Issues and Challenges to Urban Tree Health

A healthy urban forest can meet or exceed most if not all of these benefits but a healthy urban forest requires sufficient soil quantities and adequate soil conditions. Some of the issues and challenges in providing proper urban tree health and growing conditions are as follows:

1. Urban pavements need adequately compacted subsurface material. Compacted subsurface material is not an appropriate growing medium for trees and compacted soils cannot provide the sufficient air and water spaces that trees need to maintain health and mature.

2. Many species of urban trees cause heaving of pavements because the roots have inadequate room under the pavement for tree growth and root expansion.

3. Existing utilities present impediments to the placement of urban trees. Tree roots sometimes wrap around utility lines.

4. Net soil volume is essential to long-term growth of a tree. According to studies, (Urban 2010) a typical large canopy tree needs in excess of 1,000 cubic feet of loam soil to reach a large enough size that provides significant environmental benefits.

5. Designers must understand existing soil conditions. In many instances the existing soil under pavements may actually be suitable for tree growth. The question is whether the soil volume is sufficient.

6. Managing storm water at the point source is critical to meeting many newer codes as well as sustainability criteria. Impermeable pavements and underlying compacted soils prevent proper storm water management.

Providing Urban Trees with Enhanced Soil Volume

There are two methodologies for providing urban trees with enhanced soil volume and a better quality soil base. One methodology known as structural soil is comprised of a mixture of soil and stone aggregate mixed together with a polymer gel for adhesion. The second methodology is a soil cell system, which is a modular system comprised of a recycled plastic frame and deck that is filled with soil.

The Attributes of Structural Soil

Structural soil was developed by a research team at Cornell University to provide load-bearing soils under pavements so that the soil could be compacted to the required density to bear the load of the pavement while still allowing trees roots to grow in it. Structural soil is a mixture of stone aggregate (crushed gravel) and soil, with a small amount of polymer gel used to prevent the soil and stone aggregate from separating during the mixing and installation process. This soil mix can be compacted to 95% of dry density so it supports paving while still allowing for tree root growth. The mix takes advantage of the fact that there are about 20% to 25% void spaces between pieces of compacted gravel, which is where the tree roots will grow. The type of soil needed to make structural soil is a loam to clay-loam containing at least 20% clay which maximizes the soil's water and nutrient holding capacity. Structural soil should also have 3-5% organic matter. The ratio of soil to stone aggregate is approximately 80% stone to 20% soil by dry weight. This proportion ensures that there is sufficient bearing capacity in the mix and each stone touches another stone producing a rigid stone skeleton.

Observations have shown that trees growing in structural soil do exhibit drought stress symptoms. The clay-loam soil in the structural soil mixture is a low percentage (20%) of the total volume and this limits the water holding capacity of structural soil. Because of the large percentage of stone aggregate, structural soil is a very rapidly draining material with a percolation rate of approximately 24 inches per hour. Consequently structural soil does not effectively retain significant amounts of water in a meaningful period of time. This rapid percolation rate and ineffective ability to retain water inhibits the ability of structural soil to filter and remove pollutants. In a 2008 study (Xioa and McPherson 2008), structural soil was found to filter an average of 53% of the nutrient pollutants.

As structural soil is made from processed hard stone aggregates that require energy to mine, crush and ship, this may present issues with meeting sustainability criteria. Quarry activity can be damaging to local environments. Clay loam soil is often not available in many local project sites so it must be shipped to the site. Rock quarries are usually located a distance from urban areas and thus the stone aggregate must also be shipped. The stone and soil must be shipped to a site, processed and mixed and the reloaded for delivery to the project site. This can become a significant energy footprint.

The Attributes of a Soil Cell System

A soil cell system provides architects and site design professionals with a methodology that allows trees to be planted, maintained and matured in a variety of urban settings. The system provides room for the large volumes of soil that are necessary for tree growth and root expansion. The system allows for the installation of pavements over tree roots without causing damage to the roots or the health of the tree as the frame provides the necessary structural support for pavements without the need for soil compaction or the addition of large volumes of stone aggregate. The system also provides a means for tree installation around utilities as the modular frames can be installed around existing utility lines without requiring expensive re-routing.

The soil cell system is comprised of rectangular plastic modules filled with soil. This modular plastic structure supports pavements using a post-and-beam frame and a deck. The space below the deck and within the frame is filled with lightly compacted soils that can absorb and treat stormwater and that provide a large enough soil medium for the tree and the expansion of its root system.

The frames and decks are made of recycled polymer and can be stacked up to three modules high and can be spread out laterally to any required width. Because they are modular, they can be adjusted to fit irregular urban conditions. The open frame module has about 92% void space, thus making it easy fit in and around utilities. Modular underground soil cell systems are designed to meet AASHTO H-20 loading standards.

The modular frame of the soil cell system provides a structural support for pavements as well as space for increased soil volume. Image courtesy of DeepRoot Green Infrastructure

A soil cell system provides not only for increased soil volume but allows for a less compacted, and thus, better quality of soil. Due to the plastic frame in the soil cell system, the infill soils do not require stone aggregate or densely compacted soils to provide structural support for overlying pavements. Minimally compacted soils are better quality soils. They provide the air and water spaces necessary for tree root expansion and tree growth. Also, soils with air and water spaces allow storm water to penetrate through to groundwater areas thus serving as a means of storm water management. Soils in the soil cell system are usually fine-grained loamy soils that are slow draining. Soil draining soils have good capillary capability and this attribute provides a wider, more even distribution of water. Slow draining soils also hold the water for a longer period and, thus, they are able to filter pollutants before the water enters the groundwater.