Classics
The following article has been selected because it is deemed very popular or very important to the arboricultural profession and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
Note: Click on green text in each section for more information.
Case Study
Do Soil Cells Work?
By John Atkins and edited by Len Phillips
This report documents the root patterns of trees growing in soil cells. Two 4-year old trees were excavated to examine the extent of root growth within a soil cell system and determine how roots interacted within the cells.
The inspection of two Hill’s Weeping Figs (Ficus microcarpa var. ‘Hillii’) took place in December 2015 at Singleton, New South Wales, Australia. The fig trees had been planted in engineered soil cell tree vaults in August 2011. The overall vigor and health of the trees was excellent.
Original Site Conditions
The original site soil was assessed as a sandy clay loam that had been considerably modified during the construction of a building on the site. The normal definitions of a topsoil and sub-soil were not clearly defined. The existing soil was noticeably dry to a depth of 5 feet (1.5 metres). Large and strongly bonded soil peds (soil peds are natural, relatively permanent aggregates, separated from each other by voids or natural surfaces of weakness) were noticeable throughout the soil profile. The existing soil had excellent water holding capacity, reasonable macro and micro-pore spaces, and strong chemical and physical bonds between soil particles.
On a larger scale, the site has an average annual rainfall of 24 inches (645 mm) with slight seasonal variations. The climate of the region is temperate, with hot summers and cool winters. Frost may occur from June to September and temperatures can regularly exceed 100°F (40°C) for several days from December to March. The highest recorded temperature was 112°F (45.9°C) in 2014.
Apart from the fig trees installed in close proximity to each other, the only other vegetation near the trees is Kikuyu turf with some weeds. Supplementary irrigation is present and was used initially to establish the trees during the hot summer months.
Installation Method
When the trees were installed in 2011, the soil was excavated to a depth of 4.4 feet (1.3 m). A 4 inch (100 mm) deep layer of gravel was installed at the bottom of the installation pit below the soil cells. Then the soil cells were installed and backfilled with a sandy loam soil blend. Porous piping was installed within the soil cells at a depth of approximately 20 inches (500 mm) below the final surface. An air vent system was installed within the tree installation zone. Next, a root barrier, geotextile fabric, and 1-½ inches (30 mm) of gravel were placed over the soil cells. The trees were installed, a tree grate was placed over the installed tree root balls, and the concrete surface was installed to complete the project.
Excavation Method
In December 2015, two trees were removed. First, a trench was dug around the tree vault zone to permit access to a depth of 5 feet (1.5 m) and then the tree canopy was removed. This was followed by the removal of the tree grate and the root barrier. Excavation of the gravel layer down to the soil cells was finished with air excavation of soil around the root zone and first layer of soil cells. No surface roots were observed. A photo of the roots and air vent system illustrates how well the tree grew.
As the removal of soil cells proceeded, the first insight into the patterns of root growth in the cells was revealed. In some cases, the soil cells had to be cut away as roots had penetrated the cells. Air excavation around the remaining roots and soil cells for next two layers clearly showed the downward initial root growth caused by the root barrier. The diameter and large number of roots present was observed, as well as the deep penetration of roots that had grown to a depth of 27 inches (700 mm) and then they spread through the soil cell voids in a linear fashion. The roots grew in all directions within the tree vault and were not limited in their spread by the soil cell system.
One unexpected observation was that the majority of root development was just above the porous pipes while the remaining roots grew at 90 degrees below the root barriers. The proliferation of roots about the porous piping was likely due to good gaseous exchange and water availability. The pipe itself may have caused a perched water table effect in the growing media just above the well aerated zone. It is possible that the water available to the roots in this zone was present longer than in the free draining soil. This combined with good gas exchange allowed the roots to proliferate in this area. Most lateral spreading roots were present at a depth of 24 – 28 inches (600 – 700 mm) or deeper. A few lateral roots were found at a depth exceeding 6 feet (1.8 metres) and had a spread exceeding 12 feet (4 metres) in all directions. Roots of 2 inches (45 mm) in diameter were observed at 12 feet (3.7 m) from the trunk. These roots had developed to this spread and thickness in only 4 years.
The development of roots within the soil cell was more common at the top of a cell than in the middle or bottom. It is likely that the flat surface of the top of each cell acted as some type of perched water table. The typical pattern of early root growth followed the top of the cells and expanded upwards into the less rigid voids within the next layer. Small openings within the cells frequently had roots growing through them. The constriction of roots at these points did not appear to reduce root growth and increased root diameter on either side of the cells was common.
Roots were able to grow well beyond the soil cell and into the surrounding site soil. The roots grew in an initial descending pattern, then continued horizontally at the same level until they penetrated the site soil. Typically, roots were found between 1 and 3 feet (300 and 900 mm) below the soil surface at the edge of the soil cell.
When roots reached the interface between the blended soil in the soil cell vault and the site soil, they rarely deflected in direction or angle. Roots were observed to grow directly into voids at the same depth and in the same direction as they were growing in the soil cell vault.
