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The Management of Tree Root Systems in Urban and Suburban Settings II: A Review of Strategies to Mitigate Human Impacts

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Abstract

Root systems of nearly all trees in the built environment are subject to impacts of human activities that can affect tree health and reduce longevity. These influences are present from early stages of nursery development and throughout the life of the tree. Reduced root systems from root loss or constriction can reduce stability and increase stress. Natural infection of urban tree roots after severing has not been shown to lead to extensive decay development. Roots often conflict with infrastructure in urban areas because of proximity. Strategies to provide root space under pavements and to reduce pavement heaving have been developed, but strategies for prevention of foundation and sewer pipe damage are limited to increasing separation or improved construction.
Arboriculture & Urban Forestry 40(5): September 2014
©2014 International Society of Arboriculture
249
Gary W. Watson, Angela M. Hewitt, Melissa Custic, and Marvin Lo
The Management of Tree Root Systems in Urban
and Suburban Settings II: A Review of Strategies
to Mitigate Human Impacts
Arboriculture & Urban Forestry 2014. 40(5): 249–271
Abstract. Root systems of nearly all trees in the built environment are subject to impacts of human activities that can aect tree
health and reduce longevity. ese inuences are present from early stages of nursery development and throughout the life of the
tree. Reduced root systems from root loss or constriction can reduce stability and increase stress. Natural infection of urban tree
roots aer severing has not been shown to lead to extensive decay development. Roots oen conict with infrastructure in urban
areas because of proximity. Strategies to provide root space under pavements and to reduce pavement heaving have been developed,
but strategies for prevention of foundation and sewer pipe damage are limited to increasing separation or improved construction.
Key Words. Ground-Penetrating Radar; Infrastructure Damage; Root Architecture; Root Decay; Root Defects; Rooting Space; Root
Flare; Root Severing; Stability.
Tree root systems are generally shallow and wide-
spread (Day et al. 2010). Human activity around
trees frequently impacts tree root systems, decreas-
ing tree health and reducing longevity compared to
trees on natural sites. Construction and repair of
infrastructure oen severs tree roots. e presence
of buildings and pavements can restrict root systems
with detrimental eects on both the tree and the
structure. Urban landscape design and maintenance
can be very dierent than the natural environment
to which the trees are adapted. Root architecture is
altered by nursery production and transplanting,
which can aect the tree throughout its life. e
management challenge is to avoid or reduce these
impacts through proper management, including
minimizing injury to existing roots, speeding root
regrowth aer severing occurs, and maximizing
the quality and quantity of root space in design.
ROOT ARCHITECTURE AND
STABILITY
Tree stability depends heavily on both root system
architecture and the anchorage of roots in the soil.
Root/soil resistance gives rise to the characteris-
tic mass of roots and soil seen on uprooted trees,
known as the root plate. e anchorage strength of
a tree root system has four components: 1) the mass
of the roots and soil levered out of the ground, 2) the
strength of the soil and depth of root penetration
under the root plate, 3) the resistance to failure in
tension of tree roots on the windward side as the up-
ward movement of the root–soil plate causes roots
to pull out of the soil with or without rst break-
ing, and 4) the length of the lever arm (where the
roots hinge) on the leeward side, which is aected
by root diameter and resistance to bending of the
tree roots (Coutts 1983; Coutts 1986; Blackwell et
al. 1990; Kodrik and Kodrik 2002). A change in one
feature can aect several others. us, an increase
in root plate diameter will increase the weight com-
ponent, the length of lever arm between the trunk
and the roots around the perimeter of the plate,
and the area of root/soil contact under the plate.
As each of these anchorage components increases,
the greater the force needed to tip up the root plate.
Uneven distribution (large sections without roots)
reduces anchorage (Sundstrom and Keane 1999).
Environmental factors inuence root architec-
ture and stability. If roots penetrate deeper, as can
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
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250
be the case in sandy soil, the tap root and deeper
roots have more inuence on overturning resis-
tance in sandy soil compared with clayey soil (Four-
caud et al. 2008). Wind loading appears to result in
increased growth of lateral roots at the expense of
the tap root. Development of the lateral root system
may therefore ensure better anchorage of young
trees subjected to wind loading (Tamasi et al. 2005).
Root branching shortens the root plate lever
arm and makes tipping easier. e roots of nearly
all trees in urban areas have been severed during
transplanting, which creates branching at the cut
end and smaller regenerated roots. is branching
may shorten the root plate fulcrum on the leeward
side and reduce the diameter of the roots at the
perimeter of the root plate, with the possible eect
of rendering urban trees less stable than their forest
counterparts with less-branched, larger roots. How-
ever, no direct research on urban trees is available.
ROOT INJURY
Consequences of Root Severing
Analysis of published data on root spread of trees
concluded that the radius of the root system is
approximately equal to tree height (Day et al.
2010), which is oen greater than the radius of
the branches (drip line). Given the close proxim-
ity of trees to structures, pavements, and utili-
ties in most urban and suburban landscapes, tree
roots can be easily injured by soil excavation.
Root loss from severing can be considered tem-
porary when roots are able to regenerate and even-
tually replace roots that were lost. If the root space is
permanently lost (e.g., resulting from construction
of a structure or pavement in the root zone), then
the root system will not be able to replace itself, and
stress and stability concerns may never be overcome.
Root loss from trenching can aect both tree
health and stability. Trenching through the root zone
of parkway trees was considered to be responsible
for substantial tree dieback and decline over the fol-
lowing 12 years, and was the basis for development
of auguring specications in common use (Morell
1984). While generally accepted, the little research
available has not been completely supportive.
When trenches were dug for installation of new
utilities 0.5 to 3.3 m from hackberry (Celtis occiden-
talis), sweetgum (Liquidambar styraciua), sugar
maple (Acer saccharum), and honeylocust (Gleditsia
triacanthos), only on hackberry, where the trench
was only 0.5 m from the trunk (approximately 1.5
times the trunk diameter), was growth-reduced
for all four growing seasons monitored following
trenching. e trenching did not predispose the
trees to readily evident disease or insect infestations
(Miller and Neely 1993). If the trench was three times
the trunk diameter away from the trunk, or more,
no consistent growth reduction was measured. No
growth reduction or dieback was reported when pin
oak (Quercus palustris) trees were trenched on one
or two sides at a distance of three times the trunk
diameter. However, moderate dieback was noted
on trees that were trenched on three sides (Watson
1998). Street or sidewalk construction at a distance
of ve to seven times the trunk diameter from the
tree resulted in only a 4% increase in mortality and a
5% decrease in condition rating (Hauer et al. 1994).
Root loss reduces the capacity of the root system
to absorb water, most of which is transpired through
the leaves. Compensatory pruning along with severe
trenching reduced dieback from stress but was most
benecial aer the most severe root loss (Watson
1998). ese trees did not receive any irrigation
or special care, which could possibly have reduced
dieback development even without pruning.
Hamilton (1988) suggested that some species
may be more prone to uprooting aer root pruning,
based on observation. Stability of trees aer the roots
have been severed is a concern that has not been
fully addressed by research. When trenches were cut
alongside trees, tree anchorage was compromised by
trenches only when closer than 2.5 times the diam-
eter of the trunk on the tension side (Bader 2000;
Smiley 2008b; Ghani et al. 2009). e surprisingly
high anchorage of the trees with such severe root loss
was thought to be because rooting depth close to the
trunk was a major component of anchorage. Cut-
ting roots on both sides of the tree reduced the force
required to cause tree failure by two-thirds when
trees were trenched simultaneously at ve times the
trunk diameter on the tension side and about half
that distance on the compression side (approximate
location of the root plate hinge point) (O’Sullivan and
Ritchie 1993). Trees with asymmetrical or restricted
root systems may be less stable aer root severing.
ese studies suggest that vigorous trees less than
30 cm diameter may be able to tolerate roots being
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251
severed on one side as close as three times the trunk
diameter without a major loss in stability or crown
decline. Larger trees, such as those on which the
specications were based, may be less tolerant. As
surprising as it might seem that root severing did
not kill any trees or cause severe dieback in these
studies, consider that when roots are cut to form
a root ball to transplant a tree, roots are cut on all
sides at a distance of three to ve times the diam-
eter of the trunk (Anonymous 2004; Anonymous
2010). e trees are stressed, but even very large
trees recover if cared for properly. Comparison of
trenched trees in the established landscape to trans-
planted trees may be fairly realistic (Hamilton 1988).
Root Decay
Principles of Compartmentalization of Decay
in Trees (CODIT, Shigo 1977) apply to roots as
well as stems, although roots have not been stud-
ied as extensively (Shigo 1972; Shigo 1979a; Tip-
pett and Shigo 1980; Tippett and Shigo 1981;
White and Kile 1993; Robinson and Morrison
2001). Because root injuries are common and in-
juries serve as infection courts for root-rotting or-
ganisms (Tippett et al. 1982), roots have evolved
to be strong compartmentalizers (Shigo 1986).
Average values of longitudinal extension of decay
columns in roots of Sitka spruce (Picea sitchensis),
white r (Abies concolor), and Norway spruce
(Picea abies) aer articial inoculation have been
reported from 10 to 53 cm per year, (Morrison and
Redfern 1994; Garbelotto et al. 1997; Piri 1998).
Decay introduced experimentally through root
wounds within a meter of the trunk can extend into
the trunk (Redmond 1957; Garbelotto et al. 1997).
