Forestry
An International Journal of Forest Research
Forestry 2013; 86, 255 – 265, doi:10.1093/forestry/cps083
Advance Access publication 23 December 2012
Long-term effects of single-tree selection cutting on structure
and composition in upland mixed-hardwood forests
of the southern Appalachian Mountains
Tara L. Keyser1* and David L. Loftis2
1
USDA Forest Service, Southern Research Station, Bent Creek Experimental Forest, 1577 Brevard Road, Asheville, NC 28806, USA
2
USDA Forest Service (Emeritus), Southern Research Station, Bent Creek Experimental Forest, 1577 Brevard Road,
Asheville, NC 28806, USA
*Corresponding author Telephone: +1 8286675261; [email protected]
Received 31 July 2012
In 1946, this study was initiated to examine the efficacy of single-tree selection cutting in submesic to mesic
mixed-hardwood forests of the southern Appalachian Mountains. Seven stands comprising 40 ha of mixedhardwood forests plus three unmanaged stands were utilized. Species composition of the overstorey prior
to treatment was dominated by species intolerant and mid-tolerant of shade, including oak (Quercus L.)
species and yellow-poplar (Liriodendron tulipifera L.). After 10 years of improvement cuttings, the BDq
method of regulating stocking was employed. Selective cuttings occurred in 1956–1957, 1962, 1970, 1984
and 2006. Early observations suggested that control of shade-tolerant midstory species might be required to
regenerate the mid-tolerant overstory species. In 1972, shade-tolerant species between the 5 and 20 cm
d.b.h. classes present in small openings created during the 1970 entry were injected with herbicide in all
seven managed stands. In 1986, another treatment which included the complete removal of the shade-tolerant midstory occurred in two of those seven stands. After 60 years, the majority of managed stands have diameter distributions other than the traditional reverse-J shape, including concave and rotated sigmoid shapes.
Even after the treatment of shade-tolerant competitors, results suggest that single-tree selection has thus
far been ineffective at recruiting the desirable tree species that are intolerant or mid-tolerant of shade. In
fact, the density of mid-tolerant oak-hickory and shade-intolerant species in the sapling and pole size
classes in the managed stands did not differ from that in unmanaged stands. As applied in this study, the
BDq approach to single-tree selection cutting has led to the sapling and pole size classes being increasingly
dominated by shade-tolerant species and is not likely to be sustainable, in terms of both desirable species
composition and structure, in the long-term.
Introduction
The application of uneven-aged management via single-tree
selection cutting in eastern hardwood forests of North America
has produced varied results with regard to regenerating and
maintaining desired species composition. Although species composition and diversity have been maintained over the long term
following the application of single-tree selection cutting in xeric
oak (Quercus L. spp.) forests of Missouri,1 in more mesic hardwood forests, where structural and compositional complexity is
greater, the application of single-tree selection cutting has
shifted species composition away from species intolerant
and mid-tolerant of shade to more shade-tolerant species.2 – 5
For example, Schuler6 documented a significant decrease in
stand-level tree species diversity during 50 years of single-tree
selection in mixed-mesophytic forests of West Virginia, with
shade-tolerant species, such as red maple (Acer rubrum L.) and
sugar maple (Acer saccharum Marsh.), significantly increasing
in importance at the expense of mid-tolerant oak species. Similarly, in the same submesic to mesic mixed-hardwood stands
used in the present study, Della-Bianca and Beck2 concluded
that unless growing space is managed by the removal of
undesirable shade-tolerant species, the likelihood of successfully
using single-tree selection in the southern Appalachian
Mountains to regenerate ‘desirable’ (i.e. species intolerant and
mid-tolerant of shade) upland hardwood tree species was low.
The application of single-tree selection cutting in eastern hardwood stands has been predominantly accomplished by regulating
stocking according to the BDq method. The objective of using the
BDq method is to produce a theoretically ‘balanced’ uneven-aged
stand structure, which has been traditionally defined by
stand-level diameter distributions conforming to a negative exponential or reverse-J-shaped curve. This structure is a condition suggested by Meyer7 to yield a sustained volume with little to no
change in stand structure or volume over the long term. The
reverse-J shape curve, which can be created by a constant7,8 or
Published by Oxford University Press on behalf of the Institute of Chartered Foresters, 2012.
