The Field

A New Medium to Allow Urban Trees to Grow in Pavement 

The fact that trees have difficulties surviving amid the conditions of urban and suburban environments is not a surprise. Urban areas for the most part are not designed with trees in mind. Trees are often treated as if they were afterthoughts to an environment built for cars, pedestrians, buildings, roadways, sidewalks and utilities. Studies point out that trees surrounded by pavement in the most urban downtown centers live for an average of 7 years (Moll, 1989; Craul, 1992), while those in tree lawns, those narrow strips of green running between the curb and sidewalk, live for up to 32 years. These same species might be expected to live anywhere from 60 to 200 years in a more hospitable setting.

Why is this so?

Urban trees experience a virtual litany of environmental insults such as increased heat loads, de-icing salts, soil and air pollution and interference from utilities, vehicles and buildings (Bassuk and Whitlow, 1985; Craul, 1992). Yet the most significant problem that urban trees face is the scarce quantity of useable soil for root growth (Lindsey and Bassuk, 1992). A large volume of uncompacted soil, with adequate drainage, aeration, and reasonable fertility, is the key to the healthy growth of trees (Perry, 1982; Craul, 1992). The investment in soil for a healthy tree is paid back by fulfilling the functions for which it was planted. These functions may include shade, beauty, noise reduction, wind abatement, pollution reduction, stormwater mitigation, wildlife habitat and the creation of civic identity. An adequate soil volume is key considering soils are where nutrients, water and air are held in a balance that allows for root growth, water and nutrient acquisition. Simply put, when soils are inadequate, plant growth suffers and trees die prematurely.

Soil Terminology

The usefulness of any given soil is largely dictated by its texture, structure and fertility. Soil texture or the percentage of sand, silt and clay in a given soil type, is an important parameter to define. Several soil characteristics, including sand, silt, clay and organic matter make up the solid portion of soil, while water and air make up the rest. Nutrient-holding capability is regulated by the proportional amount of clays and organic material in the soil. A soil’s susceptibility to compaction will be determined by the soil’s particle size distribution and the total amounts of silts and clays in the soil. Soil hydraulic characteristics, including moisture-holding, aeration and drainage, will be determined, in part, by the types of soil particles present in the soil matrix. The compacted bearing capacity, frost heave potential, and other engineering characteristics are intrinsically tied to the soil texture. Beyond soil texture, soil structure, and the aggregation of individual sand, silt and clay particles into larger clumps called peds, heavily influences the agricultural viability of a soil. Within these aggregates, water may be held against the force of gravity, making it available to the plant’s roots. Good structure or well aggregated soils provides pores that allow water to drain and aeration of the root zone to take place. Human activities can severely damage soil structure. The process of building in a city, or even installation of a sidewalk in an otherwise rural area, necessarily dictates a high level of soil disturbance. Any construction effort requires soil excavation, cut and fill, re-grading and soil compaction. Often highly efficient heavy machinery is brought on site to accomplish this work increasing the potential for compaction of soils. There are two critical effects of soil compaction which directly impact plant growth:

1. Soil structure is destroyed, crushing the majority of large interconnected pores (macro pores) which restrict water drainage and subsequent aeration
2. As the macro pores are crushed, soils become denser, eventually posing a physical barrier to root penetration. There are numerous accounts of urban soils being literally as “dense as bricks” (Patterson, 1980).

One method of evaluating relative compactness, or the severity of soil compaction, is to measure the soil’s weight per volume, or its density. This measurement is communicated either by bulk density or by dry density. Dry density is the dry weight of soil per a given volume, often expressed as grams dry weight/cm³ (g/cm³ or Mg/m³). Soils, depending on their texture, become limiting to root growth when their dry density approaches 1.4 g cubic cm for clayey soils to 1.7 g cubic cm for sandy soils (Morris and Lowry, 1988). When roots encounter a soil so dense that they cannot penetrate it, the roots may change direction if that is possible, or be stopped from growing altogether. Very often in the urban environment, roots coming out of a newly planted root ball into compacted soil will grow from a depth of 12 or 18 inches upwards where they remain just below the surface. This superficial rooting tends to make urban trees more sensitive to drought as soils dry out in the summer (Bassuk and Whitlow, 1985).

Conversely, when a tree is planted into compacted soil and drainage is impeded through the crushing of soil macro pores, water may remain around the root zone depriving the roots of needed oxygen. This can lead to root death and an impaired ability to take up water and nutrients that are necessary for tree growth. In urban soils that are not covered by pavement, it is possible to cultivate, amend or replace compacted soils to make them more conducive to root growth. However, where soils are covered by pavement, the needs of the tree come in direct opposition to specifications that call for a highly compacted base on which to lay pavement. All pavements must be laid on well-draining compacted bases so that the pavement will not subside, frost heave, or otherwise prematurely require replacement.

What is Proctor density?

