The Evolution of Tree Soil Under Pavement

James Urban: Presentation Slide Deck, 2015

In 1978, Urban met Tom Perry, and eccentric tree lover and Harvard PhD who preached that Landscape Architects were killing trees with their designs. He taught a simple analogy that a tree was a wine glass on a dinner plate. Thirty years later Up By Roots was published to teach Landscape Architects how to build healthy soils and trees. 

Now, we have the tools to build healthy tree soil under pavement. This slide deck, packed with information, graphs, and images, walks through the evolution of tree soil under pavement, telling the story of thirty five years of research, investigation, and innovation.

Download the full slide deck here. (Note, this is a large file and will take some time to download.)

Two Different Approaches to Improve Growing Conditions for Trees

James Urban and Nina Bassuk: Arboricultural Consultant, 2013

Summary: Urban compares Silva Cells (a plastic/fiberglass structure of columns and beams that support paving above optimally compacted planting soil and transfers the force to a base layer below the structure) to Structural Soil (a mixture consisting of 80% crushed rock and 20% loam soil coating the rock, which allows soil to fill the 30% void space of the crushed rock, remaining un-compacted with the compaction force and paving loads transferred through the rock matrix), arguing against the belief that that equal amounts of the two products have similar capabilities and support similar size trees. 

Bassuk, conversely, discusses the Why's and How's of using Structural Soil to grow trees in pavement, explaining that "The need for a load-bearing soil under pavement gave rise to the development of Structural Soil, a blended soil that can be compacted to 100% peak density to bear the load of a pavement, while allowing tree roots to grow through it.

Download the full article here.

Health Assessment of the Willow Oaks on the Tryon Street Mall Charlotte, North Carolina

F.A. Bartlett Tree Expert Co., January 2010

Summary: The development of the Tryon Street Mall began in late 1983 using a new strategy called “suspended pavement” aimed at growing large maturing trees in confined areas. This project has been very successful and large trees are now shading the streetscape in Uptown. However, some willow oaks on the Mall have begun to decline in health and die in recent years.

The F.A. Bartlett Tree Expert Company working with the Bartlett Tree Research Laboratories was contracted to: 1) Visually inspect each tree on the Tryon Street Mall to determine its size, location and condition, 2) Select a subsample of the tree population and determine the causes of tree decline, 3) Report on the findings of the study, and 4) Provide recommendations to improve the health and longevity of the remaining willow oak population.

An inventory of the willow oaks on the Tryon Street Mall identified 167 willow oaks ranging from 6 to 27 inches in diameter with an average diameter of 16.25 inches. The condition of the trees was 58% Good, 34% Fair and 8% Poor. Nine trees were found to be lifting the curb or pavement around the tree. Many trees were in contact with buildings, or interfering with street lighting or signage.

A subsample of the inventoried tree population was selected for an in-depth study to determine the causes of the tree decline. The study evaluated many soil and pest related factors that could be associated with tree decline.

The cause of the willow oak decline syndrome on the Tryon Street Mall was identified as the interaction between pests and environmental factors. An infestation by oak lecanium scale on the trees is reducing energy produced in the leaves and Phytophthora root rot is affecting nutrient and water uptake. The environmental factors that are stressing the trees are a lack of water and the associated limitation of soil volume necessary to sustain root and crown growth. 

An integrated management program is recommended to improve the health of the trees and extend their
effective lifespan. Treatments should include treating the scale insect and root disease, monitoring and
managing soil moisture and nutrient levels, and managing tree growth so that the trees can live longer in
the limited space available to them.

Download the full report here.

Growth Response of Ficus Benjamina to Limited Soil Volume and Soil Dilution in a Skeletal Soil Container Study

Felix C.W. Loh, Jason Grabosky, and Nina Bassuk: Urban Forestry & Urban Greening: 2003

Abstract: The interactive effects of rooting volume and nutrient availability in a skeletal soil medium designed to meet street tree and pavement needs were observed in a containerized experiment. Bejamin fig (Ficus benjamina L.) was grown in a stone-soil blended skeletal soil material (CU-Soil TM) and compared to a loam soil. The same topsoil used as the soil component in the skeletal soil material was used as the sole component in the comparison soil-only treatment. 

Plants grown in the skeletal soil material had reduced leaf tissue N content and depressed growth compared with plants grown in non-diluted soil. No other mineral deficiencies were found. Leaf number, chlorophyll concentration, shoot weight, and root characteristics were all affected. 

Reduced growth from soil dilution could be offset by the provision of an enlarged rooting volume for root development. Large containers of skeletal soil were observed to have smaller root systems compared to equivalent net volumes of loam soil at the first two harvest dates of the study. By the end of the study, the large containers of skeletal soil were observed to have developed larger root systems compared to equivalent vet volumes of loam soil; resulting in comparable leaf N levels and total plant dry matter. Plants in skeletal soil had lower shoot: root ratios at the end of the study. Investing resources to further root growth in times of nutrient shortages is a probably plant reaction as evidenced by differences in specific root length between treatments. The study allowed a method for directly partitioning the containerization effect by having equivalent amounts of soil over two volumes. 

Download the full paper here

Testing of Structural Urban Tree Soil Materials for Use Under Pavement to Increase Street Tree Rooting Volumes

Jason Grabosky and Nina Bassuk: Journal of Arboriculture, November 1996

Abstract: Quercus robur seedlings were grown in compacted stone-soil mixes known to meet engineering standards for pavement base compaction and strength. Root penetration into these materials was greatly increased in comparison to an equally compacted clay loam, which was also a component of the test material. Oak root penetration in the clay loam decreased 50%, from 6 to 3 g dry root weight, as the bulk density increased from 1.24 to 1.55 Mg/m3. Severe root impedance was observed when clay loam bulk densities exceeded 1.5 Mg/m3 (90% standard AASHTO peak density), a situation produced after 20% of the standard compaction effort was imposed on the soil profile. At the standard AASHTO peak density for the clay loam (1.67 Mg/m3), which would be the norm in a sidewalk installation, root growth was entirely stopped. In contrast, structured stone-soil mixes compacted to 100% of their respective standard AASHTO peak densities (between 1.85 and 2.07 Mg/m3) did not restrict root penetration with mean root dry weights between 4 and 6 g and demonstrated satisfactory bearing strength (California bearing ratios between 40 and 80). 

Download the full paper here.

A New Urban Tree Soil to Safely Increase Rooting Volumes Under Sidewalks

Jason Grabosky and Nina Bassuk: Journal of Arboriculture, July 1995

Abstract: Soil compaction, which is necessary to safely support sidewalks and pavement, conflicts with urban trees' need for usable rooting space to support healthy tree growth. We have defined a rigid soil medium that will safely bear loads required by engineering standards yet still allow for rapid root exploration and growth. This was accomplished by forming a stone matrix and suspending soil within the matrix pores with the assistance of a hydrogel gluing agent. Initial studies using three stone types and various stone to soil ratios showed that the compacted stone-soil test medium (dry densities > 1700 kg/m3) increased root growth by a minimum of 320% over the compacted clay loam control (dry density of 1378 kg/m3). The proposed system can safely bear load demonstrated by California Bearing Ratios consistently exceeding 40. Discussion of a critical mixing ratio is presented as an approach for developing a specification for field installation. 

Download the full paper here.