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Forestry
variable9, 10 diminution quotient, or q, across the diameter distribution within a stand is generally considered to be indicative of
this theoretically balanced, uneven-aged structure. However,
diameter distributions other than this theoretically balanced
uneven-aged structure have been documented in both managed
and unmanaged (e.g. old-growth) uneven-aged stands. Diameter
distributions of unmanaged old-growth stands have been described
as increasing q,11 negative exponential or rotated sigmoid in
shape,12 – 15 while diameter distributions of stands managed via
single-tree selection cutting are more variable, often displaying
the negative exponential, rotated sigmoid, concave, increasing-q
and unimodal forms.4,5,15 – 17
Results from the majority of studies that quantify the effects
of single-tree selection on stand structure and composition in
eastern hardwood forests deal mostly with northern hardwood
forest types in the Lake States and New England Regions of
the US.5,9,11,13,14,18 Evidence regarding the efficacy of using the
single-tree selection method in submesic to mesic upland
mixed-hardwood forests is less abundant, primarily because of
the limited application of single-tree selection to regenerate
stands dominated by species mid-tolerant to intolerant of
shade. In general, single-tree selection has been recommended
in forest types comprised of fairly shade-tolerant species.18 Of
the studies that do report the effects of single-tree selection in
mixed-hardwood forests, results suggest that it is ineffective at
meeting goals associated with regenerating mixed species
stands in which intolerant and mid-tolerant species are the
predominant canopy tree species.2,6
In this study, we examine the effects of 60 years of single-tree
selection cutting on the structure and species composition of
upland mixed-hardwood forests of the southern Appalachian
Mountains. In the context of the overall objective, which was
to use utilize single-tree selection cutting to regenerate predominantly desirable (i.e. mid-tolerant and intolerant species) tree
species, we quantify the long-term effects of single-tree selection on (i) the abundance and relative importance of midtolerant, intolerant and tolerant tree species across stands
managed via the BDq method of single-tree selection, as well
as between managed stands and unmanaged stands; and
(ii) examine diameter distributions of stands after 60 years of
management under the BDq method of single-tree selection
and compare diameter distributions of managed stands to
those of unmanaged stands.
Methods
This study was conducted on 40 ha of the Bent Creek Experimental
Forest hereafter termed ‘The Farm Woodlot’ in Asheville, NC, US
(35.58N, 82.68W). The Bent Creek Experimental Forest lies within the
Blue Ridge Physiographic Province of the southern Appalachian Mountains. The Farm Woodlot, which is enveloped within the larger forest
area of Bent Creek Experimental Forest, was divided into seven separate
stands that range in size from 2.4 to 14.4 ha. Site index (base-age 50) for
yellow-poplar (Liriodendron tulipifera L.) ranges from 23.5 to 32 m. Altitude of the study area is 760 m. Climate is characterized by cool
winters and warm summers. Mean January and July temperature is
2.3 and 22.38C, respectively, and average annual precipitation is
1200 mm.19 Species composition of the study area is typical of
upland hardwood forest types in the southern Appalachian Mountains.
The predominant overstory species are species mid-tolerant of shade, including oak (Quercus rubra L., Q. velutina Lam., Q. coccinea Müenchh,
256
Q. alba L., Q. prinus L.) and hickory (Carya Nutt. spp.) species and
shade-intolerant yellow poplar (Liriodendron tulipifera L.). The lower
canopy layers are dominated by shade-tolerant and often nonmerchantable species, including sourwood (Oxydendrum arboreum L.),
black gum (Nyssa sylvatica Marsh.), flowering dogwood (Cornus florida
L.) and red maple (Acer rubrum L.).
