The Management of Tree Root Systems in Urban and Suburban Settings: A Review of Soil Influence on...

8/10/2019 The Management of Tree Root Systems in Urban and Suburban Settings: A Review of Soil Influence on Root Grow

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Arboriculture & Urban Forestry 40(4): July 2014

2014 International Society of Arboriculture

193

Gary W. Watson, Angela M. Hewitt, Melissa Custic, and Marvin Lo

The Management of Tree Root Systems inUrban and Suburban Settings: A Review of SoilInfluence on Root Growth

Arboriculture & Urban Forestry 2014. 40(4): 193217

Abstract.Te physical, chemical, and biological constraints o urban soils oen pose limitations or the growth o tree roots. An under-standing o the interrelationships o soil properties is important or proper management. As a result o the interdependence o soil properties,

the status o one soil actor can have an effect on all others. Preventing soil damage is most effective and preerred. Cultural practices, such as cul-tivation and mulching, can be effective in improving soil properties. Soil additives, such as biostimulant products, have not proven to be consis-tently effective through research. Te management challenge is to provide an urban environment that unctions like the natural environment. Key Words.Biostimulants; Bulk Density; Cation Exchange Capacity; Mechanical Resistance; pH; Soil Oxygen; Soil pH; Soil Salt; SoilWater; emperature.

In urban and suburban areas, the soil environ-ment oen creates numerous challenges or treeroot growth. Urban soil has been defined as, asoil material having a non-agricultural, manmadesurace layer more than 50 cm thick that has beenproduced by mixing, filling, or by contamina-tion o land surace in urban and suburban areas(Bockheim 1974). Urban soils are oen highlyaltered rom the natural state, and human activ-ity is the primary agent o the disturbance. Teygenerally have high vertical and spatial variabil-ity, modified and compacted soil structure, animpermeable crust on the soil surace, restrictedaeration and water drainage, interrupted nutrientcycling, altered soil organism activity, presence oanthropogenic materials and other contaminants,

and altered temperatures (Craul 1985; Bullock andGregory 1991; Scheyer and Hipple 2005). Tesephysical, chemical, and biological constraints ourban soils pose limitations or the growth o treeroots. Early experience gained working with theurban soils in Washington, D.C., and other difficulturban sites, led to the projection that about 80% ourban tree problems can be attributed to a poor soilenvironment, leading to synergistic effects o other

debilitating urban stress actors producing an over-all decline in plant vigor (Patterson et al. 1980).

Te resources provided by the soil environment orroot growth include adequate oxygen, water, and nutri-ents, non-limiting penetration resistance, acceptablepH range, and robust biological activity. Presence ocontaminants or pathogens can be harmul to roots.Any one o these actors can limit root growth anddevelopment, even i all others are in adequate supply.

Urban environments are quite different rom thenatural environment to which trees are adapted,yet they must provide the same resources orgrowth i trees are to maintain a healthy balancebetween the crown (supplier and user o energy,user o nutrients and water) and root system (sup-plier o water and nutrients, user o energy). Te

management challenge is to provide an urbanenvironment that unctions like the natural envi-ronment, though its appearance may be different.

Recent reviews have described root architec-ture and rhizosphere ecology in the urban envi-ronment (Day et al. 2010a; Day et al. 2010b)and serve as a oundation or this review oresearch summarizing our current understand-ing o soil management techniques or urban trees.


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higher fine-root mortality concurrent withincreased root growth (Meier and Leuschner 2008).

In wet soils, the growth o roots tends to be con-fined towards the soil surace. In dry soils, rootgrowth can be shied downward due to waterdepletion in surace soils (orreano and Morris1998). When urban soils limit rooting depth, theability o tree root systems to respond to periods odrought and high soil moisture may be very limited.

Flooding o soil usually leads to greatlyreduced root growth, and death o many o theine absorbing roots. he small root systemso looded trees relect the combined eect oreduction in root initiation and reduced growtho existing roots, as well as decay o the origi-nal root system. Because root growth is usuallydecreased more than shoot growth by high soil

moisture, drought tolerance o looded trees isreduced ater the lood waters recede. his changerelects the inability o the small root systems tosupply enough water to meet the transpirationalrequirements o the crown (Kozlowski 1985).

Responses o tree species to flooding vary widely(White 1973; Bell and Johnson 1974; Whitlowand Harris 1979). olerance can vary rom only aew hours to many days or weeks, depending onthe species, the organs directly affected, the stageo development, and external conditions, such astemperature. Roots are oen more susceptible to

oxygen deficiency than shoots (Vartapetian andJackson 1997). Broadleaved trees as a group aremuch more flood-tolerant than coniers. Oldertrees usually tolerate flooding better than seed-lings or saplings. Flooding during the dormantseason is much less harmul than flooding duringthe growing season (Heinicke 1932). Te greaterinjury and growth reduction by flooding dur-ing the growing season are associated with highoxygen requirements o growing roots with highrespiration rates (Yelenosky 1963; Koslowski 1985).

AerationRespiration by plant roots and other soil organismsconsumes oxygen and produces carbon dioxide. Inunsaturated soils, the soil air connects directly withthe aboveground atmosphere, but diffusion o gassesthrough the soil is slowed by water and soil particles.Oxygen concentrations decline and carbon dioxideconcentrations increase with depth due to the oxy-

gen demands o the roots, the soil auna, ungi, andmicrobes. Oxygen deficiency in roots will be morelikely to occur in warm soils than in cooler soils whenreduced respiration is more balanced with diffusionrates (Yelenosky 1963; Armstrong and Drew 2002).

For most species, approximately 10%12% oxy-gen in the soil atmosphere is needed or adequateroot growth (Stolzy and Letey 1964; ackett andPearson 1964; Stolzy 1974; Valoras et al. 1964;Gilman et al. 1987; Mukhtar et al. 1996), andgrowth may cease at 5% oxygen (Stolzy 1974). Soilcarbon dioxide concentration can be damaging toroots when it reaches 0.6% (Gaertig et al. 2002).

For most species, root growth is reduced orstopped when the oxygen diffusion rate (ODR) dropsbelow 0.2 g/cm2/min. Most plants are severelystressed between 0.2 and 0.4 g/cm2/min. Above

0.4 g/cm2

/min, plants grow normally (Stolzey andLetey 1964; Valoras et al. 1964; Lunt et al. 1973; Stolzy1974; Erickson 1982; Blackwell and Wells 1983).

Redox potential can also be used as a mea-sure o the oxygen status o the soil. Soil redoxpotentials o 400700 mV are generally consid-ered well aerated. Root growth o most species isstopped at a soil redox potential o 350 mV, thoughroots o more water-tolerant species (e.g., axo-dium distichum) are able to grow until the redoxpotential reaches 200 mV (Carter and Rouge1986; Pezeshki 1991; Stepniewski et al. 1991).

