Pavements, trees, and urban soils: pervious pavements as Nature-Based-Solutions to mitigate soil sealing in urban areas

The aim of this 4-year experiment was to assess the effects of three pavements, differing in permeability to water and gases, on some soil physical parameters, on growth and physiology of newly planted Celtis australis and Fraxinus ornus. Treatments were: 1) impermeable pavement (asphalt on concrete sub-base); 2) permeable pavement (curb on crushed rock sub-base); 3) porous design (porous pavement on crushed rock sub-base); 4) control (unpaved soil, kept free of weed by chemical control). Soil (temperature, moisture, oxygen content and CO2 efflux) and plant (above- and below-ground growth, leaf gas exchange, chlorophyll fluorescence, water relations) parameters were measured. All types of pavements altered water cycle compared to unpaved soil plots, but this disturbance was less intense in porous pavements than in other soil cover types. Porous pavements allowed both higher infiltration and evaporation of water than both curb and asphalt. Reduction of evaporative cooling from soil paved with permeable and impermeable pavements contributed to significant soil warming: at 20 cm depth, soils under curb and asphalt were 4 and 5 °C, respectively, warmer than soils covered by porous pavements and unpaved soils. Thus, enhancing evaporation from paved soils by using porous pavements may be an effective mitigation measure to urban heat island. CO2 greatly accumulated under impermeable and permeable pavements, but not under porous pavements, which showed CO2 efflux rates similar to control. Soil oxygen slightly decreased only beneath asphalt. Growth of newly planted C. australis and F. ornus was little affected by pavement type. Tree transpiration rapidly depleted soil moisture compared to the "not-planted" scenario, but soil moisture did not fall below wilting point (particularly in the deeper soil layers, i.e. 40-50 cm) in any treatment. While C. australis showed similar leaf gas exchange and water relations in all treatments, F. ornus showed a depression in CO2 assimilation and slight signs of stress of the photosynthetic apparatus when planted in the impermeable pavement.