water potential, and gas exchange were recorded during humid and dry periods to assess how substrate depth modulates plant-soil interactions. Results More favourable water availability and temperature with 30 cm substrate reduced drought stress and increased stomatal conductance, photosynthesis, and biomass in all shrubs compared to 10 cm substrate. However, species responded differently to substrate depth and drought intensity, with A. uva-ursi and V. vitis-idaea performing best: the former remained above its hydraulic vulnerability threshold (Ψ 50 ) in both plots, and the latter with 30 cm substrate. Despite strict stomatal control, C. vulgaris and V. myrtillus exceeded their Ψ 50 during drought in the 10 cm plot and experienced dieback. S. officinalis surpassed all Alpine species by achieving the highest gas exchange, growth and favourable water status in both substrates. Conclusions Alpine dwarf shrubs can broaden the species for temperate green roofs, but some may experience drought-induced dieback with shallow substrate. Alternating deep and shallow substrate patches may be a cost-efficient strategy to improve shrub survival and account for species-specific limitations.