The disturbance history of the Bent Creek Watershed is complex. A
broad-scale view of the disturbance regime suggests that frequent
small-scale gap dynamics20 coupled with intermediate in scale and frequency wind and ice storms are the predominant natural disturbance
agents in the southern Appalachians.21 In addition, prior to EuroAmerican settlement, purposeful burning by native peoples likely
occurred.22 By the turn of the twentieth century, however, the forests
of the southern Appalachians were heavily influenced by Euro-American
settlers. Within the Bent Creek Watershed, 104 homesteads were located
within the boundaries of what is now Bent Creek Experimental Forest,
with 23 per cent of the 2550 ha cleared for cultivation.22 Although
records indicate that The Farm Woodlot was not cleared and cultivated,
activities associated with Euro-American settlement, including grazing
(livestock and hogs), firewood collecting, burning (purposeful and wildfires) and periodic high-grading common throughout the region2
coupled with the simultaneous decline and loss of a dominant and keystone tree species,23 American chestnut [Castanea dentata (Marshall)
Borkh.] all influenced forest structure and subsequent composition
observed at the time this study was initiated.
In 1946, this study was initiated to examine the efficacy of using the
single-tree selection system to, in part, develop a theoretically balanced,
uneven-aged stand structure (inferred by the reverse-J shape distribution)
capable of sustaining periodic harvests in small woodlot-type forest
stands.2 When this study was initiated, structure and composition of the
stands were typical of forest stands throughout the region. Stands were
30–50 years of age, but contained individual low quality trees between
100 and 200 years old.2 Initial basal area of the seven stands varied
between 10.6 and 22.3 m2 ha21. Across all stands, 56 per cent of the
initial basal area was in trees ≥30.5 cm diameter at breast height (d.b.h.),
with oak species comprising the majority of the basal area in each stand.
Starting in 1946 and continuing through 1955, annual improvement
cuts were conducted. During these improvement cuts, merchantable
trees (≥25 cm d.b.h., depending on species) possessing excessive rot at
the base or top of the tree and trees with long crown ratios (i.e. trees
with excessive limbs) were targeted for removal. Yellow pines (Pinus echinata Mill., Pinus virginiana Mill., Pinus rigida Mill.) with crown ratios ,30
per cent, undesirable species (Table 1), or low quality desirable species
(Table 1) ≥12 cm d.b.h. but less than merchantable size were also
removed during the improvement cuts. Each stand received one to two
improvement cuts over this 10-year period. Documents describing
these initial improvement cuts reveal that relative to the 1946 inventory,
after 10 years of cutting, basal area of oak and yellow pine decreased by
an average of 10 and 35 per cent, respectively, suggesting these species
were targeted during the improvement cuttings. Despite the 10 per cent
reduction in oak basal area, in 1956, basal area of oak still comprised 48
per cent of the basal area across the entire 40 ha.
In the 1956 entry (which continued into 1957), efforts towards
managing for a balanced uneven-aged diameter distribution based on a
constant q 7 began and the BDq-based approach to regulating stocking
and structure was implemented. Residual basal area (m2 ha21) of the
managed diameter distribution, which included the 15– 85 cm d.b.h.
classes, was set to 13.8 m2 ha21 and maximum residual diameter was
86 cm. The q (based on 5 cm d.b.h. classes) used during the 1956 –1957
and 1962 entries was stand specific and ranged from 1.5 to 1.8. After
the 1962 entry, a standard q of 1.4 (based on 5 cm d.b.h. classes) was
selected for all seven stands. Using the q of 1.4, entries into the seven
stands comprising The Farm Woodlot occurred in 1970, 1984 and 2006.
The period between the 1984 and 2006 entry greatly exceeded the
Long-term effects of single-tree selection cutting on structure
Table 1. List of the prominent species and respective species group in
The Farm Woodlot single-tree selection study area.
Oak-hickory
Northern red oak
Black oak
Scarlet oak
White oak
Chestnut oak
Southern red oak
Mockernut hickory
Pignut hickory
Red hickory
Shade-intolerant
Sweet birch
Black cherry
Yellow-poplar
Black locust
Shade-tolerant
Flowering dogwood
Black gum
Sourwood
Red maple
American beech
Sassafras
American holly
Eastern hemlock
Species group
Classification
Quercus rubra L.
Quercus velutina Lam.
Quercus coccinea Müenchh.
