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



The physical, chemical, and biological constraints of urban soils often pose limitations for the growth of tree roots. An understanding of the interrelationships of soil properties is important for proper management. As a result of the interdependence of soil properties, the status of one soil factor can have an effect on all others. Preventing soil damage is most effective and preferred. Cultural practices, such as cultivation and mulching, can be effective in improving soil properties. Soil additives, such as biostimulant products, have not proven to be consistently effective through research. The management challenge is to provide an urban environment that functions like the natural environment.
Arboriculture & Urban Forestry 40(4): July 2014
©2014 International Society of Arboriculture
Gary W. Watson, Angela M. Hewitt, Melissa Custic, and Marvin Lo
The Management of Tree Root Systems in
Urban and Suburban Settings: A Review of Soil
Influence on Root Growth
Arboriculture & Urban Forestry 2014. 40(4): 193–217
Abstract. e physical, chemical, and biological constraints of urban soils oen pose limitations for the growth of tree roots. An under-
standing of the interrelationships of soil properties is important for proper management. As a result of the interdependence of soil properties,
the status of one soil factor can have an eect on all others. Preventing soil damage is most eective and preferred. Cultural practices, such as cul-
tivation and mulching, can be eective in improving soil properties. Soil additives, such as biostimulant products, have not proven to be consis-
tently eective through research. e management challenge is to provide an urban environment that functions like the natural environment.
Key Words. Biostimulants; Bulk Density; Cation Exchange Capacity; Mechanical Resistance; pH; Soil Oxygen; Soil pH; Soil Salt; Soil
Water; Temperature.
In urban and suburban areas, the soil environ-
ment oen creates numerous challenges for tree
root growth. Urban soil has been dened as, “a
soil material having a non-agricultural, manmade
surface layer more than 50 cm thick that has been
produced by mixing, lling, or by contamina-
tion of land surface in urban and suburban areas”
(Bockheim 1974). Urban soils are oen highly
altered from the natural state, and human activ-
ity is the primary agent of the disturbance. ey
generally have high vertical and spatial variabil-
ity, modied and compacted soil structure, an
impermeable crust on the soil surface, restricted
aeration and water drainage, interrupted nutrient
cycling, altered soil organism activity, presence of
anthropogenic materials and other contaminants,
and altered temperatures (Craul 1985; Bullock and
Gregory 1991; Scheyer and Hipple 2005). ese
physical, chemical, and biological constraints of
urban soils pose limitations for the growth of tree
roots. Early experience gained working with the
urban soils in Washington, D.C., and other dicult
urban sites, led to the projection that about 80% of
urban tree problems can be attributed to a poor soil
environment, leading to synergistic eects of other
debilitating urban stress factors producing an over-
all decline in plant vigor (Patterson et al. 1980).
e resources provided by the soil environment for
root growth include adequate oxygen, water, and nutri-
ents, non-limiting penetration resistance, acceptable
pH range, and robust biological activity. Presence of
contaminants or pathogens can be harmful to roots.
Any one of these factors can limit root growth and
development, even if all others are in adequate supply.
Urban environments are quite dierent from the
natural environment to which trees are adapted,
yet they must provide the same resources for
growth if trees are to maintain a healthy balance
between the crown (supplier and user of energy,
user of nutrients and water) and root system (sup-
plier of water and nutrients, user of energy). e
management challenge is to provide an urban
environment that functions like the natural envi-
ronment, though its appearance may be dierent.
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 foundation for this review of
research summarizing our current understand-
ing of soil management techniques for urban trees.
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
Water, oxygen, mechanical resistance, temperature,
soil reaction, cation exchange capacity, contami-
nants, and biology are soil factors that directly aect
root growth. Water absorbed by plants transports
nutrients and cools leaves through evaporation.
Soil oxygen is essential for respiration in plant
roots. Mechanical resistance physically limits root
exploration of the soil (Letey 1985). Tempera-
ture controls certain metabolic processes in roots.
Water can be a dominant controlling factor, but
all are interconnected. e inuence of each factor
on root growth will rst be reviewed individually,
followed by a review of their interactions. Because
altering one factor does aect the quality of oth-
ers, management practices to improve root growth
will consider the eects on all factors together.
e amount of water held in the soil is related to tex-
ture and structure. Sandy soils contain less than 10%
total water at eld capacity. Clay soil can contain as
much as 35% water, but more is unavailable to plant
roots. e dierence between the water content at
eld capacity and the water content at the perma-
nent wilting point is the amount of available water.
Urban soils oen have less structure and
greater bulk density than most undisturbed natu-
ral soils. e resulting reduction in pore space
reduces plant available water (Letey 1985; Craul
1992). e loss of natural soil structure is one of
the most important limitations to tree growth
in urban areas (Stewart and Scullion 1989).
Assessment of soil moisture status in the root zone
is necessary to determine the need for site improve-
ments, such as improved drainage, or supplemen-
tal irrigation. Soil moisture can be measured by a
variety of methods. e hand-feel method (Ross
and Hardy 1997) is simple and fast. If the soil
retains its shape aer compression between the
ngers, but is not sticky, the moisture content
is favorable. is method can be prone to error
since it requires experience and can be subjective.
Determining gravimetric soil water is the most
accurate, simple method not requiring special equip-
ment. Soil is weighed before and aer oven drying.
e most widely used and least-expensive water-
potential measuring device is the tensiometer.
e tensiometer establishes a quasi-equilibrium
condition with the soil water system through a
porous ceramic cup. Electrical resistance blocks
consist of electrodes encased in some type of
porous material that reaches a quasi-equilib-
rium state with the soil. ey are less sensitive in
wet soils. Time-domain reectometry and neu-
tron scatter methods can be very precise, but
require expensive, specialized equipment, and
their use in arboriculture is primarily limited to
research (World Meteorological Association 2008).
Effect on Root Growth
Fine root growth is slowed up to 90% by low soil
water content (Barnett 1986; Walmsley et al. 1991;
Kätterer et al. 1995; Torreano and Morris 1998; Mei-
er and Leuschner 2008; Olesinski et al. 2011). Root
growth decreases rapidly in most species when soil
moisture is reduced to 10%–14% on an oven-dry
basis (Newman 1966; Lyr and Homann 1967) or
-50 kPa soil moisture tension (Bevington and Castle
1985). is can result in a signicant decrease of
the root/shoot ratio (Blake et al. 1979; Meier and
Leuschner 2008), especially during periods of
active root growth (McMillin and Wagner 1995).
As soil begins to dry, the development of branch
roots is inhibited more than the growth of pri-
mary roots (Wright et al. 1992). When roots are
drought stressed, they mature rapidly toward the
tip, decreasing absorption, and reducing future
growth (Kaufmann 1968; Bilan 1974). As the eec-
tive absorbing surface is diminished, the roots do
not regain their full capacity for water uptake until
new root tips can be produced. When roots are re-
watered immediately aer cessation of elongation,
roots may not resume elongation for at least one
week. Resumption of root growth can take up to
ve weeks if water is withheld longer (Bilan 1974).
According to the optimal partitioning theory,
plants should allocate relatively more carbon and
nutrients to root growth than to aboveground
growth when plant growth is limited by water short-
age (Bloom et al. 1985). However, some research
reports have shown a decrease in root length den-
sity when water is withheld (Ruiz-Canales et al.
2006; Abrisqueta et al. 2008). is decrease may
be explained by increased ne-root turnover
Arboriculture & Urban Forestry 40(4): July 2014
©2014 International Society of Arboriculture
—higher ne-root mortality concurrent with
increased root growth (Meier and Leuschner 2008).
In wet soils, the growth of roots tends to be con-
ned towards the soil surface. In dry soils, root
growth can be shied downward due to water
depletion in surface soils (Torreano and Morris
1998). When urban soils limit rooting depth, the
ability of tree root systems to respond to periods of
drought and high soil moisture may be very limited.
Flooding of soil usually leads to greatly
reduced root growth, and death of many of the
fine absorbing roots. The small root systems
of flooded trees reflect the combined effect of
reduction in root initiation and reduced growth
of existing roots, as well as decay of the origi-
nal root system. Because root growth is usually
decreased more than shoot growth by high soil
moisture, drought tolerance of flooded trees is
reduced after the flood waters recede. This change
reflects the inability of the small root systems to
supply enough water to meet the transpirational
requirements of the crown (Kozlowski 1985).
Responses of tree species to ooding vary widely
(White 1973; Bell and Johnson 1974; Whitlow
and Harris 1979). Tolerance can vary from only a
few hours to many days or weeks, depending on
the species, the organs directly aected, the stage
of development, and external conditions, such as
temperature. Roots are oen more susceptible to
oxygen deciency than shoots (Vartapetian and
Jackson 1997). Broadleaved trees as a group are
much more ood-tolerant than conifers. Older
trees usually tolerate ooding better than seed-
lings or saplings. Flooding during the dormant
season is much less harmful than ooding during
the growing season (Heinicke 1932). e greater
injury and growth reduction by ooding dur-
ing the growing season are associated with high
oxygen requirements of growing roots with high
respiration rates (Yelenosky 1963; Koslowski 1985).
Respiration by plant roots and other soil organisms
consumes oxygen and produces carbon dioxide. In
unsaturated soils, the soil air connects directly with
the aboveground atmosphere, but diusion of gasses
through the soil is slowed by water and soil particles.
Oxygen concentrations decline and carbon dioxide
concentrations increase with depth due to the oxy-
gen demands of the roots, the soil fauna, fungi, and
microbes. Oxygen deciency in roots will be more
likely to occur in warm soils than in cooler soils when
reduced respiration is more balanced with diusion
rates (Yelenosky 1963; Armstrong and Drew 2002).
For most species, approximately 10%–12% oxy-
gen in the soil atmosphere is needed for adequate
root growth (Stolzy and Letey 1964; Tackett and
Pearson 1964; Stolzy 1974; Valoras et al. 1964;
Gilman et al. 1987; Mukhtar et al. 1996), and
growth may cease at 5% oxygen (Stolzy 1974). Soil
carbon dioxide concentration can be damaging to
roots when it reaches 0.6% (Gaertig et al. 2002).
For most species, root growth is reduced or
stopped when the oxygen diusion rate (ODR) drops
below 0.2 µg/cm2/min. Most plants are severely
stressed between 0.2 and 0.4 µg/cm2/min. Above
0.4 µg/cm2/min, plants grow normally (Stolzey and
Letey 1964; Valoras et al. 1964; Lunt et al. 1973; Stolzy
1974; Erickson 1982; Blackwell and Wells 1983).
