Many different types of tree BMP’s exist. The most prevalent types are described below.
Where existing trees exist, tree preservation is highly recommended, as existing trees are typically bigger than newly planted trees and bigger trees provide significantly more benefits than smaller trees.
Incorporating trees into traditional bioretention practices is Highly Recommended.
Accordingly, North Carolina’s bioretention standards recommend incorporating trees in all bioretention practices except for grassed cells: “A minimum of one (1) tree, three (3) shrubs, and three (3) herbaceous species should be incorporated in the bioretention planting plan unless it is a grassed cell. A diverse plant community is necessary to avoid susceptibility to insects and disease.”
Where there is not enough open space for traditional bioretention, several techniques exist to protect soil volume under pavement from traffic compaction so that this soil can be used both for bioretention and tree root growth. Examples of these techniques are discussed below.
In areas that do not have enough open space to grow large trees, techniques like structural cells (suspended pavement) can be used to extend tree rooting volume under HS-20 load bearing surfaces and create favorable conditions to grow large trees in urban areas. This rooting volume can also be used for bioretention. While suspended pavement has been built in several different ways, all suspended pavement is held slightly above the soil by a structure that “suspends” the pavement above the soil so that the soil is protected from the weight of the pavement and the compaction generated from its traffic.
One of the earliest examples of trees grown in suspended pavement is in Charlotte, North Carolina, where a reinforced concrete sidewalk was installed over the top of poured in place concrete columns. While this is an effective way to grow large trees, it is labor intensive and requires intensive surveying to ensure that column heights are precise.
A more recently developed, and less labor intensive, technique to build suspended pavement is through the use of soil cells. An example is Silva Cells, modular proprietary pre-engineered structural cells manufactured by Deeproot Green Infrastructure. Other examples are RootSpace systems manufactured by GreenBlue Urban and Stratavault systems manufactured by Citygreen. The modular design allows flexibility to size the rooting/bioretention volume as needed for each site. Underground utilities can be accommodated within these systems. Because soil in a suspended pavement system is protected from compaction from loads on pavement above the cells, a wide range of soils can be used in these systems, so soil can be tailored to desired functions (e.g. tree growth and stormwater management). Construction documents and specifications for a wide range of soil cell applications can be found on the manufacturers' websites (Deeproot Green Infrastructure, GreenBlue Urban, and Citygreen).
Despite their ability to support healthy tree growth, there may be limitations to the use of soil cells.
Rock based structural soils are typically engineered to be able to be compacted to 95 percent Proctor density without impeding root growth. Rock based structural soils are typically gap graded engineered soils with the following components.
Several types of rock based structural soils have been developed, including, for example:
Note: University of California (UC) Davis structural soil is not designed to be load-bearing and therefore should not be compacted. This structural soil is 75 percent lava rock (0.75 inches) and 25 percent loam soil (by volume) (Xiao and McPherson, 2008). Because of the lava rock, this soil stores more stormwater than other structural soils and has a very high surface area to facilitate pollutant trapping.
Care must be taken to select species tolerant of structural soil pH. For example, if limestone-based structural soil is used, trees tolerant of alkaline pH must be selected, as limestone can raise the pH of soil to 8.0 or higher (Bassuk, 2010 soil debate; Urban, 2008).
Because rock based structural soils drain quickly (greater than 24 inches per hour), designers should select tree species tolerant of extremely well drained soils (Bassuk 2010).
Because only 20 percent of the volume of a rock based structural soil is actually soil, a greater total volume of rock based structural soil is needed compared to growing the same size tree in a sandy loam soil. A study by Grabowsky et al., (2009) found the water holding capacity of a CU structural soil was about half that of a typical soil, meaning about 2 parts of structural soil are needed to provide the soil value of 1 part of loam soil. This could be reduced to a ratio of 1.5:1 if soil is planted around the root ball.
An on-going study at Bartlett Tree labs is finding that over the past 9 years, trees growing in loam soil in suspended pavement have been consistently outgrowing trees growing in equal volumes of rock based structural soils, stalite soil, and compacted soil. See File:Trees Task 4 Soil guidelines.docx for more information on these studies. Based on the above studies, Urban (2008) recommends:
More information about rock based structural soils is available online,for example, at Cornell University’s website and a joint website by Cornell University’s, Virginia Tech, Rutgers University, and University of California Davis
Sand based structural soils were first developed in Amsterdam when some trees were in poor condition because of an “unfavorable rooting environment” (Couenberg, 1993). Because the natural soils in Amsterdam (bog-peat) were non-load bearing, the top 2 meters of soil had been replaced with a medium coarse sand which had insufficient nutritional value. Amsterdam soils were developed in an effort to grow better trees but still provide adequate bearing capacity for pavement bearing light loads, such as sidewalks. The Dutch studied various mixes for tree growth, soil settlement, and several other parameters. The resulting Amsterdam Tree Soil contains medium coarse sand with 4 to 5 percent organic matter and 2 to 4 percent clay by weight and also meets other criteria, including the following.
All medium coarse sand (the layer above the Amsterdam Tree Soil) is compacted to 95 to 100 percent Proctor density. Amsterdam Tree Soil is not compacted to 100 percent density, but “is compacted until the soil has a penetration resistance between 1.5 and 2 MegaPascal (187 to 250 pounds per square inch)...Comparison of soil density values after filling with soil density at 100 percent Proctor Density has shown that soil density of Amsterdam Tree Soil after filling amounts to 70 to 80 percent Proctor Density” (Couenberg 1993). Amsterdam Tree Soil was found to settle 19 millimeters [0.75 inches] in 3 years compared to the surrounding pavement, which was acceptable according to Dutch standards (Couenberg 1993). This settling may not be acceptable to many communities in the United States due to litigation risk associated with trip and fall hazards.
The standard design in Amsterdam includes the following from bottom to top.
Soil boxes are concrete boxes designed for bioretention. They are typically proprietary products, such as, for example, the boxes made by Filterra and Contech.
Rooting volume capacity of these boxes is typically limited to large shrubs. Soil volumes provided by these boxes are typically not sufficient to grow healthy large trees. A standard 6 foot by 6 foot Filterra box, for example, provides 72 cubic feet of soil, assuming a 2 foot depth of soil. Given that the recommended soil volume for trees is 2 cubic feet of soil per 1 square foot of tree canopy, this is only enough to support a tree with a 5 foot radius canopy.
The following pages address incorporation of trees into stormwater management under paved surfaces