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Citation: Clatterbuck, W.K.; Brannon,
T.M.L.; Yost, E.C. Branch Elongation,
Bud Durability, and Wind-Generated
Crown Movement Associated with
Crown Abrasion in Deciduous Trees.
Forests 2024,15, 247. https://doi.org/
10.3390/f15020247
Academic Editor: Thomas J. Dean
Received: 19 December 2023
Revised: 18 January 2024
Accepted: 24 January 2024
Published: 28 January 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
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4.0/).
Article
Branch Elongation, Bud Durability, and Wind-Generated Crown
Movement Associated with Crown Abrasion in Deciduous Trees
Wayne K. Clatterbuck 1,*, Tyler M. L. Brannon 1,† and Edward C. Yost 2
1School of Forest Resources, University of Tennessee, Knoxville, TN 37996, USA; tbranno2@gmail.com
2
Forest Resources Research & Education Center, Institute of Agriculture, AgResearch, University of Tennessee,
Oak Ridge, TN 37830, USA; ecyost50@gmail.com
*Correspondence: wclatter@utk.edu; Tel.: +1-865-974-7346
†This work was part of the Master’s thesis of the second author Tyler M. L. Brannon.
Abstract: Trees that grow in close proximity to other trees are subject to crown and branch abra-
sion, causing mechanical injury. The loss of branch tips and buds through abrasion can affect the
architecture and growth of tree crowns. This research quantifies the impacts of crown abrasion
between neighboring trees of several deciduous species and how crown abrasion may influence
stand dynamics. Tree interactions were evaluated during the dormant and growing seasons to
determine how wind-generated movement affects crowns under foliated and un-foliated conditions.
Branch elongation was measured in tree crowns where growth was both inhibited and uninhibited
by adjacent trees. Bud durability was evaluated by growing season for species with determinate
and indeterminate shoot growth forms using a pendulum impact tester. Crown movement during
wind events was assessed by using three-axial accelerometers in the outermost points of tree crowns.
Accelerometers logged the movement of branches in the tree crown. By using both the crown sway
acceleration and associated bud durability and mass data, the possible force necessary to break or
abrade buds and branches was calculated at different wind speeds. Branch elongation was greater
for most species on the exposed side of the crown that was not affected by adjacent trees. Preformed
buds from the determinate growth form were determined to have greater durability than sustained
growth or indeterminant buds. Acceleration from wind gusts increased more rapidly as wind speed
intensified in the growing season when leaves were on the tree than in the dormant season. This
research suggests that crown abrasion contributes to the development of mixed species stands by
reducing crown size and growth therefore allowing slower-growing species with determinant growth
to stratify above faster growing trees with indeterminant growth.
Keywords: crown abrasion; crown shyness; crown dynamics; bud durability; tree sway acceleration
and force; stand development
1. Introduction
The physical damage or loss of terminal branches and buds when adjacent tree crowns
overlap is known as crown abrasion or shyness. This process occurs during wind sway
when branches from adjacent crowns collide with each other. Mechanical abrasion af-
fects growth and yield by reducing crown size and leaf area, altering growing space and
suppressing the future growth of damaged branches and crowns [1].
Most crown abrasion research is based on observations of crown dynamics in monocul-
ture conifer stands that influence crown dimensions and stand density, especially in stands
that have become overstocked. The impacts of crown abrasion have been studied in Pinus
contorta Dougl. Ex. Loud. (lodgepole pine) stands [
2
–
7
], Pinus sylvatica L. (Scotch pine)
in Europe [
8
,
9
], as well as in plantations of Pinus strobus L. (eastern white pine) [
10
] and
Pinus resinosa Ait. (red pine) [
11
]. A loss of horizonal crown size, extended vertical crowns,
and reduction of tree leaf area resulted in tall, slender trees that sway more during windy
Forests 2024,15, 247. https://doi.org/10.3390/f15020247 https://www.mdpi.com/journal/forests
Forests 2024,15, 247 2 of 16
conditions with a greater chance that branches collide with adjacent trees. Recently, Onoda
and Bando [
12
] and van der Zee et al. [
13
] used remote sensing technology to describe
horizontal and vertical expressions of crown abrasion. Putz et al. [
14
] observed crown
abrasion in Avicennia germinans L. (black mangrove) between branches on the same tree
and branches of adjacent trees. This interaction facilitated crown shyness in young trees of
a similar size that were growing in plantations.
Research on deciduous species is more limited and complicated in mixed species with
varied growth rates and habits of species. Hajek et al. [
15
], using a laser-based canopy
structural analysis, concluded that mechanical abrasion, not the competition for sunlight,
was the dominant canopy interaction in a temperate, mixed deciduous forest. Because
quantitative information on crown interactions among deciduous species is sparse, the
purpose of this research is to examine how crown abrasion might occur among different
deciduous species with an emphasis on branch elongation, bud durability, and force
estimates from branch acceleration that cause bud breakage during wind events. The
hypotheses were as follows: (1) branch growth will be more restricted in the interior of tree
crowns, (2) preformed buds will require more energy to break than sustained growth buds,
and (3) crown movements will exhibit greater acceleration during the growing season than
during the dormant season.
Species phenology between deciduous species in mixed stands differ. Thus, the degree
of crown abrasion between and among species varies. Two of these species-related features
that affect crown abrasion include preformed (determinate) or sustained (indeterminate
or recurrent) branch or bud growth and crown form where apical dominance is strong
(excurrent) or weak (decurrent). The temporal aspects of growth influence the durability of
buds and branches susceptible to abrasion. Preformed buds are more durable and older
and tend to damage recurrent buds that are less durable and younger.
The movement of a tree crown depends on the extent of static and dynamic strain
created by the wind disturbance as well as diameter and mass of the tree branch, which
varies by species. Dynamic strain is dependent on pressure from gusts of wind and the
frequency of branch oscillation in the crown. The oscillation frequency of broad-leaf trees
is seasonal whether trees are with or without leaves. Leaf mass during the growing season
with greater moisture contents can reduce the frequency of oscillation [
16
]. During the
winter, without leaves, the oscillation frequency can be two or three times greater. Despite
winter oscillation frequencies being greater, a single strong wind gust in the growing season
may be more damaging to developing buds and growth of branches, as more energy is
transferred during a collision [17].
Although previous research on crown abrasion was to qualify observations associated
with crown abrasion or crown shyness, few have quantified crown friction mechanisms and
parameters to confirm and assess how crown abrasion occurs and impacts stand and crown
development, especially in mixed-species deciduous stands. This crown abrasion research
evaluates branch elongation subject to crown abrasion, bud durability of preformed (deter-
minant) and sustained (indeterminant) bud growth, and branch (crown) movement during
dormant and growing seasons using accelerometers, and it estimates forces necessary to
sever buds.
