Branch Elongation, Bud Durability, and Wind-Generated Crown Movement Associated with Crown Abrasion in Deciduous Trees

Abstract

Trees that grow in close proximity to other trees are subject to crown and branch abrasion, 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.

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 affects 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 monoculture 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 [2,3,4,5,6,7], 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 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 (determinant) 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 incorporating 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 access locations that contained samples of available species in plantings or natural stands necessary for each of the three studies.

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. 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	n	Bud 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.

) 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 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.

2.4. Bud Durability Tests

Terminal bud samples of various species (

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. R² 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

) 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.

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 accelerometers 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 forward 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.

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 X axis² + Y axis² + Z axis² = (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. 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

). 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.

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 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 interior 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 R² 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.

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 occurred 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 R² 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 fracture 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. 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
	n	Mean (G)	n	Mean (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	-

). 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.

Maximum acceleration during dormant and growing seasons increased as wind gusts increased (

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%)	-	-	-

). 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.

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. 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

). 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 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.

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 measurement 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, representing 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).

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.

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 (Table 2 and Table 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 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.

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.

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 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.