Soil aeration is impacted by urban landscapeeatures. In undisturbed, well-drained soil, oxygenand carbon dioxide contents can be near atmo-spheric levels close to the soil surace, decreasingmost rapidly in the first 30 cm (Yelenosky 1963;Brady and Weil 1996). When not paved, vegetatedand nonvegetated urban sites can be as well-aeratedas orest stands (Gaertig et al. 2002). However, itopsoils are sealed or compacted, gas exchangebetween the soil and the atmosphere is inter-rupted (Gaertig et al. 2002). Oxygen content was

reduced to 14.5% and carbon dioxide content wasincreased to 6% at 15 cm depth under an unpavedparking lot. Te same levels were not reached until90 cm depth in the adjacent undisturbed orestsoil (Yelenosky 1963). In another study, therewere minimal differences in soil oxygen betweenpavement and tur in the top 45 cm (Hodge andBoswell 1993). However, soil oxygen measure-ments were made only 75 cm rom the edge o the


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pavement and oxygen could have diffused laterallyrom the nearby exposed soil. While it is commonlyaccepted that stone pavement with gaps allowsor aeration o the soil, there was no difference ingas diffusivity between completely sealed suraces(asphalt) and areas with flagstone or cobblestonewith gaps in between (Weltecke and Gaertig 2012).

A water table less than 50 cm deep can reduceoxygen below levels considered sufficient tosustain vigorous root growth to within 5 cmo the soil surace (Callebaut et al. 1982). Ele-vated berm soils can be more aerated than sur-rounding soils at grade (Handel et al. 1997).

Measurement

Assessment o soil oxygen can be helpul in choosingthe appropriate plant or the site, or under-

standing whether site modifications, such asimproved drainage, may be necessary. However,measuring oxygen levels in the soil can be chal-lenging: equipment can be expensive and suitedprimarily or research applications. Measurementat any moment in time may not reflect sustainedconditions, and not all measurements providethe same inormation related to root growth.

Oxygen content, expressed as a percentage,is the amount o oxygen in the soil gases (theaboveground atmosphere contains 21% oxygen).ODR measures the rate at which oxygen canmove through the soil to replace oxygen that isused by the root. ODR can be a better indicatoro soil aeration (i.e., oxygen availability to roots)than oxygen content because it is possible to havea high soil oxygen concentration, but very lowdiffusion rate (MacDonald et al. 1993). Te oxy-gen concentration in the soil atmosphere maynot vary substantially at monitoring sites overtime, or in response to changes in soil moisture.In contrast, ODR is strongly influenced by soilmoisture and bulk density. Oxygen concentra-

tion was not consistently low enough to severelyinhibit root unction at sites where trees weredeclining. At the same time, ODR values withinthe root zones o declining trees were invariablyin a range considered injurious to roots, whileODR values around vigorous trees were avor-ably high (Stolzy 1974; MacDonald et al. 1993).

Rusting pattern on steel rods can be used toassess soil anaerobism over an extended period

(Carnell and Anderson 1986; Hodge and Knott1993; Hodge et al. 1993) and has been related tofine-root development o trees (Watson 2006a).Fine-root density in soils, where rust was presenton over 60% o the steel rods, was generally threetimes greater than in soils with less than 25% rust-ing. Tis method can provide an indication o soilaeration over a period o months and up to a deptho 60 cm without the use o expensive equipment.

Effect on Root Growth

Growing root tips have high oxygen requirements,and fine-root density is oen reduced when oxy-gen availability is low (Koslowski 1985; Gaertig etal. 2002; Weltecke and Gaertig 2012). In older partso the root, the oxygen demand can be approxi-mately hal that o the tip (Armstrong and Drew

2002). Root dysunction as a result o inadequateoxygenation can modiy plant growth and devel-opment through intererence in water relations,mineral nutrition, and hormone balance (Kramerand Kozlowski 1979; Armstrong and Drew 2002).

Species vary in their root system tolerance tolow soil aeration. For example, loblolly pine (Pinustaeda) grew better at low aeration conditions (eitherhigh compaction or high water content) than pon-derosa pine (Pinus ponderosa var. scopulorum) orshortlea pine (Pinus echinata) (Siegel-Issem et al.2005). Lists o species tolerance to flooding, which

reduces soil aeration, are available (White 1973;Bell and Johnson 1974; Whitlow and Harris 1979).

In some trees, such as willow (Salix), alder(Alnus), poplar (Populus), tupelo (Nyssa), ash(Fraxinus), baldcypress (axodium), and birch(Betula), oxygen can move down to the rootsinternally through intercellular spaces. Tis oxygen-transporting tissue within roots is called aeren-chyma. It is not uncommon in the subapical parts owetland plant roots or as much as 60% o the rootvolume to be gas space or diffusion o oxygen rom

the shoot (Drew 1997; Armstrong and Drew 2002).Enough oxygen can be transported so that some isreleased into the soil immediately surrounding theroots (Hook et al. 1971; Armstrong and Read 1972).

Mechanical ResistanceBulk density is a measure o dry mass per unitvolume and used to describe limits to root growth incompacted soil. Soil strength, expressed as penetra-


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tion resistance, is a broader indicator o constraintson root growth that accounts or soil moisture, aswell as bulk density (Baver et al. 1972; Gerard etal. 1982; Ehlers et al. 1983; aylor and Brar 1991).

Parent material is the deepest and densest layerin the soil profile. As soils develop, ormation ostructure in the overlying horizons reduces bulkdensity. Clay deposition in the B horizon tends tofill existing pore spaces, making it denser as claycontent increases (Foth 1990). Roots compact thesoil nearby as they increase in size, and they alsotransmit the weight o the tree and orces generatedby the wind onto the soil (Greacen and Sands 1980).

In urban and suburban settings, soil ormationhas been interrupted by removal, grading, mix-ing, or other disturbances. Tus, urban soils canhave high bulk densities (Yang et al. 2005; Feng

et al. 2008). Urban soil mean bulk density valueso 1.6 g cm-3 have been reported, with individualvalues as high as 2.63 g cm-3 (Patterson 1977;Short et al. 1986; Jim 1998a; Jim 1998b). Teselevels o compaction restrict root growth or manywoody species, especially in finer-textured soils.

Compaction occurs very quickly. On fine- tomedium-textured soils, hal o the increase in soilbulk density and soil strength occurred in the firsttwo passes o traffic. Coarse soils were slightly moreresistant to compaction (Brais and Camire 1998).Fine-textured soils are also slower to recover than

coarse-textured soils (Page-Dumroese et al. 2006).Soil on construction sites was heavily compacted

to depths o 0.30.8 m (Randrup 1997). In a sur-vey o areas to be landscaped near new residentialand commercial construction, mean soil bulk den-sity was ound to be 1.56 g cm-3, which represents a0.5 g cm-3increase over adjacent undisturbed areas(Alberty et al. 1984). Bulk densities in enced (undis-turbed) areas ranged rom 1.05 to 1.42 g cm-3, whilein unenced areas, bulk densities were 1.56 to 1.90g cm-3; oen exceeding the 1.60 g cm-3critical bulk

density or the loam soils on the study site (Lichterand Lindsey 1994). In another study, the absenceo differences between protected and unprotectedareas was attributed to traffic occurring on areasnot meant or traffic (Randrup and Dralle 1997).

