Mohammad A RAHMAN, Astrid MOSER-REISCHL, Thomas RÖTZER and Stephan PAULEIT
Institution: Technical University of Munich – Germany
Keywords: Urban Greenspace, Ecosystem Services, Species Characteristics, Cooling Effect, Runoff Reduction
Greenspaces can moderate the effects of ongoing climate change and urbanization through the provision of ecosystem services (ES). However, due to high heterogeneity at micro-scales, there is still uncertainty concerning the magnitude, pattern and the process of mitigation. Over the last four years we continuously measured eco-physiological, dendrochronological, meteorological, edaphic variables and the heat fluxes below and above canopy of contrasting tree species planted in different street and canyon conditions in Munich as well as in the comparatively drier city of Würzburg in Germany. Species characteristics leading to different growth patterns showed influences on both shading and transpirational cooling with a decrease in air temperature up to 2 ºC in the centre of the crown and around 1 ºC at 1.5 m height from the ground. Among the species characteristics, leaf area index was the central criterion which caused surface temperature differences of up to 24 °C over asphalt and up to 9 °C over grass surfaces. Even during the hot and dry summer of 2018, tree shade provided physiologically equivalent temperature reductions of around 4 °C in Würzburg. In case of transpirational air cooling, the size of the trees, wood anatomy, climate zones and water use efficiency showed the greatest effect. Trees with higher above and below ground growth also linearly increased the soil infiltration potential. Largely, ES were also dependent on various meteorological variables and below-canopy surfaces. Trees grown over paved surfaces and within narrow-street canyons provided lower boundary layer cooling and also hindered the wind flow, therefore, only lawns within the wind channels were more efficient. Conversely, on wider street canyons, trees in combination with grasses were more efficient. Overall, trees with less water demand and lower canopy density showed higher cooling potential and runoff reduction when they were planted over grass surface than over built surfaces through the evapotranspiration of grass and an extended fine rooting system. Where larger areas of lawn are not practical, higher root biomass and infiltration at around 3 m away from the tree trunk suggested a need for larger tree planting pits to optimize the ES provision. However, there were trade-offs between different indicators for ES such as carbon gain and transpiration; number of trees and wind flow. Thus the study provided new insights on designing urban greenspaces and choice of species.