Quercus alba L.
Quercus prinus L.
Quercus falcata Michx.
Carya tomentosa (Poir.) Nutt.
Carya glabra (Mill.) Sweet
Carya ovalis (Wangenh.) Sarg.
Desirable
Desirable
Desirable
Desirable
Desirable
Desirable
Desirable
Desirable
Desirable
Betula lenta L.
Prunus serotina Ehrh.
Liriodendron tulipifera L.
Robinia pseudoacacia L.
Desirable
Desirable
Desirable
Desirable
Cornus florida L.
Nyssa sylvatica Marsh.
Oxydendrum arboreum L.
Acer rubrum L.
Fagus gradifolia Ehrh.
Sassafras albidum (Nutt.) Nees
Ilex opaca Aiton
Tsuga Canadensis (L.) Carrière
Undesirable
Undesirable
Undesirable
Undesirable
Undesirable
Undesirable
Undesirable
Undesirable
Classification as ‘desirable’ or ‘undesirable’ is according to Della-Bianca
and Beck.2
cutting cycle established after the 1962 entry, which was 10–15 years,
largely due to administrative constraints associated with planning and
implementing a commercial timber sale. Results from the early 1970s
suggested that the pattern of individual tree removal was not sufficient
to regenerate and recruit mid-tolerant species into successively larger
diameter classes.2 Consequently, in 1972, non-merchantable shadetolerant species (termed ‘undesirable’ by Della-Bianca and Beck),2 including sourwood, blackgum, red maple and dogwood, between the 5 and
20 cm d.b.h. classes present in small openings created by the removal of
a small group of overstory trees (e.g. two or three trees in close proximity
to one another) in the 1970 entry were injected with herbicide in all
seven stands. This treatment is hereafter referred to as the targeted midstory removal (TMR) treatment. Observations of increased growth and
development of the desirable species, which were in the intolerant and
oak-hickory species groups (Table 1), following this herbicide treatment
led to a second herbicide treatment in two of the seven stands.2 To
assess how effective the complete removal of undesirable tree species
would be relative to less intensive treatment of undesirable species, in
1986, individuals within the shade-tolerant species group (Table 1)
between the 5 and 20 cm d.b.h. class were injected with herbicide in two
of the seven stands (Stands D and G). This treatment is hereafter referred
to as the complete midstory removal (CMR) treatment.
Data collection
In each stand, between 3 and 15 transects (depending on stand size,
with larger stands possessing a greater number of transects) were established at random locations along each stand boundary. Individual
transects within a stand were separated by a random multiple of 20 m,
up to 80 m. All transects within a given stand originated at the
random point along a stand boundary and extended through the entire
stand in either the north–south or east–west direction. Along each transect, a series of randomly located, concentrically nested circular permanent plots was established. The number of plots within each stand varied
by stand size, and ranged from 11 plots in the smallest stand to 71 plots
in the largest stand. This corresponded to a sampling intensity (as a percentage of stand area sampled) between 10 per cent in the smallest
stand to 20 per cent in the largest stand. Plots along a transect
were separated by a random multiple of 20 m, up to 60 m. In 1997,
9 years prior to the most recent entry, and 2008, 2 years after the
most recent entry into The Farm Woodlot, species and d.b.h. of overstory
(≥12.5 cm d.b.h.) and midstory (≥2.5 but ,12.5 cm d.b.h.) trees were
recorded on 0.01 and 0.04 ha plots, respectively.
No unmanaged, control stands were set aside when this study was
established in 1946. Consequently, there are no long-term data directly
comparable to this study that describes changes in structure and composition in unmanaged stands relative to managed stands. However, in
Spring 2012, three separate stands located on Bent Creek Experimental
Forest that were similar in age and site quality to the stands in The
Farm Woodlot were sampled. Although exact information regarding
past land use of these unmanaged stands is unknown, we are fairly confident that these stands were never cultivated,22 and therefore there is a
high likelihood that past land use was relatively similar between unmanaged stands and stands located in The Farm Woodlot. Two transects
were randomly established along the boundary of each unmanaged
stand. Transects were separated by at least 50 m and extended
through the entire stand in either the north–south or east –west directions. Along each of the two transects per stand, five plots separated
by 60 m were established. On each plot, species and d.b.h. of overstory
(≥12.5 cm d.b.h.) and midstory (≥2.5 but ,12.5 cm d.b.h.) trees were
recorded on a 0.01 and 0.04 ha plot concentrically nested plot, respectively. Although the unmanaged stands are not directly comparable, they
do provide a comparison (although only a relative comparison) between
how the current structure and composition of stands managed via
single-tree selection compares to that of nearby unmanaged stands.
