Potential of thermal imaging in soil bioengineering to assess plant ability for soil water removal and air cooling

Soil bioengineering uses living vegetation to improve the ecological value and engineering performance of infrastructure slopes. Vegetation can stabilise slopes and provide co-benefits such as air cooling for potential mitigation of the urban heat island effect. This study represents the first attempt to apply the thermal imaging technique to simultaneously evaluate the abilities of plants to induce suction via transpiration (hydrological reinforcement) and cooling in the context of soil bioengineering. A pilot glasshouse experiment was performed to evaluate the use of thermal imaging to determine plant transpiration of Corylus avellana and Ilex aquifolium. Field thermal imaging was also carried out on a 20 m-long vegetated embankment where soil matric suction was measured. In addition to C. avellana and I. aquifolium, Ulex europaeus was also selected for testing in the field. U. europaeus had the greatest cooling effect, largest value of the index of stomatal conductance and the largest induced soil suction. Thermal imaging can be a useful preliminary tool to evaluate the cooling and water removal effects induced by different plant species. Whilst thermal imaging has a great potential to remotely estimate the transpiration of multiple individual plants at one time, plant surface temperature was subject to variabilities of environmental factors and above-ground plant properties that require further investigation before the technique can be widely applied.