In contrast, the natural infection of landscape tree
roots 3 to 22 cm in diameter aer severing has not
led to extensive decay development. Five to seven
years aer severing, decay extended no more than
10 cm from the severed end of roots of 7-year-old
sweetgum (Liquidambar orientalis × L. styraciua)
and plane hybrids (Platanus occidentalis × P. orien-
talis) (Santamour 1985), or 40-year-old honeylocust
(Gleditsia triacanthos var. inermis), pin oak (Quercus
palustris), tulip-tree (Liriodendron tulipifera), and
green ash (Fraxinus pennsylvanica) trees (Watson
2008). Trunk wood decay was observed only when
the root cambium had died back to, or above, the soil
surface and may have been the result of trunk injury
(cambial death) rather than the root wounding
(Santamour 1985). Although the number of research
studies is limited, these results suggest that decay
development as a result of severing roots is not an
immediate threat to the health or stability of a tree.
Santamour (1985) also reported dierences
between species in their ability to resist trunk decay
and discoloration aer root severance. Four years
aer severing roots within 0.5 m of the trunk, there
was no discoloration or decay in trunk tissues in red
maples (Acer rubrum), and 6 cm maximum in the
roots. Discoloration and decay was present in trunk
tissues of 2 of 10 black oaks (Quercus velutina) and 4
of 10 white oaks (Q. alba ) aer similar root severance.
Root size and proximity to the trunk has been
reported to aect decay development rate. Root
decay increased as root size increased on hard-
woods (Whitney 1967; Santamour 1985; Balder
et al. 1995; Balder 1999) and conifers (Piri 1998;
Tian and Ostrofsky 2007). Injury to roots close
to the trunk resulted in more extensive decay
on hardwoods (Shigo 1979b; Balder et al. 1995;
Balder 1999). Other studies do not support
these conclusions (Shigo 1991; Watson 2008).
Injury of roots in the dormant season may lead
to poorer compartmentalization and increased
decay development, but reports are inconsistent
(Santamour 1985; Balder et al. 1995; Balder 1999).
Stressing and limiting the development of roots,
particularly constriction of root diameter growth,
as results from certain root defects, predispose the
roots to Armillaria infection (Livingston 1990). e
increased success of infection by Armillaria sp. as a
result of root severance appears to be associated with
changes in the nutrient status of the roots aer they
have been damaged, rather than simply an increase
in sites for penetration (Popoola and Fox 1996).
Trees that fail due to root decay under non-
stormy conditions oen have extensive decay in
the root are (roots forming the curvature between
vertical trunk and the angled structural roots, also
known as buttress roots). Decay can develop on
the lower side of major are roots, where it can
remain undetected. Drilling is recommended to
determine the amount of sound wood. Major are
roots are considered signicantly decayed if the
thickness of the sound wood on the root is less
than 0.15 times the tree diameter (Fraedrich and
Smiley 2002). ermography can be eective in
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©2014 International Society of Arboriculture
252
approximating the decayed areas of the root col-
lar (Cellerino and Nicolotti 1998; Catena 2003).
Locating Roots
Locating roots prior to construction to avoid dam-
aging them is time-consuming and expensive if
done with hand digging. Introduction of air ex-
cavation tools has made the task considerably
more ecient (Nadezhdina and Cermak 2003).
Non-destructive, ground-penetrating radar can
be used to map larger roots. Roots 1.0 cm diam-
eter and larger, and as deep as 2 m, can be detected
(Hruska et al. 1999; Cermak et al. 2000; Butnor et
al. 2001; Butnor et al. 2003; Nadezhdina and Cer-
mak 2003; Barton and Montagu 2004). Vertical
roots and roots with less than 20% water content
could not be detected by ground-penetrating radar
(Stokes et al. 2002; Hirano et al. 2009). Two roots
located closely together cannot be individually dis-
tinguished (Hirano et al. 2009; Bassuk et al. 2011).
Resolution of roots may be best in sandy, well-
drained soils, whereas soils with high soil water
and clay contents may seriously degrade resolu-
tion and observation depth (Butnor et al. 2001).
Interference from other objects present in the
soil was sometimes found to be a problem in
early ground-penetrating radar studies (Cellerino
and Nicolotti 1998; Hruska et al. 1999). Ground-
penetrating radar was eective in structural soil,
which is 80% stone (Bassuk et al. 2011). Roots
could be mapped under concrete and asphalt
(Nadezhdina and Cermak 2003; Bassuk et al.
2011). Development of soware to reconstruct 3D
images of root system architecture from raw data
may still need improvement (Stokes et al. 2002).
Root Regeneration
Root severing can increase the rate of root growth
on one-year-old seedlings or rooted cuttings, but
the more rapid root production merely compensates
for the roots removed (Abod and Webster 1990).
e potential for water uptake is proportional to
the number of new roots produced (Carlson 1986).
When woody roots are severed, numerous new
roots are initiated at, or just behind, the cut (Wil-
cox 1955; Carlson 1974; Watson and Himelick
1982a; Gilman et al. 2010). However, a portion of
regenerated roots can originate from at least 10
cm behind the cut, depending on species (Gilman
and Yeager 1988). The ability of damaged roots to
form new roots decreased with increasing diam-
eter (Balder et al. 1995; Balder 1999). When a root
is severed, new roots that formed nearest to the cut
surface will elongate in the same direction as the
original root. New roots forming slightly behind
the cut surface tend to grow at more perpendic-
ular angles to the original root (Horsley 1971).
Initiation of new roots from severed palm
roots varies with species and distance from the
base of the trunk (Broschat and Donselman
1984). Less than one percent of all cut cabbage
palm (Sabal palmetto) roots regenerated root tips,
whereas coconut palms (Cocos nucifera) regener-
ated root tips about 50% of the time regardless
of root stub length. For other species of palms,
such as queen palm (Syagrus romanzoffiana),
royal palm (Roystonea regia), Mexican fan palm
(Washingtonia robusta), and Senegal date palm
(Phoenix reclinata) the percentage of roots sur-
viving increases with stub length (Broschat and
Donselman 1984; Broschat and Donselman
1990a). Cutting palm roots at least 30 cm from
the trunk will ensure better survival of exist-
ing roots (Broschat and Donselman 1990a).
Auxins are commonly used to promote rooting
in stem cuttings and can increase the number of
new roots initiated near the cut ends of roots.
Indole-3-butyric acid (IBA), indole-3-acetic acid
and naphthaleneacetic acid applied to roots
resulted in increased root initiation (Gossard
1942; Verzilov 1970; Lumis 1982; Magley and
Struve 1983; Prager and Lumis 1983; Struve and
Moser 1984; Fuchs 1986; Watson 1987; Al-Mana
and Beattie 1996; Percival and Gerritsen 1998;
Percival and Barnes 2004), but may reduce root
elongation (Struve and Moser 1984; Percival
and Barnes 2004). Addition of 5% sucrose to
the auxin solution enhanced the results (Fuchs
1986). Verzilov (1970) reported increased root
growth into the third season after application
but was unsure if it was a residual effect of the
auxin application or resulted from greater tree
vigor after the initial increase in root growth.
IBA treatment did not increase root initiation
of palms (Broschat and Donselman 1990b).
e rate of new root initiation is aected by
the environment. At near-optimum soil tempera-
tures, new root growth was detected in 4 to 43
Arboriculture & Urban Forestry 40(5): September 2014
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253
days depending on species (Howland and Grith
1961; Arnold and Struve 1989). Intact root tips
began to elongate before new roots were initiated
(Arnold and Struve 1989). Total new root length
was positively correlated with soil temperature
with signicantly more new root growth at 20ºC
(Andersen et al. 1986). Tree ne-root growth
was slowed by approximately half when soil tem-
peratures dropped from 20ºC to 10ºC (Tyron and
Chapin 1983). When roots are severed late in
autumn, aer soils have cooled, substantial new
root growth may not occur until the soils have
warmed again in the spring. In warmer regions,
active root growth may continue all winter. Plants
that were slightly drought-stressed prior to sev-
ering roots had greater root regeneration (Abod
and Sandi 1983), but decreased soil moisture aer
severing signicantly reduced root regeneration
(Witherspoon and Lumis 1986). Plants supplied
with adequate (non-decient) nutrients before
transplanting had a high capacity to regener-
ate roots following root severing (Abod 1990).
Annual root extension depends on species and
annual soil temperature regime. In the upper Mid-
western United States (USDA Hardiness Zone 5),
with its moderate summers and frozen soils in win-
ter, roots grow at an average annual rate of approxi-
mately 50 cm (Watson 1985; Watson 2004). In one
season under nursery conditions in Hardiness Zone
6, red oak (Quercus rubra) roots grew 53–61 cm
(Starbuck et al. 2005) and birch (Betula pendula)
roots grew 89 cm (Soleld and Pedersen 2006).
In the subtropical climate of north central Florida
(Hardiness Zone 9), where the growing season is
nearly year-round, annual root growth is up to 2 m
or more for some oak (Quercus) and citrus species
that have been studied (Castle 1983; Gilman 1990;
Gilman and Beeson 1996). As the roots continue to
increase in length, ne roots continue to increase in
density for up to ve years (Hutchings et al. 2006).