Redox potential can also be used as a mea-
sure of the oxygen status of the soil. Soil redox
potentials of 400–700 mV are generally consid-
ered well aerated. Root growth of most species is
stopped at a soil redox potential of 350 mV, though
roots of more water-tolerant species (e.g., Taxo-
dium distichum) are able to grow until the redox
potential reaches 200 mV (Carter and Rouge
1986; Pezeshki 1991; Stepniewski et al. 1991).
Soil aeration is impacted by urban landscape
features. In undisturbed, well-drained soil, oxygen
and carbon dioxide contents can be near atmo-
spheric levels close to the soil surface, decreasing
most rapidly in the rst 30 cm (Yelenosky 1963;
Brady and Weil 1996). When not paved, vegetated
and nonvegetated urban sites can be as well-aerated
as forest stands (Gaertig et al. 2002). However, if
topsoils are sealed or compacted, gas exchange
between the soil and the atmosphere is inter-
rupted (Gaertig et al. 2002). Oxygen content was
reduced to 14.5% and carbon dioxide content was
increased to 6% at 15 cm depth under an unpaved
parking lot. e same levels were not reached until
90 cm depth in the adjacent undisturbed forest
soil (Yelenosky 1963). In another study, there
were minimal dierences in soil oxygen between
pavement and turf in the top 45 cm (Hodge and
Boswell 1993). However, soil oxygen measure-
ments were made only 75 cm from the edge of the
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
pavement and oxygen could have diused laterally
from the nearby exposed soil. While it is commonly
accepted that stone pavement with gaps allows
for aeration of the soil, there was no dierence in
gas diusivity between completely sealed surfaces
(asphalt) and areas with agstone or cobblestone
with gaps in between (Weltecke and Gaertig 2012).
A water table less than 50 cm deep can reduce
oxygen below levels considered sucient to
sustain vigorous root growth to within 5 cm
of the soil surface (Callebaut et al. 1982). Ele-
vated berm soils can be more aerated than sur-
rounding soils at grade (Handel et al. 1997).
Assessment of soil oxygen can be helpful in choosing
the appropriate plant for the site, or under-
standing whether site modications, such as
improved drainage, may be necessary. However,
measuring oxygen levels in the soil can be chal-
lenging: equipment can be expensive and suited
primarily for research applications. Measurement
at any moment in time may not reect sustained
conditions, and not all measurements provide
the same information related to root growth.
Oxygen content, expressed as a percentage,
is the amount of oxygen in the soil gases (the
aboveground atmosphere contains 21% oxygen).
ODR measures the rate at which oxygen can
move through the soil to replace oxygen that is
used by the root. ODR can be a better indicator
of soil aeration (i.e., oxygen availability to roots)
than oxygen content because it is possible to have
a high soil oxygen concentration, but very low
diusion rate (MacDonald et al. 1993). e oxy-
gen concentration in the soil atmosphere may
not vary substantially at monitoring sites over
time, or in response to changes in soil moisture.
In contrast, ODR is strongly inuenced by soil
moisture and bulk density. Oxygen concentra-
tion was not consistently low enough to severely
inhibit root function at sites where trees were
declining. At the same time, ODR values within
the root zones of declining trees were invariably
in a range considered injurious to roots, while
ODR values around vigorous trees were favor-
ably high (Stolzy 1974; MacDonald et al. 1993).
Rusting pattern on steel rods can be used to
assess soil anaerobism over an extended period
(Carnell and Anderson 1986; Hodge and Knott
1993; Hodge et al. 1993) and has been related to
ne-root development of trees (Watson 2006a).
Fine-root density in soils, where rust was present
on over 60% of the steel rods, was generally three
times greater than in soils with less than 25% rust-
ing. is method can provide an indication of soil
aeration over a period of months and up to a depth
of 60 cm without the use of expensive equipment.
Effect on Root Growth
Growing root tips have high oxygen requirements,
and ne-root density is oen reduced when oxy-
gen availability is low (Koslowski 1985; Gaertig et
al. 2002; Weltecke and Gaertig 2012). In older parts
of the root, the oxygen demand can be approxi-
mately half that of the tip (Armstrong and Drew
2002). Root dysfunction as a result of inadequate
oxygenation can modify plant growth and devel-
opment through interference in water relations,
mineral nutrition, and hormone balance (Kramer
and Kozlowski 1979; Armstrong and Drew 2002).
Species vary in their root system tolerance to
low soil aeration. For example, loblolly pine (Pinus
taeda) grew better at low aeration conditions (either
high compaction or high water content) than pon-
derosa pine (Pinus ponderosa var. scopulorum) or
shortleaf pine (Pinus echinata) (Siegel-Issem et al.
2005). Lists of species’ tolerance to ooding, 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 (Taxodium), and birch
(Betula), oxygen can move down to the roots
internally through intercellular spaces. is oxygen-
transporting tissue within roots is called aeren-
chyma. It is not uncommon in the subapical parts of
wetland plant roots for as much as 60% of the root
volume to be gas space for diusion of oxygen from
the shoot (Drew 1997; Armstrong and Drew 2002).
Enough oxygen can be transported so that some is
released into the soil immediately surrounding the
roots (Hook et al. 1971; Armstrong and Read 1972).
Mechanical Resistance
Bulk density is a measure of dry mass per unit
volume and used to describe limits to root growth in
compacted soil. Soil strength, expressed as penetra-
Arboriculture & Urban Forestry 40(4): July 2014
©2014 International Society of Arboriculture
tion resistance, is a broader indicator of constraints
on root growth that accounts for soil moisture, as
well as bulk density (Baver et al. 1972; Gerard et
al. 1982; Ehlers et al. 1983; Taylor and Brar 1991).
Parent material is the deepest and densest layer
in the soil prole. As soils develop, formation of
structure in the overlying horizons reduces bulk
density. Clay deposition in the B horizon tends to
ll existing pore spaces, making it denser as clay
content increases (Foth 1990). Roots compact the
soil nearby as they increase in size, and they also
transmit the weight of the tree and forces generated
by the wind onto the soil (Greacen and Sands 1980).
In urban and suburban settings, soil formation
has been interrupted by removal, grading, mix-
ing, or other disturbances. us, urban soils can
have high bulk densities (Yang et al. 2005; Feng
et al. 2008). Urban soil mean bulk density values
of 1.6 g cm-3 have been reported, with individual
values as high as 2.63 g cm-3 (Patterson 1977;
Short et al. 1986; Jim 1998a; Jim 1998b). ese
levels of compaction restrict root growth for many
woody species, especially in ner-textured soils.
Compaction occurs very quickly. On ne- to
medium-textured soils, half of the increase in soil
bulk density and soil strength occurred in the rst
two passes of trac. Coarse soils were slightly more
resistant 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 of 0.3–0.8 m (Randrup 1997). In a sur-
vey of areas to be landscaped near new residential
and commercial construction, mean soil bulk den-
sity was found to be 1.56 g cm-3, which represents a
0.5 g cm-3 increase over adjacent undisturbed areas
(Alberty et al. 1984). Bulk densities in fenced (undis-
turbed) areas ranged from 1.05 to 1.42 g cm-3, while
in unfenced areas, bulk densities were 1.56 to 1.90
g cm-3; oen exceeding the 1.60 g cm-3 critical bulk
density for the loam soils on the study site (Lichter
and Lindsey 1994). In another study, the absence
of dierences between protected and unprotected
areas was attributed to trac occurring on areas
not meant for trac (Randrup and Dralle 1997).
To determine bulk density, a soil core of known
volume is oven dried at 105°C and weighed. Care
is exercised in the collection of cores so that the
natural structure of the soil is preserved. Any
change in structure is likely to alter pore space and
bulk density. Excavation methods are better for a
gravelly soil. A quantity of soil is excavated, dried,
and weighed, along with determining the volume
of the excavation by lling the hole with sand of
which the volume per unit mass is known, or water
in a rubber liner (Grossman and Reinsch 2002).
Penetrometers are used to measure soil strength.
Type of equipment used and soil moisture con-
tent will aect measurement. Penetrometers with
30-degree tips and diameter sizes of 12.8 and 20.3
mm are standard. e smaller cone size is for use
in harder (more resistant) soils (American Society
of Engineers 1992; Lowery and Morrison 2002).
Soil strength increases with bulk density and
decreases with soil water content (Taylor and
Burnett 1964; Eavis 1972; Blouin et al. 2008.) Fine-
textured soils are the most limiting (Gerard et al.
1982), but penetration resistance can be aected
more by water content than by texture. Penetra-
tion resistance in a dry soil (−1500 kPa) exhibited
a maximum at clay content of 35%, while in a
moist soil (−10 kPa) penetration resistance was
minimally aected by texture (Vaz et al. 2011).
Effect on Root Growth
e bulk density that limits root growth varies
with soil texture (as reviewed in Daddow and
Warrington 1983) and soil moisture (Day et al.
2000). Greater development of structure in ne-
textured soils accounts for their lower bulk density
as compared to coarse-textured soils. A bulk density
of 1.60 g cm-3 would be limiting in a clay loam, but
not in a sandy loam (Foth 1990). Summary tables
(Jones 1983; Daddow and Warrington 1983; NRCS
Soil Quality Institute 2000 (Table 1) are consistent
with reports of root restriction in individual tree
species (Minore et al. 1969; Chiapperini and Don-
nelly 1978; Webster 1978; Zisa et al. 1980; Heilman
1981; Tworoski et al. 1983; Alberty et al. 1984;
Pan and Bassuk 1985; Simmons and Pope 1985;
Reisinger et al. 1988; Watson and Kelsey 2006).
Reconstruction of soil proles from six forest sites
in greenhouse tests showed root and shoot growth
in soil from lower horizons (10–30 cm) averaged
only 41% of that in topsoil, a signicantly greater
restriction of growth than that achieved through
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
compaction of up to 0.17 g cm-3 greater than the
undisturbed eld sites (25%). Topsoil displacement
and prole 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). e 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 eld
observations (Forristall and Gessel 1955) have
shown that the capacity for root growth in com-
pacted soil oen varies among plant species. For
example, root growth of Siberian larch (Larix
sibirica), English oak (Quercus robur), western red
cedar (uja plicata), and Formosa acacia (Acacia
confusa) were little aected by soil bulk density as
high as 1.89 g cm-3, while Norway spruce (Picea
abies), Douglas r (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 can make a dierence (Minore et al. 1969).
Soil compaction can aect root distribution.
Root penetration depth can be restricted by soil
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 oen 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 conned
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).
e 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 aected
when surface soils dry during periods of drought.
There is a tendency to form more lateral roots
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
Table 1. General relationship of soil bulk density to root growth based on soil texture (adapted from NRCS Soil Quality
Institute 2000).
