Abstract
In urban areas there is a limited amount of soil space available for tree root growth. However, many systems have been developed that provide rooting space below pavement while supporting the weight of vehicles and pedestrians. Two main approaches have emerged: 1) supported pavement, and 2) structural growing media. This research was composed of two controlled studies that compare variations of these two approaches. The first was a 10-year study using elm trees that compared gravel-based structural soil (GBSS), expanded slate structural soil (ESSS), expanded slate (ES) alone, a concrete supported pavement and a compacted control. The second study was a four-year study using Liriodendron trees that compared GBSS, sand-based structural soil (SBSS), Silva Cells™, Stratacells™, an open control, and a compacted control. The results of these two studies showed that the trees growing in the supported pavement treatments with low-density soil media resulted in significantly greater tree growth and a healthier appearance. The treatments with the highly compacted soil media had less root development and less top growth. However, soil media that were highly compacted experienced less subsidence.
- Cornell Soil
- Gravel-Based Structural Soil
- Limited Soil Volume
- Sand-Based Structural Soil
- Structural Soil
- Suspended Pavement
- Urban Tree Planting
INTRODUCTION
In the 1980s, professionals began developing methods to improve tree root growth under load-bearing pavement. Since then, many systems have been advanced to provide additional rooting space for trees. Of these, two main approaches have emerged: 1) supported/suspended pavement, and 2) structural growing media. Supported pavements are structural systems designed to bridge over lightly compacted soil and transfer loads on pavement to the subsoil rather than the top layer of soil. Structural growing media are soil mixes designed to be compacted to the degree that they can serve as a subbase for pavement that can support vehicles while allowing tree root growth (Grabosky and Bassuk 1995; Smiley et al. 2006; Urban and Smiley 2014).
Combining load carrying capacity with tree root growth is challenging, since even a small increase in soil density can negatively influence root growth (Alameda and Villar 2009; Watson et al. 2014; Layman et al. 2016). The “Cornell Mix,” a mixture of rock and soil, was the first gravel-based structural soil (GBSS) growing media (Grabosky and Bassuk 1995; Grabosky and Bassuk 1996). Grabosky and Bassuk (1995) found that there was about 30% void space in 1.9-cm diameter crushed gravel, and if 20% clay loam soil was mixed with gravel, the larger diameter rocks would still touch and allow for full compaction of the material. This meets road subgrade standards, while the soil between the rocks is of low enough density to allow root growth. Others have developed modifications to this system by varying the size of the stone and the percentage of soil in the mix (Stockholm 2009; Wenz 2012; Ostberg, 2014; Solfjeld 2014). Studies have found that using larger stones did not increase the soil volume. As the soil ratio in the mix approached 30%, the soil in between the stones became more compacted, reducing root growth (Urban 2008). Expanded slate (ES) has also been promoted as a lightweight component in roof-top gardens and as a substitute for the gravel component of structural soil (Chuck Friedrich, personal communication). Expanded slate’s high level of porosity can allow water and nutrient retention that could not occur in solid rock.
GBSS was first installed in the U.S.A. around 1996 with initial good tree performance results. However, by the early 2000s, tree decline was observed and limitations were recognized (Grabosky et al. 2002; Smiley et al. 2006; Fite et al. 2014; Urban and Smiley 2014; Grabosky and Bassuk 2016). Loh et al. (2003) showed that a tree would grow well until it reached the limits of water and nutrients contained within a small amount of soil and then would begin a decline. In later work, Grabosky and Bassuk (2016) observed that one group of trees was performing similarly to nearby trees in a park, which were planted in loam soil (authors’ personal observation). This observation led to our hypothesis that tree roots may use structural soil as an “escape path” to a more favorable rooting environment nearby.