The excavator operator described the roots present in the tree vaults as “stronger” and more resistant to mechanical forces. This anecdotal statement was reinforced by the amount of difficulty he had in completely excavating the root system of the tree. He experienced fig roots in other locations to be more brittle.
Observations
The Hills Weeping Fig has been routinely observed at other locations to develop broad spreading roots at or just below the soil surface as they mature. This lateral surface root development is known to cause significant problems with pavements and hard surfaces. Here however, the root director system placed about the newly installed tree was found to direct roots in a downward pattern, minimizing the escape of surface roots.
The soil cell system combined with the use of the fabric membrane and gravel layer at the surface also limited the colonization of roots in the uppermost one foot (300 mm) of the soil. Damage to pavements and surfaces would likely be minimized in other locations using this construction method.
The mulching effect of the gravel layer and fabric membrane should be considered. Mulches are recognized for reducing moisture loss by evaporation and improving overall soil moisture content. In the trees that were excavated, the lateral spread and depth of most roots below the fabric membrane was similar to that previously observed in trees with an organic mulch layer.
The depth of root growth within the soil cell system was surprising. It is not typical of Hill’s Figs to develop roots to a depth of 5 feet (1.5 metres) in heavy clay soils, though it has been observed that they will develop roots at this depth in sands and sandy loams. The cell system encourages roots to develop at deeper levels improving root vault occupancy. The development of tree roots at lower depths is likely to improve tree stability and the overall resistance to wind throw.
Follow-up
As a result of the excavation, confirmation that improvements that were made to Citygreen's Stratacells in the subsequent generation of Stratacell manufacture were significant. Stratavault is the next generation from Stratacell. The excavation process also pointed out that the aeration pipes should be installed at deeper levels. This change will be made in all future installations.
References
Sources
The following article has been selected because it is deemed very popular or very important to the arboricultural profession and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
Note: Click on green text in each section for more information.
Case Study
Do Soil Cells Work?
By John Atkins and edited by Len Phillips
This report documents the root patterns of trees growing in soil cells. Two 4-year old trees were excavated to examine the extent of root growth within a soil cell system and determine how roots interacted within the cells.
The inspection of two Hill’s Weeping Figs (Ficus microcarpa var. ‘Hillii’) took place in December 2015 at Singleton, New South Wales, Australia. The fig trees had been planted in engineered soil cell tree vaults in August 2011. The overall vigor and health of the trees was excellent.
Original Site Conditions
The original site soil was assessed as a sandy clay loam that had been considerably modified during the construction of a building on the site. The normal definitions of a topsoil and sub-soil were not clearly defined. The existing soil was noticeably dry to a depth of 5 feet (1.5 metres). Large and strongly bonded soil peds (soil peds are natural, relatively permanent aggregates, separated from each other by voids or natural surfaces of weakness) were noticeable throughout the soil profile. The existing soil had excellent water holding capacity, reasonable macro and micro-pore spaces, and strong chemical and physical bonds between soil particles.
On a larger scale, the site has an average annual rainfall of 24 inches (645 mm) with slight seasonal variations. The climate of the region is temperate, with hot summers and cool winters. Frost may occur from June to September and temperatures can regularly exceed 100°F (40°C) for several days from December to March. The highest recorded temperature was 112°F (45.9°C) in 2014.
Apart from the fig trees installed in close proximity to each other, the only other vegetation near the trees is Kikuyu turf with some weeds. Supplementary irrigation is present and was used initially to establish the trees during the hot summer months.
Installation Method
When the trees were installed in 2011, the soil was excavated to a depth of 4.4 feet (1.3 m). A 4 inch (100 mm) deep layer of gravel was installed at the bottom of the installation pit below the soil cells. Then the soil cells were installed and backfilled with a sandy loam soil blend. Porous piping was installed within the soil cells at a depth of approximately 20 inches (500 mm) below the final surface. An air vent system was installed within the tree installation zone. Next, a root barrier, geotextile fabric, and 1-½ inches (30 mm) of gravel were placed over the soil cells. The trees were installed, a tree grate was placed over the installed tree root balls, and the concrete surface was installed to complete the project.
Excavation Method
In December 2015, two trees were removed. First, a trench was dug around the tree vault zone to permit access to a depth of 5 feet (1.5 m) and then the tree canopy was removed. This was followed by the removal of the tree grate and the root barrier. Excavation of the gravel layer down to the soil cells was finished with air excavation of soil around the root zone and first layer of soil cells. No surface roots were observed. A photo of the roots and air vent system illustrates how well the tree grew.
As the removal of soil cells proceeded, the first insight into the patterns of root growth in the cells was revealed. In some cases, the soil cells had to be cut away as roots had penetrated the cells. Air excavation around the remaining roots and soil cells for next two layers clearly showed the downward initial root growth caused by the root barrier. The diameter and large number of roots present was observed, as well as the deep penetration of roots that had grown to a depth of 27 inches (700 mm) and then they spread through the soil cell voids in a linear fashion. The roots grew in all directions within the tree vault and were not limited in their spread by the soil cell system.
