Arboriculture & Urban Forestry 33(5): September 2007 331 aging 3.2 m2 (34.6 ft2) (Table 1). Topsoil is exchanged with a sand-based tree growth substrate to a depth of ≈50 to 60 cm (≈20 to 24 in). The original soil beneath that layer is loos- ened, and installations to improve drainage may be installed. In this survey, conventional establishment, however, also covers establishment in planting stripes covered with turf or gravel, i.e., any situation where none of the three alternative approaches (structural soils, sand mixes, or super planting pits) have been applied. Structural Soils This method of extending the rooting zone to soil volumes under sealed surfaces was first described and tested by Grabosky and Bassuk (1995). Structural soil is basically a mixture of gravel and soil and should meet two requirements; the gravel fraction should provide a skeletal structure that transfers loads from paved surfaces to the subsoil, and the soil fraction in the voids between the stones should provide the possibility of root growth. In addition to opening up soil volumes under paved surfaces for root growth, structural soils were also thought to reduce sidewalk damages caused by shallow root systems as described by, for example, Kopinga (1994) and Nicoll and Armstrong (1998), because they would enable roots to explore deeper soil layers. Container growth tests by Grabosky and Bassuk (1996) and Kristoffersen (1999) revealed that roots actually would grow into structural soils. Kristoffersen (1999) established that structural soils resulted in total tree growth rates compa- rable to the sole use of topsoil as growth media. However, root/shoot ratios increased when structural soils were used, indicating that less shoot growth will be obtained in a given volume of structural soils compared with a similar volume of uncompacted pure topsoil. This altered root/shoot ratio of trees established in structural soils was also observed in a container experiment with Ficus benjamina by Loh et al. (2003), who, however, also reported lower leaf tissue N con- tent and reduced growth of plants grown in structural soils compared with plants grown in topsoil only. Smiley et al. (2006) investigated several growth and vital- ity parameters of trees grown in structural soils and report Table 1. Properties of the different establishment concepts. Establishment method Conventional Structural soil Sand mix Depth of excavation [m]y 0.60 0.60 0.60 Super planting pit 0.80z Mean surface area [m2 ]x 3.2 1.6 1.7 12 Load bearing (light traffic) No Yes Yes No zSoil is loosened to a depth of 1.2 m (4 ft). yAccording to principal dimensional sketch. xMean area of the unsealed surface of the pits in this survey. significant growth reductions of trees grown in structural soil compared with trees grown in uncompacted soil covered with suspended paving. Lemaire and Sorin (1997) report experiences from Angers, France, with structural soils composed of 35% (v/v) organic soil and 65% (v/v) stones (40/90). All plantations established using this “mélange terre-pierre” were reported to result in satisfactory development, and root growth into the stone–soil mixture was observed. Sand-Based Soils Initially described as Amsterdam tree soil by Couenberg (1994), sand-based soils are supposed to function as growth media and support paving for light traffic. They are composed of a sand fraction consisting of medium coarse sand with uniform particle sizes mixed with soil rich in de- composed, organic matter (4% to 5% w/w) and clay (2% to 4% w/w). This substrate is then filled into the planting pits and/or adjacent paved areas where root growth is desired. According to Couenberg (1994), the substrate should be filled into the pit and compacted to ≈70% to 80% proctor density in two layers of ≈40 to 50 cm (≈16 to 20 in). In contrast to encouraging results by Couenberg (1994), Kristoffersen (1999) reveals possible risks of this establish- ment method, because a sand mix containing 2.2% organic matter, compacted to 80% standard proctor (corresponding to 1.4 g/m3), had a tendency to waterlog and thus obstructed root growth. This indicates a need for more detailed specifi- cation of composition and installation of this material, in particular in regard to compaction. Super Planting Pits The design of super planting pits does not involve load- bearing layers accessible to root growth below paved sur- faces. Instead, a super planting pit offers a large, unsealed surface (>12 m2; 129.6 ft2) in combination with deep soil loosening, providing at least 15 m3 (525 ft3) of soil for each tree. A typical super planting pit profile can be described as exchanged topsoil from 0 to 60 cm (0 to 24 in), exchanged mineral base soil from 60 to 80 cm (24 to 32 in), and loosened original soil from 80 to 120 cm (32 to 48 in). Aims of the Survey Experiences with the alternative establishment methods gen- erally only cover relatively young plantations or experimental setups. Information on the actual in situ performance of ma- terials designed to perform equally well as load-bearing layer and as root zone for urban trees is still sparse. Expenses for establishment of trees with any of the three alternative methods exceed the costs of conventional tree es- tablishment, and information about growth and vitality of ©2007 International Society of Arboriculture
September 2007
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