Root growth for some species will be higher or
lower than average gures. For example, black maple
(Acer nigrum) roots grew 39 cm in a season in the
midwestern United States, which is near the expected
average. Under the same conditions, green ash (Frax-
inus pennsylvanica) grew nearly twice as much, 67
cm (Watson 2004). In general, it may require many
years to replace the roots lost when they are severed.
Fine-Root Desiccation
ere is sometimes concern that ne roots subject
to drying by excavation will be damaged. Desicca-
tion of little-leaf linden (Tilia cordata), green ash
(Fraxinus pennsylvanica), and sugar maple (Acer
saccharum) ne roots had no eect on root regen-
eration (Witherspoon and Lumis 1986; Watson
2009), though moisture content was reduced by
as much as 80% (Watson 2009). In contrast, root
growth of wild cherry (Prunus avium) and cherry
plum (P. cerasifera), and of noble r (Abies procera)
seedlings, was reduced aer desiccation treat-
ment (Symeonidou and Buckley 1997; Bronnum
2005). Susceptibility of ne roots to damage
from desiccation may be species dependent.
ALTERATION OF ROOT STRUCTURE
Root structure and tree growth rate are closely
related. For conifers (Picea sp., Abies sp., Pinus
taeda) and hardwoods (Quercus sp., Liquidambar
styraciua, Juglans nigra) studied, when one-year-
old seedlings are sorted by root morphology, indi-
viduals with a high number of laterals consistent-
ly have greater growth aer planting (Kormanik
1986; Ruehle and Kormanik 1986; Kormanik 1988;
Kormanik et al. 1989; Schultz and ompson 1997;
Kormanik et al. 1998; Gilman 1990; Ponder 2000).
Little information is available on how long this
increased growth persists, but large forest trees that
have out-competed their weaker neighbors over
a lifetime typically have many visible are roots.
Structural Root Depth
e large woody roots giving characteristic form to
the root system are commonly referred to as struc-
tural roots (Sutton and Tinus 1983). ese roots
can be too deep for many reasons. Roots of young
trees can be too deep because nursery production
systems can increase structural root depth. Prun-
ing the primary (tap) root of seedlings early in the
production of eld-grown nursery stock produces
adventitious roots at the cut end of the primary root
that grow rapidly (Johnson et al. 1984; Harris et al.
2001; Hewitt and Watson 2009). Up to 60% of the
natural lateral roots that would normally develop
into are roots located above the regenerated roots
may be lost (Hewitt and Watson 2009). e vigor-
ously growing adventitious roots at the cut end,
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
254
and loss of natural lateral roots above them, oen
replace the natural root are (swelling where roots
join the trunk also known as the trunk are) with
an “adventitious root are” deeper in the soil. e
depth of the adventitious root are is determined
by the length of the primary root aer pruning
(root shank). Even if the tree is planted at the origi-
nal depth with the gra union visible aboveground,
the adventitious root are can be 30 cm or more
below the soil surface. Other practices, such as
burying the gra union below the soil surface and
certain cultivation practices, can also contribute to
root depth. Young trees can be more susceptible
to being blown over by high winds when depth
to the rst root is excessive (Lyons et al. 1982).
e structural roots can also be too deep in con-
tainer-grown nursery stock if trees are not planted
carefully at each repotting. A dense mat of roots can
ll the soil above the woody roots that form the root
are (Fare 2006; Gilman and Harchick 2008; Gil-
man et al. 2010b), and make it impossible to plant
the woody roots at the correct depth without cutting
away a substantial portion of the roots in the ball.
Though trees may grow well enough in the
well-drained substrate of the container or high-
quality soil of the nursery field, they may struggle
to survive when planted on difficult urban sites
with heavy soils and poor drainage (Switzer 1960;
McClure 1991; Day and Harris 2008). The conse-
quences may not be seen immediately. Regener-
ated roots can grow back to the surface (Day and
Harris 2008), but the root collar will always be
too deep. Dramatic improvements in tree condi-
tion have been attributed to root collar excavation
in practice (Smiley 2006). In the only published
research study, street trees failed to show any
influence of root collar excavation on tree growth
over a four-year period (Rathjens et al. 2009).
Root systems of established trees can become
deeper when fill soil is added over them. Research
has not been able to consistently show detrimen-
tal effects on trees, though reports from prac-
tice attribute poor performance and Armillaria
and Phytophthora infections to deep roots and
soil against the trunk (Smiley 2006). After three
years, there was no consistent effect of 20 cm of C
horizon fill on overall root density, growth, or soil
respiration. Fill did disrupt normal soil moisture
patterns (Day et al. 2001). After approximately ten
years, the fill still had no effect on trunk diameter
growth. Bark of some oak trees appeared to be
decaying, but bark biopsies revealed only sapro-
phytic fungi (Day et al. 2006). A “collar rot” caused
by a Phytophthora sp. and a “basal canker” caused
by Fusarium spp. were associated with buttress
roots of planted maples that were deeper than roots
of natural, woodland maples (Drilias et al. 1982).
Installation of subterranean piping systems
or core venting systems to counter the adverse
impact of fills is sometimes recommended (Har-
ris et al. 1999). Studies of aeration pipes installed
prior to addition of fill have been inconclusive.
With or without pavement-like surface cover,
conditions under fill were not severe enough for
any “improved” effect to be measured from the
use of an aeration system. Greater trunk growth
in plots with aeration pipes was attributed to
increased soil moisture in the plot with aera-
tion pipes (MacDonald et al. 2004; Townsend et
al. 1997). These results underscore the need for
further quantitative studies of conditions cre-
ated by various fill and paving procedures to
better ascertain the usefulness of elaborate and
expensive aeration systems. Other factors asso-
ciated with raising the grade, such as soil traf-
ficking and root severance, may be responsible
for much of the tree decline attributed to fill.
A layer of crushed rock over existing soil
before lling with clay soil increased oxygen (per-
cent) and reduced carbon dioxide in the soil
beneath it compared to a comparable area where
no crushed rock was used before clay ll was
placed over the soil surface (Yelenosky 1964).
Circling Roots
Growing trees in nursery containers alters natural
root structure (Halter et al. 1993). Reports are rare
of adventitious roots developing above the circling,
kinked, or twisted form found within the container
root ball aer planting (Gilman and Kane 1990).
Circling roots on the surface of the container
root ball are widely recognized as a defect and it is
common practice to disrupt these by making sev-
eral vertical cuts, or “slashes,” on the outside of the
root ball before planting (Ellyard 1984; Blessing
and Dana 1987; Arnold 1996; Gilman et al. 1996).
Methods that disrupt circling roots do not elimi-
nate descending, ascending, and kinked roots. Con-
Arboriculture & Urban Forestry 40(5): September 2014
©2014 International Society of Arboriculture
255
tainers designed to prevent circling oen direct
roots contacting the wall down to the bottom or
up to the surface. Root deformities oen become
a permanent part of the root system (Grene 1978).
Root ball “shaving” is cutting o the outer surface
of the root ball to remove all roots on the root ball
surface. It results in a root system with roots grow-
ing more radially from the trunk (Burdett 1981; Gil-
man et al. 2010). Root growth aer planting trees
from containers without shaving was one-quarter
of that of eld-grown trees and resulted in reduced
tree stability (Gilman and Masters 2010). Growing
plants in CuCO3-treated containers resulted in the
reduced defects aer planting in the landscape (Bur-
dett 1978; Arnold and Struve 1989; Arnold 1996).
Girdling Roots
Girdling roots have a dierent origin than circling
roots caused by production containers and can be a
signicant problem for at least some species of trees
planted as eld-grown stock. Norway maples (Acer
platanoides) frequently had severely girdling roots as
mature trees (Watson et al. 1990; Wells et al. 2006).
All 50-year-old Norway maples (Acer platanoides)
had one to nine girdling roots. ere was no graing
between girdling roots and trunks (Tate 1980). Gir-
dling roots and potentially girdling roots were more
common on sugar maple (Acer saccharum) and red
maple (Acer rubrum) than on green ash (Fraxinus
pennsylvanica), honeylocust (Gleditsia triacanthos),
littleleaf linden (Tilia cordata), and Yoshino cherry
(Prunus × yedoensis) trees, 2 to 10 years aer planting.
e majority of the girdling roots can be either
small, existing laterals when transplanted, or new
laterals initiated during the rst year aer trans-
planting. Lateral roots at perpendicular angles,
close to the base of the trunk, are naturally posi-
tioned to develop into girdling roots. Growth of
these lateral roots is oen slow while the root ter-
minal is intact, but can be stimulated when the ter-
minal is severed as the tree is dug from the nursery.
Further evidence that girdling roots result from
transplanting is provided by the low incidence of
girdling roots found in nature (Watson et al. 1990).
Girdling roots have been associated with exces-
sive soil over the root system (dAmbrosio 1990; Gib-
lin et al. 2005; Wells et al. 2006), though not always
(Watson et al. 1990). One report hypothesized
through observation that girdling roots are associ-
ated with low dense crowns creating cool and moist
conditions at the base of the tree (dAmbrosio 1990).
Cross-sectional area of vessels in stem xylem
aected by the girdle was only 10% that of unaected
wood. Rays in stem wood were skewed and contained
few pits. Bark on girdled stems was compressed
from a normal thickness. e oending roots sus-
tained slight compression of cells where in contact
with the stem and appeared to remain functional.
us, girdling roots apparently cause tree decline
by reducing stem conductivity and radial commu-
nication between tissues (Hudler and Beale 1981).
