Arboriculture & Urban Forestry 36(5): September 2010 Although bulk density indicates the degree of compaction for a particular soil, it does not provide a complete picture of root inhibition for that soil. Soil texture and moisture must also be considered along with bulk density, because these proper- ties in combination determine soil strength (Taylor and Gard- ner 1963; Taylor and Ratliff 1969; Zisa et al. 1980; Daddow and Warrington 1983; Day et al. 2000). In their classic study, Daddow and Warrington (1983) used an in-depth survey of for- est soil compaction research to create a chart depicting root- growth-limiting bulk density for each soil texture (i.e., the bulk density at which root growth would essentially halt for a given soil texture). As they note, this serves as a useful proxy for soil resistance to penetration, but does not account for other factors that affect soil strength, particularly moisture. Soil strength is a function of bulk density and moisture content. As bulk density increases due to compaction, the frictional and co- hesive forces between soil particles increase and thus soil strength increases (Greacen and Sands 1980). As soil strength increases, root elongation rate decreases due to resistance of soil particles to displacement (Clark et al. 2003). The critical soil strength (mea- sured with a cone penetrometer) above which woody plant root elongation is severely restricted is in the vicinity of 2.3 MPa, de- pending on soil type and plant species (Day and Bassuk 1994). Soil moisture can alleviate excessive soil strength by lubricating soil particles and the elongating root tip. However, the moisture con- tent required to alleviate excessive soil strength is progressively greater as bulk density increases. In sandy loam soil, the volumet- ric moisture content at which soil strength fell below the critical limit was about 20% at a bulk density of 1.18 g/cm3 versus about 30% at a bulk density of 1.26 g/cm3 (Siegel-Issem et al. 2005). In compacted soil, the combination of increased volumetric water content, and decreased macroporosity limits gas diffusion and may cause root aeration stress. In silty loam soil compacted to 1.44 g/cm3 limited above 35% volumetric water content due to poor aeration (Siegel-Issem et al. 2005). In a loam soil compacted to 1.5 g/cm3 root growth of Cornus florida (flowering dogwood) is depressed in very moist soils (matric tension of 0.006 MPa and oxygen dif- fusion rates <0.5 mg cm–2 , root growth of shortleaf pine (Pinus echinata) is , min), while roots of Acer saccharinum (silver maple) are not (Day et al. 2000). However, poor aeration due to low macroporosity in compacted soil may not be an issue in unsaturated soil (Day et al. 1995; Aust et al. 1998; Day et al. 2000). Species vary in their ability to elongate roots in compacted soils. This is not simply attributable to differential ability to ex- ert pressure on the soil, although slight differences have been demonstrated among species in controlled laboratory environ- ments. For example, Materechera et al. (1991) evaluated root penetration of 22 crop species at an extreme soil strength of 4.2 MPa and found that all species had root elongation reduced between 92 and 98% and that the ability of a given species to penetrate strong soil was positively correlated with root diam- eter. At lower soil strength levels, species differences in root response to compaction can be easier to discern. For example, when soil strength is increased from 0 to 1.0 MPa, root elonga- tion of peanuts is reduced by only 29% while elongation of cotton roots is reduced by 62% (Taylor and Ratliff (1969). However, low soil strengths such as these are unlikely to be encountered in the field except under wet conditions. These data illustrate that root growth of woody plants will be restricted with any increase 195 in soil strength, rather than growing “normally” until a certain threshold is reached. In a recent study with native Australian Eucalyptus spp., root penetration decreased linearly as soil bulk density was increased from 1.0 to 1.4 g/cm3 (soil texture not de- scribed), further demonstrating the immediate reduction in root penetration when soil compaction increases (Skinner et al. 2009). Variation in species tolerance of soil compaction is currently conceived to be a complex response to the whole rooting envi- ronment. The strongest hypothesis for explaining the ability of certain tree species to tolerate compacted soil is the “root growth opportunity” hypothesis, which states that tree species tolerant of wet soils (e.g., bottomland species) can grow roots during wet periods when soil strength is low, while species less tolerant of wet soils (i.e., soil hypoxia) cannot. Thus bottomland species may be expected to have a greater root growth opportunity when soil strength is low, and thus be more adapted to soil compaction, such as is found in urban areas. Generalized models addressing this root growth opportunity were initially developed to integrate the limits of soil strength with the limits of soil water content into a single descriptor for evaluating soil quality for crop produc- tion (Letey 1985), and were eventually described as the Least Limiting Water Range (da Silva et al. 1994). Day et al. (2000) presented a similar hypothesis for urban trees and evaluated the root growth opportunity in the context of species tolerances via a study of silver maple (Acer sacharrinum) and flowering dog- wood (Cornus florida). Siegel-Issem et al. (2005) further devel- oped this approach as a measure of forest soil productivity. These last experiments evaluated the influence of soil strength, bulk density, soil moisture, and oxygen diffusion rate on seedling root growth, providing support for this hypothesis as an explanation for species response to compacted soils (Day et al. 2000; Siegel- Issem et al. 2005). Yet, response to compacted soils is influenced by a host of environmental and genetic factors and species dif- ferences are not always easily explained (Bassett et al. 2005). ROOT RESPONSE TO SOIL CHEMISTRY AND CONTAMINANTS Urban soils typically have very different environmental in- puts than rural or forested landscapes. These include anything related to intense human activity, such as de-icing salts, tire residue, engine oil, construction debris, landscape mulches, and lawn clippings. Many of these items alter soil chemistry. In addition, brownfields—land previously used for industrial, or sometimes other commercial, purposes that may have en- vironmental contaminants—are prevalent in many countries (Oliver et al. 2005). Decisions concerning brownfield devel- opment receive more attention as land becomes more scarce (e.g., Altherr et al. 2007), and the numerous economic, social, and environmental benefits of urban greenspaces are better ap- preciated. In a Canadian study, uncertainty about the effects of soil contamination and approaches to its mitigation was ranked as the most important noneconomic barrier to developing these areas as greenspace (De Sousa 2003). Chemical contaminants are also common beyond brownfields. These include de-icing salt as well as heavy metals such as Cu, Pb, and Zn that are by-products of automobile traffic (Pouyat et al. 1995; Irvine et al. 2009). Thus, there is increasing need to broaden our knowl- edge of root interactions with chemically altered urban soils. ©2010 International Society of Arboriculture
September 2010
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