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Relationship of Structural Root Depth on The Formation of Stem Encircling Roots and Stem Girdling Roots: Implications on Tree Condition

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Abstract

The study investigated the relationship between structural root depth and the formation of stem girdling roots. We also investigated the effects of stem girdling roots on tree condition. We found as depth to structural roots increases, the proportion of the stem with stem girdling roots increased. We also found a negative relationship between tree condition and stem girdling and also depth to structural roots.
Accepted Preprint Urban Forestry and Urban Greening, February 2021
Relationship of Structural Root Depth on The Formation of Stem Encircling Roots and
Stem Girdling Roots: Implications on Tree Condition
Richard J. Hauer1† and Gary R. Johnson2
1Professor of Urban Forestry, College of Natural Resources, University of Wisconsin–Stevens
Point, 800 Reserve Street, Stevens Point, WI 54481, United States, 2Professor Emeritus of Urban
and Community Forestry, Department of Forest Resources, University of Minnesota, 115 Green
Hall, 1530 Cleveland Ave. N., St. Paul, MN 55108, United States, johns054@umn.edu
Corresponding author: rhauer@uwsp.edu
Abstract. The relationships of structural root depth, stem girdling roots, stem diameter, and
boulevard width were studied on the condition of four tree species (Acer saccharum L., Celtis
occidentalis L., Fraxinus pennsylvanica Marsh, and Tilia cordata Mill.) grown as street trees.
The relationship between depth from the soil surface to the structural roots and development of
stem encircling roots and stem girdling roots was also determined. Stem girdling roots,
boulevard width, and root depth were significant predictors of tree condition. Tree condition was
greater as boulevard width increased, but stem girdling roots and structural root depth had a
negative relationship on tree condition. Depth to structural roots was positively related to the
percentage of the tree stem circumference with stem encircling roots and also for stem girdling
roots. For every cm the structural roots were below the soil surface, 3.3% of the stem was
encircled. Thus, a 10 cm root depth translates to approximately 1/3 of the stem with encircling
roots. With stem girdling roots, an approximate 1% of the stem was girdled for each cm that
structural roots were below the surface. Results from the measurement of 398 trees that were
approximately 10 to 20 years post planting provide additional justification for maintaining
structural roots at the soil surface. Results also demonstrate the importance for planning tree
planting locations with adequate boulevard widths to foster tree health. Findings have
implications with nursery production, tree planting, and arboricultural treatments to remove soil
away from tree stems and expose structural roots at planting and subsequently with established
trees.
Keywords: Abiotic Disorder; Tree Health; Tree Planting; Urban and Community Forestry
Highlights:
Study found a relationship between structural root depth and landscape tree health
Stem encirclement by tree roots increases with depth from soil surface to structural roots
Planting depth and stem girdling roots were predictors of tree condition
Greater boulevard width for street trees was associated with greater tree condition
Results reinforce a recommendation to detect and correct root depth prior to tree planting
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1 Introduction
Trees in urban and community forests have many abiotic and biotic factors that affect tree
health (Rich and Walton, 1979; Berrang et al., 1985; Drilias et al, 1982; Costello et al, 1991;
Bond, 2010; Clark and Matheny, 1991; Hauer et al., 2020). The depth that structural roots are
below the soil surface (and subsequently the tree stems) and stem girdling roots are believed to
be two important factors that affect tree health, tree growth, and longevity of landscape trees
(d’Ambrosio, 1990; Johnson and Hauer, 2000; Wells et al., 2006; Day et al, 2009; Giblin et al.,
2011). The belief that structural root depth affects the health and survival of trees is not new
(Lawson, 1618; Evelyn, 1664; Ball, 1999). Lawson (1618) observed functioning roots near the
soil surface and believed injury to trees may result from buried tree root systems. Evelyn (1664)
wrote “… never to inter your stem deeper than you found it standing; for profound burying very
frequently destroys a tree …” Girdling roots have been suggested since at least the 1930’s as a
factor that influences the decline and premature death of landscape trees (Van Wormer, 1940).
Van Wormer (1937, 1940) observed nearly all declining Acer saccharum L. trees to have severe
girdling roots. Girding roots can be quite common and Tate (1980) quantified 82% of Acer
platanoides L. had at least one girdling root. However, most trees in that study had 37.5% or less
encirclement and overall tree canopies showed little impact from stem girdling. Maple species
(A. platanoides L, Acer rubrum L, and A. saccharum) have been noted as common hosts of
girdling roots, and at least in one study all three species were confirmed as chronically inflicted
by the abnormality 3-10 years after transplanting (Watson et al., 1990). In contrast, the same
study found that encircling or girdling roots on Gleditsia triacanthos L., Fraxinus pennsylvanica
Marsh, and Tilia cordata Mill. were less common than with maple species. Girdling roots can be
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removed; however, if soil is backfilled in the excavated area post removal, they have the
potential to grow back and in position as encircling or girdling roots (Watson and Clark, 1993).