Girdling roots do not always cause rapid decline
or death of trees. Aboveground decline symptoms of
girdling roots include gradual shortening of termi-
nal growth, small leaves, early autumn color, dieback
of branches in sections of the canopy, and partial or
total absence of a root are (Gouin 1983; Holmes
1984). A survey of 416 urban Norway maples (Acer
platanoides) found that although 336 had girdling
roots, most girdling was minor and did not lead to
visible decline of the trees (Tate 1981). Red (Acer
rubrum) and sugar maples (Acer saccharum) arti-
cially girdled with angle iron to simulate a girdling
root on one side, remained alive for seven to eight
years, but Norway maples engulfed the girdling
devices and were alive aer 17 years (Holmes 1984).
Treatments consisting of cutting girdling roots,
fertilizing, and pruning foliage were evaluated
aer two years and did not alleviate aboveground
symptoms (Tate 1980). Removal of potential gir-
dling roots resulted in a detrimental eect on
twig extension (Rathjens et al. 2009). Removal
of girdling roots as an early corrective treatment
on young Norway maple trees did not eliminate
them. Multiple roots reformed from the wound
site where a single girdling root had been removed.
Despite this lack of validation by research, girdling
root removal continues to be a common prac-
tice. e best hope for eliminating girdling root
problems may be to develop root stock from trees
without girdling roots (Watson and Clark 1993).
Girdling by wires of the wire baskets used to
support root balls during shipping and handling
is a similar situation. Studies with wires girdling
stems of young trees showed no detrimental eect
of girdling (Goodwin and Lumis 1992). Examina-
tion of roots contacting wires 11 years aer plant-
ing found that root tissues reunited aer closing
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
256
around the wire and there was complete union of
vascular tissue beyond the wire (Lumis and Struger
1988). e small diameter of the wires may pose less
of a threat than larger roots similarly positioned.
Root Grafts
Root graing can be benecial or detrimental to
trees, depending on the circumstances. When root
gras between individuals of the same species
occur, the gras allow passage of solutes through
the connecting xylem (Graham and Bormann 1966;
Jane 1969). Girdled trees with no transport of car-
bohydrates from the crown to the root system can
survive for years if their roots are supported through
gras to roots of neighboring trees (Stone 1974).
Root gras among groups of elms were considered
responsible for more than 50% of Dutch elm disease
disease transmission when the disease was at its peak
in U.S. cities (Cuthbert et al. 1975). Oak wilt is also
commonly transmitted through root gras (Gibbs
and French 1980; Appel 1994). In both situations,
disrupting root gras is an important method of dis-
ease control. Both mechanical and chemical methods
of severing roots have a long history (Himelick and
Fox 1961; Neely and Himelick 1966), with more
recent variations tested (Wilson and Lester 2002).
INFRASTRUCTURE–ROOT CONFLICTS
Pavement Conflicts
When pavements are laid on a compacted soil base,
roots oen grow in the gap between the pavement
and the compacted soil under it. Moisture is high
because the pavement prevents evaporation, and
condensation can form beneath the pavement as
it cools (Kopinga 1994a; Kopinga 1994b; Wagar
and Franklin 1994). Aeration can be adequate,
especially under narrow pedestrian sidewalks
(Kopinga 1994a; DAmato 2002a). Roots enlarge
and can eventually li and crack the pavement.
Species that have a small number larger roots
could cause considerably more damage than if the
same biomass were allocated between larger num-
bers of smaller roots (Nicoll and Coutts 1997).
Potential for conicts between trees and pave-
ment is high when one or more of the following
factors are present: tree species that are large at
maturity, fast growing trees, shallow rooting habit,
trees planted in restricted soil volumes, shallow
topsoil (hardpan underneath topsoil), limited or
no base materials underneath the sidewalk, shal-
low irrigation, distances between the tree and
sidewalk of less than two to three meters, or trees
greater than 15 to 20 years old (Wong et al. 1988;
Randrup et al. 2003). Large trees in restricted
planting spaces is most commonly associated
with pavement damage (Barker 1983; Wagar and
Barker 1983; Wong et al. 1988; Francis et al. 1996;
Achinelli et al. 1997; McPherson 2000; D’Amato
et al. 2002b; Reichwein 2002; Reichwein 2003).
Research has challenged the common assump-
tion that sidewalk pavement cracks near roots are
always caused by the roots. Sidewalk damage can
result from soil conditions and age of pavement as
well as from tree roots. Older sidewalks failed more
oen. Sidewalks did not fail at higher rates where
trees were present (Sydnor et al. 2000). With no
roots present, 61% of all pavement expansion joints
were also cracked (D’Amato et al. 2002a). Roots
were more likely to be found under a cracked expan-
sion joint in the sidewalk than under an uncracked
joint, but the cracks may actually be contribut-
ing to roots growing under sidewalk pavements.
Sidewalks that fail may allow more root growth
beneath the cracks due to increased oxygen in
the soil (Sydnor et al. 2000; DAmato et al. 2002a).
Barriers are sometimes installed to prevent
root growth under pavement. Barriers have been
constructed from plastic, metal screening, and
geotextile impregnated with herbicide. Most
are eective at blocking roots between the sur-
face and the bottom of the barrier if installed
correctly. Dierences in products have some-
times been reported in the rst few years, but
may not persist with time (Smiley et al. 2009).
Installation of root barriers reduces the num-
ber and diameter of roots and causes them to grow
deeper for a limited distance on the far side. is
has been reported consistently, and in both poorly
drained (Wagar 1985) and well-drained and well-
aerated soils (Gilman 1996; Costello et al. 1997;
Nicoll and Coutts 1998; Peper 1998; Peper and
Mori 1999; Smiley 2005; Pittenger and Hodel 2009;
Smiley et al. 2009). Aer they grow under the bar-
rier, roots do grow back toward the surface within
a short distance from the barrier, but may remain
deeper long enough to reduce pavement damage.
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e eectiveness of barriers may not be permanent,
since pavement damage by 30-year-old sweet cherry
(Prunus avium) roots was associated with large
roots as deep as 40 cm below the pavement (Nicoll
and Armstrong 1997; Nicoll and Armstrong 1998).
Depth and installation of the barrier is impor-
tant. A 45 cm deep barrier reduced roots under
the pavement (Smiley 2008a) while a 30 cm barrier
of similar design did not (Gilman 2006). Barriers
need to be installed with the uppermost edge above
grade. If roots are able to grow over the top of the
barrier because of incorrect installation, deteriora-
tion of the exposed barrier material, or mulching
over the barrier, can result in signicant damage to
pavements (Smiley 2008a; Tworkoski et al. 1996).
Barriers can reduce overall root development
of trees (Wagar and Barker 1993; Barker 1995a;
Gilman 1996; Smiley et al. 2009), but in most
studies, no eect on trunk diameter growth was
reported (Barker 1995a; Barker 1995b; Tworkoski
et al. 1996; Costello et al. 1997; Peper 1998; Peper
and Mori 1999; Gilman 2006; Smiley 2008a).
ere is no evidence that root barriers will
decrease stability. Slightly more force was required
to pull over trees within root barriers. e increased
stability was attributed to deeper roots (Smiley et
al. 2000). e situation may be dierent if roots are
not able to grow under the barrier, such as on sites
with very poor soil aeration or very deep barriers.
In such a situation, the limited root system on one
or more sides could result in increased instability.
Other alternatives to root barriers have proven
to be eective in preventing roots from grow-
ing beneath pavements and causing crack-
ing and liing. Extruded polystyrene foam 10
cm thick installed directly under poured con-
crete forced roots to grow under the foam.
e expanding roots crushed the foam instead
of heaving the pavement (Smiley 2008a).
When pavements were laid on a base of coarse
gravel or brick rubble, the coarse material was
apparently not a suitable environment for root
growth between the stones, and the roots grew in
the soil underneath it. icknesses of 15 cm and
30 cm were somewhat more eective than 10 cm
(Kopinga 1994a; Gilman 2006; Smiley 2008a).
A 10 cm thick layer of structural soil beneath
the pavement is not the intended use of structural
soil, but has been used in place of gravel in prac-
tice (Smiley 2008a). Whereas the use of gravel dis-
couraged root growth, a similar 10 cm deep layer
of structural soil allowed vigorous root growth in
the soil between the coarse stones, as it is designed
to do. Roots in the stone layer resulted in extensive
pavement cracking and liing. When structural
soils are used with a minimum depth of 60 cm, or
a preferred depth of 90 cm, roots grew to the full
depth of the structural soil and were not found
exclusively at the surface (Grabosky et. al. 2001).
Certain root barrier products that are impregnated
with herbicides to reduce root growth can be eec-
tive as root barriers, but raise concerns that mycor-
rhizae could be aected. Sweetgum (Liquidambar
styraciua, endomycorrhizal) and pin oak (Quercus
palustris, ectomycorrhizal) root mycorrhizae col-
lected from within 1 cm of a chemically impreg-
nated barrier were unaected in the only reported
study (Jacobs et al. 2000). (For an extensive review
of root barrier research, see Morgenroth 2008.)
Just as disease resistance is the preferred way
to control a tree disease, developing trees with
deeper root systems would be the best way to
reduce pavement damage. Research has shown
that root systems of certain tree species that oen
cause sidewalk damage [e.g., shamel ash (Fraxinus
uhdei), zelkova (Zelkova serrata), Chinese pistache
(Pistacia chinensis)] can be selected for deep rooting
patterns. Unfortunately, when these trees were
propagated by rooting cuttings; the propa-
gated trees did not exhibit the same deep-root-
ing characteristic (Burger and Prager 2008).