Analysis procedures
Species composition
Overstory layer A split-plot analysis of variance (ANOVA) under a completely randomized design was used to analyse the effects of treatment
and species groups on basal area and trees ha21 using the most recent
inventory data (2008 for managed stands and 2012 for unmanaged
stands). Treatment (df ¼ 2), which consisted of the TMR treatment, the
CMR treatment and unmanaged controls, was the whole-plot factor
(fixed-effect) and species group (df ¼ 2), which consisted of the
oak-hickory, tolerant and intolerant groups, was the split-plot factor
(fixed effect). The interaction (df ¼ 4) between species group and treatment (fixed effect) along with a random effect for replication (or
stand) nested within treatment (df ¼ 7) were also included in the
model. Analyses were conducted separately for each of four size
classes: (i) sapling (5 and 10 cm d.b.h. classes); (ii) pole (15– 25 cm
d.b.h. classes); (iii) small sawtimber (30 –60 cm d.b.h. classes); and (iv)
large sawtimber (≥65 cm d.b.h. classes). When the interaction between
treatment and species group was statistically significant (P , 0.05), the
significance of the effect of treatment and species group was determined
using the SLICE option in Proc Mixed in SAS (SAS Institute, Inc.,), respectively. Post hoc tests of pairwise comparisons were performed using a
Bonferonni-adjusted P-value of 0.0167 (0.05/3) following significant
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Forestry
(P , 0.05) results in either the overall ANOVA (in the case of a significant
main effect(s)) or following a significant result in the SLICE analysis (in
the case of a significant interaction). For the ANOVAs, some data were
square-root or loge (trees ha21 +1) transformed to achieve normality
and/or homoscedasticity. Means and standard errors from the raw,
untransformed data are presented.
Diameter distributions
Stand-level diameter distributions in 1997 and 2008 for managed stands
and in 2012 for unmanaged stands were analysed using procedures outlined by Leak (1996) and Janowiak et al.15 An unbiased estimate of q for
each stand was obtained using simple linear regression, where the log10
of the number of trees ha21 in each 5 cm d.b.h. class was the dependent
variable and diameter class midpoint (DCM) was the independent
variable, with q calculated as the antilog of the slope coefficient raised
to width of the diameter class.24 Stand-level diameter distributions
were quantified by regressing the log10(trees ha21) in each d.b.h. class
against all possible combinations of DCM, DCM2 and DCM3 using ordinary
least squares regression.5 The best model for each stand was chosen
from the models possessing significant coefficients and the lowest root
mean square error.5 Diameter distributions were categorized following
Janowiak et al.15 according to the sign (i.e. positive or negative) of the significant coefficients associated with DCM, DCM2 and/or DCM3. Regression
analyses were conducted using the Proc GLM procedure in SAS (SAS Institute, Inc.,). An alpha ¼ 0.05 was used to assess significance.
Results
Diameter distributions
Stand attributes of managed and unmanaged stands from the
most recent inventory are presented in Table 2. At the stand
level, modelled q, assuming a simple negative exponential diameter distribution in managed stands in 1997 (9 years prior to the
most recent entry into The Farm Woodlot), varied between 1.33
and 1.48 in the TRM treatment and 1.31 and 1.38 in the CMR
treatment (Table 3). In 2008 (2 years after the most recent
entry), modelled q varied between 1.41 and 1.58 in TMR treatment and 1.32 and 1.38 in the CMR treatment. The modelled q
in unmanaged stands ranged between 1.28 and 1.37.