A buried structural root system occurs through a variety of ways. These include soil fill
placed over existing trees, excessive organic mulch over the root system especially the zone of
rapid taper, root systems buried deeply during tree planting, trees sinking into the landscape soil
profile post planting, soil deposition from flooding, during nursery production, and nursery
harvesting practices (Johnson and Hauer, 2000; Wells et al., 2006: Day et al., 2009; Gilman et al.,
2010a). Several authors report a 5 to 28 cm depth from the soil surface to structural roots in B&B
harvested trees planted in landscapes and as street trees in the northeastern USA (Smiley and
Booth, 2000; Giblin et al., 2005; Smiley, 2005; Rathjens et al. 2007). Thus, standards (e.g., Z60.1
and A300 Part 6) and practice offer approaches to develop and maintain trees.
Buried structural roots affect root system form, upward direction of root growth, and tree
stability (Gilman et al, 2010a; Giblin et al., 2011; Gilman and Grabosky et al., 2011). The
relationship of structural root depth on tree growth and survival is less clear. Reports include
little to no relationship (Watson et al, 1990; Broschat, 1995; Gilman and Grobosky, 2004; Jarecki
et al., 2005; Day et al., 2009) to reduced growth in tree height and diameter (Broschat, 1995;
Arnold et al., 2005; Giblin et al., 2005; Wells 2006; Arnold et al., 2007; Day et al., 2009). The
amount of time following when the burial of roots occurred, root depth, and contributing factors
(e.g., adventitious root formation, soil moisture, flooding, soil texture) appear as reasons for
differences in reported relationship (Gilman and Grabosky, 2004; Arnold et al., 2005; Wells et
al., 2006).
Stem tissue girdling by roots lead to anatomical changes in xylem (vessel elements, rays,
and fiber tracheid’s) and phloem tissue through compression and distortion of normal tissue
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orientation (Hudler and Beale, 1981). The anatomical change from stem compression by roots
likely leads to physiologic change in tissue function and increased water stress (Johnson and
Hauer, 2000). Tree growth and crown appearance are indirect evidence that girdling roots have a
negative relationship on tree growth and condition, but study results have been inconsistent and
do not always show indirect relationships through reduced tree growth and canopy dieback (Tate,
1980; Holmes, 1984; d’Ambrosio, 1990; Johnson and Hauer, 2000). In a study where phloem
tissue of six year old Norway spruce (Picea abies (L.) H. Karst.) trees were physically girdled at
bud break, during rapid shoot and caliper expansion, and post shoot growth, fine root biomass
was significantly reduced compared to controls, and the starch content of coarse roots was
likewise significantly reduced compared to controls (Rainer-Lethaus and Olberhuber, 2018). All
trees girdled at bud break were dead within five months, and drought-stressed trees girdled
during peak shoot and stem growth were dead within four months. Thus, girdling roots gradually
reduce the size and efficiency of transport tissues leading to dysfunction.
The size of a tree at planting may determine if deep planting is to become detrimental.
Small tree seedlings (e.g., 1 to 3 years old planting stock) outplanted in forests are often planted
deep on purpose with favorable results for tree survival, especially during dry periods of soil
moisture (Slocum and Maki, 1956; Stroempl 1990; VanderSchaaf and South, 2003; Dreesen and
Fenchel, 2008; Pinto et al., 2011). Planting seedlings 5 to 10 cm deep on well-drained soil was
also suggested to decrease tree mortality due to a lower incidence of heat damage to stem tissue
at the root collar (Stroempl, 1990; Switzer, 1960). Adventitious roots may develop above the
original root system in some tree species (Stroempl, 1990). In all, it appears seedlings might have
an inherent recovery capacity when root systems are replanted at a greater depth (Van Eerden
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and Kinghorn, 1978). This relationship is apparently lost as seedlings transition towards a sapling
stage (Gilman and Anderson, 2005).
The intent of this study was to 1) quantify the relationship of structural root depth and
stem girdling roots on tree condition, and 2) quantify the relationship of structural root depth on
encircling root and stem girdling root development. We hypothesized a relationship based on
observations prior to designing this experiment that structural root depth is positively related to
stem girdling root formation and a resultant negative relationship on tree condition. We also
hypothesized that structural root depth is positively related to stem encircling root formation.
Thus, we set out to test any relationships of structural root depth and stem girdling roots on tree
condition. We also asked the question if structural root depth explained the percentage of the
stem with stem encircling roots and/or stem girdling roots.
