Impervious and pervious pavements increase soil CO2 concentrations and reduce root production of American sweetgum (Liquidambar styraciflua)

Mature trees with large canopies help reduce problems of excessive heat and surface runoff in urban areas through shading, transpirational cooling, and interception and absorption of precipitation. Trees in paved impervious areas often suffer from a poor root zone environment. The objective of this experiment was to test if use of pervious concrete could be more beneficial for root growth and consequently a better alternative towards preserving existing mature trees during urban development. We measured root activity through CO2 flux, and also measured root production of American sweetgum under impervious concrete, pervious concrete and no pavement conditions (control). Soil under impervious concrete had lower oxygen concentrations than soil under pervious concrete and control treatments, particularly under wet conditions. Soil under pervious concrete had a 0.020 m3 m−3 (2% volumetric water content (VWC)) greater volumetric water content than soils paved with impervious concrete. Soil CO2 efflux rates and soil CO2 concentrations were extremely high under both concrete treatments. Standing live root length at the end of the experiment was greater under the control treatment than under both concrete treatments and these changes in root production were likely due to high soil CO2 concentrations under both pervious and impervious concrete. The soil type at our site, which is a Ships clay with a very low permeability rate, may have limited CO2 exchange through the pervious pavement. On this heavy clay soil, pervious concrete did not convey any measurable root growth benefits over impervious concrete.