2. Materials and Methods
2.1. Study Sites
The research was conducted during 2010–2012 at three separate locations, each in-
corporating different stages of the study: the University of Tennessee Forest Resources
Research and Education Center (FRREC) near Oak Ridge, Tennessee, the East Tennessee
Nursery (ETN) near Delano, Tennessee, and the Mississippi State University J.W. Starr
Memorial Forest (SMF) in East–Central Mississippi, 21 km south of Starkville, Mississippi.
Our purpose was not to evaluate environmental conditions between locations, but to ac-
cess locations that contained samples of available species in plantings or natural stands
necessary for each of the three studies.
Forests 2024,15, 247 3 of 16
The FRREC (84
◦
34
′
26
′′
W, 35
◦
14
′
53
′′
N; elevation, 224 m) is located in the ridge
and valley physiographic province in the southeast corner of Anderson County in East
Tennessee. The mean annual precipitation is 132 cm, and the average annual temperature
is 14
◦
C. The soils are of the Fullerton-Palio complex (Thermic, Typic Paleudults) that were
formed from cherty limestone parent material [18].
The ETN (84
◦
13
′
12
′′
W, 35
◦
59
′
38
′′
N; elevation, 267 m), operated by the Tennessee
Department of Agriculture, Division of Forestry, is located in Northern Polk County,
Tennessee. The mean annual precipitation is 145 cm, and the average annual temperature
is 14
◦
C. The soils are of the Toccoa loam series (Thermic, Typic Udifluvents) along the
alluvial floodplain of the Hiwassee River [18].
The SMF site (33
◦
29
′
N, 88
◦
91
′
W; elevation, 100.5 m) is located on the boundary of
Oktibbeha and Winston Counties, Mississippi. The mean annual precipitation is 155 cm,
and the average annual temperature 17
◦
C. The soils are Mathison silt loam (Thermic, Aeric
Fluvaquents) adjacent to the Noxubee River [18].
2.2. Background
Three separate studies evaluated the process of crown abrasion and potential impacts
of crown abrasion on stand development. The first study used branch elongation analysis
to examine growth patterns of crowded branches under two conditions in plantations: one
where crowns were interacting with interior adjacent tree crowns and the second on the
edge of the plantation where crowns were not being influenced by adjacent trees. For the
second study, a pendulum impact tester was used to evaluate bud durability, which is the
amount of energy a bud can absorb before fracture. A pendulum tester was selected over
other testing methods due to its consistency in testing and its similarities to impacts caused
by branch collisions within and between tree crowns. The third study used accelerometers
to measure the gravitational force of branches on the crown edge and to 2-dimensionally
map the energy of branch movement following the work of Rudicki and others [
3
] with
Pinus contorta.
2.3. Branch Elongation and Bud Weight Measurement
Planted stands of Quercus texana Buckley and Liriodendron tulipifera L. from the ETN
and Liquidambar styraciflua L. (sweetgum) from the SMF [
19
] were sampled for the branch
elongation analysis. These stands were planted in 1993, 1991, and 1972, respectively at
spacings ranging from 3.0 to 3.7 m. All three stands were overstocked with crowns of
adjacent trees overlapping.
Ten branches were collected from each of the three species: five interior branches
which had constant contact with adjacent tree crowns within the plantation and five
exterior branches from edge trees with no contact with adjacent tree crowns for a total of
thirty analyzed branches. An adequate sample size was deemed as five samples each for
exterior and interior branches for each species. Since the branch samples were yielding
similar length results, there was no need for additional branch samples. The sampled
branches were segmented at various intervals for aging based on annual ring counts and
on the last 30.5 cm of the branch through bud scars. The amount of branch length added
each growth year was determined. Graphs were created to compare growth differences
between interior and exterior branches. If damage from crown abrasion occurred, the
physical damage on the branch and the branch elongation data should indicate growth
differences between interior and exterior branches.
Mass and moisture content were measured for buds of various species (Table 1)
during the growing season from bud collections at FRREC and ETN. Several species
were evaluated to compare preformed buds to sustained growth buds. Although species’
growth forms are somewhat ambiguous in the literature, the following were designated as
species with preformed buds and determinant growth: Quercus falcata Michx. (southern
red oak), Quercus rubra L. (northern red oak), Quercus alba L. (white oak), Quercus texana,
Juglans nigra L. (black walnut), and Carya tomentosa (Lam.) Nutt; (mockernut hickory). The
Forests 2024,15, 247 4 of 16
indeterminate growth species evaluated were Liriodendron tulipifera,Liquidambar styraciflua,
Platanus occidentalis L. (American sycamore), and Acer rubrum L. (red maple). Mean bud
mass was calculated for each species using a balance scale. Buds were then dried at 200
◦
C
for 24 h in a drying oven, and dry mass was recorded for each bud sample. Dry weight was
subtracted from hydrated weight and divided by dry weight to determine moisture content.
Table 1. Bud parameters (standard error in parenthesis) by species during the growing season for
crown abrasion study, Tennessee, 2011. Sample size of species without data was too small to be
included in the analysis.
Species nBud Collar
Diameter (cm)
Bud Mass
(g)
Moisture
Content (%)
Acer rubrum 20 0.198 (0.009) 0.005 (0.001) 93.72 (3.29)
Carya tomentosa * 20 0.434 (0.018) 0.231 (0.027) 83.62 (0.50)
Juglans nigra * 20 0.511 (0.014) 0.084 (0.006) 66.30 (0.74)
Liquidambar
styraciflua 20 0.262 (0.011) 0.029 (0.002) 170.54 (3.78)
Liriodendron tulipifera
20 0.259 (0.006) 0.057 (0.005) 257.68 (2.94)
Quercus alba * 20 0.236 (0.007) 0.009 (0.000) 100.91 (0.48)
Quercus falcata * 20 0.272 (0.009) 0.017 (0.001) 69.80 (1.66)
Quercus rubra * 20 0.338 (0.011) 0.040 (0.003) 57.03 (2.09)
Quercus texana * 20 0.218 (0.006) 0.009 (0.001) 103.26 (2.25)
* Preformed bud growth form.
2.4. Bud Durability Tests
Terminal bud samples of various species (Table 2) were collected at the FRREC and
ETN and tested within four hours of collection to ensure freshness of buds for analysis.
Sample size during the dormant season was greater than required ensuring adequate
samples for evaluation as well as concentration for one preformed bud species (Carya) and
one sustainable growth species (Liriodendron). Fifty samples for each species during the
growing season were adequate as suggested by the similar standard errors for bud collar
diameter and bud fracture energy between growing seasons. Dormant-season samples
were collected in January 2011 and growing-season samples were collected in June 2011.