Sewer Pipe Intrusion
Tree root intrusion into sewer systems can be a sub-
stantial problem. Tree roots rarely damage pipes,
but Mattheck and Bethge (2000) hypothesize that
when a tree root encircles a pipe, wind loading
may result in enough movement to break the pipe,
especially when this occurs near material defects.
Roots can enter pipes in breaks and loose joints
and then proliferate rapidly once inside the moist,
nutrient-rich environment. Older pipes have more
root intrusions because of age and materials used.
Clay and concrete pipes without rubber gaskets in
the joints resist root intrusion the least. e most
intrusions have been into the smaller dimension
pipes, 22.5–40 cm, possibly because the larger pipes
are usually deeper in the soil and the roots may not
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
258
reach them as easily. Sandy soils are more easily
penetrated by roots reaching pipes. In poor growing
conditions, the roots seek their way into the pipes
relatively quickly, while in good growing conditions
the process is considerably slower. In general terms,
full-grown trees that have a large crown volume
and thus a high requirement for water during the
growing season have the greatest potential to cause
large-scale damage to sewage systems by root intru-
sion. Certain tree species, such as poplar (Populus),
willow (Salix), Melaleuca (Melaleuca), and Eucalyp-
tus (Eucalyptus) are more likely to cause root intru-
sion. Tree size and proximity to the sewer pipe are
also factors (Stål 1992; Lidstrom 1994; Rolf and Stål
1994; Stål 1995; Pohls et al. 2004; Ridgers et al. 2006).
Herbicides have been used to control root growth
in sewer pipes. Metam-sodium and dichlobenil in
combination is the most common. Metam-sodium is
non-systemic and does not move throughout the root
system, killing the whole plant. Dichlobenil is used
with metam-sodium because it is an eective growth
inhibitor. Air-aqueous foam is more eective than
an aqueous mixture. e amount of chemical used
in the foam application is small. Rapid breakdown
of the metam-sodium and dilution of the product
in the wastewater minimize environmental impacts,
but use is still restricted in many areas (Ahrens et
al. 1970; Leonard and Townley 1971; Leonard et al.
1974; Prasad and Moody 1974; Pohls et al. 2004).
Strategies to combat root intrusion are lim-
ited. Tree roots are less likely to grow into sewer
pipes if planted 6 m or more from existing
pipes. Slower-growing species with less aggres-
sive root systems are best. Pipe construction
can reduce intrusion by using longer pipe seg-
ments with fewer joints and proper installa-
tion (Rolf et al. 1995; Stål 1998; Randrup 2000).
Foundation Damage
Tree roots have been associated with interference
with building foundations but rarely cause direct
damage. Force from roots increasing in diameter
is small, and damage only occurs to lightly loaded
structures (Day 1991; Macleod and Cram 1996).
Roots in the vicinity of shallow foundations on
soils with a high shrink-swell capacity can con-
tribute to soil moisture depletion during drought,
causing the soil to shrink and the building foun-
dation to settle and crack (Day 1991). Records
in England show that the incidence of failure of
foundations on shrinkable clay soils is greater by
a factor of ten than on other soils (Pryke 1979).
Tree genera vary in the amount their root sys-
tems can spread and contribute to building subsid-
ence. [Roots cannot be reliably identied to species
through anatomical features (Cutler et al. 1987).]
e distance between damaged foundations and
the tree with roots contributing to the damage was
recorded for over 11,000 trees in the Kew Tree Root
Survey. e average distance at which foundation
damage was recorded varied from 2.5 m for cypress
(Cupressus) to 11 m for poplar (Populus) with dam-
age from most species occurring between 5 m and
7 m (Cutler and Richardson 1989). Depth of water
extraction by roots may be restricted by soil condi-
tions. Sharp changes in water and air permeability
retarded rooting and water extraction beyond the
upper 0.5 m of soil (Misra and Sands 1993). Species
such as ash (Fraxinus), with relatively poor stomatal
control of water loss, may accelerate soil drying,
and therefore shrinkage (Stewart and Sands 1996).
Coutts (1979) suggests that since roots will
grow where conditions are most favorable,
and urban landscapes oen have pavements
and other features that restrict root growth in
areas away from buildings, the most favorable
soil may be between the tree and the building.
Control of roots with barriers is not considered an
acceptable solution. Roots can grow under or over
the barrier if not properly installed (as previously
stated), or through cracks that may develop over
time (Marshall et al. 1997). When roots are deected
laterally, there is a tendency to resume the original
direction of growth once past the barrier (Wilson
1967), unless the barrier is long (Riedacker 1978).
Pruning is ineective in controlling water use.
Crown thinning did not reduce total tree water use
or soil drying. A crown reduction of over 70% by vol-
ume aected water use for only a single season (Hipps
2004). e only way to ensure that there will not be
a recurrence of the subsidence event aer repair is
to remove the tree (O’Callaghan and Kelly 2005).
Two solutions to the problem are to plant the
tree well away from the structure or to use deep-
ened perimeter footings to restrict roots from
gaining access beneath the foundation (Day 1991).
A combination of these is employed in the Brit-
ish National House Building Council guidelines,
Arboriculture & Urban Forestry 40(5): September 2014
©2014 International Society of Arboriculture
259
which provide recommendations based on shrink-
ability of the soils, the depth of the foundation,
and the water demand and mature height of the
tree. On a highly shrinkable soil, if a high water
demand tree is located a distance equal to its height
away from the foundation, the foundation should
be 1.5 m deep. At half of that distance, a 2.5 m
deep foundation is recommended (Biddle 1998).
ROOT SPACE REQUIREMENTS
When trees are planted in paved areas, the limited
root space available in planting pits will ultimately
limit the size and longevity of the tree (Fluckiger and
Braun 1999). Average tree life expectancy in a side-
walk pit can be as little as ten years (Kopinga 1991;
Nowak et al. 2004). Root restriction can reduce shoot
elongation and decrease root dry weight:leaf area
ratio. Imbalanced root:shoot ratios caused the devel-
opment of internal water stress and plant senescence
(Tschaplinski and Blake 1985; Vrecenak et al. 1989;
Rieger and Marra 1994; Ismail and Noor 1996).
Crown spread and trunk diameter of trees grow-
ing in parking lots is reduced as surface area of non-
paved surfaces is reduced (Grabosky and Gilman
2004). Ninety-six percent of parking lot trees with at
least 28 m3 of soil were in good condition, compared
to only 60% in less than 14 m3 of soil. However,
over 80% of the trees had been planted in the last
12 years (Kent et al. 2006), and condition of trees
is likely to deteriorate as the trees grow and reach
the limits of even the most generous root space.
Soil Volume and Quality
Variables to consider when determining how much
root space is needed includes the quality of the
soil present (water and nutrient storage capacity),
how much evaporation and transpiration is ex-
pected, and how oen the tree will receive rainfall
or irrigation. As a general guideline for temperate
climates, if above- and belowground environmen-
tal extremes are not severe, the root space recom-
mendations vary from 0.15 to 0.7 m3 of soil for
each square meter of crown projection area of the
expected mature size of the tree (Kopinga 1985;
Lindsey and Bassuk 1991; Lindsey and Bassuk 1992;
Urban 1992; Urban 2008). Similar estimates have
not been developed for arid and semi-arid climates.
A computer model has been developed that
uses climatological data to estimate the soil vol-
ume necessary to provide moisture in growing
conditions likely to be encountered for an area.
e example used is New York City, New York,
U.S., with a 6 m crown diameter tree and 17 m3
of soil, as recommended by Lindsey and Bassuk
(1991). e tree, without irrigation, would face
a water decit every other year. With 27.4 m3 of
soil the tree would face a decit only once in 10
years, but with only 4.3 m3 of root space soil, the
tree would need irrigation every h day to face
a decit only once in 10 years (DeGaetano 2000).
Using a dierent method, Blunt (2008) calculated
that under UK weather conditions, a mature tree
(size and species not specied) would require at
least 50 m3 of high-quality soil with soil moisture
recharged by rainfall or irrigation ten times dur-
ing the growing season to avoid drought stress.
When soil volume is restricted, soil quality
becomes very important. High-quality soil and
intensive maintenance can compensate for limited
root space volume to a limited extent. When soil
was amended with organic matter to 60 cm depth,
root development was greater than when just the
upper 15 cm was amended (Smith et al. 2010).
It is generally accepted that when soil volumes
are combined and shared by several trees, the per-
formance of the trees seems to be better than when
trees are in several smaller, individual planting
pits of the same total volume. Research to sup-
port this observation is limited. Condition of live
oaks (Quercus virginiana) was better in shared
planting spaces but not lacebark elm (Ulmus
parvifolia) or red maple (Acer rubrum). Maples
performed poorly in all root spaces, and other
factors may have been more limiting than shared
root space. e elms performed well even in very
limited, non-shared root spaces and may be less
sensitive to small root spaces (Kent et al. 2006).
