Comparing the infiltration potentials of soils beneath the canopies of two contrasting urban tree species

With ongoing urbanization and climate change our cities are continuously becoming warmer and susceptible to flash flooding. A wide array of green infrastructure including urban trees is getting attention to mitigate those problems; however, magnitude, patterns and processes of hydrological functional capacity of different tree species at micro-scales are largely unknown. We measured morphology, above and below ground growth of two ecologically contrasting street tree species at two streets in Munich, Germany: Robinia pseudoacacia L. and Tilia cordata Mill. during 2016-17. Additionally, tree transpiration, meteorological and edaphic variables including soil hydraulic conductivity was measured on two streets. Overall the soil physical properties including bulk density, particle size distribution and meteorological variables were not significantly different at the two sites. A higher soil infiltration rate (0.42 cm/min) under the canopies of R. pseudoacacia compared to the T. cordata (0.29 cm/min) was mainly attributed to the higher annual growth rate of R. pseudoacacia and showed a significant correlation (r = 0.62) with infiltration rate. Subsequently, R. pseudoacacia showed increased fine root biomass compared to T. cordata (121 g/m² and 27 g/m² respectively). However, T. cordata trees transpired three times more compared to R. pseudoacacia which indicated that they might have a deeper rooting system. The study showed that fast growing species such as R. pseudoacacia can enhance infiltration through its intense fine rooting system creating channels in the upper soil. Moreover, grass cover also plays a role both in infiltration and ground evapotranspiration compared to the built surfaces and can enhance the processes if planted in combination with less water using species. Secondly, higher root biomass and infiltration further away from the tree trunk suggest a need for larger tree planting pits whereas species such as T. cordata are better suited in paved cut-out pits inducing vertical rooting depth.