In 1997, the diameter distributions of the five TMR stands
were classified as concave, negative exponential or rotated
sigmoid in shape (Table 4, Figure 1). In 2008, three out of the
five TMR stands possessed diameter distributions that were
concave in shape, while the remaining two stands were classified
as a negative exponential and rotated sigmoid (Table 4, Figure 1).
When data from all five TMR stands were pooled, the shape of
the diameter distribution was rotated sigmoid in form in 1997
Table 2. Trees ha21, basal area (m2 ha21) and quadratic mean
diameter (Dq) from the most recent inventory of stands in managed
and unmanaged stands.
Treatment
Trees ha21
Basal area (m2 ha21)
Dq (cm)
TMR (n ¼ 5)
CMR (n ¼ 2)
Unmanaged (n ¼ 3)
1041+51
1005+37
1104+368
18.5+0.5
18.7+1.1
44.3+10.3
17.2+0.8
18.1+1.0
24.9+3.6
Values represent the mean+SE of trees ≥2.5 cm d.b.h.
258
Table 3. Unbiased estimates of q for managed stands in 1997 (9 years
prior to the most recent entry) and 2008 (2 years following the most
recent entry) derived through simple linear regression.
Treatment
1997
TMR
Stand B
Stand C
Stand E
Stand F
Stand H
Overalla
CMR
Stand D
Stand G
Overall
Unmanaged
Buell
Ingles
South Ridge
Overall
2008
q
R2
Q
R2
1.48
1.33
1.45
1.43
1.43
1.43
0.92
0.90
0.89
0.88
0.83
0.92
1.46
1.41
1.47
1.45
1.58
1.42
0.94
0.82
0.92
0.91
0.87
0.93
1.31
1.38
1.35
0.74
0.87
0.84
1.32
1.38
1.35
0.77
0.93
0.88
–
–
–
–
–
–
–
–
1.37
1.29
1.28
1.30
0.84
0.85
0.73
0.91
The q in unmanaged stands was obtained using 2012 inventory data.
Regressions were performed using all trees ≥5 cm d.b.h. class, with q
calculated according to Leak.24
a
Overall q refers to the modelled q following the pooling of data from all
stands in a given treatment.
Table 4. Characterization of diameter distributions in the TMR, CMR and
unmanaged stands.
Treatment
TMR
Stand B
Stand C
Stand E
Stand F
Stand H
Overalla
CMR
Stand D
Stand G
Overall
Unmanaged
Buell
Ingles
South Ridge
Overall
a
1997 diameter
distribution
2008 diameter
distribution
Concave
Concave
Rotated sigmoid
Negative exponential
Concave
Rotated sigmoid
Concave
Concave
Concave
Negative exponential
Rotated sigmoid
Concave
Rotated sigmoid
Rotated sigmoid
Rotated sigmoid
Rotated sigmoid
Rotated sigmoid
Rotated sigmoid
–
–
–
–
Negative exponential
Concave
Increasing q
Negative exponential
Overall diameter distribution refers to the diameter distribution
assignment following the pooling of data from all stands within a given
stand type.
Long-term effects of single-tree selection cutting on structure
Figure 1 Stand-level diameter distributions of stands in The Farm woodlot single-tree selection study area. Stands B, C, E, F and H received the TMR
treatment. Stands D and G received the CMR treatment. Triangles denote 2008 observed; broken lines denote 2008 modelled; circles indicate 1997
observed; continuous lines indicate 1997 modelled.
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Forestry
Figure 2 Treatment-level (TMR, CMR and unmanaged) diameter distributions. Triangles denote 2008 observed; broken lines denote 2008 modelled;
† 1997 observed; continuous lines indicate 1997 modelled.
Table 5. Results of the split-plot ANOVA for trees ha21 and basal area
(m2 ha21) with respect to treatment (df ¼ 2), species group (df ¼ 2) and
the interaction between treatment and species group (df ¼ 4).