2 Methods
2.1 Study site and species studied
Four tree species were studied for the relationships of distance from the soil line mark on
the tree trunks to structural roots on tree condition and any relationship(s) on the development of
stem encircling roots and stem girdling roots (Appendix A). Three species, Acer saccharum
Marshall (sugar maple), Fraxinus pennsylvanica Marshall (green ash), and Tilia cordata Mill.
(littleleaf linden) were located in boulevards (i.e. the area between the curb and sidewalk) along
street segments in Minneapolis, MN USA 44.9778° N, 93.2650° W. A fourth tree species, Celtis
occidentalis L. (hackberry) was randomly selected from a boulevard tree population along street
segments in Rochester, MN USA 44.0121° N, 92.4802° W. In both communities a street segment
is a contiguous distance (approximately 100 to 200 m) between two street intersections (city
block). The sites (i.e. boulevards) and species were chosen based on prior observations with
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diagnosing declining trees suggesting a relationship between tree condition and the development
of stem encircling roots and stem girdling roots as a function of structural root depth.
The species selected were trees commonly planted within the urban forests of the study
locations. Trees were initially identified within the study locations by selecting trees that were
established post transplanting (at least two years after transplanting), approximately 10 to 20
years of age, and between 7.6 cm and 22.9 cm stem diameter at 1.37 m above the ground. The
size and location (street segments) of the study population was obtained from tree inventories
provided by the two communities and three times the number of trees by species were located
and vetted for species identification and targeted size accuracy, resulting in approximately 300
eligible trees per species. Subsequently, every third tree by species along the street segments
subsequently were included in the study.
2.2 Condition rating system
A rating system was used to determine an overall tree condition ranking based on the
visual appearance of the tree stem, tree canopy, and leaf condition (Johnson and Fallon, 2009).
Each of the three tree locations (stem, canopy, leaf) were ranked 0 (poor), 1 (fair), 2 (good), and
3 (excellent). Ranking guidelines were:
3 - No obvious problems or defects; tree appears healthy and normal as evidenced by
other like species in the communities.
2 - Minor problems and/or defects which are recoverable and/or repairable; tree has
noticeable abnormal condition determined to be minor and that the tree will most likely
recover.
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1 - Significant problems which may be difficult to recover and/or repair; tree is in
advanced decline and requires immediate attention to decrease possibility of
irrecoverable defects.
0 - Irrecoverable defects and/or problems; the tree has major problems and recovery is
not expected.
For leaves, characteristics such as unusual leaf color (e.g. chlorotic), leaf size smaller than
normal for the species, scorched leaves, and early or unusual leaf drop were used in ranking
(Johnson and Fallon, 2009). In the canopy we used early autumn color, branch/twig dieback,
canopy thinning, and stagheading (i.e. death of large, structural branches) with the ranking
assignment. Stem condition included cracking, cambial dieback, and abnormal lean to rank trees
(Figure 1). The 12-point ranking system that used equal weighting for each of the three tree
locations (e.g., leaf, canopy, and stem) was further adjusted to a 0 to 100% scale. The tree
condition ranking was conducted “blind,” prior to the determination of the depth to structural
roots and the assessment of the tree root system. The root system was excluded from the
condition rating since it was a study variable of interest as either a dependent or independent
variable depending on the research questions presented later. The “blind” above-ground
condition ratings were conducted by the same principle investigators for each species.
2.3 Field root examination method and measurements
Tree roots were non-destructively exposed by hand removal of soil, with vacuum
extraction of soil to a distance of 10 to 15 cm from the tree stems as needed. Soil removal was
done to ascertain the depth (cm) from the soil surface to the structural roots (aka, first-order
roots, primary roots, skeletal roots, scaffold roots) and to visually examine for the presence of
stem encircling roots and stem girdling roots (Table 1). The presence or absence of stem
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encircling roots and stem girdling roots was noted for all trees in the study (Figure 2). The total
percentage of the stem circumference with stem encircling roots and stem girdling roots was
calculated for C. occidentalis (n=96), F. pennsylvanica (n=101), and T. cordata (n=101) by
measuring the total length of stem encircling roots and stem girdling roots, the buried stem
circumference above the flare at the root-stem transition zone, and dividing total length by
circumference (Figure 3). In cases that the length was greater than the stem circumference due to
multiple encircling and/or girdling roots, the product was set at 100% for a totally encircled or
girdled tree. A. saccharum (n=100) only had the presence or absence of stem encircling roots or
stem girdling roots recorded. Boulevard width, stem diameter (at 1.37 m), and tree species were
recorded (Table 1). All measurements and assessments occurred during the growing season after
full leaf development and prior to normal fall leaf color change and leaf drop (mid-May to late
September).