Growing-season samples were collected after the completion of the first leaf flush for
determinate growth species following the method of Romberger [
20
]. No lateral buds were
sampled, as they were assumed not to be directly affected by crown abrasion.
Table 2. Dormant- and growing-season bud durability tests by species using a pendulum impact
test with average bud collar diameter (cm) and energy (Joules) needed to fracture the bud as the
variables for the crown abrasion study, Tennessee, 2011. Standard errors are given in parentheses for
each species. R
2
value is given to explain the amount of variation in bud fracture energy explained by
bud collar diameter. Species without data were not included in the analysis for that season. Asterisk
(*) is considered preformed bud growth form.
Dormant
Season
Growing
Season
n
Bud Collar
Diameter
(cm)
Bud Fracture
Energy
(Joules)
R² n
Bud Collar
Diameter
(cm)
Bud Fracture
Energy
(Joules)
R2
Acer rubrum 77 0.229 (0.005) 0.014 (0.001) 0.07 52 0.183 (0.005) 0.013 (0.002) 0.51
Carya tomentosa * 177 0.465 (0.008) 0.184 (0.009) 0.52 52 0.417 (0.014) 0.114 (0.010) 0.52
Juglans nigra * - - - - 52 0.427 (0.013) 0.063 (0.006) 0.44
Liquidambar styraciflua 77 0.277 (0.011) 0.037 (0.004) 0.80 52 0.267 (0.005) 0.02 (0.001) 0.10
Liriodendron tulipifera 181 0.292 (0.003) 0.045 (0.002) 0.31 52 0.216 (0.006) 0.013 (0.001) 0.26
Platanus occidentalis 73 0.414 (0.009) 0.053 (0.003) 0.47 - - - -
Quercus alba * 77 0.295 (0.008) 0.049 (0.004) 0.39 51 0.249 (0.006) 0.034 (0.003) 0.28
Quercus falcata * - - - - 52 0.259 (0.004) 0.027 (0.003) 0.19
Quercus rubra * - - - - 52 0.292 (0.007) 0.037 (0.005) 0.42
Quercus texana * 74 0.231(0.004) 0.017 (0.001) 0.27 51 0.180 (0.003) 0.017 (0.001) 0.06
Forests 2024,15, 247 5 of 16
Terminal bud durability refers to the amount of energy a bud can absorb before
fracture [
21
]. Durability was tested with a Tinius Olsen Model 92T Impact Tester (Tinius
Olson Inc., Horsham, PA, USA). The impact tester is a pendulum that swings and strikes
the sample. The amount of energy that is absorbed by the sample is measured in Joules.
Buds were braced with a block of wood to ensure the break occurred at the bud collar. If
samples were not braced, the sample could break at any weak point of the twig or stem
below the base of the bud.
Bud collar diameter and the amount of energy required to break the bud were recorded.
Most of the Quercus spp. had several buds or bud clusters such that a single bud could not
be isolated for striking by the pendulum. Thus, more than one bud in a cluster may have
been severed. However, only complete strike throughs (one strike) were included in the
data. With multiple buds, the bud with the largest collar diameter was measured.
Bud durability data were analyzed using mixed model analysis of variance in SAS©
software 9.2 (SAS Institute, Cary, NC, USA). A Completely Randomized Design (CRD)
analysis was used at an alpha level of 0.05 to test differences between seasonality. Mean
comparisons between species’ bud collar diameter and bud break energy were evaluated
using Tukey’s Honestly Statistical Difference (HSD).
2.5. Tree Sway Accelerations
Data were collected during various windstorm events during the dormant season of
2010–2011. Growing season data were collected after full leaves were out in June 2011. Tree
sway accelerations for a single Quercus texana tree were investigated in a plantation that was
planted in the spring of 1993 at the ETN. The trees in the plantation averaged 6.1 m tall with
the live crown beginning at 2.7 m above the ground and branches expanding 4.9 m from the
stem. A tree in the southwestern corner of the plantation was selected for the placement of
tri-axis accelerometers (SparkFun Electronics, Niwot, CO, USA) based on crown symmetry,
accessibility, and exposure to prevailing wind. The research study only had resources to
monitor the sway of one tree with four accelerometers, an anemometer, and recording
devices due to the irregularity of wind events (unknown time periods) during the growing
and dormant seasons. These data from one tree were considered exploratory in describing
tree crown response to windstorms.
Branches at least six meters long in each cardinal direction were selected, and ac-
celerometers were attached 0.6 m from the end of the branches to ensure that the weight
of the device had minimal influence on branch movement. Universal Serial Bus (USB)
cords (9.1 m) connected the accelerometers to the laptop data recorder at the base of the
tree. Cords were ziplocked along the branch to the bole of the tree and then to the base
of the tree to minimize cord movement. Placement of the accelerometer 0.6 m from the
end of the branch only approximated the movement at the end of the branch. Only one
accelerometer at a time could be accessed by the data recorder. Data from the four cardinal
directions could not be conducted synchronously. The accelerometer data were recorded
and analyzed together; then, they were synchronized with the wind data.
Accelerometers recorded gravitational force (G) on each axis (X,Y, and Z) and were set
to record at +/
−
4 G. The X-axis recorded left to right acceleration, the Y-axis recorded for-
ward and backward, and the Z-axis recorded upward and downward. The accelerometers
recorded at 10 hertz, resulting in data recording 10 times per second.
A Windlog
TM
Wind Data Logger (RainWise, Boothwyn, PA, USA) anemometer was
used to record local wind data near the plantation. The anemometer was placed 2.7 m in
height above the ground to record wind speed and top wind gust. The device was set to
record once every minute while accelerometer data were being recorded. All wind data
reported were from top wind gusts and were recorded in kilometers per hour (KPH). Gusts
cause the most force in tree movement. Top wind gust speeds were divided into categories:
0 to 8.1, 8.2 to 16.1, 16.2 to 24.1, 24.2 to 32.2, 32.3 to 40.2, 40.3 to 48.3, 48.4 to 56.3, and 56.4 to
64.4 KPH.
Forests 2024,15, 247 6 of 16
The accelerometer data were analyzed using mixed model analysis of variance in SAS©
software 9.2. ANOVA was used to distinguish differences in branch movement between the
growing and dormant seasons. A Randomized Block Design (RBD) was used at an alpha
level of 0.05 with blocking on accelerometers to test seasonality (growing and dormant)
differences on the acceleration of each axis. An RBD for growing and dormant seasons was
evaluated using HSD in each wind speed category. Data were transformed using a rank
transformation. Data from the accelerometer were an average for every 10 observations.
