Susan D. Day

Can urban tree roots improve infiltration through compacted subsoils for stormwater management

Global land use patterns and increasing pressures on water resources demand creative urban stormwater management. Strategies encouraging infiltration can enhance groundwater recharge and water quality. Urban subsoils are often relatively impermeable, and the construction of many stormwater detention best management practices (D-BMPs) exacerbates this condition. Root paths can act as conduits for water, but this function has not been demonstrated for stormwater BMPs where standing water and dense subsoils create a unique environment. We examined whether tree roots can penetrate compacted subsoils and increase infiltration rates in the context of a novel infiltration BMP (I-BMP). Black oak (Quercus velutina Lam.) and red maple (Acer rubrum L.) trees, and an unplanted control, were installed in cylindrical planting sleeves surrounded by clay loam soil at two compaction levels (bulk density = 1.3 or 1.6 g cm(-3)) in irrigated containers. Roots of both species penetrated the more compacted soil, increasing infiltration rates by an average of 153%. Similarly, green ash (Fraxinus pennsylvanica Marsh.) trees were grown in CUSoil (Amereq Corp., New York) separated from compacted clay loam subsoil (1.6 g cm(-3)) by a geotextile. A drain hole at mid depth in the CUSoil layer mimicked the overflow drain in a stormwater I-BMP thus allowing water to pool above the subsoil. Roots penetrated the geotextile and subsoil and increased average infiltration rate 27-fold compared to unplanted controls. Although high water tables may limit tree rooting depth, some species may be effective tools for increasing water infiltration and enhancing groundwater recharge in this and other I-BMPs (e.g., raingardens and bioswales).

Transpiration and Root Development of Urban Trees in Structural Soil Stormwater Reservoirs

Stormwater management that relies on ecosystem processes, such as tree canopy interception and rhizosphere biology, can be difficult to achieve in built environments because urban land is costly and urban soil inhospitable to vegetation. Yet such systems offer a potentially valuable tool for achieving both sustainable urban forests and stormwater management. We evaluated tree water uptake and root distribution in a novel stormwater mitigation facility that integrates trees directly into detention reservoirs under pavement. The system relies on structural soils: highly porous engineered mixes designed to support tree root growth and pavement. To evaluate tree performance under the peculiar conditions of such a stormwater detention reservoir (i.e., periodically inundated), we grew green ash (Fraxinus pennsylvanica Marsh.) and swamp white oak (Quercus bicolor Willd.) in either CUSoil or a Carolina Stalite-based mix subjected to three simulated below-system infiltration rates for two growing seasons. Infiltration rate affected both transpiration and rooting depth. In a factorial experiment with ash, rooting depth always increased with infiltration rate for Stalite, but this relation was less consistent for CUSoil. Slow-drainage rates reduced transpiration and restricted rooting depth for both species and soils, and trunk growth was restricted for oak, which grew the most in moderate infiltration. Transpiration rates under slow infiltration were 55% (oak) and 70% (ash) of the most rapidly transpiring treatment (moderate for oak and rapid for ash). We conclude this system is feasible and provides another tool to address runoff that integrates the function of urban green spaces with other urban needs.

Influence of urban land development and subsequent soil rehabilitation on soil aggregates, carbon, and hydraulic conductivity.

Urban land use change is associated with decreased soil-mediated ecosystem services, including stormwater runoff mitigation and carbon (C) sequestration. To better understand soil structure formation over time and the effects of land use change on surface and subsurface hydrology, we quantified the effects of urban land development and subsequent soil rehabilitation on soil aggregate size distribution and aggregate-associated C and their links to soil hydraulic conductivity. Four treatments [typical practice (A horizon removed, subsoil compacted, A horizon partially replaced), enhanced topsoil (same as typical practice plus tillage), post-development rehabilitated soils (compost incorporation to 60-cm depth in subsoil; A horizon partially replaced plus tillage), and pre-development (undisturbed) soils] were applied to 24 plots in Virginia, USA. All plots were planted with five tree species. After five years, undisturbed surface soils had 26 to 48% higher levels of macroaggregation and 12 to 62% greater macroaggregate-associated C pools than those disturbed by urban land development regardless of whether they were stockpiled and replaced, or tilled. Little difference in aggregate size distribution was observed among treatments in subsurface soils, although rehabilitated soils had the greatest macroaggregate-associated C concentrations and pool sizes. Rehabilitated soils had 48 to 171% greater macroaggregate-associated C pool than the other three treatments. Surface hydraulic conductivity was not affected by soil treatment (ranging from 0.4 to 2.3 cm h(-1)). In deeper regions, post-development rehabilitated soils had about twice the saturated hydraulic conductivity (14.8 and 6.3 cm h(-1) at 10-25 cm and 25-40 cm, respectively) of undisturbed soils and approximately 6-11 times that of soils subjected to typical land development practices. Despite limited effects on soil aggregation, rehabilitation that includes deep compost incorporation and breaking of compacted subsurface layers has strong potential as a tool for urban stormwater mitigation and soil management should be explicitly considered in urban stormwater policy.