Another structural growing media is the sand-based structural soil (SBSS). This is a loamy sand of controlled gradation (Robert Pine, personal communication). This concept is similar to the tree soil developed for the city of Amsterdam (Couenberg 1994). However, in Amsterdam their tree soil was only intended to be compacted to 70 to 80% of the soil’s maximum density as determined by the use the standard Proctor compaction test (a test in which soil moisture is optimized to achieve maximum compaction, here after referred to as “Proctor”) and was never intended to be used as a load-bearing soil (Couenberg, personal communication). Root growth of many tree species is inhibited by soil densities of 85% Proctor and greater. Tree root growth is impeded for nearly all species at about 90% Proctor (Urban 2008). Many road subgrade and building foundation support standards call for soils compacted to 90 to 95% Proctor (Urban 2008).
Variations of the SBSS design have been developed and installed in many landscapes. Our research group has noticed tree growth issues with SBSS plantings over the last decade. In 2013, we noticed trees in SBSS significantly underperforming nearby trees in open loam soil as well as trees in supported pavement plantings (authors’ personal observation). Kristoffersen (1999) observed that fully compacted sand soil, similar to Amsterdam soil, performed about the same as a compacted subsoil. Rahman (2013) found that a non-compacted loam soil performed significantly better than compacted sand soil.
Supported pavement systems were first used in the United States in the early 1980s (authors’ personal observation). In these early systems, the pavement was supported by concrete posts or concrete ledges along the side of the planting area. Since then, several plastic support systems have been developed and commercialized. The concept is to support the pavement so that lightly compacted soil can be installed in the space beneath the pavement. Since the soil is not heavily compacted, root growth tends not to be impeded.
With limited and confined soil volume, consideration needs to be given to the amount of soil available in the space. GBSS provide about 20% of the volume for soil, and supported pavement systems provide about 90% of the volume (DeepRoot—Silva Cell™ product literature). However, if the goal is to provide an escape path for the roots, soil volume is of little importance.
The goal of this research was to compare the growth and health of trees growing in limited soil volumes that were either a supported pavement system or a structural growing media covered with concrete pavement.
MATERIALS AND METHODS
We established two plots at the Bartlett Tree Research Laboratory in Charlotte, North Carolina, U.S.A. Charlotte has a temperate climate with an average of 1118 mm of rain per year. Summers are hot (average July high temperature is 32° C) and humid, while winters are cold (January average low temperature is −0.6° C).
Study 1
The first plot was installed in 2004 and consisted of five treatments, with each treatment replicated three times (Figure 1). Each treatment consisted of four trees, two of each species, with a soil space of 3 m × 3 m for each tree. Treatments were blocked by rows and replicated three times for a total of 60 trees. With the exception of the supported pavement treatment, each treatment was randomly assigned within a row, creating a randomized block design. Because of the different construction techniques used in the supported pavement treatment, all of the supported pavement treatments were located in a single row.
Study 1 plot design.
The trees installed in the plot were Snowgoose cherry (Prunus serrulata ‘Snowgoose’) and Bosque lacebark elm (Ulmus parvifolia ‘Bosque’). These species were selected because they are medium-sized at maturity and root aggressively. Tree caliper measured at 15 cm above soil grade averaged 3.8 cm when installed. Wire baskets and burlap were removed from the top of the root balls at planting. The cherry suffered high mortality rates in some treatments in 2007 and were all removed that year. Cherries were replaced with Magnolia grandiflora ‘Little Gem.’ Data from the cherries and magnolias are not presented in this paper.
Each tree was provided a root space that was 0.6 m deep and 3 m2 resulting in 5.4 m3. Root escape was limited by surrounding the growing area with a combination of Biobarrier™ around the sides and Typar™ 3301 geotextile below (Berry Global, Old Hickory, TN). Below the growing area, a 15-cm layer of #57 stone and a perforated drain line ran to an outfall. Density of soil mixes was determined during the installation process by an independent construction consulting firm (ESP Associates P.A., Fort Mill, SC) using a Troxler 3450 nuclear density gauge and volume/weight techniques.