Expanding Root Space
Soils under pavements can be very dicult for
root growth. e pavement itself can have mixed
eects on the root environment beneath it. Soil
moisture can be greater under pavement than
surrounding unpaved areas because of reduced
evaporation (Hodge and Boswell 1993; Arnold
and McDonald 2009). Maximum summer soil
temperatures under pavement exposed to sun
can be up to 10ºC warmer than nearby unpaved
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
260
areas and exceed levels that injure tree roots
(Halverson and Heisler 1981; Graves and Dana
1987). e soil compaction necessary to support
stable pavement oen restricts root growth. Sev-
eral approaches have been used to provide suit-
able conditions for root growth under pavements
without compromising stability of the pavement.
Pervious Paving
It has been suggested that pervious paving materials
could improve the soil environment beneath pave-
ments for better tree growth, but research has not yet
shown this to be consistently true. Soil oxygen was
insucient for root growth (less than 12% oxygen)
for prolonged periods beneath two of ve pervious
paving products tested on park footpaths (Couen-
berg 2009). Dierences in soil oxygen and moisture
between impervious and pervious concrete pave-
ments are inconsistent (Morgenroth and Buchan
2009; Viswanathan et al. 2011). Pervious concrete
plots had greater soil moisture in deeper layers in
some seasons, but not in summer when it would be
most benecial, and there was no dierence in tree
growth rates, leaf water potential, or gas exchange
(Volder et al. 2009). e narrow pavements (less
than 1.5 m wide) used in these studies may allow
water and oxygen to diuse under the pavement
from the edges of the solid pavement, just as eec-
tively as through the pores of the pervious pavement.
To function correctly, pervious concrete pavement
systems must have underlying soil that percolates
well, which should also be benecial for roots. If soil
beneath the porous pavement is too compacted, the
resulting poor soil aeration and penetration resis-
tance are more likely to factor in limiting tree perfor-
mance than the pavement (Viswanathan et al. 2011).
Structural Soils
Soils designed to support pavement without settling
are oen called load-bearing, skeletal, or structural
soils. To expand root space under pavement in this
way, the soil must provide a favorable environment
for root growth while supporting the pavement.
e rst soil of this type developed was called
Amsterdam Tree Soil. Specications call for 91%–
94% medium coarse sand, 4%–5% organic matter,
and 2%–4% clay (by weight). Phosphorous and
potassium are added as necessary. e organic mat-
ter provides a source of nitrogen (Couenberg 1993).
e soil mix is carefully compacted to a 70%–80%
Proctor density when installed, and aeration is pro-
vided through spaces in the pavers placed over the
soil. Callery pear (Py rus calleryana) trees grew almost
twice as rapidly in Amsterdam Tree Soil compared
to standard pavement construction, and 50% faster
than those grown in grass (Rahman et al. 2011).
Stone–soil mix structural soils create a network
of interconnected spaces between the stones
that can be lled with soil for root growth.
When mixed and installed properly, structural
stone-soil mixes compacted to 1.85 g cm-3, and
greater, and did not reduce macropore space or
restrict root penetration in the soil between the
stones (Grabosky and Bassuk 1996; Grabosky
et al. 2009). In a container study, structural soil
held 7%–11% moisture by volume, similar to a
loamy sand, and had high inltration and good
drainage and aeration (Grabosky et al. 2009),
but no eld measurements have been reported.
Early tests of structural soil mixes in containers
showed that stone–soil mixes could support better
root and top growth than compacted soils or typical
road base materials (Grabosky and Bassuk 1995;
Kristoersen 1999). Growth was limited by net
soil volume rather than the total volume of the
stone–soil mix (Loh et al. 2003). e root:crown
ratio was greater in stone mixes than topsoil alone,
indicating a larger root system was needed for
absorption of water and nutrients when the soil
was spread out in the mix (Kristoersen 1999).
Results of eld studies have been mixed. At
three and ten years aer installation, growth (DBH,
height, canopy width) of trees planted in structural
soil under pavement was equal to trees planted
at the same time in a lawn adjacent to the side-
walk (Grabosky et al. 2002; Grabosky and Bassuk
2008). However, the trees planted in structural soil
were within a few feet of an adjacent open-lawn
area and the possibility that their roots may have
grown into that soil volume was not addressed in
the report. Other reports show that trees planted
in non-compacted soils in open planters (Bühler
et al. 2007) or covered by suspended pavement
(Smiley et al. 2006) will outperform structural soil
mixes. Stone–soil mixes can be a useful compromise
in situations where high quality non-compacted
soils cannot be used, but will not produce the
same results in an equal volume of quality soil.
Arboriculture & Urban Forestry 40(5): September 2014
©2014 International Society of Arboriculture
261
Structural soils may increase tree anchorage. Trees
were more stable in structural soils than traditional
tree pits due to greater root length in gravel-based
skeletal soil (Bartens et al. 2010). is is supported by
a computer model in which a 20% soil to 80% gran-
ite chip mix was optimum for withstanding wind
forces required to uproot trees (Rahardjo et al. 2009).
Suspended Pavement
If the pavement is suspended above the soil, the soil
does not have to be compacted to support it. Sus-
pended pavements range from elaborate designs con-
structed on-site to simpler and smaller precast con-
crete structures. Trees grew better in non-compacted
soils under suspended pavement than in compacted
soil or two structural soil types (Smiley et al. 2006).
e study design did not include non-compacted
soil without pavement over it, though experience has
shown that trees will grow even better in open soil.
Root Paths
Root paths are narrow trenches installed in a com-
pacted subgrade under pavement to provide a path
for roots to grow from restricted planting pits to
open spaces on the other side of the pavement.
Commercially available strip-drain material is usu-
ally installed in the trench and then backlled with
loam soil (Costello and Jones 2003; Urban 2008). It
could take several years for roots to grow through
the root path and access the soil beyond. ere is
not yet any research to show that roots are able to
eectively take advantage of the paths to access
the soil beyond the pavement and improve tree
growth and longevity, or that if roots do utilize
the paths that they will not li the pavement.
Soil conditions suitable for root growth under
pavements also provide some level of stormwater
storage (Day and Dickinson 2008). If signi-
cant, this could be additional justication for
the higher cost of the expanded root space.
ENHANCING ROOT DEVELOPMENT
Irrigation
Trees are not irrigated in their natural environment.
Healthy, established urban trees with adequate root
space of quality soil are not typically dependent
on irrigation if they are adapted to the climate in
which they are growing. Little research is available
on irrigation needs of established urban trees. A
greenhouse study of ponderosa pine (Pinus pon-
derosa) showed that when water stress occurred
during active root growth, the root:shoot ratio was
reduced. When water stress occurred during active
shoot growth, the root:shoot ratio was increased
(McMillin and Wagner 1985; Silva et al. 2012).
Transpiration rates and pan evaporation are
strongly correlated for woody species. Transpira-
tion of larger trees is approximately 20% of pan
evaporation (Knox 1989; Lindsey and Bassuk 1991).
Because of more direct sunlight on the south side
of the tree there may be greater water stress on the
south side of the tree (Watson and Himelick 1982b;
von der Heide-Spravka and Watson 1990). Increased
irrigation may be appropriate on the south side of
larger trees to compensate. Trickle irrigation can
concentrate root development within the wet zones
near the emitters (Levin et al. 1979; Mitchell and
Chalmers 1983; Fernandez et al. 1991; Watson et
al. 2006; Sokalska et al. 2009). Less frequent irri-
gation with the same amount of water can result
in a wider distribution of roots (Levin et al. 1979).
In the summer, soils moist from irrigation and
drainage changes can be a major cause of oak (Quer-
cus) mortality in Mediterranean climates (Costello
et al. 2011). e moisture and warm soil tempera-
tures create conditions favorable to the develop-
ment of root and crown rot diseases (Swiecki 1990).
Controlled studies of irrigation needs of large
trees subject to root severing and loss are dicult
to conduct, but studies on irrigation of transplanted
trees with substantial root loss can provide informa-
tion. Newly planted trees have reduced growth if
subjected to water stress aer transplanting (Haase
and Rose 1993). Applying excessive irrigation may
reduce root growth and increase the time needed
for the tree to develop enough of a root system to
survive without irrigation (Gilman et al. 2009).
Proper irrigation can reduce secondary stress-
related problems, such as bark cracks, sunscald,
and injury from borers (Roppolo and Miller 2001).
Fertilization
Total tree root system development is greater when
soil nutrients are low (Kodrik and Kodrik 2002).
Fertilization may not stimulate root growth unless
low levels are already limiting root growth (Philip-
son and Coutts 1977). An increase in soil fertility
Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings
©2014 International Society of Arboriculture
262
is commonly associated with a reduction in the
root:shoot ratio; that is, shoot growth increases
more than root growth (Ingestad 1960; Philip-
son and Coutts 1977; Coutts and Philipson 1980;
Nambiar 1980; Yeager and Wright 1981; Glea-
son et al. 1990; Warren 1993; Lloret et al. 1999;
Jose et al. 2003; Qu et al. 2003; Rytter et al. 2003).
Fertility can alter the distribution of roots. Fine
roots will grow preferentially in pockets of nitro-
gen rich soil (Wahlenberg 1929), by stimulat-
ing the growth of lateral roots (May et al. 1964;
Hackett 1972; Eissenstat and Caldwell 1988; Witt
1997). Root growth may be increased even more
when nitrogen availability is low outside the
pocket (Krasowski et al. 1999). Application of
nitrogen to a part of the root system has a strictly
localized eect and does not increase overall root
growth or alter the shoot:root ratio (Smith 1965;
Drew et al. 1973; Drew and Saker 1975; Coutts and
Philipson 1976; Carlson 1981; Carlson and Pre-
sig 1981; Friend et al. 1990; Sheri and Nambiar
1995). Enhanced growth of one part of the root
system can reduce growth in the other (Weller
1966; Phillipson and Coutts 1977). Severe soil
compaction reduced nitrogen fertilizer uptake
and was presumably related to the reduced uptake
by a smaller root system (Jordan et al. 2003).