The equation Xaxis
2
+Yaxis
2
+Zaxis
2
= (Mean Vector)
2
was used to combine all axes
into one mean vector that included the effect of gravity for each wind category.
Separate figures were created to illustrate maximum acceleration for each wind gust
category and rate of change between categories. Maximum acceleration was calculated
by taking the greatest range of acceleration for each wind speed category and pairing it
with wind gust data. A 2-dimensional energy plot was also created for each wind gust
category for both seasons. To better understand oscillation differences between dormant
and growing seasons, similar data were graphed as a function of time.
Forces in Newtons (N) were calculated to address the possibility of abrasion. Joules
recorded from the pendulum impact tester are units of work and cannot be used to calculate
the minimal force required for bud failure. The drop height of the pendulum was 61 cm
and was used to calculate minimal force by applying the equation F = U
×
0.6069 m, where
F is the force and U is the energy as described in the Tinius Olson Pendulum Impact Tester
Manual (http://www.tiniusolson.com, accessed on 24 October 2023).
Forces in Newtons possible for each wind category were calculated using the bud
mass and branch accelerations from the growing-season data for each species. Acceleration
for each wind category was from wind gusts and represented the maximum acceleration
observed. Maximum force possible from observed wind gusts was calculated using the
equation F = M
×
A, where F is force, M is bud mass, and A is branch acceleration.
Maximum force possible was compared to the minimal force required to cause bud failure
to assess severing of buds.
3. Results and Discussion
3.1. Branch and Bud Elongation
3.1.1. Branch Analysis
Liriodendron tulipifera and Liquidambar styraciflua in plantations had greater branch
elongation on exterior limbs than on the interior side where limbs were in contact with
adjacent trees (Table 3). Quercus texana was an exception with a shorter branch length on
the exterior side of the plantation as compared to branches on the interior side. All interior
samples exhibited missing growth years and were most frequent in Liquidambar styraciflua.
Table 3. Mean length of five interior and five exterior branches located in plantations for each of three
species, Tennessee, 2011. Asterisk (*) is considered preformed bud growth form.
Species
Interior Branches Exterior Branches
Mean Length
(cm)
Range
(cm)
Mean Length
(cm)
Range
(cm)
Liquidambar
styraciflua 170 137–190 377 356–417
Liriodendron tulipifera
246 186–338 363 340–384
Quercus texana * 351 298–490 247 208–318
Branch growth was not consistent in crowded crowns as compared to those growing
freely for the three species sampled in monoculture plantations. This provided support
for the first hypothesis that branch growth will be more restricted in the interior than
the exterior of tree crowns. Direct branch growth comparisons between species were not
possible since each plantation was a different age, planted at slightly different spacings,
and on different site productivities. Each species also had different growth rates and crown
Forests 2024,15, 247 7 of 16
forms. However, branch elongation for each species on the interior and exterior sides of
the crown could be assessed.
With Liriodendron tulipifera and Liquidambar styraciflua, growth was restricted on the in-
terior branches adjacent to other tree crowns. On the interior branch samples of Liriodendron
tulipifera, elongation declined with the onset of crown closure. Indicators of crown abrasion
were observed, including damaged branches, missing growth rings, and irregular forked
branches where elongation continued at a different angle but at a much slower rate. Similar
attributes were recognized with Liquidambar styraciflua. The appearance of the branch was
deformed and twisted with further elongation, often breaking at a previous abrasion. This
physical branch abrasion and sunlight being more restricted when approaching canopy
closure contributed to limiting the interior branch growth.
The Quercus texana plantation had the densest canopy, which may have resulted in the
reduction in wind effects or decrease in wind speed which allowed branches to overlap and
continue growing with little inhibition. Quercus texana was the only species sampled to have
a shorter branch length on the exterior samples likely due to its preformed growth form
of branches and denser crowns. Although interior branches were longer, some branches
exhibited restricted growth at the branch tip (multiple buds) from breakage. As with
the other two species, interior branch elongation rates were decreasing as well, probably
because of denser crowns and a progressive lack of sunlight, with crown closure affecting
growth rates.
3.1.2. Bud Analysis
Carya tomentosa had the greatest bud mass of all species sampled, and Acer rubrum
had the least (Table 1). Most of the other species had a bud mass between 0.01 g and 0.08
g. Although Juglans nigra had the greatest mean bud collar diameter, it did not have the
greatest mean bud mass. Carya tomentosa had a smaller mean bud collar but almost tripled
the mean mass of the Juglans nigra bud. The shape of the bud probably influences the
bud mass for these two species. Juglans nigra has a trapezoid-shaped bud with the largest
portion positioned at the bud collar, while Carya tomentosa is shaped more like a rhombus
with most of its mass in the center of the bud. Quercus alba,Quercus texana, and Acer rubrum
had the lowest mean bud mass with their smaller buds (Table 1).
The moisture content of buds varied between species ranging from 66 to 258 percent.
Liriodendron tulipfera and Liquidambar styraciflua had greater moisture contents of the buds
sampled in this study (Table 1). Both species have the sustained, indeterminate growth
form and increased moisture content, which makes the bud more pliable, reducing rigidity.
Buds of both species were green and flexible, but Liriodendron tulipifera, in particular, is
composed of a tender leaf stipule protecting leaf primordia. The flexibility and faster
growth of Liriodendron tulipifera may have obscured some twig and bud damage from
branch abrasion that was visually apparent with Liquidambar styraciflua.
3.2. Bud Durability Tests
The mean diameter of the bud collars was different between the dormant and growing
seasons for all species (p= 0.001) (Table 2). The mean energy required to fracture the buds
of all species sampled varied between seasons (p= 0.001) and was different between species
within each season (p= 0.001). Liquidambar styraciflua had the greatest R
2
value in winter
and the second lowest in summer. In the dormant season, 80% of the variation in energy
required for bud fracture for Liquidambar styraciflua could be explained by the diameter of
the bud collar (p= 0.001). For the growing season, only 10% of the variation in the energy
requirements could be explained by the bud collar diameter. This trend of decreasing
variation from winter to summer is consistent across most species (Table 2).
Carya tomentosa and Juglans nigra have the largest buds among the species sampled
(Table 2). Quercus texana and Acer rubrum have the smallest bud diameters in both the
dormant and growing seasons. The determinant growth form species generally have buds
that are larger than the indeterminant growth species in both seasons.
Forests 2024,15, 247 8 of 16
Bud structure varies for each species. Some species, such as Carya, have a single
terminal bud. Other species, such as Quercus, have several terminal buds in a cluster
emanating from the same point. Several buds could serve as protection for interior buds
within a cluster. Having several buds allows the tree to continue terminal growth even
if one or more of the exterior terminal buds are damaged or abraded. Species such as
Liquidambar styraciflua without the cluster of terminal buds would not have this ability. If
Quercus were growing adjacent to a Liquidambar styraciflua while both were abrading each
other, over time, Quercus would have the opportunity for greater elongation [22].