Concrete was installed over the plots with an 80-cm diameter opening centered on each tree trunk.
Treatments were as follows:
Gravel-based structural soil (GBSS)—composed of 80% gravel 2.5 to 3.5 cm in diameter and 20% sandy clay loam soil. A hydrogel (powdered, horticultural grade Terra-Sorb™) was sprayed on the gravel before mixing with soil. The mix was installed in 20-cm thick layers (lifts) and compacted with an impact compactor (Wacker Neuson BS50-4) to 95% Proctor.
Expanded slate structural soil (ESSS)—composed of 80% expanded slate (Carolina Stalite, Salisbury, NC) 1.5 to 2 cm diameter mixed with 20% sandy clay loam. The expanded slate was wetted before mixing to better hold the soil. The mix was installed in 30-cm lifts and compacted with a vibratory plate compactor (Wacker Neuson WP1550) to 95% Proctor.
Expanded slate (ES) alone was installed in 30-cm lifts and compacted with a vibratory plate compactor to 95% Proctor.
Supported pavement—native sandy clay loam was de-compacted using a backhoe excavator after tree planting using the method proposed by Rolf (1991). Concrete posts were installed at the corners to support the concrete pavement that was installed at the soil surface.
Compacted control (CC)—native sandy clay loam was installed in 20-cm layers and compacted with an impact compactor to 95% Proctor.
Elm growth (height, spread, and trunk diameter) and color (visual rating and SPAD meter) was evaluated over a ten-year period. Tree growth was measured annually at the end of the growing season. Measurements collected were: caliper at 15 cm above grade, tree height, and crown spread. Foliage color was rated periodically using a visual assessment.
The number of cracks in the concrete surrounding the tree opening thought to be associated with tree roots were counted in 2013.
Study 2
The second study was installed in 2014. Here, smaller plots were used and smaller trees were installed. In this study there were six treatments that were replicated six times in a randomized block design (Figure 2).
Study 2 plot design.
Soil was excavated from two parallel trenches, and plots were separated from one another and the adjacent undisturbed soil alongside the trench with plywood walls on four sides. The walled space was 0.5 m deep and 1.5 m on each side, creating a 1.1 m3 of rooting volume. The plots were lined with a continuous sheet of Typar™ 3301 geotextile fabric (Berry Global, Old Hickory, TN) to restrict roots from escaping the plots while allowing water drainage. Below the growing area, a 15-cm layer of #57 stone and a perforated drain line ran to an outfall.
Treatments were applied to six replicates as follows:
Open control—the sandy clay loam soil excavated from the trench was put back into the plot. Soil was compacted only by people walking across the soil surface to approximately 80% Proctor.
Compacted control (CC)—the excavated soil was put back into the plots in 20-cm lifts and compacted to 95% Proctor using an impact compactor (Wacker Neuson BS50-4).
Silva Cells™—the modular post and deck Silva Cell™ (DeepRoot Green Infrastructure LLC, San Francisco, CA) structure was constructed in each plot by a representative of the manufacturer. Since the dimensions of the plot were not identical to the size of the Stratacell™, the Silva Cells™ were cut into sections that filled the space (Figure 3). Parts of three Silva Cells™ were installed in each plot. The soil excavated from the trench was installed within the plot structure. It was compacted only by human weight to approximately 80% Proctor.
Figure 3.Installation plan drawing for Silva Cells™.
Stratacells™—the modular structure of Stratacell™ (Citygreen, Sydney, NSW Australia) was installed by a representative of the manufacturer. Sixteen Stratacells™ were installed in each plot (Figure 4). The excavated soil was installed within the Stratacell™ structure. The soil was not compacted.
Figure 4.Installation plan drawing for Stratacells™.