2.4 Research models and statistical approach
Structural root depth and the incidence of stem girdling roots was tested for significance
on tree condition a priori through two different linear regression models. The first full model
was tree condition = stem diameter + boulevard width + % stem girdling root + root depth +
species. This model was tested with F. pennsylvanica, and T. cordata which both had % stem
girdling root quantified, and boulevard width measured. A second full model tested tree
condition = stem diameter + presence of stem girdling root + root depth + species. Presence of
stem girdling root was coded as 0 or 1 for presence and included all four tree species. A
relationship between root depth and percentage of stem surrounded by stem encircling roots was
tested as encircling root = stem diameter + root depth + species. A similar full model was tested
for stem girdling root = stem diameter + root depth + species. The species coefficient was coded
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as 0 or 1 for each species and each added individually into all full models. Tests for
independence, homoscedasticity, and normality used residual plots and no violations observed.
SPSS version 25 (IBM Corp. Armonk, NY: IBM Corp.) was used for all statistical analyses with
α = 0.05 as a test for significance with final model inclusion of study variables.
3 Results
Stem diameter ranged between 7.6 and 22.9 cm with a mean 16.4 (0.22 SE) (Table 2). As
a percent of stem circumference, stem encircling roots averaged 64.3% (2.3 SE) and stem
girdling roots averaged 7.2% (0.89 SE), with both ranging between 0% and 100% (Table 2,
Figure 4). Trees with structural roots at the surface averaged 6% encircling and no stem girdling
roots were observed. Stem encircling roots increased to 53% with just 1 to 3 cm of soil over the
uppermost structural surface and stem.
girdling roots average 4% at this depth. Trees with structural roots 26 cm or more below the
surface had 100% stem encircling roots and a mean 38% stem girdling roots. Overall tree
condition ranged between 16.7% and 100.0% with a mean 75.6% (0.91 SE). Root depth to
structural roots varied between 0.0 and 29.2 cm and a mean 8.5 cm (0.33 SE) found.
3.1 Models predicting tree condition
Both tree condition models differed with variables that predicted tree condition (Tables 2,
3). The first model with stem girdling roots on a percentage basis showed boulevard width
(p<0.001) and stem girdling roots (p=0.012) as significant predictors of tree condition (F =
12.495, df = 2,196, p<0.0001, Adj R2 = 0.10). All tree species were not significant (p>0.25) and
stem diameter (p=0.24) and root depth (p=0.14) were not significant, thus these were not retained
in the final model (Table 3). A second model treating stem girdling roots as a bivariate (1 =
presence) showed linden (p=0.19) and stem diameter (p=0.26) as not significant in the initial
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model. The final model (F = 44.274, df = 5,392, p<0.0001, Adj R2 = 0.35) showed stem girdling
roots (p=0.004), root depth (p=0.002), F. pennsylvanica (p=0.048), A saccharum (p=0.034), and
C. occidentalis (p<0.001) as significant predictors of tree condition (Table 4). Tree condition
declined as the percent of the stem circumference with stem girdling roots increased (Figure 1).
Boulevard width was positively related to tree condition. Root depth was not significant in model
1, but in model 2 it negatively affected tree condition. Trees with a greater depth to structural
roots were thus lower in condition.
3.2 Models predicting development of stem encircling roots and stem girdling roots
Several parameters explained root encircling of tree stems (Tables 5, 6). Root depth was a
significant predictor of root encircling (p<0.001) and stem girdling roots (P<0.001). As root
depth increased, the percentage of the stem with encircling or girdling roots increased. Stem
diameter was also a significant predictor of stem encircling roots (p=0.004) and stem girdling
roots (p<0.001). With stem encircling roots, a negative relationship was found, thus as stem
diameter increased the percentage of stem with stem encircling roots decreased. In contrast, the
relationship was positive with stem girdling roots and larger trees had a greater stem percentage
compressed by root tissue. The finding is intuitive in that as the diameters of tree stems and roots
increase through growth stem compression may occur. And then by definition an encircling root
becomes a stem girdling root. Likewise, smaller diameter trees have a greater stem encircling
roots abundance as roots are in proximity but not yet causing stem girdling. No species were
significant predictors (all P>0.10) and boulevard width was also not significant (p>0.10) in stem
girdling root development. The models also showed T. cordata trees were more likely to have
stem encircling roots and C. occidentalis less likely to have stem encircling roots.
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Discussion
This study tested the relationships of structural root depth on the development of
abnormal root systems (e.g., stem encircling roots and stem girdling roots) and tree condition.