One unexpected observation was that the majority of root development was just above the porous pipes while the remaining roots grew at 90 degrees below the root barriers. The proliferation of roots about the porous piping was likely due to good gaseous exchange and water availability. The pipe itself may have caused a perched water table effect in the growing media just above the well aerated zone. It is possible that the water available to the roots in this zone was present longer than in the free draining soil. This combined with good gas exchange allowed the roots to proliferate in this area. Most lateral spreading roots were present at a depth of 24 – 28 inches (600 – 700 mm) or deeper. A few lateral roots were found at a depth exceeding 6 feet (1.8 metres) and had a spread exceeding 12 feet (4 metres) in all directions. Roots of 2 inches (45 mm) in diameter were observed at 12 feet (3.7 m) from the trunk. These roots had developed to this spread and thickness in only 4 years.
The development of roots within the soil cell was more common at the top of a cell than in the middle or bottom. It is likely that the flat surface of the top of each cell acted as some type of perched water table. The typical pattern of early root growth followed the top of the cells and expanded upwards into the less rigid voids within the next layer. Small openings within the cells frequently had roots growing through them. The constriction of roots at these points did not appear to reduce root growth and increased root diameter on either side of the cells was common.
Roots were able to grow well beyond the soil cell and into the surrounding site soil. The roots grew in an initial descending pattern, then continued horizontally at the same level until they penetrated the site soil. Typically, roots were found between 1 and 3 feet (300 and 900 mm) below the soil surface at the edge of the soil cell.
When roots reached the interface between the blended soil in the soil cell vault and the site soil, they rarely deflected in direction or angle. Roots were observed to grow directly into voids at the same depth and in the same direction as they were growing in the soil cell vault.
The excavator operator described the roots present in the tree vaults as “stronger” and more resistant to mechanical forces. This anecdotal statement was reinforced by the amount of difficulty he had in completely excavating the root system of the tree. He experienced fig roots in other locations to be more brittle.
Observations
The Hills Weeping Fig has been routinely observed at other locations to develop broad spreading roots at or just below the soil surface as they mature. This lateral surface root development is known to cause significant problems with pavements and hard surfaces. Here however, the root director system placed about the newly installed tree was found to direct roots in a downward pattern, minimizing the escape of surface roots.
The soil cell system combined with the use of the fabric membrane and gravel layer at the surface also limited the colonization of roots in the uppermost one foot (300 mm) of the soil. Damage to pavements and surfaces would likely be minimized in other locations using this construction method.
The mulching effect of the gravel layer and fabric membrane should be considered. Mulches are recognized for reducing moisture loss by evaporation and improving overall soil moisture content. In the trees that were excavated, the lateral spread and depth of most roots below the fabric membrane was similar to that previously observed in trees with an organic mulch layer.
The depth of root growth within the soil cell system was surprising. It is not typical of Hill’s Figs to develop roots to a depth of 5 feet (1.5 metres) in heavy clay soils, though it has been observed that they will develop roots at this depth in sands and sandy loams. The cell system encourages roots to develop at deeper levels improving root vault occupancy. The development of tree roots at lower depths is likely to improve tree stability and the overall resistance to wind throw.
Follow-up
As a result of the excavation, confirmation that improvements that were made to Citygreen's Stratacells in the subsequent generation of Stratacell manufacture were significant. Stratavault is the next generation from Stratacell. The excavation process also pointed out that the aeration pipes should be installed at deeper levels. This change will be made in all future installations.
References
- Coder, K., “Root Growth Control: Managing Perceptions and Realities” pp 51 – 82 from “The Landscape Below Ground II: Proceedings of a Second International Workshop on Tree Root Development in Urban Soils”, International Society of Arboriculture, Champaign, IL USA. 1998.
- Percival, G.C. & C. N. Sheriffs, “Identification of drought tolerant woody perennials using chlorophyll fluorescence”, Journal of Arboriculture, International Society of Arboriculture IL USA, 2002.
- Kelsey, P., “Soil mixes for Urban Sites” The Landscape Below Ground II: Proceedings of a Second International Workshop on Tree Root Development in Urban Soils, International Society of Arboriculture, Champaign, IL USA. pp 154 – 165, 1998.
- Maxwell, K. & Johnson, G. N, “Chlorophyll fluorescence – a practical guide” Journal of Experimental Botany, Vol 51, No. 345, pp 659 668. 2000.
- Percival, G. C.,“Workshop on Chlorophyll fluorescence – notes” Bartlett Tree Research Company, Reading UK, 2005.
Sources
- Atkins, John, “Roots in Citygreen Stratacells® An evaluation of root growth patterns”, Treeology, 2015.
- Citygreen StrataCells