Carya tomentosa absorbed the most energy from the impact test before fracture oc-
curred in both the dormant and growing seasons (Table 2). In the dormant season, a clear
pattern was not apparent between the energy required to fracture buds of determinant and
indeterminant species. However, all determinant growth species required more energy to
sever these buds during the growing season than indeterminant species, except for Quercus
texana. Most indeterminants were forming buds continuously (recurrent flushing) during
the growing season, but these buds varied in size, and most were undergoing elongation.
The determinant species all had newly formed buds after one bud flush, resulting in a
stouter bud that did not show signs of further elongation.
For species collected in both seasons, all R
2
values changed between those seasons
(Table 2). Acer rubrum and Liquidambar styraciflua had the most change. Acer rubrum in
the dormant season exhibited a slight relationship between bud durability and bud collar
diameter. Only 7% of the variation in energy could be explained by bud collar diameter.
The growing season data contributed 51% of the variation in bud durability. Alternatively,
Liquidambar styraciflua dropped from a strong positive relationship (80%) between energy
and bud collar diameter in the dormant season to a weak relationship (10%) in the growing
season. Seasonal variation in bud durability is inconsistent with bud collar diameter for
these two indeterminant growth species.
Preformed (determinate) bud growth species generally require more energy to frac-
ture the bud than sustained (indeterminant) growth species (Table 2), lending support to
the second hypothesis that preformed buds would require a greater force to break than
sustained buds. The exception in this study was Quercus texana, which was the weakest
preformed species tested and had energy values in the same range as sustained growth
species. The cluster of Quercus texana terminal buds may have influenced the energy values
measured. Even with its preformed growth form, Quercus texana is also known to be one of
the faster-growing bottomland oak species [23].
3.3. Tree Sway Accelerations
3.3.1. Acceleration
The mean acceleration on each axis was significantly different between seasons within
the 0 to 8.1, 16.2 to 24.1, and 24.2 to 32.2 KPH wind categories (p= 0.001), but it was not
different for the 8.2 to 16.1 KPH category (p= 0.999) (Table 4). The growing season was
consistently greater than the dormant season in mean acceleration for all wind categories.
Within seasons, the mean acceleration did not show a clear pattern between wind speeds,
presumably because of variations in wind gusts (Table 4). The standard deviation increased
as wind speed increased for each season on every axis. Data in the growing season had
greater deviations from the mean at lower wind speeds, as compared to the dormant-season
data. On every axis, the 16.2 to 32.2 KPH wind speed categories during the growing season
had three times the deviation compared to the dormant season.
Forests 2024,15, 247 9 of 16
Table 4. Mean branch accelerations and standard deviation (in parenthesis) of Quercus texana in
Tennessee, 2011, for each wind speed category during the dormant and growing seasons for the X-,
Y-, and Z-axis. Mean vector branch acceleration is for the Z-axis only. Sample size (n) is the same for
each of the axes. Observations were averaged every 10 counts to obtain per second measurements.
Wind events above 32.4 KPH (kilometers per hour) were not captured in the growing season.
Wind
Speed
(KPH)
X-Axis Y-Axis Z-Axis Vector
Dormant
Season
Growing
Season
Dormant
Season
Growing
Season
Dormant
Season
Growing
Season
Dormant
Season
Growing
Season
nMean (G) nMean (G) Mean
(G)
Mean
(G)
Mean
(G)
Mean
(G) Mean (G) Mean (G)
0–8.1 99 0.008
(0.005) 298 0.012
(0.007)
0.214
(0.004)
0.522
(0.008)
0.955
(0.004)
0.741
(0.014) 0.958 0.821
8.2–16.1 480 0.008
(0.006) 480 0.041
(0.012)
0.216
(0.004)
0.364
(0.008)
0.958
(0.006)
0.816
(0.008) 0.964 0.800
16.2–24.1 180 0.121
(0.005) 415 0.074
(0.045)
0.450
(0.004)
0.835
(0.024)
0.851
(0.006)
0.789
(0.026) 0.941 1.325
24.2–32.2 480 0.009
(0.013) 157 0.321
(0.030)
0.674
(0.010)
0.313
(0.031)
0.767
(0.012)
0.844
(0.035) 1.043 0.913
32.3–40.2 414 0.136
(0.017) - - 0.037
(0.010) -0.948
(0.017) - 1.054 -
40.3–48.3 419 0.038
(0.020) - - 0.477
(0.008) -0.866
(0.012) - 0.979 -
48.4–56.3 117 0.143
(0.024) - - 0.361
(0.011) -0.953
(0.025) - 1.059 -
56.4–64.4 180 0.078
(0.026) - - 0.462
(0.015) -0.820
(0.030) - 0.892 -
Maximum acceleration during dormant and growing seasons increased as wind gusts
increased (Table 5). The Z-axis experienced the most acceleration in almost all wind
categories for both seasons. As wind gusts increased in the dormant season, the rate
of change in acceleration fluctuated. During the growing season, the rate of change in
acceleration was more consistent.
Table 5. Maximum branch acceleration (G) and rate of change (in parenthesis) of Quercus texana
within each wind gust category for the X-, Y-, and Z-axes during the dormant and growing seasons
for the crown abrasion study, Tennessee, 2011. Wind speed categories are given in kilometers per
hour (KPH). Wind gust events were not captured in the growing season.
Wind
Speed
(KPH)
Dormant Season Growing Season
X Y Z X Y Z
0–8.1 0.02 (-) 0.06 (-) 0.07 (-) 0.14 (-) 0.08 (-) 0.19 (-)
8.2–16.1 0.19 (912%) 0.09 (57%) 0.24 (225%) 0.22 (56%) 0.17 (117%) 0.44 (131%)
16.2–24.1 0.24 (32%) 0.09 (−3%) 0.25 (5%) 0.45 (104%) 0.33 (92%) 0.92 (110%)
24.2–32.2 0.62 (154%) 0.24 (178%) 0.79 (215%) 0.99 (119%) 0.68 (108%) 1.63 (77%)
32.3–40.2 1.32 (113%) 0.40 (69%) 0.88 (11%) - - -
40.3–48.3 1.32 (0%) 0.62 (56%) 1.38 (56%) - - -
48.4–56.3 1.50 (13%) 0.65 (5%) 1.83 (33%) - - -
56.4–64.4 1.40 (−7%) 0.57 (−12%) 2.11 (15%) - - -
Crown movements displayed greater acceleration in the growing season than in the
dormant season for each axis. However, the maximum acceleration that occurred during
wind gusts in the growing and dormant seasons (Table 5) is a more effective measure
of branch movement than the mean acceleration (Table 4) within each wind category. Most
accelerations occurred in small bursts and were often negated by using the mean acceleration.