Measurement

o determine bulk density, a soil core o knownvolume is oven dried at 105C and weighed. Care

is exercised in the collection o cores so that thenatural structure o the soil is preserved. Anychange in structure is likely to alter pore space andbulk density. Excavation methods are better or agravelly soil. A quantity o soil is excavated, dried,and weighed, along with determining the volumeo the excavation by filling the hole with sand owhich the volume per unit mass is known, or waterin a rubber liner (Grossman and Reinsch 2002).

Penetrometers are used to measure soil strength.ype o equipment used and soil moisture con-tent will affect measurement. Penetrometers with30-degree tips and diameter sizes o 12.8 and 20.3mm are standard. Te smaller cone size is or usein harder (more resistant) soils (American Societyo Engineers 1992; Lowery and Morrison 2002).

Soil strength increases with bulk density and

decreases with soil water content (aylor andBurnett 1964; Eavis 1972; Blouin et al. 2008.) Fine-textured soils are the most limiting (Gerard et al.1982), but penetration resistance can be affectedmore by water content than by texture. Penetra-tion resistance in a dry soil (1500 kPa) exhibiteda maximum at clay content o 35%, while in amoist soil (10 kPa) penetration resistance wasminimally affected by texture (Vaz et al. 2011).


Te bulk density that limits root growth varieswith soil texture (as reviewed in Daddow andWarrington 1983) and soil moisture (Day et al.2000). Greater development o structure in fine-textured soils accounts or their lower bulk densityas compared to coarse-textured soils. A bulk densityo 1.60 g cm-3would be limiting in a clay loam, butnot in a sandy loam (Foth 1990). Summary tables(Jones 1983; Daddow and Warrington 1983; NRCSSoil Quality Institute 2000 (able 1) are consistentwith reports o root restriction in individual treespecies (Minore et al. 1969; Chiapperini and Don-

nelly 1978; Webster 1978; Zisa et al. 1980; Heilman1981; woroski et al. 1983; Alberty et al. 1984;Pan and Bassuk 1985; Simmons and Pope 1985;Reisinger et al. 1988; Watson and Kelsey 2006).

Reconstruction o soil profiles rom six orest sitesin greenhouse tests showed root and shoot growthin soil rom lower horizons (1030 cm) averagedonly 41% o that in topsoil, a significantly greaterrestriction o growth than that achieved through


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compaction o up to 0.17 g cm-3 greater than theundisturbed field sites (25%). opsoil displacementand profile disturbance may be more damaging thansoil compaction (Williamson and Neilsen 2003).

Soil strength, not bulk density, was ound tobe the critical impedance actor controlling rootpenetration (aylor and Burnett 1964; Zisa 1980).

Reduced survival and growth o sugar maple (Acersaccharum Seneca Chie) and callery pear (Pyruscalleryana Redspire) in compacted soil were dueto mechanical impedance, rather than limitedaeration and drainage (Day et al. 1995). Te criti-cal limit o soil strength above which woody plantroots will likely be greatly restricted is 2.5 MPawhen measured with a standard penetrometer(aylor et al. 1966; Greacen and Sands 1980; Zisaet al. 1980; Ball and OSullivan 1982; Abercrombie1990; Day and Bassuk 1994; Blouin et al. 2008).

Root growth decreases as compaction and soilstrength increase (Youngberg 1959; aylor et al.1966; Sands et al. 1979; Bengough and Mullins1990; Jordan et al. 2003; Blouin et al. 2008). Bothcontrolled studies (Minore et al. 1969) and fieldobservations (Forristall and Gessel 1955) haveshown that the capacity or root growth in com-pacted soil oen varies among plant species. Forexample, root growth o Siberian larch (Larixsibirica), English oak (Quercus robur), western redcedar (Tuja plicata), and Formosa acacia (Acaciaconfusa) were little affected by soil bulk density as

high as 1.89 g cm-3, while Norway spruce (Piceaabies), Douglas fir (Pseudotsuga menziezii), little-lea linden (ilia cordata), and tallow lowrel (Litsea

glutinosa) were the least capable o growing rootsin compacted soil (Forristall and Gessel 1955;Korotaev 1992; Liang et al. 1999). As little as 0.14g cm-3can make a difference (Minore et al. 1969).

Soil compaction can affect root distribution.Root penetration depth can be restricted by soil

bulk density (Halverson and Zisa 1982; Nambiarand Sands 1992; Laing et al. 1999). I not allparts o a root system are equally exposed tocompaction, compensatory growth by unim-peded parts o the root system may compensate,and the distribution but not the total length oroots may be altered (Unger and Kaspar 1994).

Individual root tips can penetrate only thosesoil pores that have a diameter greater than thato the root. Roots oen grow into root channelsrom previous plants, worm channels, structuralcracks, and cleavage planes, thereby tapping alarger reservoir o water and mineral nutrients. Invery compacted soils, root growth may be confinedalmost entirely to these pores and cracks (ayloret 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). I not present, roots may undergo redirectiono growth rom deeper layers toward uncom-pacted surace soil when downward growth isrestricted by high bulk density (Waddington andBaker 1965; Heilman 1981; Gilman et al. 1987).Te net result is the prolieration, i not concen-tration, o roots at a shallow depth (Gilman etal. 1982; Weaver and Stipes 1988; Jim 1993a).Such a shallow root system will be more affectedwhen surace soils dry during periods o drought.

here is a tendency to orm more lateral rootswith increasing soil strength (Gilman et al. 1987;

Misra and Gibbons 1996). Length o primary andlateral roots o shining gum (Eucalyptus nitens)was reduced 71% and 31%, respectively, with anincrease in penetrometer resistance rom 0.4 to4.2 MPa. High mechanical resistance will alsotend to increase the root diameter behind theroot tip (aylor et al. 1966; Eavis 1972; Russell1977; Bengough and Mullins 1990; Misra andGigbons 1996), and the growth and shape o

Table 1. General relationship of soil bulk density to root growth based on soil texture (adapted from NRCS Soil QualityInstitute 2000).

Soil texture Ideal bulk densities Bulk densities that may affect Bulk densities that restrict(g cm-3) root growth (g cm-3) root growth (g cm-3)

Sands, loamy sands 1.80Sandy loams, loams 1.80Sandy clay loams, clay loams 1.75Silts, silt loams 1.75Silt loams, silty clay loams 1.65Sandy clays, silty clays, some 1.58clay loams (35%45% clay)Clays (>45% clay) 1.47


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root cells are altered (Pearson 1965). Dierencesamong species in their ability to penetratestrong soil layers appear to be due to dier-ences in root diameter (Clark et al. 2003).

TemperatureUrban soils can be warmer due to surround-ing pavements and lack o vegetation cover.Unvegetated playground soils in Central Park(New York City, New York, U.S.) were 3.13Cwarmer than an adjacent wooded area (Mountet al. 1999). Maximum summer soil tempera-tures under pavement in the northern UnitedStates were 32C34C, and up to 10C warm-er than nearby unpaved areas (Halverson andHeisler 1981; Graves and Dana 1987). In exas,U.S., summer soil temperatures under pave-

ment exceeded 48C, 10C warmer than unpavedareas, and remained above 35C or all but a shorttime at night. emperatures are highest underdark pavements (Arnold and McDonald 2009).


