The 2009 ASHS Annual Conference

Placing impermeable pavement around existing trees can cause declines in tree growth and health due to reduced water infiltration and soil surface gas exchange.Using pervious concrete, with greater water infiltration and gas permeability, could be a good alternative. The purpose of this study was to test soil respiration, root growth and soil extract chemistry in root zones of American sweetgum (Liquidambar styraciflua) as affected by pavement. The experimental setup consisted of twenty-five sweetgum trees, with root zones covered by standard concrete (five plots), pervious concrete (10 plots) or left uncovered (control, 10 plots). Each plot was outfitted with access points for soil respiration measurements. Soil respiration was measured monthly on two access holes per plot, with three sub-measurements per access hole.

Data collected from February 2008 to January 2009 indicated that soil respiration correlated with seasonal soil temperatures. We observed tremendous variability in soil CO₂ efflux rates within treatments. Maximum rates of CO₂ efflux per plot were extremely high in both concrete treatments (up to 350 µmol CO₂ m^-2 s^-1), while maximum rates in the control treatment reached up to 45 µmol CO₂ m^-2 s^-1 per plot. This was likely an experimental artifact as collar depth (15 cm) may have exceeded the capacity of the equipment. Rates were generally higher for standard concrete than pervious and considerably higher for both concrete treatments than the control. It is likely that pavement caused a build up of CO₂ in the root zone, resulting in high soil CO₂ efflux rates at the vents. Soil oxygen concentrations in control and pervious plots decreased with increasing volumetric water content (VWC). At field capacity (35% VWC), impervious plots had a lower soil oxygen concentration (7% O₂) than pervious plots (15% O₂) and the control (17% O₂).

Soil samples were taken at 0-10 and 10-20 cm in the exposed soil by the tree trunk. Dissolved organic carbon (DOC) and nitrogen (DON) in soil extracts were significantly higher under the pervious concrete than the control at both depths. Soil extract chemistry under impervious concrete plots was not significantly different from that under control plots or pervious concrete plots. Neither treatment affected extractable nitrate, ammonium, or pH at either depths.

Thus impervious concrete reduced oxygen infiltration under high soil moisture conditions and likely increased CO₂ concentrations in the soil, which could potentially lead to a more stressful environment for tree roots compared to pervious concrete and control plots.