198 Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings Table 1. General relationship of soil bulk density to root growth based on soil texture (adapted from NRCS Soil Quality Institute 2000). Soil texture Sands, loamy sands Sandy loams, loams Sandy clay loams, clay loams Silts, silt loams Silt loams, silty clay loams Sandy clays, silty clays, some clay loams (35%–45% clay) Clays (>45% clay) Ideal bulk densities (g cm-3 ) <1.60 <1.40 <1.40 <1.30 <1.10 <1.10 <1.10 compaction of up to 0.17 g cm-3 greater than the undisturbed field sites (25%). Topsoil displacement and profile disturbance may be more damaging than soil compaction (Williamson and Neilsen 2003). Soil strength, not bulk density, was found to be the critical impedance factor controlling root penetration (Taylor and Burnett 1964; Zisa 1980). Reduced survival and growth of sugar maple (Acer saccharum ‘Seneca Chief’) and callery pear (Pyrus calleryana ‘Redspire’) in compacted soil were due to mechanical impedance, rather than limited aeration and drainage (Day et al. 1995). The criti- cal limit of soil strength above which woody plant roots will likely be greatly restricted is 2.5 MPa when measured with a standard penetrometer (Taylor et al. 1966; Greacen and Sands 1980; Zisa et al. 1980; Ball and O’Sullivan 1982; Abercrombie 1990; Day and Bassuk 1994; Blouin et al. 2008). Root growth decreases as compaction and soil strength increase (Youngberg 1959; Taylor et al. 1966; Sands et al. 1979; Bengough and Mullins 1990; Jordan et al. 2003; Blouin et al. 2008). Both controlled studies (Minore et al. 1969) and field observations (Forristall and Gessel 1955) have shown that the capacity for root growth in com- pacted soil oſten varies among plant species. For example, root growth of Siberian larch (Larix sibirica), English oak (Quercus robur), western red cedar (Thuja plicata), and Formosa acacia (Acacia confusa) were little affected by soil bulk density as high as 1.89 g cm-3 , while Norway spruce (Picea abies), Douglas fir (Pseudotsuga menziezii), little- leaf linden (Tilia cordata), and tallow lowrel (Litsea glutinosa) were the least capable of growing roots in compacted soil (Forristall and Gessel 1955; Korotaev 1992; Liang et al. 1999). As little as 0.14 g cm-3 Soil compaction can affect distribution. Root penetration depth can be restricted by soil ©2014 International Society of Arboriculture Bulk densities that may affect root growth (g cm-3 ) 1.69 1.63 1.60 1.60 1.55 1.49 1.39 Bulk densities that restrict root growth (g cm-3 ) >1.80 >1.80 >1.75 >1.75 >1.65 >1.58 >1.47 bulk density (Halverson and Zisa 1982; Nambiar and Sands 1992; Laing et al. 1999). If not all parts of a root system are equally exposed to compaction, compensatory growth by unim- peded parts of the root system may compensate, and the distribution but not the total length of roots may be altered (Unger and Kaspar 1994). Individual root tips can penetrate only those soil pores that have a diameter greater than that of the root. Roots oſten grow into root channels from previous plants, worm channels, structural cracks, and cleavage planes, thereby tapping a larger reservoir of water and mineral nutrients. In very compacted soils, root growth may be confined almost entirely to these pores and cracks (Taylor et al. 1966; Eis 1974; Patterson 1976; Gerard et al. 1982; Ehlers et al. 1983; Hullugalle and Lal 1986; Wang et al. 1986; Bennie 1991; van Noordwijk et al. 1991). If not present, roots may undergo redirection of growth from deeper layers toward uncom- pacted surface soil when downward growth is restricted by high bulk density (Waddington and Baker 1965; Heilman 1981; Gilman et al. 1987). The net result is the proliferation, if not concen- tration, of roots at a shallow depth (Gilman et al. 1982; Weaver and Stipes 1988; Jim 1993a). Such a shallow root system will be more affected when surface soils dry during periods of drought. There is a tendency to form more lateral roots can make a difference (Minore et al. 1969). root with increasing soil strength (Gilman et al. 1987; Misra and Gibbons 1996). Length of primary and lateral roots of shining gum (Eucalyptus nitens) was reduced 71% and 31%, respectively, with an increase in penetrometer resistance from 0.4 to 4.2 MPa. High mechanical resistance will also tend to increase the root diameter behind the root tip (Taylor et al. 1966; Eavis 1972; Russell 1977; Bengough and Mullins 1990; Misra and Gigbons 1996), and the growth and shape of
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