We found that depth to structural roots was a strong predictor of trees developing stem encircling
roots and stem girdling roots. Both root depth and stem girdling roots combined were associated
with relationships on tree condition. The significance with these findings and other published
results follows.
4.1 Structural root depth
Structural root depth occurs both naturally (e.g., soil deposition from flooding) and
artificially (e.g., soil added on top or tree planting depth). Adventitious root development in
riparian species is a natural survival adaptation to soil burying of tree roots, and trees buried in
riparian areas have a uniform cylindrical shape rather than a flare at the soil surface (Sigafoos,
1964; Wilford et al., 2004). This same cylindrical shape has been found for planted landscape
and street trees (Tate, 1981a; Tate 1981b; Johnson and Hauer, 2000; Gilman et al., 2010a).
Species vary in response to burying with Dech and Maun (2006) finding adventitious root
development stimulated by burying in Salix cordata Michx. and Populus balsamifera L., both
riparian species. Two upland conifers, Pinus strobus L. and Picea glauca (Moench) Voss, lacked
burial tolerance. Taxodium distichum (L.) Rich., a riparian species, was also found to develop
adventitious roots, which was surmised as a survival strategy (Tremmel and Martin, 2000). What
is unclear, however, is even if a species is an effective adventitious root developer, is there a
minimum soil moisture content necessary for adventitious root development? The riparian
species in nature grow in a bottomland situation and more likely will have access to soil
moisture. Species ability to develop adventitious roots may change with age, with Quercus
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virginiana Mill showing limited to no development in larger saplings (Gilman and Anderson,
2005).
In terms of the relationship of depth to structural roots and the frequency of stem
encircling and girdling roots, findings from this study were largely consistent with several
similar studies despite the variety of species and sizes investigated (Wells et al, 2006; Day and
Harris, 2008; Gilman et al., 2010a, 2010b). Having stated that, not all studies found that
relationship. Watson et al. (1990) were unable to discern a significant relationship between
planting depth and the number of stem encircling or girdling roots, despite the fact that their
study included three of the four species in this study with similar sizes. However, the numbers of
sampled trees per species may have contributed to those differences. The Watson et al. (1990)
study included 15 Acer saccharum, 10 Fraxinus pennsylvanica, and 10 Tilia cordata, whereas
this study included 100 Acer saccharum, 101 Fraxinus pennsylvanica, and 101 Tilia cordata.
Most root depth studies have been with Angiospermous deciduous trees. In palms, deep
planting is regularly done for perceived stability post planting. Phoenix roebelenii O’Brien
planted at their original depth had greater root development than those planted 15 to 90 cm deep.
The deepest treatment had 40% survival after 15 months post-transplanting. Thus, the
relationships of buried roots systems occur from both monocots and dicots. Monocots such as
palms also regularly develop adventitious roots which could aid survival from deep planting.
Root systems are opportunistic and grow in areas suitable for growth. Wagar (1985)
observed that trees planted deeply had roots that grew to the surface. Horizontal root growth then
resumed at the surface, however the direction (e.g., towards or away from stem) was not
reported. In this study we observed Tilia cordata and Acer saccharum had roots that grew
vertically to the surface and did not always radiate away from the stem upon resumption of
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horizontal growth. A “random like” horizontal direction occurred with some roots growing
towards the stem (Johnson and Hauer, 2000).
When horizontally growing lateral roots are deflected up to 30 from the original plane of
growth, they tend to radiate in the original horizontal direction after growing beyond the
deflecting barrier (Wilson 1967). If the deflection was greater than 30, however, the root did not
fully return to the original plane of growth. When the barrier was 60 to 90, the angle of
deflection was approximately 35 to 45. This raises an interesting question with deep structural
roots: “what direction do lateral roots proceed after returning to a horizontal direction from a
vertical ascent?” Thus, if the lateral roots radiate toward rather than away from the stem, this
could result in a stem girdling root situation. The horizontal direction that the roots follow after a
vertical ascent is uncertain and was not ascertained in this study.
Soil oxygen concentration affects root growth. As depth from the soil surface increases,
soil oxygen levels are likely to decrease to levels that affect root respiration. Lemon and
Erickson (1952) measured oxygen diffusion rates (ODR) of three different soil textures (fine to
coarse) at depths to 20 cm. For each 2.5 cm in depth, soil ODR declined at significant rates,
especially with fine textured soil such as a clay. At 20 cm, the ODR in a clay soil approached
zero. MacDonald et al. (1993) found a correlation between soil ODR and the relative health of
oaks in a California landscape, to wit, the lower ODR was associated with declining mature oaks,
higher ODR was associated with healthy mature oaks. Both research studies noted that ODR is a
function of soil texture, soil bulk density (compaction), and soil moisture. Oxygen diffusion rates
decline the fastest with depth when the soil is a compacted, more so with fine textured soil with a
higher moisture content. Thus, opportunistic root growth into soil closer to the surface may
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explain root growth from lower soil depths towards the surface and later growth towards a buried
stem and lead to stem encircling or stem girdling roots.