Soil texture Ideal bulk densities Bulk densities that may aect Bulk densities that restrict
(g cm-3) root growth (g cm-3) root growth (g cm-3)
Sands, loamy sands <1.60 1.69 >1.80
Sandy loams, loams <1.40 1.63 >1.80
Sandy clay loams, clay loams <1.40 1.60 >1.75
Silts, silt loams <1.30 1.60 >1.75
Silt loams, silty clay loams <1.10 1.55 >1.65
Sandy clays, silty clays, some <1.10 1.49 >1.58
clay loams (35%–45% clay)
Clays (>45% clay) <1.10 1.39 >1.47
Arboriculture & Urban Forestry 40(4): July 2014
©2014 International Society of Arboriculture
root cells are altered (Pearson 1965). Differences
among species in their ability to penetrate
strong soil layers appear to be due to differ-
ences in root diameter (Clark et al. 2003).
Urban soils can be warmer due to surround-
ing pavements and lack of vegetation cover.
Unvegetated playground soils in Central Park
(New York City, New York, U.S.) were 3.13°C
warmer than an adjacent wooded area (Mount
et al. 1999). Maximum summer soil tempera-
tures under pavement in the northern United
States were 32°C–34°C, and up to 10°C warm-
er than nearby unpaved areas (Halverson and
Heisler 1981; Graves and Dana 1987). In Texas,
U.S., summer soil temperatures under pave-
ment exceeded 48°C, 10°C warmer than unpaved
areas, and remained above 35°C for all but a short
time at night. Temperatures are highest under
dark pavements (Arnold and McDonald 2009).
Effect on Root Growth
Biological activity in the soil, and therefore root
growth, varies with temperature (Lloyd and Taylor
1994). Root growth occurs over a wide range of
temperatures, but is much slower at low and high
temperatures. Reported minimum temperatures
for root growth range from 2°C to 11°C (Lyr and
Homann 1967; Soleld and Pedersen 2006).
Sugar maple (Acer saccharum) roots began to
grow in spring as soils warmed to 5°C, but initial
root growth may be quite slow at such low tem-
peratures. Active root growth has been reported
to begin when soil temperatures reach 10°C–15°C
(Nambiar et al. 1979; Carlson 1986; Harris et al.
1995; Soleld and Pedersen 2006). Optimum tem-
peratures for root growth have been reported at
18°C–32°C (Lyr and Homan 1967; Larson 1970;
Nambiar et al. 1979; Struve and Moser 1985; Head-
ley and Bassuk 1991; Harris et al. 1995; Soleld
and Pedersen 2006; Richardson-Calfee et al. 2007).
e high temperature at which root injury begins
to occur is around 34°C (Graves and Wilkins 1991;
Graves 1994; Graves 1998; Wright et al. 2007). Roots
of most woody species are killed at 40°C–50°C
(Wong et al. 1971). Maximum temperatures for
active growth have been reported at 25°C–38°C,
depending on the species (Proebsting 1943; Wong et
al. 1971; Gur et al. 1972; Graves et al. 1989a; Graves
et al. 1989b; Graves 1991; Martin and Ingram 1991;
Graves and Aiello 1997; Arnold and McDonald
2009). Direct heat injury of roots can occur when
the soil remains above 32°C for extended periods of
time (Graves 1998), and the longer the duration of
high temperatures, the more root growth is reduced
(Graves et al. 1989b; Graves and Wilkins 1991). Hon-
eylocust (Gleditsia triacanthos) is the only temperate
tree species reported to sustain growth at root-zone
temperatures above 32°C (Graves et al. 1991).
e root tissues of most woody plants can be
killed at soil temperatures of -5°C to -20°C (Havis
1976; Studer et al. 1978; Santamour 1979; Pellett
1981; Lindstrom 1986; Bigras and Dumais 2005),
although roots of black spruce (Picea mariana) were
not aected by temperatures as low as -30°C (Bigras
and Margolis 1996). Young roots are less freeze-tol-
erant than mature roots (Bigras and Dumais 2005).
Soil pH
Plant performance is strongly aected by nutrient
availability, which in turn is inuenced by soil pH
(acidity or alkalinity). Most nutrients are available
at optimal levels in slightly acid to neutral soils
(pH between 5.5 and 7.2), and trees generally
grow best in this pH range. Soil pH can be mea-
sured with electronic meters or colorimetric
tests based on color of solutions or strips.
Urban soils tend to have higher soil pH than
their natural counterparts. In Berlin, Germany,
a pH of 8 was observed streetside, compared to a
pH of less than 4 within a forest a short distance
from the street (Chinnow 1975). Over half of soils
sampled in Hong Kong, China, were rated strongly
(pH 8.5–9) to very strongly (pH 9–9.5) alkaline,
while surrounding soils were acidic at pH 4–5 (Jim
1998b). Streetside soils of Syracuse, New York,
U.S., had a pH range of 6.6 to 9.0 with an average
of about 8.0 (Craul and Klein 1980). Urban soils
of Philadelphia, Pennsylvania, U.S., ranged from
3.7 to 9.0 with a mean of 7.6 (Bockheim 1974).
Elevated pH values have been attributed to the
application of calcium or sodium chloride as road and
sidewalk deicing compounds in northern latitudes,
irrigation with calcium-enriched water (Bockheim
1974), and the surface weathering of concrete and
limestone buildings and sidewalks (Bockheim 1974;
Messenger 1986; Okamoto and Maenaka 2006).
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
Effect on Root Growth
The effects of pH on root growth are primarily
related to nutrient availability. Some nutri-
ents, such as iron and manganese, become
less available in alkaline soils (pH above 7.2)
because of chemical changes caused by the
alkalinity. Other nutrients, such as phospho-
rous, become less available in highly acid
soils (pH less than 5.5). When the pH is 4.5
or less, aluminum toxicity can restrict root
growth (Foth 1990; Jim 1993b). In most plant
systems, aluminum toxicity has a direct ef-
fect on root growth by inhibiting cell division
in the root apical meristem (Kochian 1995).
A nutrient deciency caused by sub-optimum soil
pH could actually stimulate root growth in order to
explore larger volumes of soil to acquire additional
nutrients and alleviate deciency symptoms (Inges-
tad and Lund 1979; Ericssson and Ingestad 1988).
Cation Exchange Capacity
Cation exchange capacity (CEC) is a measure
of the nutrient-holding (adsorption) power of
the soil. Once adsorbed, cationic minerals are
not easily lost when the soil is leached by water
and therefore provide a nutrient reserve for
plant roots. CEC is highly dependent upon soil
texture and organic matter content. In general,
the more clay and organic matter in the soil, the
higher the CEC. Small clay soil particles have a
large, negatively charged surface area for their
size and hold relatively large amounts of ions.
Organic matter particles have even more nega-
tive surface charges on the surface than clay for
nutrient exchange. Sandy soils have low CEC
due to their low organic matter and clay content.
CEC is usually greatest at the surface
where organic matter accumulates. Increas-
ing clay with depth can act to counterbalance
the decrease in organic matter and reduction
of CEC. The CEC of most soils increases with
pH (Craul 1992; Brady and Weil 1996).
CEC is determined by laboratory testing,
and methods vary with the soil type. Reported
urban soil CEC values have been 5–12 cmol/
kg (Short et al. 1986; Jim 1998b). Normal values
vary, from 5 cmol/kg to 25 cmol/kg, depending
on texture, organic matter content, and pH
(Foth 1990; Landon 1991; Brady and Weil 1996).
Salt in soil inhibits plant water uptake by lowering
the osmotic pressure of soil water (Prior and
Berthouex 1967). is reduces the water uptake
of trees and symptoms of decline mimic those
of drought (Herrick 1988). Once salt enters the
roots, it upsets the osmotic balance within root
cells (Janz and Polle 2012) and is toxic to the
endomycorrhizae (Guttay 1976). e increased
sodium on the cation exchange sites also breaks
down soil structure (Holmes 1961; Hutchinson
and Olson 1967), decreasing the permeability and
water-holding capacity of the soil. All of these
factors may contribute to a decline in tree health.
Damage from salt-contaminated soil occurs fre-
quently in urban areas where large amounts of salt
are used for 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 of deicing salts is highest. Sodium levels were
5.4 times higher and chloride was 15 times higher
in the center of newly installed, narrow, raised
medians along an urban highway aer one winter,
compared to the center of wide medians along the
same roadway. e high levels were attributed to
proximity to high speed trac and its associated
spray and splash (Hootman et al. 1994). Elevated
levels of sodium have been reported in the soil
up to 30 m from the highway and elevated levels
of soil chlorine to a distance of 61 m (Langille
1976; Hofstra et al. 1979; Simini and Leone 1986).
In contrast, rural highway studies show salt levels
decline rapidly with distance to pavement (Her-
rick 1988; Cunningham et al. 2008). e release
of salts from rapid-release forms of fertilizer can
also elevate soil salt levels (Jacobs et al. 2004).
Reclaimed wastewater (RWW) and ground-
water used to irrigate urban plantings in arid
climates can be highly saline. Sodium and chlo-
ride are the major chemical constituents in RWW
that are potentially detrimental to plants (State
of California 1978; Schaan et al. 2003). Com-
pared with sites irrigated with surface water,
sites irrigated with RWW exhibited up to 187%
higher electrical conductivity (EC) and 481%
higher sodium adsorption ratio (SAR) (Qian
and Mecham 2005; Schuch et al. 2012). Soil
types play a role on soil salinization as much as
Arboriculture & Urban Forestry 40(4): July 2014
©2014 International Society of Arboriculture
water quality. The highest salinity was found in
clay and the lowest in sand (Miyamoto 2012).
e best method for assessing soil salinity is to
measure the electrical conductivity of soil solution
extracts. Conductivity of 2 dS/m (deci-Siemens/
meter) is considered harmful to salt-sensitive plants
(Foth 1990; Jacobs and Timmer 2005). All but
very salt-tolerant plants will be aected at 4 dS/m.
Czerniawska-Kusza et al. (2004) found necrosis and
chlorosis in leaves at levels of 132 µg Na+/g of soil.
Soil chloride ion concentrations of up to 200 µg/g
are not considered harmful to plants (Jim 1998a).
Deicing salt can cause the death of surface roots
in roadside trees (Wester and Hohen 1968; Krap-
fenbauer et al. 1974; Guttay 1976; Jacobs et al. 2004;
Madji and Persson 1989), though the risk of root
damage associated with salt concentrations levels
appears to be dependent on species, age of root
system, and soil moisture availability (Jacobs and
Timmer 2005). Damage may result from osmotic
and/or specic ion eects (Dirr 1975). Root rot
caused by Phytophthora sp. can increase with soil
salinity as well (Blaker and MacDonald 1985; Blaker
and MacDonald 1986). Indirect damage occurs
when sodium displaces other ions from soil cation
exchange sites reducing their availability, and breaks
down soil structure leading to soil compaction
(Herrick 1988; Dobson 1991; Hootman et al. 1994).