Source
Trees ha21
Treatment
Species group
Treatment×species
group
Basal area (m2 ha21)
Treatment
Species group
Treatment×species
group
Saplinga Poleb
Small
sawtimberc
Large
sawtimberd
0.2059
0.6817
0.0001 ,0.0001
0.3968
0.0421
0.0028
,0.0001
0.0011
0.0780
0.0006
0.0155
–
–
–
0.0034
,0.0001
0.0018
0.0500
0.0011
0.0115
0.5584
,0.0001
0.0577
Values represent P-values for the type three fixed effects.
a
Includes the 5 and 10 cm d.b.h. classes.
b
Includes the 15 –25 cm d.b.h. classes.
c
Includes the 30–60 cm d.b.h. classes.
d
Includes d.b.h. classes ≥65 cm.
and concave in form in 2008 (Table 4, Figure 2). For the two
stands that received the CMR treatment, the diameter distributions were rotated sigmoid in shape in both 1997 and 2008.
Although diameter distributions of unmanaged stands were
variable (Table 4, Figure 1), when data from the unmanaged
stands were pooled, the diameter distribution was classified as
a negative exponential (Table 4, Figure 2).
Species composition
Overstory layer
The split-plot ANOVA displayed a significant main effect of
species group on the number of trees ha21 in the sapling size
class (Table 5), with the number of trees ha21, averaged across
treatments, in the tolerant group 4.5 times that in either the
oak-hickory or intolerant groups (Table 6). In the pole, small
260
sawtimber and large sawtimber size classes, the results of the
split-plot ANOVA on trees ha21 revealed a significant species
group by treatment interaction (Table 5). Consequently, interpretation of main effects outside of this interaction is not possible. However, for the pole size class, the only difference in the
number of trees ha21 among treatments was in the intolerant
species group, where trees ha21 was significantly greater in the
CMR than unmanaged stands (Table 6). Within a treatment, no
differences in trees ha21 among species groups were observed
in the CMR treatment, while a decreasing number of trees ha21
associated with decreasing levels of shade tolerance was
observed in the TMR and unmanaged stands. In the small sawtimber size class, treatment differences in trees ha21 were
observed in the oak-hickory and tolerant species groups. In the
oak-hickory and tolerant species groups, the number of small
sawtimber sized trees ha21 was significantly greater in the unmanaged stands than either the TMR or CMR stands (Table 6).
Regarding trees ha21 in the large sawtimber size class, significant differences among treatments were observed in the
oak-hickory group only, where unmanaged stands possessed a
greater number of trees ha21 than either the TMR or CMR treatments. In unmanaged stands, trees ha21 of the large sawtimber
size class were significantly greater in the oak-hickory group than
either the intolerant or tolerant species groups. In general, trends
and differences in basal area were comparable with those
observed in the analysis of trees ha21 (Tables 5 and 6).
Relative to the theoretical target derived from the BDq parameters used in this study, there was a particularly large deficit
of trees in the oak-hickory and intolerant species groups in the
pole and small sawtimber size classes, regardless of treatment
(Figure 3). Using the BDq curve as a guide to stocking, there
should be 194 and 101 trees ha21 in the pole and small sawtimber size classes, respectively (Figure 3). However, averaged
across the TMR and CMR treatments, only 58 and 55 oak-hickory
and intolerant trees ha21 in pole and small sawtimber size
classes, respectively, were observed after the 2008 entry. Even
when taking tolerant species into consideration, there is still a
deficit in the pole and sawtimber size classes. The only size
class where trees ha21 met and actually exceeded the BDq
target was the sapling size class. In this size class, the combined
oak-hickory and intolerant species groups approximated the
Long-term effects of single-tree selection cutting on structure
Table 6. Trees ha21 and basal area (m2 ha21) by species group in the TMR, CMR and unmanaged stands.