4.2 Tree lawn width and stem girdling root development
We found no relationship between the distance between the curb and sidewalk and stem
girding root development. This finding is consistent with Tate (1980, 1981b) who also found no
relationship between tree lawn width and girdling root development. Regardless, space to grow
street trees is important and a predictor for tree health (Hauer et al., 2020; Hilbert et al., 2020).
4.3 Tree condition, growth, and stability
Tree survival and tree condition are factors associated with stem girdling roots and root
depth. We found trees with stem girdling roots to have a 10% lower tree condition rating and all
things considered would likely be a stress factor, leading to secondary (opportunistic) biotic
agents and tree decline (Clark and Matheny, 1991). And we also found as the percentage of the
stem with stem girdling roots increased, tree condition decreased. Stem and collar cankers were
associated as secondary decline factors with Acer saccharum in deeply planted (15 to 25 cm)
street trees (Drilias et al., 1982). Soil against trees stems can also lead to pathogen colonization
in landscape trees with deep structural roots (Smiley, 2005).
Reduced tree survival was found with Prunus × yedoensis Matsum. with 50% of deeply
planted trees dead after 2 years compared to no mortality with at grade planted trees (Wells et al.,
2006). Day and Harris (2008) found greater mortality (40%) in Corylus colurna L. after 8 years
in trees planted 30 cm deep compared to 0% for those at grade and 15 cm. Reduce growth was
also found for trees 7.6 cm planted deep Fraxinus pennsylvanica and Platanus occidentalis L.,
Lagerstroemia indica × Lagerstroemia fauriei, and Nerium oleander L. versus trees above 7.6
cm) grade or above (Arnold et al., 2007). Results were varied by planting depth with trees
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planted 5.0 and 7.6 cm deep having greater survivability than trees planted at grade or 15.2 cm
deep (Browne and Tilt, 1992)
Tree growth provides a mechanism to indirectly assess tree health with the premise
growth is an end result of carbon assimilation (Pallardy, 2008). Studies vary in the relationship of
planting depth on growth difference with no difference (Gilman and Anderson, 2005; Gilman et
al., 2010a) versus reduced growth (Arnold et al., 2005; Gilman et al., 2010a; Gilman and
Grabosky, 2011). Differences seem to relate to the length of the study and species used. For
example, Arnold et al. (2005) found Koelreuteria bipinnata A.R. Franchet (a hypoxia intolerant
species) had greater mortality than Fraxinus pennsylvanica (a hypoxia tolerant species),
however, both species had less growth than trees planted at or above grade (7.6 cm) than planted
deep (7.6 cm).
While not a part of this study, tree stability may be impacted by planting depth. Deeply
planted (20 cm) Malus domestica Borkh. were more prone to wind shaking trees loose than trees
planted at the grade from the nursery (Lyons et al., 1983). Deeply planted peach trees were less
stable than those with the first main-order root system at grade (Lyons and Yoder, 1981). Thus,
deep planting may affect tree stability (Lyons, et al., 1987; Arnold et al., 2005, Arnold et al.,
2007).
4.4 Study limitations
This investigation used trees that were approximately one to two decades old. This age
class was selected based on preliminary observations suggesting a relationship with structural
root depth and tree condition, stem encircling roots, and stem girdling roots. It is possible that
trees as they become older could compensate for stem compression by adaptive growth of stem
tissue in another location. While possible, we found that as a tree grows, stem encircling roots
15
become girdling roots and the percent of stem tissue compressed increases with an approximate
10% greater stem compression for the largest versus smallest trees in the study. Trees with stem
girdling roots had a lower overall condition rating (68.7%) versus those without (79.0%).
We defined a stem girdling root as one causing some degree of stem compression
including bark and functional sapwood. At the initial stage of stem contact, compression might
only consist of minimal tissue dysfunction. With time, compression becomes more severe and
acute relationships on tree health and condition become a potential end result. Thus, the actual
relationship of stem girdling roots changes over time. An absolute examination of tissue
compression was beyond the scope of this study.
Finally, trees typically have multiple stress factors (e.g., soil compaction, water deficits,
de-icing salts, insects, disease, root damage, stem damage) at any one time or over the course of
time that affect tree health (Clark and Matheny, 1991). While being able to quantify all factors
affecting tree condition would likely increase the overall predictive nature of a model, this was
beyond the scope of this paper. None-the-less, results from this study suggest that planting depth
and presence of girdling roots should be considered in models that test and predict the decline of
trees in urban and suburban landscapes.
