The 2012 ASHS Annual Conference

There are numerous benefits to preserving existing mature trees in urbanizing areas, such as larger amounts of shade, greater transpirational cooling and greater air pollution removal. However, covering root zones of mature trees with gas and water impervious materials often leads to rapid death of mature trees in urbanizing areas as result of anoxia and drought stress. Pervious pavements may improve gas and water exchange with the underlying soil and thus provide a more favorable environment for root growth. We studied the potential use of pervious concrete as a way to preserve mature trees after development. We monitored soil CO₂ efflux, root production and death, and tree diameter growth over a two-year period in plots with 20-year old Liquidambar styraciflua that were exposed to three soil covering treatments: 1) no pavement (control); 2) impervious concrete and; 3) pervious concrete. We found higher soil CO₂ concentrations and soil CO₂ fluxes in both concrete treatments when compared to the control. Soil O₂ concentration was lower in the impervious plots compared to the control and pervious plots at high soil water contents, but never was lower than 15%. There was a strong negative effect of both concrete pavements on standing root length, but no difference between pervious and impervious pavement. Both concrete pavements reduced root lifespan compared to the control. Overall tree diameter growth was not significantly affected by any of the pavement treatments. We conclude that in our system, where pervious concrete was used on a heavy clay subsoil with low permeability, there was no measurable tree growth or root production benefit of using pervious concrete over impervious concrete.