Trees growing in soils with high salt levels tended
to have more twig dieback and less twig growth
than those growing in soils with lower salt levels
(Berrang et al. 1985). Sodium chloride and other
salts accumulating in the root zone may instigate and
exacerbate street tree decline (Hootman et al. 1994).
Heavy metals is a term generally used to describe a
group of metallic elements that can be toxic to plants
and animals. Some, such as copper, molybdenum,
and zinc are essential trace elements, but exces-
sive levels can be toxic (Prasad 2004). Heavy metal
contamination tends to be greater toward the city
center and in areas of commercial and industrial
land use (Carey et al. 1980; Blume 1989; Wang and
Zhang 2004). City center and wasteland soils gen-
erally had enhanced heavy metal concentrations
to at least 30 cm depth (Linde et al. 2001). Soils
on the National Mall in Washington, D.C., U.S.,
had elevated levels of lead, zinc, nickel, copper,
and cadmium (Short et al. 1986). Concentrations
of heavy metals in roadside soils decrease with
distance from trac and depth in the soil prole.
e contamination has been related to the com-
position of gasoline, motor oil, and car tires, and
to roadside deposition of the residues of these
materials (Lagerwerf and Specht 1970; Madji and
Persson 1989). Long-term sewage sludge appli-
cation may result in the accumulation of Zn, Cu,
and Ni in the soil and plant (Bozkurt et al. 2010).
Soil heavy-metal data has been published for
several cities (Lagerwerf and Specht 1970; Carey
1980; Blume 1989; Jim 1998a). Levels of many ele-
ments were higher on urban sites than suburban
and rural sites up to 10 times or more. No plant
damage was reported with these higher levels.
Soil Biology
Soil organisms are an important component of a
healthy soil that promotes root growth. e ratio
of fungal to bacterial biomass is oen near 1:1 in
grass and agricultural soil ecosystems. With reduced
disturbance, fungi become more plentiful, and
the ratio of fungi to bacteria increases over time.
Forests tend to have fungal-dominated microora.
e ratio of fungal to bacterial biomass may be 5:1
to 10:1 in deciduous forests and 100:1 to 1000:1 in
coniferous forests (Soil and Water Conservation
Society 2000). Assessing abundance of soil bacteria
and fungi and mycorrhizal colonization of roots
requires extensive skill and laboratory equipment.
e zone of soil adjacent to plant roots with a
high population of microorganisms is the rhizo-
sphere. Bacteria feed on sloughed-o plant cells
and the proteins and sugars released by roots. e
protozoa and nematodes that “graze” on bacteria
are also concentrated near roots. us, much
of the nutrient cycling and disease suppression
needed by plants occurs immediately adjacent to
roots (Soil and Water Conservation Society 2000).
Rhizosphere pH can be up to two units dierent than
the rest of the soil (Marschner and Römbeld 1996).
Mycorrhizae are symbiotic relationships that
form between common soil fungi and plants. e
benets of mycorrhizal associations of tree roots are
well established (Smith and Read 1997). e fungi
colonize the root system of a host plant, providing
increased nutrient absorption capabilities, while the
plant provides the fungus with carbohydrates from
photosynthesis. Mycorrhizae oer the host plant
increased protection against certain pathogens.
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
Urban planting sites are oen considered to be of
poor soil quality, but mycorrhizal inoculum (spores)
was more abundant in urban soil than in forest soil
in one study (Wiseman and Wells 2005). Some
mycorrhizal fungi colonizing littleleaf linden (Tilia
cordata) roots were common to both street trees
and forest trees. Others were not. Colonization lev-
els were high on both street and forest trees (Nielsen
and Rasmussen 1999; Timonen and Kauppinen
2008). Native desert trees had greater colonization
by arbuscular mycorrhizal fungi (AMF) than resi-
dential landscape trees, and AMF species composi-
tion diered at the two site types (Stabler et al. 2001).
Interdependence of Soil Factors
As a result of the interdependence of soil proper-
ties, the status of one soil factor can have an eect
on all others; an understanding of their interre-
lationships is important for proper management.
Water and Air
Increasing soil moisture reduces soil aeration
when water replaces the air normally held in the
pores of the soil. Water slows the diusion of
oxygen to 1/10,000 of that in air, and it reduces
its concentration to about 1/32 of that in air. e
net result is an eective resistance to ow that
is around 320,000 times greater in saturated soil
than that of air (Armstrong and Drew 2002).
Water and Compaction
Compaction can decrease the number of days of
available water in clay-loam soil. However, compac-
tion can increase the number of days that water is
available in a sandy loam soil (Gomez et al. 2002).
Tree roots can grow successfully in signicantly
compacted soils provided soil moisture is readily
available (Zisa et al. 1980; Pittenger and Stamen
1990; Bulmer and Simpson 2005; Siegel-Issem et
al. 2005). Resistance to penetration in a clay loam
soil was found to decrease from 3.5 MPa (limiting)
to 2.1 MPa (non-limiting) when volumetric soil
moisture increased from approximately 27%
to 40% (Day et al. 1995). Roots of spotted gum
(Corymbia maculata) and red-owering gum (C.
cifolia) were able to penetrate soil compacted to a
bulk density of 1.6 g cm-3 at 7% soil moisture, but
when moisture was increased to 10% roots could
penetrate soils of 1.8 g cm-3 (Smith et al. 2001).
Species can vary in their ability to capitalize
on reduced penetration resistance of wet soils.
Silver maple (Acer saccharinum) roots can
grow in moderately compacted soil when high
soil water content decreases soil strength, even
though aeration is low, whereas dogwood (Cornus
orida) roots are unable to grow under the
same low aeration conditions (Day et al. 2000).
Air and Compaction
One of the main eects of high bulk density is a
restricted oxygen supply (Yelenosky 1963; Yelenosky
1964; Rickman et al. 1966). Oxygen is less restricted
when the soil is dry and less pore space is lled
with water (Day 1995). Oxygen diusion rate was
lowest in soils with high bulk density (MacDonald
et al. 1993). Compaction from a bulk density of
1.04 g cm-3 to 1.54 g cm-3 reduced gas diusion by
38% when soil was dry. In wet soil, however, com-
paction reduced diusion by 82% (Currie 1984).
Plant response to oxygen level has been shown to
interact with mechanical impedance (Gill and Miller
1956). In general, soil compaction can have a strong
inhibitory eect on root penetration when the oxy-
gen level is high, but no signicant eect at a low
oxygen level because root growth is already reduced
by lack of aeration (Tackett and Pearson 1964;
Hopkins and Patrick 1969; da Silva and Kay 1997).
Anaerobic conditions are likely to limit root
growth in compacted ne-textured and poorly
drained soils, whereas mechanical impedance is
more 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 increase
incidence of soil-borne diseases. Root diseases are
favored when soils are water-saturated (Hansen et al.
1979). Saturated soil and low oxygen supply causes
a reduction in root initiation, growth of existing
roots, and an increase in decay of roots, largely as a
result of invasion of Phytophthora sp. Fungi, which
tolerate low soil aeration (Stolzy et al. 1965; Sena
Gomes and Kozlowski 1980; Blaker and McDon-
ald 1981; Benson et al. 1982; Stolzy and Sojka 1984;
Benson 1986; Duniway and Gordon 1986; Gray
and Pope 1986; Ownley and Benson 1991). Armil-
laria root disease, also known as shoestring root
rot, causes most damage on trees that are stressed
Arboriculture & Urban Forestry 40(4): July 2014
©2014 International Society of Arboriculture
by one or more abiotic or biotic factors. ese may
include drought, soil compaction, and other soil
problems common on urban sites (Worall 2004).
Root Development and Nutrient Uptake
When soil factors limit root development there can
be a direct impact on nutrient uptake. Nutrient
deciencies can occur when there is insu-
cient uptake by the roots and use by the crown. If
improved soil conditions allow the root system
to expand and explore a larger soil volume and
supply of nutrients, the tree may overcome the
deciency and symptoms may dissipate (Inges-
tad and Lund 1979; Ericssson and Ingestad 1988).
e eectiveness of management practices to en-
hance soil as a medium for root growth can aect
all soil factors and is inuenced by soil physical
properties. Soils classied as having poor physi-
cal conditions are those that require very careful
management to maintain conditions favorable for
root growth. Soils with good physical conditions
require less careful management (Letey 1985).
Prevention of soil compaction is preferred. Treat-
ments to alleviate compaction can be expensive,
difficult to apply, sometimes ineffective, and may
injure roots (Howard et al. 1981). When only
acted upon by natural forces, 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 Elioff 1998;
Blouin et al. 2005). Fine-textured soils are slower
to recover than coarse-textured soils. Surface
soils will recover most rapidly (Page-Dumroese
et al. 2006). When compaction severely reduced
soil aeration and root growth after a logging
operation, after 14 years, recovery was limited
to the top 4 cm of soil. After 18 years, recovery
reached a depth of 18 cm. Only after 24 years was
recovery detected throughout the rooting zone
(von Wilpert and Schaffer 2006). Factors, such
as a fluctuating water table, freeze–thaw 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 prevent
soil compaction during construction. In contrast,
plywood did not protect the underlying soil from
compaction (Donnelly and Shane 1986; Lichter and
Lindsey 1994). Fencing can be an eective way to pre-
vent soil compaction on a construction site (Lichter
and Lindsey 1994), but must be monitored and main-
tained to be eective (Randrup and Dralle 1997).
The use of organic amendments, such as bio-
solids, animal manure, or compost, generally
reduces the bulk density of compacted soils (Cog-
ger 2005; Garcia-Orene et al. 2005), although
this is not always the case (Patterson 1977).
The proposed mechanisms for this phenome-
non are that the high density substrate is simply
being diluted with a low-density material (the
amendment) or that the amendment physically
increases porosity (Clapp et al. 1986; Cogger
2005). Organic amendments can increase root
growth (Beeson and Keller 2001; Davis et al. 2006),
microbial activity (van Schoor et al. 2008) and
CEC. Composted organic matter is most effective,
as the humus component has the greatest CEC.
Incorporation of certain types of biochar can
increase CEC (Chan et al. 2007; Laird et al.
2010), but research on this topic is still limited.
Inorganic soil amendments have been used to
improve soil properties and resist compaction.
Sintered y-ash and expanded slate amendments
resulted in lower bulk densities and increased
pore space aer being incorporated into the soil
(Patterson 1977). Amendment with mixtures
of gravel, expanded clay, and lava rock improved
the soil aeration and soil moisture in clay
loam and silty loam soils (Braun and Fluckiger
1998). ese studies did not assess the eect
of soil changes on root systems performance.
Hydrophilic polymer gels (hydrogels) are some-
times added to the soil to increase available water.
Research has not shown that the use of hydrogels
can consistently increase root growth of trees
(Hummel and Johnson 1985; Keever et al.