Fertilization may be necessary to maintain
appropriate vigor and growth rates of urban trees if
natural nutrient cycling is interrupted through the
removal of fallen leaves and branches. In an Eastern
deciduous hardwood forest, nitrogen in fallen litter
was measured at 0.27–0.46 kg N/100 m2/yr (Wells
et al. 1972; Larcher 1975). Arboricultural best man-
agement practices (Smiley et al. 2007; ANSI 2011)
recommend 0.96–1.44 kg N/100 m2, but allow up
to 2.88 kg N/100m2. ese rates far exceed nutri-
ents lost through litter removal and may not be
appropriate for slower growing mature trees. Lawn
fertilization alone may more than replace nutrients
lost by removal of litter (Osmond and Hardy 2004).
Root Stimulants
Paclobutrazol, uniconazole, and flurprimidol are
gibberellin-inhibiting growth regulators used pri-
marily to reduce shoot growth of trees, but can
also increase root growth under certain circum-
stances (Numbere et al. 1992). Paclobutrazol may
promote root initiation (Davis et al. 1985). Pin
oak (Quercus palustris) and white oak (Quercus
alba) fine-root densities were increased signifi-
cantly throughout the root system by a basal soil
drench of paclobutrazol. The treatment may be
effective in stabilizing slowly declining trees
with insufficient fine-root development (Wat-
son 1996; Watson and Himelick 2004). Fine-root
density was not affected by paclobutrazol treat-
ment in a high quality soil environment from
long-term mulched application where root den-
sity may have been high initially, limiting the
ability of paclobutrazol to increase them further
(Watson 2006). Species may differ in their
response to paclobutrazol. Growth of green ash
(Fraxinus pennsylvanica) roots was unaffected
by paclobutrazol treatment (Watson 2004).
e ability of paclobutrazol to increase root growth
may depend on root–leaf area ratio. Paclobutrazol
applied at planting doubled root growth on black
maple (Acer nigrum) in the rst season before
crown growth was reduced by the paclobutrazol
treatment, but not the second when crown growth
was greatly reduced (Watson 2004). e large reduc-
tion in top growth may have been responsible for
the lack of root stimulation in the second year.
Gilman (2004) also reported that paclobutrazol
had no eect on root growth of transplanted live
oaks (Quercus virginiana) at a rate that reduced
top growth. Root pruning can enhance the growth
regulation eects of paclobutrazol treatment and
slow root growth (Martinez-Trinidad et al. 2011).
Soil applications of sugar solutions have been
tested to increase root growth. Root growth is
oen but not always increased and may depend
on tree species, kinds of sugars, and application
rates included in the trials (Percival 2004; Percival
et al. 2004; Percival and Fraser 2005; Percival
and Barnes 2007; Martinez-Trinidad et al.
2009). Measurable increases in tree vitality are
uncommon, even on small experimental plants.
ere is extensive published research from a
broad spectrum of plant sciences that can be applied
to the prevention and mitigation of human impacts
on urban tree root systems. e majority of litera-
ture available on structural soils, tree root archi-
tecture, root locating methods, and root defects
has been produced in the last 25 years. At the same
time, advances have been made in understanding
topics such as infrastructure conicts and fertiliza-
Arboriculture & Urban Forestry 40(5): September 2014
©2014 International Society of Arboriculture
263
tion practices, but these advances are still not fully
understood. Arboricultural science is young and
growing. ere is hardly a topic that would not ben-
et from extensive additional research. e wide
variety of species and environmental circumstances
in urban landscapes makes it especially challenging.
Acknowledgments. We would like to thank the International
Society of Arboriculture and the ISA Science and Research Com-
mittee for funding this literature review.
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Gary W. Watson (corresponding author)
e Morton Arboretum
4100 Illinois Route 53
Lisle, Illinois 60532, U.S.
gwatson@mortonarb.org
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.
Zusammenfassung. Das Wurzelsystem von nahezu allen Bäu-
men in bebauten Bereichen ist den Einüssen von menschlichen
Aktivitäten ausgesetzt, welche die Baumgesundheit beeinussen
und die Langlebigkeit reduzieren. Diese Einüsse sind von der
Frühphase der Baumschulentwicklung bis durch das ganze Leben
der Bäume präsent. Durch Baumaßnahmen oder Wurzelverlust re-
duzierte Wurzelsysteme können die Stabilität beeinussen und den
Stress vergrößern. Natürliche Infektionen von Wurzeln bei Straßen-
bäumen durch Abtrennen führten nachweislich nicht zu extensiver
Fäulnis. Wurzeln geraten wegen ihrer Nähe zur Infrastruktur in
Konikt mit der urbanen Umgebung. Strategien zur Bereitstellung
von Wurzelraum unter den Pasterächen und zur Reduzierung
von angehobenen Pasterächen wurden entwickelt, aber die Strat-
egien zum Schutz von Fundament- und Abwasserrohrschäden sind
begrenzt auf wachsende Separation oder verbesserte Konstruktion.
Resumen. Los sistemas de raíces de casi todos los árboles en el
entorno construido están sujetos a los impactos de las actividades
humanas que pueden afectar su salud y reducir su longevidad. Estas
inuencias están presentes desde las primeras etapas de desarrollo
en viveros y luego durante toda la vida del árbol. Sistemas de raíces
reducidos, pérdida o constricción de las mismas pueden disminuir
la estabilidad del árbol y aumentar el estrés. La infección natural
de las raíces de los árboles urbanos después de su ruptura no se ha
demostrado que conduzca a un extensivo decaimiento. Las raíces
a menudo entran en conicto con la infraestructura en las zonas
urbanas debido a la proximidad. Se han desarrollado alternativas
para proporcionar espacio para las raíces bajo las aceras y reducir la
aglomeración, pero las estrategias para la prevención de daños por
pavimentación y tubería de alcantarillado se limitan a aumentar la
separación o la mejora de la construcción.
... Consistently, early research on root severance found that tree losses can increase up to 44% following severance (Morell, 1984) and that leaf wilting can occur within a few hours after roots are damaged (Hamilton, 1988). After reviewing a series of studies on root severance, however, Watson et al. (2014) found that the impact of excavation on tree health was much lower than expected and that symptoms may not be visible for several years after the roots have been severed. ...
... Other studies found progressive reductions in growth over time and a low resilience (i.e. the ability to recover to the pre-damage condition) displayed by woody trees to excavation and hypothesized increased sensitivity of trees with severed roots to water stress (Wajja-Musukwe et al., 2008;North et al., 2017). A widespread conclusion of these studies was that root severance presumably caused stress, but neither the stress was measured directly, nor its effects on plant physiology characterized (Watson, 1998;Watson et al., 2014). ...
... Few studies have evaluated so far the early impact of root severance on tree mechanical stability, but none, to our knowledge, has clarified the capacity of trees to re-establish their safety factor after a relief period that allowed trees to recover the severed roots (Forcaud et al., 2008;Watson et al., 2014). When the winching test was conducted again, four years after severance, differences in bending moment between control and severed trees were maintained or even increased. ...
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Urban forests provide critical environmental benefits, but the resilience of these socio-ecological systems to stresses like pest and disease outbreaks relies on tree health and diversity. Despite this, low species diversity continues to be a challenge in urban forest management. Using a participatory research approach in central Florida (United States), we selected and tested underutilized native tree species (Celtis laevigata Willd., Ilex vomitoria Aiton, Taxodium ascendens Brongn., Ulmus alata Michx., and Viburnum obovatum Walter) in two urban settings (streetscape and park) in four communities (total n = 200). Our collaborative process was organized into five steps, including a 2-year monitoring period to assess mortality and health through establishment. At the end of the trial, 156 trees survived with annual mortality rates differing by species and plot type. Taxodium ascendens had the highest annual mortality of the five species trialed. Overall, U. alata and V. obovatum showed the greatest potential in central Florida urban settings. Our tree selection process can guide others who want to create forward-thinking and diverse planting lists. Furthermore, this project demonstrates that co-production of knowledge involving members of local municipalities, practitioners, and researchers can be an effective strategy for selecting and testing underutilized tree species.
... Mature trees with well-developed root systems (North et al., 2015) within constrained planting areas (Watson et al., 2014;Hilbert et al., 2020b) are therefore more likely to lead to surface displacement. ...
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It is well-accepted that urban trees provide many benefits to society, but there are costs associated with their establishment and maintenance. Some indirect costs of juxtaposing trees with urban infrastructure are linked to the way in which tree roots interact with hard surfaces such as footpaths (sidewalks), which can result in expensive repairs and in some instances, tree removal. There is a need to understand the complex interactions between tree roots and infrastructure, to inform strategic planting and balance the needs of all stakeholders. In this short communication, we introduce a simple, cost-effective method for quantifying footpath displacement using Arduino robotics and provide the schematics and coding as an open-source tool. Using an ultrasonic sensor, the robot generates a 2.1 m long, two-dimensional profile of a given surface. The accuracy of the robot is validated with objects of known size and was subsequently field tested using 15 Liquidambar styraciflua growing in a suburban street. The robot allowed us to quantify the maximum (highest vertical point) and total (the area under the curve) displacements in the footpath surface. Trunk diameter and proximity to the footpath were significant predictors of displacement at P < 0.05, supporting the findings of other researchers. A larger dataset is required for more generalisable results, but the robot produced reliable data in this proof-of-concept field test.
