iii Nina Bassuk, Gary Raffel, and Miles Schwartz Sax Root Growth of Accolade™ Elm in Structural Soil Under Porous and Nonporous Asphalt After Twelve Years ............................................................................................................................. 297 Abstract. Accolade™ Elm trees were planted in CU Structural Soil® overlaid with porous or nonporous asphalt in 2005. At three separate points (2012, 2015, and 2016) over the last twelve years, root densities were measured with Ground Penetrating Radar to a depth of 30 inches (76.2 cm) beneath the asphalt. Roots under the porous asphalt were more numerous and tended to grow deeper in the structural soil profile. Shoot growth was reduced in trees that grew under the nonporous asphalt beginning in the eighth year after planting. CU Structural Soil® is a viable medium for tree growth and stormwa- ter capture when paved with porous asphalt. Keywords. Ground Penetrating Radar; Porous Asphalt; Root Growth; Shoot Growth; Structural Soil; Urban Trees. E. Thomas Smiley, James Urban, and Kelby Fite Comparison of Tree Responses to Different Soil Treatments Under Concrete Pavement ............ 303 Abstract. In urban areas there is a limited amount of soil space available for tree root growth. However, many systems have been developed that pro- vide rooting space below pavement while supporting the weight of vehicles and pedestrians. Two main approaches have emerged: 1) supported pave- ment, 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. Keywords. Cornell Soil; Gravel-Based Structural Soil; Limited Soil Volume; Sand-Based Structural Soil; Structural Soil; Suspended Pavement; Urban Tree Planting. Gregory M. Moore, Susan Bendel, and Peter B. May Root Penetration of Polyvinyl Chloride (PVC) Stormwater and Sewer Pipes ................................. 315 Abstract. Two experiments investigated factors influencing root penetration of polyvinyl chloride (PVC) pipes. Eucalyptus leucoxylon, Allocasuarina littoralis, Lophostemon confertus, Callistemon salignus, Acer palmatum, and Pyrus calleryana seedlings were grown in containers containing 150-mm lengths of sealed 75-mm PVC stormwater pipe with cracks 0.04 mm, 0.66 mm, or 1.48 mm wide on their upper surface. The buried pipes contained water, water and stormwater, soil, or soil and stormwater. There were six replicates and 432 plants. There was no significant difference in the mass of roots entering the pipes for the two larger crack widths with 70% of pipes penetrated and strong growth inside the pipes. While the roots of all species penetrated cracks greater than 0.66 mm, only roots of C. salignus, E. leucoxylon, and L. confertus penetrated 0.04 mm cracks. Roots penetrated 50 to 60% of pipes containing soil, or soil and stormwater, and 40% of pipes containing water, or water and stormwater were penetrated. The plants with roots penetrating pipes containing water and stormwater grew tallest. No roots penetrated the welded caps of the stormwater pipes. A second experiment using E. leucoxylon, Melaleuca ericifolia, Ficus macrophylla, A. littoralis, and Salix fragilis investigated root penetration of different sized holes in polycarbon- ate plates. The plates, installed in containers with growing medium above and below, had either 2 × 4 mm holes, 8 × 2 mm holes, 127 × 0.5 mm holes, or a mixture of holes (1 × 4 mm, 2 × 2 mm and 32 × 0.5 mm holes), total pore area in all being 25.14 mm2 . Below the plates, the growing medium was capillary irrigated with stormwater or water. All species grew through 0.5-mm holes and had strong root growth below the plates. When irrigated with stormwater, all species were taller and had greater biomass, and most species had a greater root mass below the plates. In general and regardless of hole size, the more holes in the plates, the more roots penetrated them. Properly installed PVC pipes are impenetrable, but the width and number of openings in a pipe influence the capacity for penetration and subsequent root growth so protocols minimizing damage to pipes should be enforced. Since species have different capacities for penetrating stormwater pipes, appropriate species selection for urban environments where damaged pipes may occur could reduce incidences of pipe damage. Keywords. Root Penetration; PVC Pipes; Tree Roots and Pipes; Street Trees and Pipes. Kelby Fite, Liza Holmes, and Elden LeBrun Comparing the Missouri Gravel Bed and a Wood Chip Production Method for Tree Growth ......... 328 Abstract. Tree root defects from current nursery production practices influence short- and long-term tree performance and survivability. The Missouri Gravel Bed (MGB) system, a production method using gravel as a substrate, has been used to prevent many of these defects from occurring. MGB pro- duction involves planting bare root stock into a bed of gravel with frequent drip irrigation in order to produce a root system with relatively few defects. MGB production methods have also been purported to allow for summer transplanting of many species, as opposed to traditional dormant transplanting. ©2019 International Society of Arboriculture
November 2019
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