1989; Tripepi et al. 1991; Walmsley et al. 1991;
Winkelmann and Kendle 1996; Huttermann et
al. 1999; Gilman 2004; Abbey and Rathier 2005).
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
Cultivation has been used with mixed results to
improve soil properties and promote tree root de-
velopment. Deep cultivation by ripping prior to
planting decreased bulk density and soil penetration
resistance (Rolf 1991; Rolf 1993; Moat and Boswell
1997; Lincoln et al. 2007) and increased both the
maximum root depth and total number of roots
compared with the untreated control for Italian
alder (Alnus cordata), Japanese larch (Larix kaemp-
feri), Austrian pine (Pinus nigra), and European
white birch (Betula pendula) (Sinnett et al. 2008). In
other cases, ripping had no eect on rooting depth
(Nieuwenhuis et al. 2003) or was reported to be ef-
fective for less than a year (Moat and Boswell 1997).
ere was no reduction in soil strength from
surface soil cultivation with an air excavation
tool aer one year on three of four sites. Com-
post incorporation with air cultivation did
result in a reduction of soil strength that per-
sisted for at least three years (Fite et al. 2011).
Cultural techniques that improve soil tilth, aera-
tion, and drainage reduce conditions favorable to
root disease (Juzwik et al. 1997), and also improve
host resistance by reducing or avoiding stress asso-
ciated with anaerobic conditions (Sutherland 1984).
e benets of organic mulch are well estab-
lished (Chalker-Scott 2007) and continue to be
reinforced. A review of published mulch research
studies showed surface mulch improved soil
physical properties and tree physiology, but
there was no improvement in chemical or bio-
logical properties (Scharenbroch 2009). Improve-
ment of soil properties will enhance root growth.
Over time, organic mulches can reduce soil
bulk density (Donnelly and Shane 1986; Cogger
et al. 2008) and increase organic matter content
(Watson et al. 1996; Johansson et al. 2006; Fite
et al. 2011). Mulch can increase water infiltra-
tion (Donnelly and Shane 1986; Cogger et al.
2008), reduce evaporation from the soil surface,
and increase moisture availability (Litzow and
Pellett 1983; Iles and Dosmann 1999; Arnold et
al. 2005; Cogger et al. 2008; Singer and Martin
2008; Fite et al. 2011). Mulch allowed a 50%
reduction in irrigation while still maintaining
acceptable growth and appearance (Montague
et al. 2007). Mulch also insulates soil from tem-
perature extremes (Montague et al. 1998; Iles
and Dosmann 1999; Singer and Martin 2008).
In December, soil under mulch was 6°C warmer
than exposed sod or bare soil (Shirazi and Vogel
2007). In temperate climates, the soil may warm
more slowly if new mulch is applied before the
soil warms in spring (Myers and Harrison 1988).
Organic surface mulch generally improves shoot
and root growth (Kraus 1998; Ferrini et al. 2008;
Arnold and McDonald 2009; Scharenbroch 2009).
Adding wood chip mulch to the surface of 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 chips
can result in a 30%–300% increase in ne-root
development in the top 15 cm of soil (Fraedrich
and Ham 1982; Green and Watson 1989; Himelick
and Watson 1990). Mulches may not be benecial
for some desert plants (Singer and Martin 2009).
When a mulch layer is maintained for several
years, a partially decomposed organic layer develops
that holds moisture and minimizes evaporation
from the soil beneath. A dense mat of roots can
form in the layer of mulch as well as in the soil
beneath it (Bechenbach and Gourley 1932; Watson
1988). e roots in the mulch will not be at any
greater risk of desiccation, since the well-established
mulch layer can hold more water than the soil itself,
without decreasing aeration to the soil beneath
it (Watson 1988; Himelick and Watson 1990).
Mulch reduces root competition for soil mois-
ture and nutrients from lawn grasses (Richardson
1953; Gilman 1989; Kraus 1998). In addition
to competition for water and nutrients, some
lawn grasses may be able to reduce the growth
of the trees through production of allelopathic
chemicals. Root growth of forsythia (Forsythia
intermedia) was suppressed by ryegrass and red
fescue leachates (Fales and Wakefield 1981). Fes-
cues have also been shown to stunt the growth
of southern magnolia (Magnolia grandiflora)
(Harris et al.1977), river redgum (Eucalyptus
camaldulensis) (Meskimen 1970), black wal-
nut (Juglans nigra) (Todhunter and Beineke
1979), and sweetgum (Liquidambar styraciflua)
(Walters and Gilmore 1976), but specific
effects on root systems were not reported.
Arboriculture & Urban Forestry 40(4): July 2014
©2014 International Society of Arboriculture
While mulching has many benets for soil
quality and root health, there are some potential
drawbacks. One concern about mulching is that it
creates conditions ideal for certain disease-causing
fungi. Fraedrich and Ham (1982) did not nd
any enhancement of the soil-borne pathogenic
fungi, Pythium spp. and Fusarium spp. during
their one-year study. Austrian pine saplings that
were mulched with fresh needles and shoot tips
from Sphaeropsis tip blight diseased trees devel-
oped more than twice the percentage of blighted
tips. ere was no Botryosphaeria canker or Armil-
laria root rot disease development when redbud
(Cercis canadensis) and red oak (Quercus rubra)
saplings, respectively, were mulched with wood
chips from diseased trees (Jacobs 2005). A decrease
in growth the rst year aer mulching, and an
increase in the second year has been attributed
to nitrogen immobilization in the rst year fol-
lowed by release the next (Hensley et al. 1988;
Truax and Gagnon 1993; Erhart and Hartl 2003).
A layer of mulch can intercept rain water before
it reaches the roots if the amount of water is small
or the mulch is thick (Gilman and Grabosky
2004; Arnold et al. 2005; Johansson et al. 2006).
Although 25 cm or more of coarse textured organic
mulch does not adversely aect soil oxygen or ne
root development (Watson and Kupkowski 1991;
Greenly and Rakow 1995), as little as 5 cm of ne-
textured organic mulch, or compost, can reduce
soil oxygen to less than 10% under wet conditions,
which can aect root function (Hanslin et al. 2005).
Compressed air soil injection treatments have
generally been ineective in relieving compac-
tion or increasing soil aeration (Yelenosky 1964;
Smiley et al. 1990; Hodge 1991; MacDonald et al.
1993; Rolf 1993). Soil texture may have a strong
inuence on the results. Reports of success in
reducing bulk density or increasing porosity were
in loamy soils (Rolf 1993; Lemaire et al. 1999).
A traditional approach to aeration of compacted
soil around trees is vertical mulching (i.e., drilling
a pattern of holes in the root zone soil). Research
on vertical mulching has provided mixed results.
Holes 5 cm diameter, 45 cm deep, with or without
sand-bark mix backll, provided no benet to
Chinese wingnut trees (Pterocarya stenoptera)
(Pittenger and Stamen 1990). Similar results were
seen in sugar maple (Acer saccharum) when the
holes were lled with perlite backll (Kalisz et al.
1994). However, roots of Monterey pine (Pinus
radiata) were able to utilize 10 mm diameter
vertical perforations to grow the same depth as
uncompacted controls, while root growth of trees
on compacted soil without perforations was sup-
pressed (Nambiar and Sands 1992; Sheri and Nam-
biar 1995). Largeleaf linden (Tilia platyphyllos) and
planetree (Platanus × Acerifolia) roots colonized the
majority of the depth of 10 cm diameter, 60 cm deep
holes lled with a mix of coarse sand, composted
organic materials, and fertilizer, and grew deeper
than in adjacent site soils (Watson et al. 1996).
Root growth in larger trenches lled with
compost-amended soil was increased relative
to undisturbed soil, but root growth was not
increased in the soils adjacent to the trenches
aer 2, 4, and 14 years. Soil aeration was not
measured and may not have been limiting in the
undisturbed and not compacted soil adjacent to
the trenches (Watson et al. 1996; Watson 2002).
pH Adjustment
Neutral to slightly acid pH is optimum for most
plants. Applications of lime are used to raise soil
pH. Aluminum sulfate and sulfur can help to lower
pH, although high rates of aluminum sulfate may
cause injury to some plants, particularly in broad-
leaf evergreens. e injury is believed to be caused
by excessive aluminum. Ammonium sulfate may
be as eective as aluminum sulfate, but neither is
as eective as granular sulfur (Messenger 1984).
Ammonium sulfate is sometimes used if nitrogen
application is needed along with pH reduction,
but applying enough to lower the pH would likely
apply a quick release form of nitrogen in excess of
best management practices (Smiley et al. 2007).
Enhancing root development may improve
uptake of available nutrients. Improving soil qual-
ity using methods such as cultivation, addition of
organic amendments, and mulching can enhance
root systems (see above). Basal drench applica-
tion of paclobutrazol, a tree growth regulator,
increased ne-root development and relieved
interveinal chlorosis commonly attributed to
iron deciency of pin oak (Quercus palustris)
on alkaline soils (Watson and Himelick 2004).
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
Salt Mitigation
Soil salt accumulation can be reduced through
design and engineering. Deicing salt accumula-
tion in road median planters can be prevented
by using wider planters with higher walls set
farther from high-speed roads. e raised planters
did not receive salt-laden runo, splash, plowed
snow, or direct application from salt spreaders
(Rich and Walton 1979; Hootman et al. 1994).
Leaching of sodium from deicing salt applica-
tion to roadways can be rapid in well-drained
soils with adequate natural precipitation (Prior
and Berthouex 1967; Cunningham et al. 2008).
High soil salts and wet soils tended to occur
together since poor drainage restricts the normal
leaching of soil salts (Berrang et al. 1985). In arid
regions, natural precipitation will not usually
leach salt from the soil (Schuch et al 2008). Under
low moisture conditions, moisture moves to the
surface and evaporates and salt moves upward
also to accumulate near the surface (Prior and
Berthouex 1967). Flushing soil with water to
remove salt and adding gypsum (CaSO4) and
fertilizers appear to be the best treatments for
salt contaminated urban soils (Dobson 1991).
Selection of resistant species and cultivars
can also minimize damage from salt in soils.
e majority of published studies evaluate
only shoot sensitivity, but growth of root
systems of crapemyrtle (Lagerstroemia) cultivars
varied in sensitivity to soil salt (Cabrera 2009).
Application of commercial products to en-
hance root growth has been increasing. Soil
application of mycorrhizal fungi have proven
beneficial to trees in soils lacking the appropri-
ate fungi, such as on strip-mining reclamation
sites and in sterilized nursery beds (Smith and
Read 1997). Native mycorrhizal fungi levels
can be low in arid regions (Dag et al. 2009).