3.3.2. Force
Carya tomentosa required the most force to cause bud failure at 0.19 N (Table 6). Juglans
nigra also required a large amount of force before failure occurred at 0.1 N. Although the
bud of Carya tomentosa is the most durable of the species sampled, it is also the first of the
Forests 2024,15, 247 10 of 16
preformed species to experience enough force to cause a bud to break. The breakage is
attributed to the greater mass of Carya buds. Quercus texana had the lowest mean minimal
force of the preformed species at 0.01 N, similar to those species with sustained growth.
The sustained, indeterminant-growth species had a lower mean minimal force than the
preformed-growth, determinant species.
Table 6. Growing season mean minimal force for bud break in Newtons (N) for each species for
the crown abrasion study, Tennessee, 2011. Maximum possible force (N) was calculated for each
kilometers per hour (KMH) wind speed category. Red values represent force at which bud failure is
possible. Species without data were not included in the analysis. Asterisk (*) is considered preformed
(determinant) bud growth form.
Species Minimal
Force (N)
Estimated Force Possible (N)
Wind Categories (KPH)
0–8.1 8.2–16.1 16.2–24.1 24.2–32.2
Acer rubrum 0.021 0.011 0.002 0.004 0.008
Carya tomentosa * 0.187 0.044 0.101 0.212 0.375
Juglans nigra * 0.104 0.016 0.037 0.077 0.136
Liquidambar styraciflua 0.031 0.005 0.012 0.026 0.046
Liriodendron tulipifera 0.022 0.011 0.025 0.052 0.092
Quercus alba * 0.056 0.002 0.004 0.008 0.015
Quercus falcata * 0.043 0.003 0.007 0.016 0.028
Quercus rubra * 0.060 0.008 0.017 0.037 0.065
Quercus texana * 0.012 0.002 0.004 0.008 0.015
The lowest wind speed category yields no force that is powerful enough to cause
breakage (Table 6). In the wind speed category of 8.2 to 16.1 KPH, Liriodendron tulipifera
branches begin to have forces that cause bud failure. At 16.2 to 24.1 KPH, Carya tomentosa
began to experience enough force to also cause mechanical failure. At the greatest wind
speed category recorded in the growing season, only Acer rubrum,Quercus alba, and Quercus
falcata did not receive enough force to cause breakage, probably because of a smaller
bud mass.
3.3.3. Wind Speed and Accelerations
Most wind events are a result of differences in temperatures across a large area of
land, often arriving with cold fronts [
24
]. Wind is a force that is variable, not constant
or consistent. The data collected in this case study suggested that wind speed influences
maximum accelerations, but not mean accelerations. Greater accelerations were masked
among smaller accelerations once the mean was calculated, even in the mean vector
calculations where all three axes were included (Table 4).
The gravity of the Earth caused the accelerometer to give a constant reading of 1.0 G
when the axis was level. However, branch orientation differed on every axis of the mea-
surement devices, resulting in inconsistent baseline readings. Each branch tested tended to
return from its displacement to a place of rest, even in a steady wind which could cause
mean accelerations for the three axes not to be representative of branch movement.
The X- and Z-axes were the two axes that experienced the most movement, repre-
senting left and right and upward and downward movements, respectively. The Y-axis
was limited by the forward and backward movements of the stem of the tree. A taller tree
would experience a greater amount of sway (Y-axis).
Forests 2024,15, 247 11 of 16
3.3.4. Acceleration during Wind Gusts
Seasonality influenced tree sway, as there were differences in tree movement in the
presence or absence of leaves (Table 5). During the dormant season, maximum accelerations
increased more sharply when wind speeds were more than 24.1 KPH. At less than 24.1 KPH,
0.25 G or less was recorded. At 32.2 KPH, accelerations were greater than 1.3 G. Between
56.4 and 64.3 KPH, accelerations almost doubled to 2.1 G. During the growing season,
acceleration increased more steadily than during the dormant season. The growing-season
wind speed of 16.2 to 24.1 KPH had accelerations approaching 1.0 G, which did not occur in
the dormant season until winds reached 32.3 to 40.2 KPH. With winds of 16.2 to 32.2 KPH
during the growing season, acceleration doubled that recorded in the dormant season. The
growing-season acceleration caused the crown to be more affected by wind because of the
extra drag and mass produced by the leaf cover.
Wind gusts produced the greatest branch acceleration. Crown abrasion would likely
occur if there was damage and rubbing from low wind speeds between adjacent and
overlapped branches for an extended time period. A large wind gust could then fracture or
sever the weaker branch.
3.3.5. Energy Maps and Seasonality
Energy maps were constructed using the branch acceleration of the X- and Z-axes for
each of the wind speed categories in the dormant and growing seasons. Figure 1(16.2 to
24.1 KPH) and Figure 2(48.4 to 56.3 KPH) are presented as examples for discussion. Wind
gust events above 32.2 KPH were not captured during the growing season.
Forests 2024, 15, x FOR PEER REVIEW 12 of 17
Figure 1. Dormant- and growing-season Quercus texana branch acceleration on the Z-axis as a func-
tion of acceleration on the X-axis for the crown abrasion study, Tennessee, 2011. Acceleration is from
the 16.2 to 24.1 KPH wind gust category.
Figure 2. Dormant-season Quercus texana branch acceleration on the Z-axis as a function of acceler-
ation on the X-axis for the crown abrasion study, Tennessee, 2011. Acceleration is taken from the
48.4 to 56.3 KPH wind gust category.
The third hypothesis that crown movements will exhibit greater acceleration during
the growing season than during the dormant season was supported by the tree sway data.
Seasonality influenced the energy paerns of the branches as depicted in Figure 1. Greater
energy and branch movement response occurred during the growing season compared to
the dormant season for all wind speed categories (Tables 2 and 5). Figure 1 with wind
gusts of 16.2 to 24.1 KPH shows similar energy cluster areas, but the growing season map
had traces of energy that loop more from the cluster than the dormant-season map. This
looping was common for all wind speeds with deviations from the cluster being greater
at greater wind speeds (Figure 2). Energy, as shown in Figure 1 in the growing season,
does not occur on the left side of the cluster. A drastic cutoff occurred when a moving
branch was physically stopped by an adjacent branch. As wind speeds increase to greater
Figure 1. Dormant- and growing-season Quercus texana branch acceleration on the Z-axis as a function
of acceleration on the X-axis for the crown abrasion study, Tennessee, 2011. Acceleration is from the
16.2 to 24.1 KPH wind gust category.