Conclusion
In situ studies are always confounded by unintentional and sometime unseen variables.
However, the data collected from this study combined with the robustness of the sample size, and
the analysis of said data presents a conclusion that is unlikely to be coincidental and is supported
by past research. Incremental additions of soil over the structural root systems of younger trees
(approximately 20 years or less) did affect the condition of these four species, did increase the
frequency of stem encircling roots, and did increase the frequency of stem girdling roots. How
16
tree root systems come to be excessively buried by soil is varied and a challenge to correct.
Many of the problems with deeply buried structural roots and tree stems can be corrected or
avoided at planting time, but only addressing planting practices is a very limited management
perspective. Sound management practices should include regular monitoring and correction of
any situations that place additional depths of soil or pre-soil (aka, organic mulches) against the
stems and over the structural roots of trees, especially those trees in their first two decades of life
and service to the landscapes.
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21
Figure 1. Acer platanoides with significant stem compression from stem girdling roots and extensive
sapwood decay as evidenced by the fungal fruiting structures.
Figure 2. A root examination of this Tilia cordata revealed a single stem girdling root (SGR), identified
by the compression of the tree trunk that had resulted from the SGR placement. Visual confirmation of
stem compression was a requirement for a root to be identified as a SGR.
22
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Figure 3. A non-destructive root examination of a Tilia cordata revealing multiple layers of stem
encircling roots, and no evidence of the original structural roots at this depth.
23
Figure 4. Relationship of depth from soil surface to structural roots on percentage encircling
roots (within 15 cm and no stem compression) and stem girdling roots (causing stem
compression) of stem circumference. (Bars are standard errors)
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Table 1. Data variables and definitions used in this study.
Variable Definition
Bouelvard Width The distance in m between a street curb back edge and the sidewalk
Condition A 12 point rating of the tree condition/health from above-ground plant parts of stem,
canopy, and leaf with each assigned a score of 0 (poor), 1 (fair), 2 (good), or 3
(excellent) for 0 to 9 total points used to scale 0 to 100%
Encircling Root Presence or absence of encircling/tangential roots within 15.2 cm (6
inches) of stem, but not yet causing compression of tissue,
recorded as yes/no and total length in cm
Percent Encircled Percent encircling root = total encircing root length / total stem circumferance
Percent Girdled Percent stem girdling root = total stem girdling root length / total stem circumferance
Root Depth Distance between the soil surface and the top of the structural root system in cm
Stem Circumferance The total distance arround the stem above the root flare at the root-stem transition
zone
Stem Diameter Diameter in cm of the tree trunk at 1.37 m
Stem Girdling Root Presence or absence of roots causing stem compression, recorded
as yes/no and total length
Street Segment A contiguous distance (approximately 100 to 200 m) between two street intersections
(city block)
Tree Number Unigue record of each tree and study location
Tree Species One of four tree species (Acer saccharum, Celtis occidentalis,
Fraxinus pennsylvanica, and Tilia cordata) in this study
Table 2. Study sample descriptive findings.
Variable (unit) N Mean Std. Error
of Mean
Standard
Deviatio
n
Minimu
m
Maximum
Boulevard Width (m) 19
9
1.55 0.02 0.28 0.76 2.13
Condition (%) 39
8
75.65 0.91 18.16 16.70 100.0
Encircling Root (%) 29
8
64.34 2.23 38.41 0.00 100.0
Strutural Root Depth (cm) 39
8
8.45 0.33 6.50 0.00 29.21
Stem Diameter (cm) 39
8
16.36 0.22 4.34 7.60 22.90
Stem Girdling Root (%) 29
8
7.20 0.89 15.39 0.00 100.0
24
25
Table 3. Model 1 testing the relationship of stem diameter, percent stem girdling root, boulevard
width, and root depth on tree condition of Fraxinus pennsylvanica and Tilia cordata.