Sand-based structural soil (SBSS)—a 5-cm layer of gravel (#57 stone) was installed at the base of the plot above the geotextile. On top of this, a uniformly blended mixture of sand, sandy clay loam, and compost with a ratio by volume of 4S:1L:1.5C was installed in 20-cm lifts and compacted to 95% Proctor using an impact compactor (Pine and Swallow Environmental, Groton, MA).
Gravel-based structural soil (GBSS)—a #57 gravel was sprayed with a suspension of a hydrogel (powdered, horticultural grade Terra-Sorb™) and then blended with 20% sandy clay loam soil. It was compacted to 95% Proctor in 15-cm lifts using an impact compactor (Bassuk et al. 2015).
The soil density was verified during the installation process by an independent construction-consulting firm (ESP Associates P.A., Fort Mill, SC) using a Troxler 3450 nuclear density gauge and volume/weight techniques.
While the volume of the space available for each treatment was the same (1.1 m3), the amount of non-compacted soil that was installed varied based on the space occupied by support structure (Silva Cells™ and Stratacells™) and the density of the soil within each treatment (Table 1).
The amount of non-compacted soil installed in each plot. The space available in each plot was 1.1 m3. Numbers with the same letter indicate that there are no significant differences among treatments (SNK p = 0.05).
Containerized, 18-mm caliper Liriodendron chinense were bare rooted and installed on August 19, 2014 in the center of each plot. A 5-cm thick layer of fiber-reinforced concrete was then poured over the entire plot surface. A 20-cm diameter hole in the concrete was centered on each tree. The open controls had a 0.1 m concrete border around the edges resulting in a 1.3-m square opening.
A soil moisture sensor (Spectrum SMEC 300) was installed in one replicate of each treatment. They were connected to a WatchDog 2400 Mini Station data logger (Spectrum Technologies, Aurora, IL). Irrigation water was automatically applied when soil moisture levels dropped below 10% VWC in 2014 and 2015, and 5% VWC in 2016. Water was applied for ten minutes, up to four times per day from two one-gallon per minute emitters (Rainbird SW-10) in all plots except the SBSS plot. In that plot, at the request of the treatment sponsor, an equal amount of water was emitted from a loop of perforated tubing (Rainbird ET63-100S). No irrigation was applied to any treatment in 2017. No data is presented from these monitors.
Tree growth was measured annually at the end of the growing season. Stem diameter measurements were collected at 15 cm above grade using a diameter tape, tree height was measured with a surveyor pole, and crown spread in two perpendicular directions was measured with a tape measure. Foliar color was assessed periodically using a visual inspection and a SPAD 502 chlorophyll meter (Spectrum Technologies, Aurora, IL).
On October 23, 2017, all leaves were removed from the trees, the trees were severed at the root collar, and concrete pavement that covered the plot was removed. Soil moisture in the newly exposed soil was measured at 15cm depth intervals using a Fieldscout TDR 350 with 12-cm rods (Spectrum Technologies, Aurora, IL) midway between the center of the tree and the corner of the plot. Root systems were excavated using high pressure air and water October 24 to 25, 2017.
The number of roots that were greater than 1.2 cm diameter at the trunk were counted. Horizontal root spread and vertical root depth were measured. Horizontal spread was measured in two directions, one parallel to the length of the plot, and the second across the width of the plot. Root system depth was measured from the soil surface to the deepest root. The tops and root systems of each freshly harvested tree were weighed. Root system weight included coarse and fine roots.
Data for both studies were analyzed using IBM® SPSS software and the ANOVA procedure. Separation of means was conducted with the Student-Newman-Keuls (SNK) due to the low number of replicates and moderate tree-to-tree variability. For multiyear measurements, data were analyzed using repeated measures of ANOVA. The supported pavement treatment in Study 1 was included in the analysis with the other treatments.