Forests 2024,15, 247 12 of 16
Forests 2024, 15, x FOR PEER REVIEW 12 of 17
Figure 1. Dormant- and growing-season Quercus texana branch acceleration on the Z-axis as a func-
tion of acceleration on the X-axis for the crown abrasion study, Tennessee, 2011. Acceleration is from
the 16.2 to 24.1 KPH wind gust category.
Figure 2. Dormant-season Quercus texana branch acceleration on the Z-axis as a function of acceler-
ation on the X-axis for the crown abrasion study, Tennessee, 2011. Acceleration is taken from the
48.4 to 56.3 KPH wind gust category.
The third hypothesis that crown movements will exhibit greater acceleration during
the growing season than during the dormant season was supported by the tree sway data.
Seasonality influenced the energy paerns of the branches as depicted in Figure 1. Greater
energy and branch movement response occurred during the growing season compared to
the dormant season for all wind speed categories (Tables 2 and 5). Figure 1 with wind
gusts of 16.2 to 24.1 KPH shows similar energy cluster areas, but the growing season map
had traces of energy that loop more from the cluster than the dormant-season map. This
looping was common for all wind speeds with deviations from the cluster being greater
at greater wind speeds (Figure 2). Energy, as shown in Figure 1 in the growing season,
does not occur on the left side of the cluster. A drastic cutoff occurred when a moving
branch was physically stopped by an adjacent branch. As wind speeds increase to greater
Figure 2. Dormant-season Quercus texana branch acceleration on the Z-axis as a function of accelera-
tion on the X-axis for the crown abrasion study, Tennessee, 2011. Acceleration is taken from the 48.4
to 56.3 KPH wind gust category.
The third hypothesis that crown movements will exhibit greater acceleration during
the growing season than during the dormant season was supported by the tree sway data.
Seasonality influenced the energy patterns of the branches as depicted in Figure 1. Greater
energy and branch movement response occurred during the growing season compared
to the dormant season for all wind speed categories (Tables 2and 5). Figure 1with wind
gusts of 16.2 to 24.1 KPH shows similar energy cluster areas, but the growing season map
had traces of energy that loop more from the cluster than the dormant-season map. This
looping was common for all wind speeds with deviations from the cluster being greater
at greater wind speeds (Figure 2). Energy, as shown in Figure 1in the growing season,
does not occur on the left side of the cluster. A drastic cutoff occurred when a moving
branch was physically stopped by an adjacent branch. As wind speeds increase to greater
than 24.2 KPH, whether in the dormant or growing seasons, more branch movements
(and energy) would cause branch damage to occur when adjacent crowns abrade each
other. Although the wind speed categories displayed more branch movements along the
X-axis than the Z-axis, the upward and downward movements (Z-axis) increased at greater
wind speeds.
3.3.6. Oscillations
Dewit and Reid [
17
] observed that wind during the dormant season caused tree
crowns to be displaced less but more frequently, while growing-season winds caused
greater displacement with less frequency. Our findings in this case study are similar
(Figure 3). During the dormant season, the period is shorter than during the growing
season. The waves in the dormant season are narrow because the wind moves a branch
a short distance, and the branch quickly returns to its neutral state. Alternatively, the
growing season waves are more stretched or elongated, as it takes longer for the branch to
complete its movement. The leaf mass catches more wind. The amplitude of the growing
season acceleration is consistently larger than during the dormant season. The acceleration
trend lines between the growing and dormant seasons are similar because the waves and
amplitude compensate for each other. These branch-movement patterns are similar for the
Y- and Z-axes. Crown abrasion is more prevalent in the growing season as branches of
adjacent trees oscillate with greater accelerations.
Forests 2024,15, 247 13 of 16
Forests 2024, 15, x FOR PEER REVIEW 13 of 17
than 24.2 KPH, whether in the dormant or growing seasons, more branch movements (and
energy) would cause branch damage to occur when adjacent crowns abrade each other.
Although the wind speed categories displayed more branch movements along the X-axis
than the Z-axis, the upward and downward movements (Z-axis) increased at greater wind
speeds.
3.3.6. Oscillations
Dewit and Reid [17] observed that wind during the dormant season caused tree
crowns to be displaced less but more frequently, while growing-season winds caused
greater displacement with less frequency. Our findings in this case study are similar (Fig-
ure 3). During the dormant season, the period is shorter than during the growing season.
The waves in the dormant season are narrow because the wind moves a branch a short
distance, and the branch quickly returns to its neutral state. Alternatively, the growing
season waves are more stretched or elongated, as it takes longer for the branch to complete
its movement. The leaf mass catches more wind. The amplitude of the growing season
acceleration is consistently larger than during the dormant season. The acceleration trend
lines between the growing and dormant seasons are similar because the waves and am-
plitude compensate for each other. These branch-movement paerns are similar for the Y-
and Z-axes. Crown abrasion is more prevalent in the growing season as branches of adja-
cent trees oscillate with greater accelerations.
Figure 3. Standardized dormant and growing season Quercus texana branch acceleration on the X-
axis as a function of time f rom the 16. 2 to 32.2 KPH wind gust category for the crown abrasion study,
Tennessee, 2011. Data were recorded at 10 observations per second. Time represents observations.
Figure 3. Standardized dormant and growing season Quercus texana branch acceleration on the X-axis
as a function of time from the 16.2 to 32.2 KPH wind gust category for the crown abrasion study,
Tennessee, 2011. Data were recorded at 10 observations per second. Time represents observations.
3.4. Potential for Crown Abrasion
Spring and summer storm fronts can easily create winds above 32.2 KPH. The National
Oceanic and Atmospheric Administration (NOAA) does not classify a storm as “severe”
until it reaches wind speeds of at least 93.3 KPH. During calm weather periods when
branch elongation is rapid, particularly in the early growing season, crowns can grow and
overlap before the arrival of the next wind event. Accelerometers were not placed at the
tips of the branches; thus, all force calculations can be considered conservative. Greater
movement and acceleration are expected at the end of the branch. Based on the results of
this study, crown abrasion occurs during wind events when branches of adjacent trees are
in contact during the growing season.
Crown abrasion becomes inevitable as growing space between trees diminishes with
tree growth and crown expansion. Canopies of even-aged monocultures do not have crown
stratification compared with mixed species stands [
6
,
10
,
11
]. These monocultures have
similar growth rates and bud durability. The intra-species competition would lead to few
dominants, resulting in an absence of crown stratification. As crown canopies close, trees
continually battle for growing space, increasing lateral branch damage in the progressively
limited growing space as reported in Pinus contorta [
4
] and Pinus sylvestris [
9
] stands. These
conditions often lead to stand stagnation with tree sway and resulting crown abrasion
or shyness, reducing the size of tree crowns throughout the stand. Thinnings would be
necessary to allow for the continued crown expansion of desired trees.