Unstandardized Standardized
Coefficients Coefficients t-test Statistics Correlations
t- Zero-
Model Variables B Std. Error Beta value Sig. order Partial
Initial full Model (R2 =.127 R2adj =.109, std. error of est.=11.245, F(4,194)=7.095, p<.0001)
(Intercept) 57.353 5.778 9.926 0.000
Stem Diameter (cm) -0.215 0.182 -0.081 -1.176 0.241 -0.094 -0.084
Stem Girdling Root (%) -0.124 0.069 -0.130 -1.807 0.072 -0.140 -0.129
Boulevard Width (m) 12.581 2.848 0.299 4.417 0.000 0.290 0.302
Root Depth (cm) -0.217 0.145 -0.107 -1.498 0.136 -0.136 -0.107
Final a priori Model (R2 =.113, R2adj =.104, std. error of est.=11.277, F(2,196)=12.495, p<.0001)
(Intercept) 51.546 4.469 11.534 0.000
Stem Girdling Root (%) -0.164 0.065 -0.171 -2.527 0.012 -0.140 -0.178
Boulevard Width (m) 12.935 2.847 0.307 4.543 0.000 0.290 0.309
Table 4. Model 2 testing the relationship of stem diameter, presence of stem girdling root,
boulevard width, and root depth on tree condition of Acer saccharum, Celtis occidentalis,
Fraxinus pennsylvanica, and Tilia cordata.
Unstandardized Standardized
Coefficients Coefficients t-test Statistics Correlations
t- Zero-
Model Variables B Std. Error Beta value Sig. order Partial
Initial full Model (R2 =.363 R2adj =.353, std. error of est.=14.606, F(6,391)=37.13, p<.0001)
(Intercept) 81.145 3.352 24.207 0.000
Stem Girdling Root1-4.625 1.700 -0.119 -2.720 0.007 -0.265 -0.136
Root Depth (cm) -0.398 0.121 -0.144 -3.281 0.001 -0.252 -0.164
Stem Diameter (cm) -0.194 0.172 -0.046 -1.126 0.261 -0.033 -0.057
F. pennsylvanica1-3.924 2.068 -0.094 -1.898 0.058 -0.214 -0.096
A. saccharum1-4.245 2.084 -0.102 -2.037 0.042 -0.225 -0.102
C. occidentalis119.036 2.145 0.449 8.876 0.000 0.558 0.410
Final a priori Model (R2 =.361 R2adj =.353, std. error of est.=14.611, F(5,392)=44.27, p<.0001)
(Intercept) 78.024 1.884 41.425 0.000
Stem Girdling Root1-4.875 1.686 -0.126 -2.892 0.004 -0.265 -0.145
Root Depth (cm) -0.379 0.120 -0.137 -3.152 0.002 -0.252 -0.157
F. pennsylvanica1-4.090 2.064 -0.098 -1.982 0.048 -0.214 -0.100
A. saccharum1-4.417 2.079 -0.106 -2.124 0.034 -0.225 -0.107
26
C. occidentalis118.865 2.140 0.445 8.815 0.000 0.558 0.407
1 bivariate indicator with 1 = presence
Table 5. Relationship of stem diameter and root depth on percentage of stem circumference with
root encircling in Celtis occidentalis, Fraxinus pennsylvanica, and Tilia cordata.
Unstandardized Standardized
Coefficients Coefficients t-test Statistics Correlations
t- Zero-
Model Variables B Std. Error Beta value Sig. order Partial
Final a priori Model (R2 =.629 R2adj =.624 std. error of est.=23.542, F(4,293)=124.40, p<.0001)
(Constant) 52.648 6.071 8.672 0.000
Stem Diameter -0.908 0.309 -0.106 -2.941 0.004 -0.206 -0.169
Root Depth (cm) 3.275 0.208 0.579 15.782 0.000 0.680 0.678
T. cordata115.518 3.329 0.192 4.662 0.000 0.428 0.263
C. occidentalis1-22.031 3.396 -0.268 -6.487 0.000 -0.492 -0.354
1 bivariate indicator with 1 = presence
Table 6. Relationship of stem diameter and root depth on percentage of stem circumference with
stem girdling roots of Celtis occidentalis, Fraxinus pennsylvanica, and Tilia cordata.
Unstandardized Standardized
Coefficients Coefficients t-test Statistics Correlations
t- Zero-
Model Variables B Std. Error Beta value Sig. order Partial
Final a priori Model (R2 =.194 R2adj =.189 std. error of est.=13.857, F(2,295)=35.61, p<.0001)
(Constant) -11.118 3.323 -3.346 0.001
Stem Diameter 0.618 0.181 0.180 3.412 0.001 0.130 0.195
Root Depth (cm) 0.962 0.119 0.424 8.066 0.000 0.403 0.425
27
Appendix A. Graphical example roots systems after soil removal and appearance of the soil line,
stem encircling roots, stem gridling roots, and structural roots of study trees.
28
Stem
Girdling
Root
Soil
Line
Stem
Encircling
Root
Soil
Line
Stem
Girdling
Root
Soil
Line
Structural
Root
Soil
Line
Stem
Encircling
Root
Structural
Root
Soil
Line
Stem
Girdling
Root
Soil
Line
Stem
Girdling
Root
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