RESULTS
Study 1
There were no difference in tree size or color at the time of planting. Beginning in 2011, there were significant growth differences among the treatments that continued until the end of the study. The supported pavement treatment trees were significantly larger in all growth metrics than other treatments (Photograph 1, Figures 5 and 6). The ESSS treatment tended to have the least amount of growth. The color ratings as expressed by SPAD readings varied by year and were not significantly different. Visual rating of foliage color was significantly higher for the supported pavement treatment from 2004 through 2012 (Figure 7). The GBSS treatment was not different from the supported pavement treatment in 2004 and 2005.
Study 1 five years after planting in 2009. Treatments are supported pavement, expanded slate structural soil (ESSS), compacted control (CC), and gravel-based structural soil (GBSS). The unlabeled plot was a replacement for the expanded slate (ES) plot where the trees died. The smaller plants are magnolias that replaced the original cherry trees in 2007.
Mean elm trunk diameter measured at 15 cm above grade at the end of the growing season. The supported pavement treatment had significantly larger caliper than all other treatments in 2006 and 2007, and from 2010 through 2014, the ESSS treatment calipers were significantly lower than all other treatments in 2006 and 2007 and again from 2009 through 2014 (SNK P = 0.05).
Mean elm height measured at the end of the growing season. There was significantly more height growth with the supported pavement treatment from 2010 through 2014, while there was significantly less growth in the ESSS treatment in 2012 and 2014 (SNK P = 0.05).
Mean visual foliar color rating based on a 0 to 10 scale (0 = dead, 10 = dark green). Bars with the same letter indicate that there are no significant differences among treatments (SNK P = 0.05).
In 2007, there was high mortality in the ES plot, so all trees in that treatment and all the cherries were removed and replaced with ‘Little Gem’ magnolias.
The number of cracks counted in the concrete surrounding the tree were significantly higher in the GBSS treatment than the other treatments (Table 2).
Concrete cracking associated with soil treatments and tree root growth. Means followed by the same letter do not differ significantly (SNK p = 0.05).
Study 2
Tree Growth and Health
There were no differences in tree size or color at the time of planting in 2014. At the end of the 2015, the Stratacell™, Silva Cell™, and open control tree size started to diverge from the other treatments (Photograph 2, Figures 8 and 9). In 2016 and 2017, the Silva Cell™, Stratacell™, and the open control caliper (Figure 8), height (Figure 9), and spread (data not presented) were significantly larger than the other treatments.
Study 2 two years after planting in 2016. One of the six replicates is labeled by treatment.
Mean Liriodendron trunk diameter measured at 15 cm above grade at the end of the growing season. In 2016 and 2017, data separated into two significantly different groups, and the Silva Cell™, Stratacell™, and open control were larger than the other three treatment trees (SNK P = 0.05).
Mean Liriodendron height measured at the end of the growing season. In 2016 and 2017, data separated into two significantly different groups, and the Silva Cell™, Stratacell™, and open control were larger than the other three treatment trees (SNK P = 0.05).
With foliar color ratings, the mean SPAD readings tended to be higher with the Silva Cell™, Stratacell™, and open control (data not presented). In 2015, the Silva Cell™ and Stratacell™ readings were significantly higher than the other treatments. In 2016, the compacted control and GBSS treatment were significantly lower. In 2017, the GBSS reading was significantly lower.
With the mean visual color ratings, the SBSS was consistently the lowest of the treatments, and the Stratacell™, Silva Cell™, and open control were the highest (Figure 10).
Mean September foliar color visual rating based on a 0 to 5 scale (0 = dead, 10 = dark green). Bars with an asterisk above do not different significantly from other means in that year (SNK P = 0.05).
Root Growth
The number of roots over 1.2 cm diameter near the trunk was significantly larger in the Silva Cell™ treatment than other treatments, and there were significantly fewer large roots in the compacted control, GBSS, and SBSS treatments than the other treatments (Figure 11).
The mean number of roots close to the base of the Liriodendron that were greater than 1.2 cm in diameter. Bars that share a letter do not differ significantly (SNK P = 0.05).