However, growth rate of urban trees has gen-
erally been unaffected when treated with com-
mercial inoculants at planting (Morrison et
al. 1993; Martin and Stutz 1994; Roldan and
Albaladejo 1994; Querejeta et al. 1998; Gilman
2001; 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 of the natural mycorrhizal inoculum, as
well as suitability of the introduced inoculum to
the ecological conditions of the site, are important
factors in the success or failure of the intro-
duced inoculum (LeTacon et al. 1992). Endemic
fungi species may replace the inoculated species
over time (Garbaye and Churin 1996). Mycor-
rhizae can develop without introduced inocula-
tion in a favorable soil environment if natural
inoculum is present (Wiseman and Wells 2009).
e quality of the inoculum may be a factor in
success of inoculations. Mycorrhizal coloniza-
tion of roots rarely exceeded 5% aer treatment
with commercial inoculants, but was up to 74%
when treated with a fresh, lab-cultured inocu-
lant (Wiseman et al. 2009; Fini et al. 2011).
Paclobutrazol (PBZ), a growth regulator used
primarily to reduce shoot growth of trees, can
also increase root growth under certain cir-
cumstances. Mycorrhizal colonization of root
tips was unaected by PBZ treatment, show-
ing that mycorrhizae are not reduced by the
fungicidal properties of PBZ (Watson 2006b).
Application of organic products, such as
humates and plant extracts, have shown limited
benet to root growth of trees. Dose and species
responses vary widely (Laiche 1991; Kelting et
al. 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 Percival
2006; Broschat and Elliot 2009; Percival 2013).
Compost teas are liquids containing soluble
nutrients and species of bacteria, fungi, protozoa,
and nematodes extracted from compost. Com-
post teas are being used to enhance soil biology
and provide nutrients, sometimes as an alterna-
tive to fertilization, but research support for their
eectiveness is lacking (Scharenbroch et al. 2011).
Sucrose can increase root:shoot ratios by down-
regulating genes used for photosynthesis (Percival
and Fraser 2005). Applied as a root drench, it
enhanced root vigor when applied at up to 70 g/L
in some studies (Percival 2004; Percival and Fra-
ser 2005; Percival and Barnes 2007), but not others
(Martinez-Trinidad et al. 2009). In most of these
studies, the sugar was applied to the soil at least twice.
Healthy soils with favorable physical and
chemical characteristics will support active soil
Arboriculture & Urban Forestry 40(4): July 2014
©2014 International Society of Arboriculture
biology naturally. Improving soil conditions is
preferred over addition of compost teas, bios-
timulants, mycorrhizal fungi, and other means.
One of the most important soil functions is
to serve as a medium for root growth. Physical,
chemical, and biological soil characteristics all
have an eect on tree roots. A thorough under-
standing of how these soil characteristics aect
root growth is necessary to properly manage soils
for optimum root growth. Although most urban
soils are substantially altered from the natural
state, or even completely manufactured, urban
soils must still provide the necessary resources for
root growth. Highly disturbed soils require very
careful management to maintain conditions favor-
able for root growth. Management practices aimed
at preventing soil damage or restoring aspects of
the natural soil environment have the strongest
research to support their eectiveness in improv-
ing root growth in urban and suburban settings.
Acknowledgements. We would like to thank the International
Society of Arboriculture and the ISA Science and Research Com-
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Gary W. Watson (corresponding author)
e Morton Arboretum
4100 Illinois Route 53
Lisle, Illinois 60532, U.S.
Angela M. Hewitt
e Morton Arboretum
4100 Illinois Route 53
Lisle, Illinois 60532, U.S.
Melissa Custic
e Morton Arboretum
4100 Illinois Route 53
Lisle, Illinois 60532, U.S.
Marvin Lo
e Morton Arboretum
4100 Illinois Route 53
Lisle, Illinois 60532, U.S.
Arboriculture & Urban Forestry 40(4): July 2014
©2014 International Society of Arboriculture
Zusammenfassung. Die physischen, chemischen und biolo-
gischen Zusammensetzungen von städtischen Böden enthalten
o Begrenzungen für das Wachstum von Wurzeln. Für ein gutes
Management ist ein Verständnis für die Beziehungen der Bode-
nanteil sehr wichtig. Als ein Resultat dieser gegenseitigen Abhän-
gigkeiten kann der Status eines Bodenfaktors alle anderen beein-
ussen. Die Vermeidung von Bodenschäden ist sehr eektiv und
erstrebenswert. Pegemaßnahmen, wie Kultivierung und Mulchen
können eektiv die Bodeneigenschaen verbessern. Zusätze für
den Boden, wie Produkte zur Biostimulation, haben sich in der
Forschung als nicht zuverlässig eektiv erwiesen. Die Herausfor-
derung an das Management ist, eine urbane Umwelt zu liefern, die
nahezu wie eine natürliche Umgebung funktioniert.
Resumen. Las restricciones físicas, químicas y biológicas de los
suelos urbanos suelen plantear limitaciones para el crecimiento de
las raíces de los árboles. La comprensión de las interrelaciones de
las propiedades del suelo es importante para un adecuado mane-
jo. Como resultado de la interdependencia de las propiedades del
suelo, el estado de uno de los factores del suelo puede tener un
efecto sobre todos los demás. La prevención de daños en el suelo
es preferida; las prácticas culturales tales como el cultivo y el acol-
chado son apropiadas en la mejora de las propiedades del suelo. Los
aditivos del suelo, tales como productos bioestimulantes, no han
demostrado ser consistentemente ecaces a través de la investig-
ación. El desafío del manejo es proporcionar un entorno urbano
que funcione como el medio ambiente natural.
... In urban environments, tree root growth patterns can differ significantly from similar species in forest or agricultural environments (Day, Wiseman, Dickinson, & Harris, 2010a). When urban soil limits tree rooting, root response to adverse conditions is reduced (Watson, Hewitt, Custic, & Lo, 2014b); for example, compact soil, high pH, high temperature, low humidity, and the presence of contaminants can alter root growth, morphology, and physiology (Day, Wiseman, Dickinson, & Harris, 2010b). In this sense, compact soil quickly limits root development (Hirons & Thomas, 2018) and together with insufficient irrigation it prevents radical tree growth (Roman et al., 2015); lack of humidity is considered the leading cause of death of newly planted trees (Lell, 2006). ...
... En entornos urbanos, los patrones de crecimiento de las raíces de los árboles se pueden diferenciar considerablemente de las especies similares en ambientes forestales o agrícolas (Day, Wiseman, Dickinson, & Harris, 2010a). Cuando el suelo urbano limita el enraizamiento de los árboles, la respuesta de las raíces a condiciones desfavorables es reducida (Watson, Hewitt, Custic, & Lo, 2014b); por ejemplo, el suelo compacto, pH elevado, temperatura alta, humedad baja y la presencia de contaminantes pueden alterar el crecimiento, morfología y fisiología de las raíces (Day, Wiseman, Dickinson, & Harris, 2010b). En este sentido, el suelo compacto limita rápidamente el desarrollo de las raíces (Hirons & Thomas, 2018) y junto con el riego insuficiente impiden el crecimiento radical de los árboles (Roman et al., 2015); la falta de humedad se considera la causa principal de muerte de los árboles recientemente plantados (Lell, 2006). ...
... An extensive root system is important for plant survival in dry soils ( Kozlowski, Kramer, & Pallardy, 1991); therefore, roots can grow further away from the plant when the soil does not have sufficient moisture (Harris, Clark, & Matheny, 2004), because they grow towards where resources are available (Topa, 2004) and soil compaction is not restrictive (Watson et al., 2014b). In contrast, when nutrients and moisture in the soil are sufficient, trees can obtain these resources very close to the stem (Zanetti, Vennetier, Mériaux, & Provansal, 2014). ...
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The ahuehuete (Taxodium mucronatum Ten.), National Tree of Mexico, is frequently found in urban green areas, in conditions of restricted humidity and compacted soils. These characteristics negatively affect growth and survival. The goal of the research was to evaluate root growth of young ahuehuete trees by the effect of the frequency of irrigation and loosening of the soil surrounding the planting strain. Twenty four trees 2 m high were planted in an urban area. The experiment was established as a completely random design with factorial arrangement: a) irrigation frequency (frequent [once weekly] and spaced [once every two weeks]) and b) treatment of the soil surrounding the plantation strain (soil with and without loosening). The growth of the root system was monitored for 12 months through digital photographs, obtained from rhizotrons installed on a side wall of each plantation strain. The original compaction of the site did not present restrictive levels for growth; therefore, the surrounding loosening did not significantly improve (P > 0.1) short-term root growth. Root length (267.75 to 453.28 cm) showed no statistically significant differences for the irrigation and soil factors and their interaction; however, the number of roots was affected by the interaction of the factors (P ≤ 0.1). Trees with frequent irrigation and soil without loosening developed a higher number of roots (190.5). The interaction of irrigation frequency and soil condition influences the number of roots, but not the length.
... Such soils in recently developed areas are also often compacted, resulting in physical impedance of root growth and reduced aeration (Lehmann and Stahr, 2007;Scharenbroch and Catania, 2012), and may be subject to liming effects from concrete and masonry leachates and soil disturbance (Doichinova et al., 2006;Scheberl et al., 2019). Responding to these common soil limitations is thus a priority in urban forestry (Watson et al., 2014). ...
... Alkalinization has been observed when N-fixing plant shoots and leaves are left on the soil to break down (Yan et al., 1996), and this might also account for the increases in clover treatment soil pH in this study. However, overall the soil pH remained in a range of 6.9-7.2, which is near optimal for most urban forestry applications (Watson et al., 2014). ...
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The use of biochar with N-fixing species has been shown to enhance productivity of agricultural systems, both in N-fixing crops, as well as in mixed-species systems. Here we investigate the potential for the use of a granulated mixed conifer biochar and a sugar maple biochar in combination with N-fixing companion plants in an urban forestry context. A factorial greenhouse experiment compared growth responses in silver maple (Acer saccharinum L.) saplings planted with two biochar types as soil amendments, alone and in combination with two N-fixing plant companions (Trifolium repens L. and Dalea purpurea Vent.). Both biochar types enhanced tree growth; however, a maple feedstock biochar resulted in greater increases than a granulated conifer-feedstock biochar. N-fixing companion species also increased tree growth, although the faster-growing Trifolium reduced soil moisture content and reduced sapling growth in the absence of biochar. The highest tree growth performance and total N uptake was obtained with a combination of both biochar and N-fixing plants, with a ~30 % increase in biomass compared to controls for the granulated conifer biochar, and a ~55 % increase for the maple biochar. We conclude that biochar additions in combination with N-fixing companion species have considerable promise in an urban forestry context, but that optimization of the system in terms of biochar type and species combination is an important consideration.