In order to create predictably compacted base course materials, a test that is typically used called ‘Proctor density’ or ‘peak density’ assures that the base below the pavement is compacted sufficiently to meet the wear that it will receive. For any type of soil or aggregate, Proctor density is defined by ASTM D 698-91 method D protocol. The soil type to be used is tested with the same amount of compactive effort, 56 blows from a 5.5 pound hammer free-falling 12 inches for each 3 layers in a 6 inch diameter mold of 4.6 inch depth, at different moisture content. As the soil moisture content increases, the standard Proctor effort will result in a higher soil dry density as water in the soil acts as a lubricant, allowing soil particles to pack and nest closer to one another. The end result is an increase in dry density, or dry-weight per volume, of the sample. Eventually, there will come a moisture level where the water in the soil actually holds the soil solids apart, resulting in a lower dry density after the standardized compaction effort. This relationship of soil dry density resultant from a standardized compact effort, over a range of moisture contents, can be graphed as a moisture-density curve. The maximum estimated dry density from the moisture-density curve is defined as 100% Proctor density (see graph below). The actual dry density at 100% Proctor will vary depending on soil texture or stone aggregate size distribution. In the field, it is often required that soils or bases under pavement be compacted to within 95% Proctor density. This means that soils are at dry densities greater than 1.8 or 1.9 g cm³. Thus, soils that must support pavement are often too dense for root growth. It is not surprising then that urban trees surrounded by pavement have the shortest life span in cities (Moll, 1989 Craul, 1992). These paved areas also tend to be those that need trees the most to mitigate the heat island microclimate that exist in downtown areas.

How much soil does a tree need?

If it is recognized that urban trees are desired and necessary to the health and livability of our cities, how much useable soil is necessary to allow them to fulfill their design functions? Research at Cornell’s Urban Horticulture Institute (UHI) has shown that a reasonable ‘rule of thumb’ for most of the United States, except for the desert southwest, is to plan for two cubic feet of soil per every square foot of crown projection. The crown projection is the area under the drip line of the tree (Lindsey and Bassuk, 1992). If the tree canopy is viewed as symmetrical, the crown projection can be calculated as the area of a circle (Pi x radius squared). For example: for a tree with a canopy diameter of 20 feet, the crown projection would be, 3.14 (10 squared), or 3.14 (100) = 314 square feet. Using the ‘rule of thumb,’ an estimate can be calculated that the tree needs approximately 600 cubic feet of soil to support it. Assuming a useable rooting depth of 3 feet, one way of dimensioning the space needed for this tree would be 20′ x 10′ x 3′, or 600 cubic feet. It is clear that the typical 4′ x 5′ tree opening in the sidewalks, or the 6′ x 6′ tree pit, is inadequate to allow the tree to fulfill its function in the landscape.

Where can one find enough soil?

Under the sidewalk there is a potential for a large volume of soil that would be adequate to allow trees to reach their ‘design size’ as long as the soil volume for each tree was connected and continuous, giving each tree a chance to share soil with its neighboring tree. Looking at the forest as a model, trees may be spaced reasonably close together as long as they share a large common soil volume to support their needs. Therefore, the task is to find a soil that meets a pavements design requirements while simultaneously allowing for unimpeded root growth under the pavement. To do this, the authors envisioned a gap graded soil system that could be compacted to 100% Proctor density while still allowing roots to grow through it. The primary component of this soil system is a uniformly sized, highly angular crushed stone or crushed gravel ranging from 3/4 to 1 1/2 inches in diameter with no fine materials. If this single-sized stone is compacted, the stones would form an open stone structure with about a 40 percent porosity. For a similar single-sized spherical stone, a structure with 33 percent porosity would be produced. Friction between the stones at contact points would “lock in” forming the load-bearing structure of the mixture. The second component of this mixture is a soil which fills the stone voids. As long as we do not prevent the stone structure from forming by adding too much soil, the soil in the voids will remain largely non-compacted and root penetrable.

Literature Cited

Bassuk, N. L. and T. H. Whitlow. “Evaluating street tree microclimates in New York City.” Proc. 5th METRIA Conference (May 1985): 18-27.

Craul, P. J. Urban Soil in Landscape Design. New York: John Wiley & Sons, Inc., 1992.

Grabosky, J. Identification and testing of load bearing media to accommodate sustained root growth in urban street tree plantings. M.S. Thesis, Cornell University, 1995.

Grabosky, Jason, Edward Haffner, and Nina Bassuk. “Plant Available Moisture in Stone-soil Media for Use Under Pavement While Allowing Urban Tree Root Growth.” Arboriculture & Urban Forestry 35, no. 5 (2009): 271-278.

Lindsey, P. and N. Bassuk. “Redesigning the urban forest from the ground below: A new approach to specifying adequate soil volumes for street trees.” Arboricultural Journal 16 (1992): 25-39.

Moll, G. “The State of our Urban Forests.” American Forests, November/December 1989.

Morris, L.A., and R.F. Lowry. “Influence of Site Preparation on Soil Conditions Affecting Stand Establishment and Tree Growth.” Southern Journal of Applied Forestry 12, no. 3 (1988): 170-178.

Patterson, J. C., J. J. Murray, and J. R. Short. “The impact of urban soils on vegetation.” Proc. 3rd METRIA Conference (1980): 33-56.

Perry, T. O. “The ecology of tree roots and the practical significance thereof.” Arboricultural Journal 8 (1982): 197-211.

by Nina Bassuk, Cornell University; Peter Trowbridge, FASLA, Cornell University; and Jason Grabosky, PhD, Rutgers University