There was no significant difference in root spread when measured across the pavement, but root growth parallel to the pavement was significantly longer with the Silva Cell™ treatment. There was less root growth in that direction with the compacted control, SBSS, and GBSS treatments (Figure 12).
Mean maximum length of root growth measured parallel to the direction of the concrete or across (perpendicular) to the concrete. There were no significant differences among treatments measured across the plot. There were significant differences in root length parallel to the length of the plot. Bars that share a letter do not differ significantly (SNK P = 0.05).
Roots grew significantly more deeply into the soil with both the Stratacell™ and Silva Cells™ (Figure 13). The compacted control had significantly less root penetration.
Mean maximum depth of root growth. Bars that share a letter do not differ significantly (SNK P = 0.05).
Tree Part Weights
There were significant differences in the weight of the above- and belowground parts of the tree based on treatment (Figure 14). Weights separated into two groups, with the Silva Cell™, Stratacell™, and open control being significantly heavier, and the compacted soil, SBSS, and GBSS being lighter.
Mean weight of the above- and belowground portions of the tree. Bars that share a letter do not differ significantly within the measurement type (SNK P = 0.05).
Soil Conditions
The compacted control treatment had a higher volumetric water content than the other treatments, and the SBSS treatment was the driest (Figure 15). Generally, there was more water at the soil surface than in the lowest level measured. Water content could not be measured in the GBSS treatment using the TDR tool.
Mean soil volumetric moisture content as measured with a Fieldscout TDR 350 with 12-cm rods (Spectrum Technologies, Aurora, IL). Bars that share a letter do not differ significantly (SNK P = 0.05).
There was a noticeable amount of soil subsidence beneath the bottom of the pavement in some treatments (Table 3). The highly compacted treatments (compacted control, SBSS, and GBSS) had less subsidence than the treatments that were not compacted to 95% Proctor.
Mean soil subsidence, measured from the bottom of the concrete to the top of the soil in cm. Means followed by the same letter do not differ significantly (SNK P = 0.05).
CONCLUSIONS
In general, the greatest amount of tree growth and healthier trees were seen in non-compacted, low-density soil media treatments, either under supported pavement or with an open soil surface. There were no growth or health differences based on the system that supported the pavement.
Lesser growth was seen in trees growing in compacted soil media. When the soil media was compacted, regardless of the substrate, there was less growth and a generally less healthy appearance aboveground. This is consistent with other studies that have shown the negative impacts of soil compaction (Kristoffersen 1999; Smiley et al. 2006; Rahman 2013; Fite et al. 2014; Urban and Smiley 2014).
In Study 1, the supported pavement treatment trees grew trees larger and generally appeared healthier than the other treatments. Trees planted in the compacted soil treatment did surprisingly well considering the soil density. When the cherry trees were removed from this treatment, it was seen that the roots did not penetrate the compacted soil but rather grew upward and outward from the edge of the root ball, likely finding lower-density soil just below the pavement and at the edge of the plots.
Trees in the ESSS plot grew less well than trees in the GBSS plot. This may be due to the smaller size of the stone, which could result in less space between stones and a higher level of soil compaction (Grabosky and Bassuk 1995; Grabosky and Bassuk 1996).
Trees planted in the 100% expanded slate (ES) media had a high mortality rate, resulting in the plot being removed from the study in 2007. This mortality was most likely based on the amount of water retained by the porous stone (personal observation). That treatment would not be considered acceptable in most urban situations unless water was carefully managed.
Concrete cracking was significantly greater in the GBSS treatment. Smiley (2008) showed that a GBSS treatment is more likely to result in concrete sidewalk cracks based on the trees production of fewer, but larger diameter roots.
In Study 2, the tree growth differences based on soil media treatments were more obvious than in Study 1. This may be attributed to the tree species selection. Liriodendron are fast growing in the Charlotte, NC region, are known to have a root system that does best in low-density, sandy soils, and are inhibited by higher densities (Francis 1979).