Biological activity in the soil, and thereore rootgrowth, varies with temperature (Lloyd and aylor1994). Root growth occurs over a wide range otemperatures, but is much slower at low and hightemperatures. Reported minimum temperatures

or root growth range rom 2C to 11C (Lyr andHoffmann 1967; Soleld and Pedersen 2006).Sugar maple (Acer saccharum) roots began togrow in spring as soils warmed to 5C, but initialroot growth may be quite slow at such low tem-peratures. Active root growth has been reportedto begin when soil temperatures reach 10C15C(Nambiar et al. 1979; Carlson 1986; Harris et al.1995; Soleld and Pedersen 2006). Optimum tem-peratures or root growth have been reported at18C32C (Lyr and Hoffman 1967; Larson 1970;Nambiar et al. 1979; Struve and Moser 1985; Head-

ley and Bassuk 1991; Harris et al. 1995; Soleldand Pedersen 2006; Richardson-Calee et al. 2007).

Te high temperature at which root injury beginsto occur is around 34C (Graves and Wilkins 1991;Graves 1994; Graves 1998; Wright et al. 2007). Rootso most woody species are killed at 40C50C(Wong et al. 1971). Maximum temperatures oractive growth have been reported at 25C38C,depending on the species (Proebsting 1943; Wong et

al. 1971; Gur et al. 1972; Graves et al. 1989a; Graveset al. 1989b; Graves 1991; Martin and Ingram 1991;Graves and Aiello 1997; Arnold and McDonald2009). Direct heat injury o roots can occur whenthe soil remains above 32C or extended periods otime (Graves 1998), and the longer the duration ohigh temperatures, the more root growth is reduced(Graves et al. 1989b; Graves and Wilkins 1991). Hon-eylocust (Gleditsia triacanthos) is the only temperatetree species reported to sustain growth at root-zonetemperatures above 32C (Graves et al. 1991).

Te root tissues o most woody plants can bekilled at soil temperatures o -5C to -20C (Havis1976; Studer et al. 1978; Santamour 1979; Pellett1981; Lindstrom 1986; Bigras and Dumais 2005),although roots o black spruce (Picea mariana) werenot affected by temperatures as low as -30C (Bigras

and Margolis 1996). Young roots are less reeze-tol-erant than mature roots (Bigras and Dumais 2005).

Soil pHPlant perormance is strongly affected by nutrientavailability, which in turn is influenced by soil pH(acidity or alkalinity). Most nutrients are availableat optimal levels in slightly acid to neutral soils(pH between 5.5 and 7.2), and trees generallygrow best in this pH range. Soil pH can be mea-sured with electronic meters or colorimetric

tests based on color o solutions or strips.Urban soils tend to have higher soil pH thantheir natural counterparts. In Berlin, Germany,a pH o 8 was observed streetside, compared to apH o less than 4 within a orest a short distancerom the street (Chinnow 1975). Over hal o soilssampled in Hong Kong, China, were rated strongly(pH 8.59) to very strongly (pH 99.5) alkaline,while surrounding soils were acidic at pH 45 (Jim1998b). Streetside soils o Syracuse, New York,U.S., had a pH range o 6.6 to 9.0 with an averageo about 8.0 (Craul and Klein 1980). Urban soils

o Philadelphia, Pennsylvania, U.S., ranged rom3.7 to 9.0 with a mean o 7.6 (Bockheim 1974).

Elevated pH values have been attributed to theapplication o calcium or sodium chloride as road andsidewalk deicing compounds in northern latitudes,irrigation with calcium-enriched water (Bockheim1974), and the surace weathering o concrete andlimestone buildings and sidewalks (Bockheim 1974;Messenger 1986; Okamoto and Maenaka 2006).


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he eects o pH on root growth are primarilyrelated to nutrient availability. Some nutri-ents, such as iron and manganese, becomeless available in alkaline soils (pH above 7.2)because o chemical changes caused by thealkalinity. Other nutrients, such as phospho-rous, become less available in highly acidsoils (pH less than 5.5). When the pH is 4.5or less, aluminum toxicity can restrict rootgrowth (Foth 1990; Jim 1993b). In most plantsystems, aluminum toxicity has a direct e-ect on root growth by inhibiting cell divisionin the root apical meristem (Kochian 1995).

A nutrient deficiency caused by sub-optimum soilpH could actually stimulate root growth in order toexplore larger volumes o soil to acquire additional

nutrients and alleviate deficiency symptoms (Inges-tad and Lund 1979; Ericssson and Ingestad 1988).

Cation Exchange CapacityCation exchange capacity (CEC) is a measureo the nutrient-holding (adsorption) power othe soil. Once adsorbed, cationic minerals arenot easily lost when the soil is leached by waterand thereore provide a nutrient reserve orplant roots. CEC is highly dependent upon soiltexture and organic matter content. In general,the more clay and organic matter in the soil, thehigher the CEC. Small clay soil particles have alarge, negatively charged surace area or theirsize and hold relatively large amounts o ions.Organic matter particles have even more nega-tive surace charges on the surace than clay ornutrient exchange. Sandy soils have low CECdue to their low organic matter and clay content.

CEC is usually greatest at the suracewhere organic matter accumulates. Increas-ing clay with depth can act to counterbalancethe decrease in organic matter and reduction

o CEC. he CEC o most soils increases withpH (Craul 1992; Brady and Weil 1996).

CEC is determined by laboratory testing,and methods vary with the soil type. Reportedurban soil CEC values have been 512 cmol/kg (Short et al. 1986; Jim 1998b). Normal valuesvary, rom 5 cmol/kg to 25 cmol/kg, dependingon texture, organic matter content, and pH(Foth 1990; Landon 1991; Brady and Weil 1996).

ContaminantsSalt in soil inhibits plant water uptake by loweringthe osmotic pressure o soil water (Prior andBerthouex 1967). Tis reduces the water uptakeo trees and symptoms o decline mimic thoseo drought (Herrick 1988). Once salt enters theroots, it upsets the osmotic balance within rootcells (Janz and Polle 2012) and is toxic to theendomycorrhizae (Guttay 1976). Te increasedsodium on the cation exchange sites also breaksdown soil structure (Holmes 1961; Hutchinsonand Olson 1967), decreasing the permeability andwater-holding capacity o the soil. All o theseactors may contribute to a decline in tree health.