... Tree roots take the brunt of soil problems, bringing limited root spread, branching and density, obstructed and contorted rooting pattern, and circling and girdling roots. Some roots penetrate the tiny gap between paving and buried soil surface to induce pavement cracking and heaving (Randrup et al., 2001;Day et al., 2010;Watson et al., 2014). ...
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Tree growth in urban areas is beset by multiple stresses, of which the soil component has imposed severe constraints. In the tropics, the inherently infertile and acidic soils present challenges that demand dedicated solutions. Yet, the key edaphic issues have remained poorly understood and often inadequately evaluated and improved. Where site soil is poor or absent, a soil specification usually prescribes a fabricated soil mix. Unfortunately, these documents frequently include non-essential attributes or exclude essential ones. The specified ranges and thresholds tend to detach from soil science concepts or are irrelevant to local conditions. Some projects would adopt specifications prepared for other local or international sites without modifications to match site requirements. Errors tend to be copied and propagated in different documents. Knowledge transfer from researchers to practitioners can improve the soil specifications to resolve a significant and chronic weakness in urban forestry. Five representative local documents and five from other countries were critically evaluated in detail to distill 21 principal concerns and 20 principles of soil specification design. The knowledge base informed the development of a rationalized and improved soil specification for urban forestry in the tropics. It included the fabricated topsoil mix and fabricated subsoil mix to meet most planting needs, and a dedicated fabricated lightweight mix for planting on rooftops with limited load-bearing capacity. Further explanations justified using local raw materials, soil sampling strategy, soil properties, recommended ranges, and standard laboratory testing methods. Other tropical regions can modify the proposed specification to fit local circumstances and specific landscape needs.
... However, high mortality rates observed in these conditions often limit the effectiveness of tree planting programs programs (Widney et al., 2016). In landscapes where soils have been disturbed during construction, like highway roadsides, the establishment period for trees is often prolonged due to poor soil quality (Jim, 1998b;Watson et al., 2014aWatson et al., , 2014b. During highway road construction, topsoil is removed and stockpiled for extended periods of time, the subsoil is compacted and then a portion of the topsoil is reapplied (Craul, 1985;Jim, 1998b). ...
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... Watson et al. [43,44] suggest that managing trees on urban sites requires an integration of multiple approaches (e.g., mulch, irrigation, aeration, prevention of compaction) to overcome site and soil limitations. The results of the present study indicate that plant growth regulators (PGR's) such as paclobutrazolmay provide arborists with an additional tool to manage trees in stressful locations. ...
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Paclobutrazol is a tree growth regulator that is frequently applied by arborists to control tree growth in utility rights of way. Paclobutrazol is also marketed to mitigate tree stresses associated with urban environments. In this study we applied paclobutrazol as a soil drench to honeylocust (Gleditsia triacanthos L var. inermis (L.) Zab.) and Callery pear (Pyrus calleryiana Decne.) trees planted as street trees on two sites in Lansing, Michigan USA. We evaluated physiological and morphological responses for two years after treatment. Application of paclobutrazol increased SPAD chlorophyll index of trees of both species in both years, compared to untreated control trees. Application of paclobutrazol increased leaf water potential of trees on one study site (Downtown) but not the other (Old Town). Paclobutrazol increased gas exchange (net photosynthesis and stomatal conductance) of Callery pear trees on one of four measurement dates (gas exchange was not measured on honeylocust trees). Leaf size of Callery pear trees was reduced following paclobutrazol application whereas leaf size of honeylocust trees was unaffected by paclobutrazol. These results indicate that paclobutrazol can help to reduce stress of trees and improve physiological function under urban conditions. However, paclobutrazol should be viewed as part of a suite of options for arborists and landscapers to manage trees on stressful sites, rather than as a replacement for proper overall care.
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The performance of laboratory X‐ray computed tomography (XCT) for the non‐destructive imaging of root wood was evaluated. Lateral roots of oriental cherry (Prunus serrulata var. spontanea) and Japanese zelkova (Zelkova serrata) were severed in spring and maintained in soil for 6 months. Without sectioning, XCT revealed the phloem, xylem and vascular cambium structures in the root wood. A virtual transverse section showed a ring of woundwood covering the severed, lateral root of the two trees. Different levels of X‐ray absorption were evident around the cut surfaces of P. serrulata; however, they were rarely detected in Z. serrata. More adventitious roots were observed on Z. serrata than on P. serrulata. Distinct white spots in the rays were only detected in Z. serrata. These results suggest that XCT has potential applications in forest pathology, providing virtual sections of wound closure, wood density distribution, organ redifferentiation, and mineral deposition in root wood.
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Irrigation frequency and volume effects were evaluated on recently installed #3 container grown shrubs of three taxa, Ilex cornuta Lindl. & Paxt. ‘Burfordii Nana’, Pittosporum tobira Thunb. ‘Variegata’, and Viburnum odorotissimum Ker Gawl. Irrigation frequency and volume had no effect on Pittosporum at any time for any measured root or shoot parameter. Irrigation frequency and volume had no effect on Ilex and Viburnum canopy biomass, root biomass, root dry weight:canopy dry weight ratio, and stem water potential at any time after planting. Canopy growth was affected by irrigation treatment only for Viburnum plants installed in May 2004, and growth response to more frequent irrigation only occurred while plants were irrigated, with no lasting impact on growth once irrigation ceased. Root spread and root spread:canopy spread ratio for only one species, Ilex, were influenced by irrigation treatment. Applying excessive irrigation volume (in this case 9L) reduced root dry weight: shoot dry weight ratio for Ilex and could increase the time needed for plants to grow enough roots to survive without irrigation. Our study found only slight influences on shrub growth from the tested values of irrigation frequency and volume regardless of the time of year when data was collected. This indicates that these shrubs can be established with 3 liters irrigation applied every 4 days until roots reach the edge of the canopy under the mostly above normal rainfall conditions of this study. Applying more volume or irrigating more frequently did not increase survival or growth. Canopy growth and plant quality data combined with past research suggest that establishment of these shrub species may be more influenced by environmental conditions such as rainfall than by the irrigation frequency and volume used in this test.
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The influence of two commercially available auxin products (IBA talc, IBA:NAA solution) in combination with a water-retaining polymer applied to the root system of two transplant-sensitive tree species, silver birch (Betula pendula Roth.) and beech (Fagus sylvatica L.) under field conditions was investigated. The efficacy of the auxins on growth was quantified by recording root and shoot growth and survival rates at week 8 and 20 after bud break. Improvements in tree vitality were assessed by measurement of leaf photosynthetic rates, chlorophyll fluorescence and chlorophyll content. Application of an IBA talc improved growth and vitality of beech but had little beneficial effects on birch. A combination of liquid IBA:NAA (50:1 dilution) and a water-retaining polymer at transplanting proved most effective for root regeneration, growth and tree vitality of both species compared to other treatments. Regardless of species, applications of a water-retaining polymer alone had no significant effect on tree survival rates or tree vitality. However, growth of birch was significantly reduced compared to controls indicating a detrimental effect of polymer application alone on this species. Results show that commercially available auxin products in combination with a water-retaining polymer can be used to reduce transplant losses and improve tree vitality and growth over a growing season in two difficult-to-transplant species. Selection of an appropriate auxin(s), however, is important as effects on growth and vitality can vary between tree species.
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Exposed fine roots are subject to desiccation, which may affect their survival as well as new root growth following bare root transplanting. Fine roots of dormant 1-year-old green ash (Fraxinus pennsylvanica) and sugar maple (Acer saccharum) seedlings, subjected to desiccation treatments of 0, 1, 2, or 3 hours in December and March, lost up to 82 percent of their water. Root electrolyte leakage, a measure of cell damage, tripled after three hours of desiccation. The increase was moderately, but significantly, greater in March for both species. Desiccation treatments had no effect on fine root survival. Growth of new roots (RGP) was also unaffected by desiccation treatments. RGP of maple was greater in March than December, but not ash.
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Metham (sodium methyldithiocarbamate), applied alone or in combination with dichlobenil (2,6-dichlorobenzonitrile) in foam, was effective in killing roots of eucalyptus ( Eucalyptus camaldulensis Dehnhardt) or willow ( Salix hindsiana Benth.). An air-aqueous (19 to 1) foam of these herbicides was at least 20 times as effective as the aqueous mixture alone. Killing of the root with metham was rapid and extended above the lower treated portion, with the extent of necrosis resulting from translocation of the herbicide varying with concentration of metham that was used. The amount of the root killed with dichlobenil was limited to the treated area regardless of concentration. Four weeks were required to control the larger roots. Root killing with metham proceeded via both the aqueous and vapor phases. Results from labeling trees with ¹⁴ C-assimilates indicated that neither translocation nor accumulation were greatly affected by metham or dichlobenil except in the tissues actually killed. However, transport and accumulation into untreated roots were reduced for a few weeks by dichlobenil. Similar results were obtained with cotton ( Gossypium hirsutum L. ‘Acala’) treated with dichlobenil.