Treatment-based growth and health differences in general separated into two groups. The open control, Stratacell™, and Silva Cell™ treatment trees grew significantly larger than the compacted soil, GBSS, and SBSS treatments. When there were differences in foliar color, it followed the same pattern with the open control, Stratacell™, and Silva Cells™ being a healthier green color.
Root growth differences were seen in the mean maximum root depth, counts of roots greater than 1.2 cm diameter, and in the overall weight of the root system. With the count of larger roots, the greatest number were found in the Silva Cell™ treatment, followed by the open control and Stratacell™. With root depth, the Silva Cell™ and Stratacell™ treatments had significantly more roots deeper in the soil. Overall root weight was significantly greater with the Silva Cell™, Stratacell™, and open control treatments.
Root differences are potentially based on soil density and oxygen availability in the soil profile. Soil moisture does not appear to be a factor, since the highest and lowest soil moisture level were found in the compacted soil and SBSS treatments, respectively, both of which had lower levels of root development. The Silva Cell™ and Stratacell™ treatments had moderate moisture contents, and the open control had higher moisture level near the surface. This high moisture at the surface can be attributed to the non-coved surface that the other treatments did not have.
The soil beneath the pavement subsided more when the soil was not heavily compacted. The compacted control, SBSS, and GBSS treatments had significantly less subsidence than the other treatments. The Stratacell™ treatment experienced much more subsidence than all other treatments. This was attributed to the redistribution of soil with the infiltration of rainwater. It was noticed that with the first three large rain events after tree planting, the trees in this treatment were moved deeper into the soil as the soil beneath the root ball was washed into other portions of the Stratacell™ structure. Those affected trees were replanted at the original depth after the addition of more soil. In one replicate a shed snake skin was found partially under the pavement, which points to potential problems with having a large gap.
SUMMARY
Soil treatments that provided a low-density growing media resulted in the largest and healthiest trees. There are multiple ways to achieve the goal of low-density soil beneath pavement including not paving over the soil surface and providing a bridge over the surface with low-density soil beneath. Our research did not point to a “best product” to achieve this goal. Rather, this study reinforces the idea that any of the methods that provide support for the intended load and keep the load off the growing media worked well. When designing and installing a supported pavement system, it should be kept in mind that low-density soils will self-compact to some degree, resulting in a subsidence and the formation of a gap between the soil surface and the bottom of the pavement. That gap can provide habitat for unwelcome urban wildlife.
ACKNOWLEDGEMENTS
We would like to acknowledge the contributions to the research by: Robert Bartlett Jr., James Ingram, Dr. Bruce Fraedrich, Dr. Chad Rigsby, Liza Holmes, Sean Henry, Chris Bechtel, Scott Hicklin, Elden Lebrun, Matt Story, Chris Brackett, Jason Patterson, Jason Ball, Dr. Lisa Calfee, Laura Johnson, Greg Paige, Jarod Faas, Emily Faas, Ethan Stewart, and Imogene Mole of the Bartlett Research Laboratories in Charlotte, NC; Jerry Dunaway, Bill Hawkins, Arthur Cashin, Matt Kocian, Roger Bergh, and Brian Whitiker of the Fiberweb Polymer Group in Old Hickory, TN; Chuck Fredrick and Debbie Stringer of Carolina Stalite Co.; Ben Gooden, Joe Gooden, and Craig Melvin of CityGreen; Robert Pine of Pine and Swallow Environmental in Groton, MA; Al Key, Brenda Guglielmina, and Graham Ray of the Deep Root Company in San Francisco, CA; Dr. Gary Watson of the Morton Arboretum in Lisle, IL; Dean and Dale Campbell of Campbell Construction; Donald McSween, retired from the City of Charlotte, NC; and Dr. Jason Grabosky of Rutgers University, NJ.
- © 2019, International Society of Arboriculture. All rights reserved.
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