Damage rom salt-contaminated soil occurs re-quently in urban areas where large amounts o saltare used or deicing roads and pavements. Sodium

chloride is the most common deicer applied. Park-ways, street tree planter boxes, highway medians,and roadsides are locations where soil accumula-tion o deicing salts is highest. Sodium levels were5.4 times higher and chloride was 15 times higherin the center o newly installed, narrow, raisedmedians along an urban highway aer one winter,compared to the center o wide medians along thesame roadway. Te high levels were attributed toproximity to high speed traffic and its associatedspray and splash (Hootman et al. 1994). Elevatedlevels o sodium have been reported in the soil

up to 30 m rom the highway and elevated levelso soil chlorine to a distance o 61 m (Langille1976; Hostra et al. 1979; Simini and Leone 1986).In contrast, rural highway studies show salt levelsdecline rapidly with distance to pavement (Her-rick 1988; Cunningham et al. 2008). Te releaseo salts rom rapid-release orms o ertilizer canalso elevate soil salt levels (Jacobs et al. 2004).

Reclaimed wastewater (RWW) and ground-water used to irrigate urban plantings in aridclimates can be highly saline. Sodium and chlo-

ride are the major chemical constituents in RWWthat are potentially detrimental to plants (Stateo Caliornia 1978; Schaan et al. 2003). Com-pared with sites irrigated with surace water,sites irrigated with RWW exhibited up to 187%higher electrical conductivity (EC) and 481%higher sodium adsorption ratio (SAR) (Qianand Mecham 2005; Schuch et al. 2012). Soiltypes play a role on soil salinization as much as


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water quality. he highest salinity was ound inclay and the lowest in sand (Miyamoto 2012).

Te best method or assessing soil salinity is tomeasure the electrical conductivity o soil solutionextracts. Conductivity o 2 dS/m (deci-Siemens/meter) is considered harmul to salt-sensitive plants(Foth 1990; Jacobs and immer 2005). All butvery salt-tolerant plants will be affected at 4 dS/m.Czerniawska-Kusza et al. (2004) ound necrosis andchlorosis in leaves at levels o 132 g Na+/g o soil.Soil chloride ion concentrations o up to 200 g/gare not considered harmul to plants (Jim 1998a).

Deicing salt can cause the death o surace rootsin roadside trees (Wester and Hohen 1968; Krap-enbauer et al. 1974; Guttay 1976; Jacobs et al. 2004;Madji and Persson 1989), though the risk o rootdamage associated with salt concentrations levels

appears to be dependent on species, age o rootsystem, and soil moisture availability (Jacobs andimmer 2005). Damage may result rom osmoticand/or specific ion effects (Dirr 1975). Root rotcaused by Phytophthora sp. can increase with soilsalinity as well (Blaker and MacDonald 1985; Blakerand MacDonald 1986). Indirect damage occurswhen sodium displaces other ions rom soil cationexchange sites reducing their availability, and breaksdown soil structure leading to soil compaction(Herrick 1988; Dobson 1991; Hootman et al. 1994).

rees growing in soils with high salt levels tended

to have more twig dieback and less twig growththan those growing in soils with lower salt levels(Berrang et al. 1985). Sodium chloride and othersalts accumulating in the root zone may instigate andexacerbate street tree decline (Hootman et al. 1994).

Heavy metals is a term generally used to describe agroup o metallic elements that can be toxic to plantsand animals. Some, such as copper, molybdenum,and zinc are essential trace elements, but exces-sive levels can be toxic (Prasad 2004). Heavy metalcontamination tends to be greater toward the city

center and in areas o commercial and industrialland use (Carey et al. 1980; Blume 1989; Wang andZhang 2004). City center and wasteland soils gen-erally had enhanced heavy metal concentrationsto at least 30 cm depth (Linde et al. 2001). Soilson the National Mall in Washington, D.C., U.S.,had elevated levels o lead, zinc, nickel, copper,and cadmium (Short et al. 1986). Concentrationso heavy metals in roadside soils decrease with

distance rom traffic and depth in the soil profile.Te contamination has been related to the com-position o gasoline, motor oil, and car tires, andto roadside deposition o the residues o thesematerials (Lagerwer and Specht 1970; Madji andPersson 1989). Long-term sewage sludge appli-cation may result in the accumulation o Zn, Cu,and Ni in the soil and plant (Bozkurt et al. 2010).

Soil heavy-metal data has been published orseveral cities (Lagerwer and Specht 1970; Carey1980; Blume 1989; Jim 1998a). Levels o many ele-ments were higher on urban sites than suburbanand rural sites up to 10 times or more. No plantdamage was reported with these higher levels.

Soil BiologySoil organisms are an important component o a

healthy soil that promotes root growth. Te ratioo ungal to bacterial biomass is oen near 1:1 ingrass and agricultural soil ecosystems. With reduceddisturbance, ungi become more plentiul, andthe ratio o ungi to bacteria increases over time.Forests tend to have ungal-dominated microflora.Te ratio o ungal to bacterial biomass may be 5:1to 10:1 in deciduous orests and 100:1 to 1000:1 inconierous orests (Soil and Water ConservationSociety 2000). Assessing abundance o soil bacteriaand ungi and mycorrhizal colonization o rootsrequires extensive skill and laboratory equipment.

Te zone o soil adjacent to plant roots with ahigh population o microorganisms is the rhizo-sphere. Bacteria eed on sloughed-off plant cellsand the proteins and sugars released by roots. Teprotozoa and nematodes that graze on bacteriaare also concentrated near roots. Tus, mucho the nutrient cycling and disease suppressionneeded by plants occurs immediately adjacent toroots (Soil and Water Conservation Society 2000).Rhizosphere pH can be up to two units different thanthe rest o the soil (Marschner and Rmbeld 1996).

Mycorrhizae are symbiotic relationships thatorm between common soil ungi and plants. Tebenefits o mycorrhizal associations o tree roots arewell established (Smith and Read 1997). Te ungicolonize the root system o a host plant, providingincreased nutrient absorption capabilities, while theplant provides the ungus with carbohydrates romphotosynthesis. Mycorrhizae offer the host plantincreased protection against certain pathogens.


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Urban planting sites are oen considered to be opoor soil quality, but mycorrhizal inoculum (spores)was more abundant in urban soil than in orest soilin one study (Wiseman and Wells 2005). Somemycorrhizal ungi colonizing littlelea linden (iliacordata) roots were common to both street treesand orest trees. Others were not. Colonization lev-els were high on both street and orest trees (Nielsenand Rasmussen 1999; imonen and Kauppinen2008). Native desert trees had greater colonizationby arbuscular mycorrhizal ungi (AMF) than resi-dential landscape trees, and AMF species composi-tion differed at the two site types (Stabler et al. 2001).

Interdependence of Soil FactorsAs a result o the interdependence o soil proper-ties, the status o one soil actor can have an effect

on all others; an understanding o their interre-lationships is important or proper management.

Water and Air

Increasing soil moisture reduces soil aerationwhen water replaces the air normally held in thepores o the soil. Water slows the diffusion ooxygen to 1/10,000 o that in air, and it reducesits concentration to about 1/32 o that in air. Tenet result is an effective resistance to flow thatis around 320,000 times greater in saturated soilthan that o air (Armstrong and Drew 2002).