... Yet, belowground resources might also become more limiting in urban environments. While human management of urban vegetation might, to some extent, alleviate belowground resource supply, for instance, through supplement of water and nutrients (29,44), plant root health is often compromised by soil contamination, water shortage, and reductions in litterfall and decomposition because of surface sealing and soil compaction (42,45). A healthy balance between the crown and root systems will be critical to sustain urban vegetation growth under more extreme climate conditions in the future. ...
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Urban environments, regarded as “harbingers” of future global change, may exert positive or negative impacts on urban vegetation growth. Because of limited ground-based experiments, the responses of vegetation to urbanization and its associated controlling factors at the global scale remain poorly understood. Here, we use satellite observations from 2001 to 2018 to quantify direct and indirect impacts of urbanization on vegetation growth in 672 worldwide cities. After controlling for the negative direct impact of urbanization on vegetation growth, we find a widespread positive indirect effect that has been increasing over time. These indirect effects depend on urban development intensity, population density, and background climate, with more pronounced positive effects in cities with cold and arid environments. We further show that vegetation responses to urbanization are modulated by a cities’ developmental status. Our findings have important implications for understanding urbanization-induced impacts on vegetation and future sustainable urban development.
... Los cambios en la estructura de los suelos urbanos en comparación con los suelos naturales se presentan como uno de los principales factores limitantes para el crecimiento y la supervivencia de los árboles (Lemay and Lemay, 2015). Cómo el sistema radicular y el vigor de los ejemplares se ve afectado en el ambiente urbano, ha sido materia de investigación entre otros de Urban J. (2008); Watson and Hewitt (2012); Watson et al. (2014). ...
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RESUMEN Desde la creación de la ciudad moderna hacia finales del siglo XVIII y durante todo el XIX, se introdujo vegetación de manera planificada, sistematizada y organizada como un proceso restaurador ante la destrucción que la misma ciudad hace de la naturaleza. El arbolado fue promovido como elemento organizador y democratizador del paisaje asociado al concepto de vida al aire libre. Además, se incrementó la demanda de bienes y servicios vinculados a la arboricultura afianzando definitivamente la especia lización de los profesionales idóneos. Para cumplir con la plenitud de sus potencialidades (funciones sociales, comunitarias, medioambientales y económicas) el arbolado en la ciudad debe ser valorado, planificado y gestionado adecuadamente. Esto exige la interrelación de conocimientos científicos, técnicos, administrativos, económicos y sociológicos que satisfagan una gestión equilibrada del bosque urbano. La formación técnica en gestión del arbolado desarrollada en la Tecnicatura en Jardinería ha dado lugar a un proceso concreto y a la vez sumamente enriquecedor y creativo de enseñanzaaprendizaje. ABSTRACT Since the creation of the modern city towards the end of the 18th century and throughout the 19th cen tury, vegetation has been presented in a planned, systematized and organized manner as a restoration process in the face of the destruction that the city itself makes of nature. Urban trees were promoted as an element that organizes and democratizes the landscape associated with the concept of outdoor life. In addition, the demand for goods and services linked to arboriculture is strengthened by definitively consol idating the specialization of qualified professionals. In order to fulfill its full potential (social, community, environmental and economic functions), trees in the city must be valued, planed and properly managed. This requires the interrelation of scientists, technical, administrative, economic and sociological knowl edge that satisfies a balanced management of the urban forest. The technical training in Tree Manage ment Developed in the Technology in Gardening has given rise to a concrete and at the same time highly enriching and creative process of teaching and learning.
... En comparación con los suelos naturales, los cambios en la estructura de los suelos urbanos constituyen uno de los principales factores que limitan el crecimiento y la supervivencia de los árboles (Lemay y Lemay, 2015). Por esta razón, la forma en que el ambiente urbano afecta el crecimiento del sistema radical y el vigor de los ejemplares ha sido materia de investigación de diferentes autores (Urban, 2008;Watson y Hewitt, 2012;Watson et al., 2014). ...
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Este Manual propone un nuevo enfoque para una antigua disciplina. Cada capítulo ofrece fundamentos que desde la arboricultura moderna valoran el entendimiento de la biología del árbol como un sistema vivo, complejo y altamente eficiente. Esta nueva perspectiva requiere la formación de profesionales idóneos, calificados y diversificados, que posibiliten una gestión equilibrada, inteligente y sensible del árbol como individuo y del bosque urbano como universo. Por otro lado demanda una permanente actualización y colaboración, a fin de promover la creación de equipos interdisciplinarios capaces de diseñar y gestionar planes estratégicos para satisfacer las metas exigibles para la sostenibilidad del arbolado como componente esencial de la infraestructura verde urbana. Este Manual surge como resultado del trabajo en áreas de gestión pública de arbolado urbano, del desarrollo docente en el ámbito universitario y de la interacción en distintas asociaciones profesionales. Su contenido está ideado para cubrir los saberes del técnico que debe gestionar el arbolado en el ámbito público o privado, así como también al estudiante de alguna de las carreras de Ciencias Agropecuarias que se oriente en la formación en Silvicultura Urbana.
... Developing soil standards for tree establishment and performance, including geographically relevant soil research and specifications were highlighted as pressing needs. While it is well understood that tree establishment and performance in urban settings is often challenging because of poor soil quality (Day and Bassuk 1994;Conlin and van den Driessche 1996;Jim 1998b;De Lucia et al. 2013;Watson et al. 2014;Layman et al. 2016), it is often neglected (Haan et al. 2012;McGrath et al. 2020) as well as understudied. Major gaps in knowledge include soil structure, aggregate stability, bulk density, porosity (Jim 1998a; Puskás and Farsang 2009), soil volume restriction, and soil sealing (Jim 2017;Just et al. 2018). ...
For cities to grow their urban forest canopy the formula appears rather straightforward: the right trees, plus the right conditions, plus the right care equals success. These simplified “tree chain of custody” steps, however, represent activities within a complex value-chain in Canada. Given that there is heightened demand for urban tree planting as natural climate solutions become the norm, how can we prepare the value-chain to meet these demands? To answer this question, we outline the pathways by which trees presently go from nurseries into urban and peri-urban areas. Delineating the actors, roles, and present barriers to success exposes the complexity of the process and relationships in the value-chain, as there are distinct phases with multiple actor groups involved who influence, and are influenced, by one another. We explore the issues that pose prominent challenges to, as well as opportunities for, the value-chain. Emergent themes include communication, forecasting demand and timing, underpricing and undervaluing tree establishment, lack of awareness on the importance of soils, juvenile tree health, species selection, and gaps in evidence-based decision support tools. The touchstones of science and innovation, collaboration, and knowledge mobilization are pertinent for the value-chain in Canada to draw upon to navigate the future.
... Generally, the roots of trees in urban areas are bounded by the soil properties. So, most of the stress is generated from soil and root system (Watson et al. 2014). ...
... The critical PR seems to be dependent on soil tillage; for instance, in an Oxisol regardless of cropping succession systems, Moraes et al. (2014) observed a critical value of 2 MPa for conventional tillage, 3 MPa for minimum tillage with chiseling, and to 3.5 MPa for notillage with continuous, biological pores. Roots often grow into root channels from previous plants, worm channels, structural cracks, and cleavage planes (Watson et al., 2014), where differences in root diameter may explain variances in species' ability to penetrate compacted soil layers (Clark et al., 2003). Penetration resistance is most limiting in finetextured (Gerard et al., 1982), but water content has a greater effect than texture. ...
Efficient soil tillage methods to create a favorable environment for commercial forest require proper evaluation of soil compaction with depth, by using operative indicators of physical quality for diverse soils. Our aim was to investigate the spatial variability of field-measured penetration resistance (PR), and then determine the relationships between penetrability and mechanical and hydrologic properties of Planosol, Nitisol, and Regosol used for eucalyptus production. Low, intermediate, and high compaction states were defined based on field PR spatial variability, and in those areas with distinct compaction states we determined bulk density, degree-of-compactness, macroporosity, water retention and availability, and least limiting water range. The soil compaction states are related to composition or capacity (mass/volume) properties, most consistently with soil bulk density and relative compaction. Whether using simple measurements such as penetrability and bulk density, or more complex, integrative properties (e.g. degree-of-compactness and least limiting water range), tillage recommendation would be the same. Namely, deep tillage for Nitisol, and shallower tillage for Planosol and Regosol. In practical terms, the first soil requires deep subsoiling, and second and third soil demand shallower subsoiling associated or not with ridging. In conclusion, mapping of soil compaction states based on PR data can be used for best management practices and tillage recommendation for forest installation.
Long-term, multi-decade research on planted tree survival in urban settings is sparse. One understudied urban environment is highway rights-of-way (ROW), lands adjacent to high-speed, unsignalized roadways. We conducted a re-inventory of tree planting cohort in northern Illinois, U.S. on a 48 km-long highway near Chicago which were 10-, 21-, and 30-years old to evaluate long-term patterns of survival and diversity. Using each randomly selected planting site along the highway as a unit of observation and analysis, we compared the number of trees documented in record drawing to the number of trees currently alive to determine percent survival. We evaluated 224 planting sites which originally contained 2,944 trees and collected data about the planting site location. For the oldest cohort, 26% of trees were still alive in 2018 (median survival by species = 16%, Q1 = 0%, Q3 = 48%), while 31% of the 21-year-old cohort (med. = 6%, Q1 = 0%, Q3 = 47%) and 86% of the 10-year-old cohort were still alive (med. = 85%, Q1 = 74%, Q3 = 96%). The survival of the 21- and 30-year-old cohort matches urban tree survival estimates by other researchers, while the 10-year-old survival is higher than expected. The only planting location characteristic that significantly affected survival was traffic islands (areas between the highway and entrance/exit ramps). Species with low drought tolerance were less likely to be alive for the 10-year-old cohort. Waterlogging tolerant species were more likely to be alive in the 10-year-old cohort. Since some species in the 21- and 30-year-old cohorts had very low survival, the tree species richness and diversity s in study areas declined between the initial record drawings and reinventory. This study demonstrates the challenges of maintaining long-term survival and diversity in the highway ROW and emphasizes the importance of species selection.
