240 The ideal urban soil for tree growth should mirror an ideal healthy topsoil and be comprised of 45% min- eral solids, 5% organic matter solids, and 25% each of air and water (Trowbridge and Bassuk 2004). Such a soil allows for root penetration and deep rooting due to a high level of aggregation and a high proportion of macropores that allow water percolation through the soil and entry of air as the water drains. A soil with good structure can be damaged if the macropore pat- tern is altered by disturbance of the aggregates, and even in well-aggregated soils with increasing depth, the volume of large pores and available oxygen typi- cally decrease (Perry and Hennen 1989; Acquaah 1999; Watson 2006). While bulk density can be decreased with organic matter, which increases aggregation and promotes structure, it generally increases with soil depth due to reduced organic matter and associated microorganisms and the weight of the soil above (Urban 2008). Total porosity can be as low as 30% in heavily trafficked compacted soils and as high as 95% in some peats (Handreck and Black 1999). Soils compact due to compressive and vibrational forces, which degrade soil structure by crushing macro- pores and filling pore spaces as soil particles become densely packed after the mechanical force exceeds the shear strength of the soil (Trowbridge and Bassuk 2004). Compaction increases soil density and strength by increasing the forces holding the soil together. It prevents soil settling, complicates diffusion pathways, slows infiltration rates, and slows the soil’s ability to drain quickly, reducing the level of air in the soil (Watson and Kelsey 2006; Bühler et al. 2007; Sinnett et al. 2008). Compaction usually occurs in the upper 15 cm, directly affecting the root zone, and can vary over short distances and with depth (Pittenger and Stamen 1990). The reduction in macropores, decreased soil aeration, and altered soil water status can create an anaerobic rhizosphere leading to root and tree death (Smith and Moore 1996; Acquaah 1999; Roberts et al. 2006; Hascher and Wells 2007). If the diffusion rate is low, roots may be confined to the soil surface (Urban 2008), and root elongation, root and shoot dry weights, and leaf area can be affected by high levels of compaction (Tubeileh et al. 2003; Bühler et al. 2007). Vertical and horizontal root growth can be hin- dered, resulting in small, shallow root systems. When roots encounter compacted soil, they gener- ally grow parallel to or away from the compacted zone, but growth may slow or stop (Perry and Hennen 1989; Trowbridge and Bassuk 2004). If lateral roots ©2019 International Society of Arboriculture Moore et al.: Growth and Establishment of Australian Street Trees can fit through the pores of compacted soil, then growth continues while the main axis of the root is constrained (Coder 1998), resulting in increased branching and radial thickening of roots, which can exert greater force and penetrate further into compacted soil (Coder 1998; Gregory 2006; Day et al. 2010). Trees growing in compacted soil tend to have spreading root systems often less than 10 cm deep (Smith and Moore 1996). Species with high root:shoot ratios have a greater ability to penetrate compacted soil (Watson and Himelick 1997; Sæbø et al. 2003; Jutras et al. 2010). Heavy machinery can compact soil by the load of the wheels exerting a vertical force, wheel slippage causing shear stress, and the force of the engine vibrating through the tires (Smith and May 1996; Watson 2006; Hascher and Wells 2007). The use of such machinery should be avoided in wet soils, and care must be taken to avoid compacting street tree planting sites (Rolf 1991). Compaction is the most common form of soil damage and is difficult to ame- liorate, as only part of the soil deformation can be reversed (Kozlowski 1999). Rectifying compaction, especially on construction sites, is difficult, as it can require cultivation to the depth of the subsoil which machinery usually cannot reach (Roberts et al. 2006; Gregory et al. 2007). Preventing compaction is the best strategy, as it is more effective and less costly than alleviating it (Handreck and Black 1999; Kozlo- wski 1999; Urban 2008). This paper reports an experiment and survey aimed at determining whether there were differences in the growth and establishment of trees grown in com- pacted and uncompacted soils. The experiment inves- tigated whether there were differences in canopy and root growth, differences between the north/south and east/west canopy and root dimensions, and differ- ences in root length or depth within and between spe- cies growing in compacted or uncompacted soils. The survey aimed to examine the health, growth, and the soil conditions under which street trees planted 24 to 36 months earlier were growing. Both the experi- ment and survey collected data on tree size and con- dition, leaf area and chlorophyll fluorescence, and soil bulk density and penetrative resistance. MATERIALS AND METHODS Experiment Eight species commonly planted in suburban Mel- bourne were selected from a study of street trees in suburban Melbourne (Beer et al. 2001):
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