Water and Compaction

Compaction can decrease the number o days oavailable water in clay-loam soil. However, compac-tion can increase the number o days that water isavailable in a sandy loam soil (Gomez et al. 2002).

ree roots can grow successully in significantlycompacted soils provided soil moisture is readilyavailable (Zisa et al. 1980; Pittenger and Stamen1990; Bulmer and Simpson 2005; Siegel-Issem etal. 2005). Resistance to penetration in a clay loam

soil was ound to decrease rom 3.5 MPa (limiting)to 2.1 MPa (non-limiting) when volumetric soilmoisture increased rom approximately 27%to 40% (Day et al. 1995). Roots o spotted gum(Corymbia maculata) and red-flowering gum (C.

ficifolia) were able to penetrate soil compacted to abulk density o 1.6 g cm-3at 7% soil moisture, butwhen moisture was increased to 10% roots couldpenetrate soils o 1.8 g cm-3 (Smith et al. 2001).

Species can vary in their ability to capitalizeon reduced penetration resistance o wet soils.Silver maple (Acer saccharinum) roots cangrow in moderately compacted soil when highsoil water content decreases soil strength, eventhough aeration is low, whereas dogwood (Cornus

florida) roots are unable to grow under thesame low aeration conditions (Day et al. 2000).

Air and Compaction

One o the main effects o high bulk density is arestricted oxygen supply (Yelenosky 1963; Yelenosky1964; Rickman et al. 1966). Oxygen is less restrictedwhen the soil is dry and less pore space is filledwith water (Day 1995). Oxygen diffusion rate waslowest in soils with high bulk density (MacDonaldet al. 1993). Compaction rom a bulk density o

1.04 g cm-3to 1.54 g cm-3reduced gas diffusion by38% when soil was dry. In wet soil, however, com-paction reduced diffusion by 82% (Currie 1984).

Plant response to oxygen level has been shown tointeract with mechanical impedance (Gill and Miller1956). In general, soil compaction can have a stronginhibitory effect on root penetration when the oxy-gen level is high, but no significant effect at a lowoxygen level because root growth is already reducedby lack o aeration (ackett and Pearson 1964;Hopkins and Patrick 1969; da Silva and Kay 1997).

Anaerobic conditions are likely to limit rootgrowth in compacted fine-textured and poorlydrained soils, whereas mechanical impedance ismore likely to limit root growth in compacted coarse-textured and well-drained soils (Webster 1978).

Soil Conditions and Root Disease

Poorly aerated and poorly drained soil can increaseincidence o soil-borne diseases. Root diseases areavored when soils are water-saturated (Hansen et al.1979). Saturated soil and low oxygen supply causesa reduction in root initiation, growth o existing

roots, and an increase in decay o roots, largely as aresult o invasion o Phytophthorasp. Fungi, whichtolerate low soil aeration (Stolzy et al. 1965; SenaGomes and Kozlowski 1980; Blaker and McDon-ald 1981; Benson et al. 1982; Stolzy and Sojka 1984;Benson 1986; Duniway and Gordon 1986; Grayand Pope 1986; Ownley and Benson 1991). Armil-laria root disease, also known as shoestring rootrot, causes most damage on trees that are stressed


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by one or more abiotic or biotic actors. Tese mayinclude drought, soil compaction, and other soilproblems common on urban sites (Worall 2004).

Root Development and Nutrient Uptake

When soil actors limit root development there canbe a direct impact on nutrient uptake. Nutrientdeficiencies can occur when there is insuffi-cient uptake by the roots and use by the crown. Iimproved soil conditions allow the root systemto expand and explore a larger soil volume andsupply o nutrients, the tree may overcome thedeficiency and symptoms may dissipate (Inges-tad and Lund 1979; Ericssson and Ingestad 1988).

MANAGEMENT PRACTICES TOIMPROVE THE SOIL ENVIRONMENT

Te effectiveness o management practices to en-hance soil as a medium or root growth can affectall soil actors and is influenced by soil physicalproperties. Soils classified as having poor physi-cal conditions are those that require very careulmanagement to maintain conditions avorable orroot growth. Soils with good physical conditionsrequire less careul management (Letey 1985).

PreventionPrevention o soil compaction is preerred. reat-ments to alleviate compaction can be expensive,diicult to apply, sometimes ineective, and mayinjure roots (Howard et al. 1981). When onlyacted upon by natural orces, return to the initial,uncompacted state is slow (Hatchell et al. 1970;Froehlich and McNabb 1984; Froehlich et al. 1986;Corns and Maynard 1998; Stone and Elio 1998;Blouin et al. 2005). Fine-textured soils are slowerto recover than coarse-textured soils. Suracesoils will recover most rapidly (Page-Dumroeseet al. 2006). When compaction severely reducedsoil aeration and root growth ater a logging

operation, ater 14 years, recovery was limitedto the top 4 cm o soil. Ater 18 years, recoveryreached a depth o 18 cm. Only ater 24 years wasrecovery detected throughout the rooting zone(von Wilpert and Schaer 2006). Factors, suchas a luctuating water table, reezethaw cycles(Fleming et al. 1999; Stone and Kabzems 2002),and vegetation regrowth (Page Dumroese et al.2006), may accelerate a bulk density decrease.

Mulch or gravel over geotextile can preventsoil compaction during construction. In contrast,plywood did not protect the underlying soil romcompaction (Donnelly and Shane 1986; Lichter andLindsey 1994). Fencing can be an effective way to pre-vent soil compaction on a construction site (Lichterand Lindsey 1994), but must be monitored and main-tained to be effective (Randrup and Dralle 1997).

Amendmentshe use o organic amendments, such as bio-solids, animal manure, or compost, generallyreduces the bulk density o compacted soils (Cog-ger 2005; Garcia-Orene et al. 2005), althoughthis is not always the case (Patterson 1977).he proposed mechanisms or this phenome-non are that the high density substrate is simply

being diluted with a low-density material (theamendment) or that the amendment physicallyincreases porosity (Clapp et al. 1986; Cogger2005). Organic amendments can increase rootgrowth (Beeson and Keller 2001; Davis et al. 2006),microbial activity (van Schoor et al. 2008) andCEC. Composted organic matter is most eective,as the humus component has the greatest CEC.Incorporation o certain types o biochar canincrease CEC (Chan et al. 2007; Laird et al.2010), but research on this topic is still limited.

Inorganic soil amendments have been used toimprove soil properties and resist compaction.Sintered fly-ash and expanded slate amendmentsresulted in lower bulk densities and increasedpore space aer being incorporated into the soil(Patterson 1977). Amendment with mixtureso gravel, expanded clay, and lava rock improvedthe soil aeration and soil moisture in clayloam and silty loam soils (Braun and Fluckiger1998). Tese studies did not assess the effecto soil changes on root systems perormance.

Hydrophilic polymer gels (hydrogels) are some-

times added to the soil to increase available water.Research has not shown that the use o hydrogelscan consistently increase root growth o trees(Hummel and Johnson 1985; Keever et al.1989; ripepi et al. 1991; Walmsley et al. 1991;Winkelmann and Kendle 1996; Huttermann etal. 1999; Gilman 2004; Abbey and Rathier 2005).