Full-text available
Long-term, multi-decade research on planted tree survival in urban settings is sparse. One understudied urban environment is highway rights-of-way (ROW), lands adjacent to high-speed, unsignalized roadways. We conducted a re-inventory of tree planting cohort in northern Illinois, U.S. on a 48 km-long highway near Chicago which were 10-, 21-, and 30-years old to evaluate long-term patterns of survival and diversity. Using each randomly selected planting site along the highway as a unit of observation and analysis, we compared the number of trees documented in record drawing to the number of trees currently alive to determine percent survival. We evaluated 224 planting sites which originally contained 2,944 trees and collected data about the planting site location. For the oldest cohort, 26% of trees were still alive in 2018 (median survival by species = 16%, Q1 = 0%, Q3 = 48%), while 31% of the 21-year-old cohort (med. = 6%, Q1 = 0%, Q3 = 47%) and 86% of the 10-year-old cohort were still alive (med. = 85%, Q1 = 74%, Q3 = 96%). The survival of the 21-and 30-year-old cohort matches urban tree survival estimates by other researchers, while the 10-year-old survival is higher than expected. The only planting location characteristic that significantly affected survival was traffic islands (areas between the highway and entrance/exit ramps). Species with low drought tolerance were less likely to be alive for the 10-year-old cohort. Waterlogging tolerant species were more likely to be alive in the 10-year-old cohort. Since some species in the 21-and 30-year-old cohorts had very low survival, the tree species richness and diversity s in study areas declined between the initial record drawings and reinventory. This study demonstrates the challenges of maintaining long-term survival and diversity in the highway ROW and emphasizes the importance of species selection.
Full-text available
Seedlings of Acer platanoides, A. rubrum, Quercus palustris, and Q. rubra were subjected to soil-applied sodium chloride (NaCl) solutions of 0.0, 1.1, and 5.0 N NaCl once every month beginning in October and ending in April. In May, the trees were evaluated for damage, harvested and dried. Growth measurements and shoot Na and Cl content were analyzed. For all four species, plants in the November through February/March salt treatments sustained little plant damage and reduction in growth. The October application of NaCl resulted in heavy plant damage and reduced growth in each species, while April NaCl applications produced similar results in A. rubrum and Q. palustris alone. Shoot Na and Cl content were greater in plants in the October, March, and April salt treatments. In a second experiment, actively-growing, greenhouse-grown plants of the four species were subjected to either a fertilizer solution plus 0.25 N NaCl at every irrigation or a single application of 1.1 N NaCl followed by normal irrigation thereafter. A. platanoides lost its resistance to soil-applied NaCl by mid summer, while A. rubrum and Q. palustris were sensitive to a high dosage of NaCl applied at this time and Q. rubra was resistant. In both experiments, there were significant interactions between the time of NaCl application and the periodicity of plant growth, soil temperature, precipitation, and leaching of the salt from the soil as well as genetic factors, which affected the amount the salt injury sustained by trees.
Curbside sugar maples in the New Haven area developed decline symptoms and died at an annual rate of 20 to 33%. The severity of symptoms could be correlated with concentrations of sodium in the leaves, twigs, and sap of the trees. Even though symptom severity is negatively correlated with foliage nitrogen, the symptoms could not be alleviated by the application of nitrogen in the root zone. Symptom severity could not be correlated with chloride ion, plant parasitic nematodes, or concentrations of the metallic pollutants lead, zinc, copper, nickel and cadmium. There was significant negative correlation between curb height and symptom severity.
Rooting zone soil pHs and foliar nutrient imbalances were determined for chlorotic pin oaks, white oaks, and red maples. Soil pHs were significantly different between green and chlorotic tree sites of each species to a depth of 18-22 inches. Nutrient imbalances consisted of high phosphorus, potassium, and magnesium depending on season and species; and low manganese, iron, copper, and zinc depending on season and species. Abatement of chlorosis was accomplished by soil acidification with or without nutrient or mulch additions. Acidifications with sulfuric acid rapidly reduces the pH of alkaline soils to desired levels which may persist in treated subsoil zones for as long as four years.
Parkways, street tree planter boxes, and highway medians and roadsides are locations where soil accumulation of deicing salts is highest. Sodium chloride is the most common deicer applied in the United States. Sodium chloride and other salts accumulating in the root zone may instigate and exacerbate street tree decline. Salts affect soil aggregate stability, porosity, and water and nutrient uptake in trees. Data collected in Chicago, Illinois show much higher soil sodium (1,272 jxg/g) and chloride (348 |ig/g) in the center of newly installed, narrow, raised medians along Lake Shore Drive after one winter, compared to the center of wide medians along the roadway (236 ng/g sodium and 23 (ig/g chloride). Proximity to high speed traffic and its associated spray and splash were reasons for this. In suburban Downers Grove, Illinois, grade level street tree planter soils had extremely high levels of sodium (1,426 |ig/g to 2,277 |ig/g) compared to adjacent raised planter soils. The raised planters did not receive saltladen runoff, splash, plowed snow, or direct application from salt spreaders.
Aerated compost tea (ACT) is gaining interest as a nutrient amendment for urban trees. This study examined the effects of ACT, synthetic fertilizer, and deionized water on 15 biochemical properties with two soil types. Significant effects for pH, Mg2+, Na+, C, N, and C/N ratio were not observed among treatments. No differences between dilute ACT (ACTd) at 22.4 kL ha-1 and water were detected. Soil K+ was greater with ACT concentrate (ACTc) at 224 kL ha-1 compared to 30-10-7 fertilizer at 195 kg N ha-1 with A horizon soils. Soil K+, NH4 +, and microbial respiration were greater with ACTc compared to water in A soils. Soil P (A soils only), NO3 - (Bt soils only), dissolved organic N, microbial biomass N, and N mineralization were greater with fertilizer compared to ACT. Increases in denitrification were seen with ACTc compared to fertilizer and water in the first 24 hours (+4 to +12 mg N2O kg-1), but greater increases were observed with fertilizer at hours 48 and 96 (+65 to +127 mg N2O kg-1). Greatest improvements in soil fertility were observed with fertilization. Minor improvements in soil fertility were observed with ACTc, and denitrification losses were lower with ACTc compared to the fertilizer.
Amendment of backfill soil at planting with peat moss, fired montmorrilonite clay or a “superabsorbent” gel had no significant positive influence on growth and establishment of container grown Liquidambar styraciflua L., sweet gum, plants placed in well-drained Arredondo fine sand soil. A cost estimate indicated the addition of amendments to backfill soil would increase installation costs 27 to 30% over those for control plants.
The effectiveness of several commercial mycorrhizal inoculants on the growth and development of Liquidambar styraciflua (sweetgum) was evaluated. Plants were grown in a nursery potting mix and were inoculated with the mycorrhizal products at the manufacturer's recommended rate. The growth response of mycorrhizal and nonmycorrhizal plants was analyzed at two harvests (8 and 14 weeks after transplanting). Significant differences were found in the growth of L. styraciflua to mycorrhizal colonization with the different commercial products. Fourteen weeks after transplanting, inoculation with products 1 (Earth Roots), 2 (MycoApply endo), and 3 (VAM 80) enhanced the growth of sweetgum relative to the nonmycorrhizal plants. However, plants inoculated with products 2 and 3 had greater leaf area, dry mass and relative growth rates than those inoculated with product 1. Plants of L. styraciflua inoculated with product 4 were less responsive to mycorrhizal colonization and only increased their leaf area relative to the non-inoculated controls. Testing both the infectivity and effectiveness of mycorrhizal fungi is recommended for the successful application of mycorrhizal technology in horticultural practices.
This study investigated the effects of vesicular-arbuscular mycorrhiza (VAM) inoculation on the growth of landscape trees and shrubs under high-fertility nursery growing conditions. Four species of 1 year old trees, and rooted cuttings of nine species of shrubs, were inoculated with Glomus intraradices, or Glomus fasiculatum, or served as non-inoculated controls. The trees were transplanted to two high fertility, non-sterile field locations. Inoculation significantly increased the level of colonization in Acer platanoides, Sorbus aucuparia, Malus, and Fraxinus pennsylvanica, but did not enhance growth. The shrubs were containerized in a peat and bark medium with two levels of controlled release fertilizer. VAM inoculation significantly increased the level of colonization in Spiraea × bumalda, Syringa × chinensis, Prunus × cistena, and Cornus alba, while Weigela, Cotoneaster dammeri, and Potentilla parvifolia became well colonized without inoculation. Forsythia ovata and Viburnum opulus did not become significantly colonized. The growth of Syringa was stimulated by VAM inoculation during consecutive seasons, irrespective of fertilizer level. The growth of Prunus at the lower fertilizer level was significantly stimulated by inoculation even though control plants became highly colonized without VAM inoculation. Two years after inoculation, five species were transplanted to a second, non-sterile, field site to monitor the effect of inoculation on post transplant growth. G. intraradices significantly enhanced S. aucuparia caliper growth in the second year post-transplant.
Root-zone temperature (RZT) is an important environmental factor affecting growth and performance of woody ornamental plants in the landscape. Research was conducted to compare the effects of RZT on survival, growth, and root morphology of a difficult-to-transplant species, mountain laurel (Kalmia latifolia L.), and an easy to transplant species, Japanese holly (Ilex crenata Thunb.). Seedlings of mountain laurel or micropropagated liners of mountain laurel (Kalmia latifolia L. ‘Sarah’) and rooted stem cuttings of Japanese holly (Ilex crenata Thunb. ‘Compacta’) were grown hydroponically for 12 weeks in controlled environment conditions under long days at 9-hr days/15-hr nights of 26/22C (79/72F) with RZTs of 16, 24, or 32C (61, 75, or 90F). Compared to 16 and 24C (61 and 75F), percent survival of mountain laurel was reduced by a RZT of 32C (90F), whereas percent survival of Compacta holly was unaffected by RZT. Root dry weight of mountain laurel was reduced 72% at 32C (90F) while top dry weight was unaffected by RZT. Top and root dry weights of Compacta holly were unaffected by RZT. Root: top ratio of mountain laurel was reduced by increasing RZT, whereas root: top ratio of Compacta holly was unaffected by RZT. Root area of mountain laurel and Compacta holly were reduced 80 and 64%, respectively, at 32C (90F) compared with 16C (61F). Number of lateral roots in the apical 2 cm (0.8 in) of primary roots of both taxa increased with increasing RZT. Results of this research indicate that reducing RZT in the landscape may increase survival and root growth of transplanted mountain laurel.
Biostimulants are used to reduce the stress associated with non-dormant (summer dug) harvest of field-grown nursery stock; however, the effectiveness of biostimulant treatment is uncertain. This study tested the effects of three application methods of Bioplex™ (a commonly used biostimulant) to container-grown red oak seedlings on whole plant transpirational water use and growth before and after root pruning. Root pruning was used to simulate field harvest; it removed 59% of the seedling's total root surface area. Bioplex™ application by foliar spray, soil drench or a combination of foliar spray and soil drench, significantly reduced whole plant transpirational water use by 15% for three days after application, relative to untreated control seedlings. Root pruning significantly reduced whole plant transpiration, compared to non-root-pruned seedlings, and had a greater effect on transpiration than any Bioplex™ treatment. The previous season's Bioplex treatment had no effect on the spring growth flush following fall root pruning. Root pruning in fall significantly reduced root and total plant dry weights the following spring. Although Bioplex™ applications significantly reduced transpiration for three days after application, there does not seem to be any long-term beneficial effect when used to mediate summer digging transplant stress.