Forests 2024,15, 247 14 of 16
In mixed-species stands, crown abrasion is much more complex. A myriad of different
species, each with different growth rates, growth habits, and ecological requirements on
a variety of site productivities and spacings, yield various crown interactions and stand
development patterns. Factors inherent to different species such as crown form (excurrent
or decurrent), growth form (preformed or sustained), light tolerance, and phenology will
impact the degree of crown abrasion. Typically, mixed species stands have multiple canopy
layers even when they are even-aged [
25
] because they are composed of different species
and growth rates. Crown stratification and stand development patterns in pure and mixed
stands are described by Oliver and Larson [1].
Stand development and crown closure incur some crown abrasions that influence
growth. One example is the development of Quercus pagoda Raf. (cherrybark oak)—
Liquidambar styraciflua stands in minor river bottoms in Mississippi, USA. Liquidambar
styraciflua initially outgrows Quercus pagoda, composing a dense overstory that leads to intra-
specific crown abrasion within Liquidambar styraciflua, allowing for sunlight to infiltrate.
Meanwhile, the growth rate of Quercus pagoda residing in the lower canopy increases
with the addition of more sunlight from the abraded overstory. The stouter branches of
the belligerent Quercus pagoda continually abrade the lower branches of the Liquidambar
styraciflua (inter-specific abrasion), creating an unrestricted path for Quercus pagoda to
ascend into the overstory. Quercus pagoda eventually surpasses the height of declining
Liquidambar styraciflua, spreading its crown and dominating the overstory suppressing
the Liquidambar styraciflua that was once taller. These two instances of crown abrasion,
intra-specific among trees in the Liquidambar styraciflua overstory and then the ascendence
of Quercus pagoda into the overstory through inter-specific abrasion, occurred during this
stand development process. Liquidambar styraciflua has sustained branch growth, fast initial
growth rate as an intolerant species, and excurrent growth form, which are quite different
from Quercus pagoda with preformed branch growth, more intermediate light tolerance,
slower initial growth rate, and decurrent crown form. This stand development pattern
associated with crown abrasion for these two species was present in both natural [
22
] and
planted stands [
26
]. Similar crown abrasion effects in mixed-species deciduous stands have
been reported in Germany [
15
] and in New England, USA [
27
], with mixed deciduous
species, in mixed conifer stands in the Pacific Northwest, USA [
28
], and with mixed conifer–
deciduous species in Japan [12].
4. Conclusions
Crown abrasion, as a mechanism of stand development of mixed species stands, has
not been studied extensively. Therefore, methods to assess crown abrasion are lacking,
especially in deciduous forests. Evidence from these three studies (analysis of branch
elongation, bud durability, and tree sway acceleration) corroborates that crown abrasion
occurs and influences tree and stand development.
4.1. Branch Elongation and Bud Analysis
Crowns that collide with neighboring crowns restrict branch elongation. The growth
disruption of branches occurred more frequently on the interior side of trees (next to adjacent
tree crowns) than on the exterior edges where branches were free to grow and adjacent trees
were not present to influence growth. All sample plantations exhibited crown abrasion.
4.2. Bud Durability
The differences in bud durability between growing and dormant seasons suggest that
buds do not have the ability to absorb as much energy in the growing season as they do in
the dormant season. During the growing season, the actively growing buds are tender due
to a higher moisture content, making them more susceptible to abrasion than the hardened,
stout buds of the dormant season. Species with preformed buds and buds that occur in
clusters tended to absorb more energy, and, thus, they are more durable than species with
sustained growth buds. In general, all species decreased in bud collar diameter size after
Forests 2024,15, 247 15 of 16
bud breakage and new growth began. Although various strong relationships were present
for some species between bud collar diameter and bud durability, this relationship was not
consistent across all species or seasons.
4.3. Tree Sway Acceleration and Force
Branch movement was variable at different wind speeds during both the dormant
and growing seasons. Branch acceleration increasingly deviated from the mean as wind
speeds increased. Wind gusts caused differences in branch acceleration as the wind speed
increased. The presence of leaves caused a greater difference in the mean and maximum
acceleration during the growing season due to resistance (drag). When the same wind
speed was compared for both seasons, the growing-season wind created greater movement
in the crown than the dormant-season wind.
The minimal force required for bud breakage was calculated during the growing
season for several species. The amount of force was derived by using the bud mass
from the bud durability study and acceleration from the tree sway acceleration study.
As wind gust speeds increased, the amount of force that occurred on terminal buds also
increased. For most of the species tested, wind gust speeds of 24.2 to 32.2 KPH generated
enough force to cause a bud to break. In the spring, when new branch elongation and
terminal buds are moist and tender, a strong storm can easily reach wind gusts well above
32.2 KPH. Generally, buds of sustained-growth species break at a lower force than buds of
the preformed growth species.
4.4. Implications
Few studies have investigated the mechanics associated with physical crown abrasion.
This research elucidates that branch acceleration, bud durability, bud mass, and force
from branch movement are variable among species. Crown abrasion can produce a wide
range of stand development patterns, depending on species composition. The crown
forms, phenology, growth habits, and growth rates differ among tree species and influence
crown abrasion. This research provides insight for further research on stand development
processes and patterns in mixed-species stands.
Author Contributions: Conceptualization, W.K.C.; methodology, software, validation, formal analy-
sis, investigation, resources, and data curation, W.K.C. and T.M.L.B.; writing—original draft prepara-
tion, writing—review and editing, W.K.C., T.M.L.B. and E.C.Y.; visualization, W.K.C. and T.M.L.B.;
supervision, project administration, and funding acquisition, W.K.C. All authors have read and
agreed to the published version of the manuscript.
Funding: This research was funded internally by the School of Forest Resources, Institute of Agricul-
ture, University of Tennessee, Knoxville, TN, USA.
Data Availability Statement: Information and data about this research are available at TRACE:
Tennessee Research and Data Exchange at the following weblink: https://trace.tennessee.edu/utk_
gradthesis/1136, accessed on 2 June 2023.
Acknowledgments: The authors gratefully acknowledge the following organization for their co-
operation in providing and maintaining study sites for the research: (1) Forest Resources Research
and Education Center, AgResearch, Institute of Agriculture, University of Tennessee, Oak Ridge,
TN, USA; (2) East Tennessee Nursery, Tennessee Department of Agriculture, Division of Forestry,
Delano, TN, USA; and (3) John W. Starr Memorial Forest, College of Forest Resources, Mississippi
State, University, Mississippi State, MS, USA.
Conflicts of Interest: The authors declare no conflicts of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
Forests 2024,15, 247 16 of 16
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