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CultivationCultivation has been used with mixed results toimprove soil properties and promote tree root de-velopment. Deep cultivation by ripping prior toplanting decreased bulk density and soil penetrationresistance (Rol 1991; Rol 1993; Moffat and Boswell1997; Lincoln et al. 2007) and increased both themaximum root depth and total number o rootscompared with the untreated control or Italianalder (Alnus cordata), Japanese larch (Larix kaemp-

feri), Austrian pine (Pinus nigra), and Europeanwhite birch (Betula pendula) (Sinnett et al. 2008). Inother cases, ripping had no effect on rooting depth(Nieuwenhuis et al. 2003) or was reported to be e-ective or less than a year (Moffat and Boswell 1997).

Tere was no reduction in soil strength romsurace soil cultivation with an air excavation

tool aer one year on three o our sites. Com-post incorporation with air cultivation didresult in a reduction o soil strength that per-sisted or at least three years (Fite et al. 2011).

Cultural techniques that improve soil tilth, aera-tion, and drainage reduce conditions avorable toroot disease (Juzwik et al. 1997), and also improvehost resistance by reducing or avoiding stress asso-ciated with anaerobic conditions (Sutherland 1984).

MulchTe benefits o organic mulch are well estab-

lished (Chalker-Scott 2007) and continue to bereinorced. A review o published mulch researchstudies showed surace mulch improved soilphysical properties and tree physiology, butthere was no improvement in chemical or bio-logical properties (Scharenbroch 2009). Improve-ment o soil properties will enhance root growth.

Over time, organic mulches can reduce soilbulk density (Donnelly and Shane 1986; Coggeret al. 2008) and increase organic matter content(Watson et al. 1996; Johansson et al. 2006; Fite

et al. 2011). Mulch can increase water iniltra-tion (Donnelly and Shane 1986; Cogger et al.2008), reduce evaporation rom the soil surace,and increase moisture availability (Litzow andPellett 1983; Iles and Dosmann 1999; Arnold etal. 2005; Cogger et al. 2008; Singer and Martin2008; Fite et al. 2011). Mulch allowed a 50%reduction in irrigation while still maintainingacceptable growth and appearance (Montague

et al. 2007). Mulch also insulates soil rom tem-perature extremes (Montague et al. 1998; Ilesand Dosmann 1999; Singer and Martin 2008).In December, soil under mulch was 6C warmerthan exposed sod or bare soil (Shirazi and Vogel2007). In temperate climates, the soil may warmmore slowly i new mulch is applied beore thesoil warms in spring (Myers and Harrison 1988).

Organic surace mulch generally improves shootand root growth (Kraus 1998; Ferrini et al. 2008;Arnold and McDonald 2009; Scharenbroch 2009).Adding wood chip mulch to the surace o red maple(Acer rubrum) and sugar maple (A. saccharum)grown in sandy loam and clay loam, respectively,increased growth above- and belowground (Frae-drich and Ham 1982). Mulching with wood chipscan result in a 30%300% increase in fine-root

development in the top 15 cm o soil (Fraedrichand Ham 1982; Green and Watson 1989; Himelickand Watson 1990). Mulches may not be beneficialor some desert plants (Singer and Martin 2009).

When a mulch layer is maintained or severalyears, a partially decomposed organic layer developsthat holds moisture and minimizes evaporationrom the soil beneath. A dense mat o roots canorm in the layer o mulch as well as in the soilbeneath it (Bechenbach and Gourley 1932; Watson1988). Te roots in the mulch will not be at anygreater risk o desiccation, since the well-established

mulch layer can hold more water than the soil itsel,without decreasing aeration to the soil beneathit (Watson 1988; Himelick and Watson 1990).

Mulch reduces root competition or soil mois-ture and nutrients rom lawn grasses (Richardson1953; Gilman 1989; Kraus 1998). In additionto competition or water and nutrients, somelawn grasses may be able to reduce the growtho the trees through production o allelopathicchemicals. Root growth o orsythia (Forsythiaintermedia) was suppressed by ryegrass and red

escue leachates (Fales and Wakeield 1981). Fes-cues have also been shown to stunt the growtho southern magnolia (Magnolia grandif lora)(Harris et al.1977), river redgum (Eucalyptuscamaldulensis) (Meskimen 1970), black wal-nut (Juglans nigra) (odhunter and Beineke1979), and sweetgum (Liquidambar styraciflua)(Walters and Gilmore 1976), but speciiceects on root systems were not reported.


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While mulching has many benefits or soilquality and root health, there are some potentialdrawbacks. One concern about mulching is that itcreates conditions ideal or certain disease-causingungi. Fraedrich and Ham (1982) did not findany enhancement o the soil-borne pathogenicungi, Pythium spp. and Fusarium spp. duringtheir one-year study. Austrian pine saplings thatwere mulched with resh needles and shoot tipsrom Sphaeropsis tip blight diseased trees devel-oped more than twice the percentage o blightedtips. Tere was no Botryosphaeriacanker orArmil-laria root rot disease development when redbud(Cercis canadensis) and red oak (Quercus rubra)saplings, respectively, were mulched with woodchips rom diseased trees (Jacobs 2005). A decreasein growth the first year aer mulching, and an

increase in the second year has been attributedto nitrogen immobilization in the first year ol-lowed by release the next (Hensley et al. 1988;ruax and Gagnon 1993; Erhart and Hartl 2003).

A layer o mulch can intercept rain water beoreit reaches the roots i the amount o water is smallor the mulch is thick (Gilman and Grabosky2004; Arnold et al. 2005; Johansson et al. 2006).Although 25 cm or more o coarse textured organicmulch does not adversely affect soil oxygen or fineroot development (Watson and Kupkowski 1991;Greenly and Rakow 1995), as little as 5 cm o fine-

textured organic mulch, or compost, can reducesoil oxygen to less than 10% under wet conditions,which can affect root unction (Hanslin et al. 2005).

AerationCompressed air soil injection treatments havegenerally been ineffective in relieving compac-tion or increasing soil aeration (Yelenosky 1964;Smiley et al. 1990; Hodge 1991; MacDonald et al.1993; Rol 1993). Soil texture may have a stronginfluence on the results. Reports o success in

reducing bulk density or increasing porosity werein loamy soils (Rol 1993; Lemaire et al. 1999).

A traditional approach to aeration o compactedsoil around trees is vertical mulching (i.e., drillinga pattern o holes in the root zone soil). Researchon vertical mulching has provided mixed results.Holes 5 cm diameter, 45 cm deep, with or withoutsand-bark mix backfill, provided no benefit toChinese wingnut trees (Pterocarya stenoptera)

(Pittenger and Stamen 1990). Similar results wereseen in sugar maple (Acer saccharum) when theholes were filled with perlite backfill (Kalisz et al.1994). However, roots o Monterey pine (Pinusradiata) were able to utilize 10 mm diametervertical perorations to grow the same depth asuncompacted controls, while root growth o treeson compacted soil without perorations was sup-pressed (Nambiar and Sands 1992; Sheriff and Nam-biar 1995). Largelea linden (ilia platyphyllos) andplanetree (PlatanusAcerifolia) roots colonized themajority o the depth o 10 cm diameter, 60 cm deepholes filled with a mix o coarse sand, compostedorganic materials, and ertilizer, and grew deeperthan in adjacent site soils (Watson et al. 1996).

