City Trees

July/August 2013

City Trees is a premier publication focused on urban + community forestry. In each issue, you’ll learn how to best manage the trees in your community and more!

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Can Urban Soil be Rehabilitated to Improve Stormwater Infiltration? by David Mitchell, M.S. Candidate, Department of Forest Resources and Environmental Conservation, Virginia Tech, Blacksburg, Virginia The March/April 2013 City Trees featured an urban forestry research update from Drs. Susan Day and Eric Wiseman and their colleagues at Virginia Tech. Findings from one of the projects discussed in that update, Urban Soil Rehabilitation and Soil Profile Rebuilding, led to inquiry into how soil rehabilitation affects stormwater infiltration. Soil Profile Rebuilding: Starting Principles Urban spaces typically have very high proportions of impervious surfaces: pavement, rooftops, sidewalks–but compacted soil can act just like pavement when it comes to stormwater. Healthy, non-compacted soils absorb water before creating runoff and are loose enough to allow tree roots to grow. Improved infiltration is one of the reasons that Soil Profile Rebuilding (SPR; specifications available at:, has been under study at Virginia Tech. SPR is not a planting technique per se, but a soil rehabilitation method applied to an entire site that uses deep tillage, compost, and trees to create a self-sustaining soil improvement system. should be used over the largest compacted area possible, as long as it does not damage existing tree roots or underground utilities or reduce stability of structures. SPR and Stormwater: Experimental Design Virginia Tech is collaborating with the Arlington, Virginia government to explore whether SPR can be used to increase the stormwater-related ecosystem services of street trees and the soil they inhabit. My advisor, Dr. Susan Day, has worked with Arlington Urban Forester Vincent Verweij and Capital Project Manager Christine Simpson to set up 25 experimental plots in the city. The plots for our experiment were installed in October of 2012 as part of a traffic calming project. Planting areas (bump-outs and medians) were created in spaces that had been parallel parking spots close to intersections and turning lanes. Because the planting areas had long been covered with pavement, the underlying soil was very compacted and disturbed, which was a perfect condition for studying SPR, as the technique is intended to rehabilitate highly compacted and disturbed soil. There are 36 trees in the Arlington study: 16 Katsura trees (Cercidiphyllum japonicum), 13 Scarlet oaks With SPR, the deep tillage uses a "scooping and dumping" action with a backhoe that incorporates compost into the subsoil to a depth of two feet. This breaks up compaction and reintroduces carbon and nutrients into the soil to allow for tree root growth and soil structure to develop. This loosening of the subsoil is followed by the addition of topsoil to the surface that is part of standard practice in most localities. The SPR approach Virginia Tech's Soil Rehabilitation Experiment Site • Photo by Yujuan Chen 8 Dr. Day and collaborator Christine Simpson at a control plot • Photo by David Mitchell City Trees

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