Root growth in larger trenches filled withcompost-amended soil was increased relative

to undisturbed soil, but root growth was notincreased in the soils adjacent to the trenchesaer 2, 4, and 14 years. Soil aeration was notmeasured and may not have been limiting in theundisturbed and not compacted soil adjacent tothe trenches (Watson et al. 1996; Watson 2002).

pH AdjustmentNeutral to slightly acid pH is optimum or mostplants. Applications o lime are used to raise soilpH. Aluminum sulate and sulur can help to lowerpH, although high rates o aluminum sulate maycause injury to some plants, particularly in broad-lea evergreens. Te injury is believed to be causedby excessive aluminum. Ammonium sulate maybe as effective as aluminum sulate, but neither isas effective as granular sulur (Messenger 1984).Ammonium sulate is sometimes used i nitrogenapplication is needed along with pH reduction,but applying enough to lower the pH would likelyapply a quick release orm o nitrogen in excess obest management practices (Smiley et al. 2007).

Enhancing root development may improve

uptake o available nutrients. Improving soil qual-ity using methods such as cultivation, addition oorganic amendments, and mulching can enhanceroot systems (see above). Basal drench applica-tion o paclobutrazol, a tree growth regulator,increased fine-root development and relievedinterveinal chlorosis commonly attributed toiron deficiency o pin oak (Quercus palustris)on alkaline soils (Watson and Himelick 2004).


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Salt MitigationSoil salt accumulation can be reduced throughdesign and engineering. Deicing salt accumula-tion in road median planters can be preventedby using wider planters with higher walls setarther rom high-speed roads. Te raised plantersdid not receive salt-laden runoff, splash, plowedsnow, or direct application rom salt spreaders(Rich and Walton 1979; Hootman et al. 1994).

Leaching o sodium rom deicing salt applica-tion to roadways can be rapid in well-drainedsoils with adequate natural precipitation (Priorand Berthouex 1967; Cunningham et al. 2008).High soil salts and wet soils tended to occurtogether since poor drainage restricts the normalleaching o soil salts (Berrang et al. 1985). In aridregions, natural precipitation will not usually

leach salt rom the soil (Schuch et al 2008). Underlow moisture conditions, moisture moves to thesurace and evaporates and salt moves upwardalso to accumulate near the surace (Prior andBerthouex 1967). Flushing soil with water toremove salt and adding gypsum (CaSO

4) and

ertilizers appear to be the best treatments orsalt contaminated urban soils (Dobson 1991).

Selection o resistant species and cultivarscan also minimize damage rom salt in soils.Te majority o published studies evaluateonly shoot sensitivity, but growth o root

systems o crapemyrtle (Lagerstroemia) cultivarsvaried in sensitivity to soil salt (Cabrera 2009).

BiostimulantsApplication o commercial products to en-hance root growth has been increasing. Soilapplication o mycorrhizal ungi have provenbeneicial to trees in soils lacking the appropri-ate ungi, such as on strip-mining reclamationsites and in sterilized nursery beds (Smith andRead 1997). Native mycorrhizal ungi levels

can be low in arid regions (Dag et al. 2009).However, growth rate o urban trees has gen-erally been unaected when treated with com-mercial inoculants at planting (Morrison etal. 1993; Martin and Stutz 1994; Roldan andAlbaladejo 1994; Querejeta et al. 1998; Gilman2001; Ferrini and Nicese 2002; Appleton et al. 2003;Abbey and Rathier 2005; Corkidi et al. 2005; Bros-chat and Elliot 2009; Wiseman and Wells 2009).

Vigor o the natural mycorrhizal inoculum, aswell as suitability o the introduced inoculum tothe ecological conditions o the site, are importantactors in the success or ailure o the intro-duced inoculum (Leacon et al. 1992). Endemicungi species may replace the inoculated speciesover time (Garbaye and Churin 1996). Mycor-rhizae can develop without introduced inocula-tion in a avorable soil environment i naturalinoculum is present (Wiseman and Wells 2009).

Te quality o the inoculum may be a actor insuccess o inoculations. Mycorrhizal coloniza-tion o roots rarely exceeded 5% aer treatmentwith commercial inoculants, but was up to 74%when treated with a resh, lab-cultured inocu-lant (Wiseman et al. 2009; Fini et al. 2011).

Paclobutrazol (PBZ), a growth regulator used

primarily to reduce shoot growth o trees, canalso increase root growth under certain cir-cumstances. Mycorrhizal colonization o roottips was unaffected by PBZ treatment, show-ing that mycorrhizae are not reduced by theungicidal properties o PBZ (Watson 2006b).

Application o organic products, such ashumates and plant extracts, have shown limitedbenefit to root growth o trees. Dose and speciesresponses vary widely (Laiche 1991; Kelting etal. 1997; Kelting et al. 1998a; Kelting et al. 1998b;Ferrini and Nicese 2002; Fraser and Percival

2003; Gilman 2004; Sammons and Struve 2004;Abbey and Rathier 2005; Barnes and Percival2006; Broschat and Elliot 2009; Percival 2013).

Compost teas are liquids containing solublenutrients and species o bacteria, ungi, protozoa,and nematodes extracted rom compost. Com-post teas are being used to enhance soil biologyand provide nutrients, sometimes as an alterna-tive to ertilization, but research support or theireffectiveness is lacking (Scharenbroch et al. 2011).

Sucrose can increase root:shoot ratios by down-

regulating genes used or photosynthesis (Percivaland Fraser 2005). Applied as a root drench, itenhanced root vigor when applied at up to 70 g/Lin some studies (Percival 2004; Percival and Fra-ser 2005; Percival and Barnes 2007), but not others(Martinez-rinidad et al. 2009). In most o thesestudies, the sugar was applied to the soil at least twice.

Healthy soils with avorable physical andchemical characteristics will support active soil


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biology naturally. Improving soil conditions ispreerred over addition o compost teas, bios-timulants, mycorrhizal ungi, and other means.

One o the most important soil unctions isto serve as a medium or root growth. Physical,chemical, and biological soil characteristics allhave an effect on tree roots. A thorough under-standing o how these soil characteristics affectroot growth is necessary to properly manage soilsor optimum root growth. Although most urbansoils are substantially altered rom the naturalstate, or even completely manuactured, urbansoils must still provide the necessary resources orroot growth. Highly disturbed soils require verycareul management to maintain conditions avor-able or root growth. Management practices aimedat preventing soil damage or restoring aspects o

the natural soil environment have the strongestresearch to support their effectiveness in improv-ing root growth in urban and suburban settings.

Acknowledgements. We would like to thank the InternationalSociety o Arboriculture and the ISA Science and Research Com-mittee or unding this literature review.

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