Treatment
Sapling
TMR
CMR
Unmanaged
Pole
TMR
CMR
Unmanaged
Small sawtimber
TMR
CMR
Unmanaged
Large sawtimber
TMR
CMR
Unmanaged
Basal area (m2 ha21)
Trees ha21
Oak-hickory
Intolerant
Tolerant
156+57a
165+79a
20+0a
117+39a
183+19a
27+18a
482+63b
427+3b
597+246b
35+8a
27+3
31+4a
43+9Ba
33+12B
122+13Aa
1+1B
5+3AB
25+13Aa
19+4ABa
34+1B
8+3Ab
14+7b
20+8
8+7b
111+9b
66+37
158+52c
Oak-hickory
–
–
–
1.1+0.3a
0.9+0.1a
1.2+,0.1a
Intolerant
–
–
–
Tolerant
–
–
–
0.5+0.1a
0.9+,0.1a
0.3+0.1a
3.0+0.2b
1.7+1.0b
4.7+1.5b
9+3Bb
9+4B
54+15Ac
6.1+1.3Ba
5.2+1.7B
17.3+1.7Aa
2.0+1.2b
3.2+1.8
1.3+1.0b
0.8+0.3Ab
0.9+0.3A
5.4+1.6Ab
0+0
0+0
1+1b
0.5+0.2B
1.7+1.0B
10.4+5.6Aa
0.7+0.2
1.8+1.4
0.5+0.5b
0.0+0.0
0.0+0.0
0.3+0.3b
2+0.4
5+4
1+1b
Values represent the mean+1 SE. Means followed by the same letter are not significantly different. Uppercase letters indicate across treatment
differences within a given species group. Lowercase letters indicate within treatment differences among species groups. Analysis of basal area for
the sapling size class was not performed because of the negligible amount of basal area in the sapling size class.
target and when considering total trees ha21 actually exceeded
the theoretical target derived by the BDq parameters (Figure 3).
Discussion
Diameter distributions
After 60 years of applying single-tree selection cutting using
various forms of the BDq approach, the target q of 1.4 has
been closely approached, would imply the stands in this study
closely approximate a balanced, uneven-aged structure implied
by the reverse-J-shaped distribution. However, further analyses
revealed that following the most recent entry, only one out of
the seven stands (stand F) managed via single-tree selection
displayed structural characteristics associated with a reverse-J
shaped, or negative exponential, distribution. The departure
from the reverse-J-shaped distribution has been fairly consistent
in this study. For example, 1 year prior to the 1984 entry, which
was the second to last entry, the diameter distribution of The
Farm Woodlot as a whole displayed characteristics of a rotated
sigmoid2 and in 1997, 9 years prior to the most recent entry,
all but one stand possessed diameter distributions other than
the negative exponential. This lack of conformity to the negative
exponential distribution, however, does not imply that all of
these stands are unbalanced. For example, stands with rotated
sigmoid diameter distributions have been referred to as balanced
despite not following the characteristic reverse-J-shaped curve.14
Similarly, the Arbogast structure common in northern hardwood
forest types is considered balanced despite a non-constant q
across the diameter distribution.9,10,18 With the exception of
the increasing-q distribution, the diameter distributions observed
in managed stands were also found to occur in unmanaged
stands. When averaged over treatment, managed stands were
rotated sigmoid (CMR treatment) or concave (TMR treatment)
in shape compared with the reverse-J shaped distribution of unmanaged stands.
The concave distribution observed in the majority of TMR
stands has been referred to as an unbalanced11 and an unsustainable structure.25 In a stratified mixed forest structure
where growth rates and ecological characteristics vary among
species,26 the concave pattern observed in the TMR stands is
likely an artefact of the sawtimber size classes being dominated
by faster-growing intolerant and mid-tolerant species coupled
with the sapling and pole size classes being predominantly
shade-tolerant and slower-growing species.11 This concave
pattern can also result from overcutting in the pole size classes
coupled with an excess of overstory trees.27 Given the complete
removal of the shade tolerant midstory in the CMR treatment,
which was all located in the sapling and pole size classes, it
was surprising that the diameter distribution of the CMR stands
did not appear more concave in shape.
The rotated sigmoid shape observed in the two CMR stands
has been documented in both unmanaged stands12,13,28 and
stands managed via single-tree selection.4,5,14 These rotated
sigmoid distributions are thought to arise due to a variety of
reasons, including a U-shaped mortality curve and unequal
diameter growth across the diameter distribution (Goff and
West, 1975).29,30 In this study, it is likely the unequal diameter
increment is responsible for the development of the rotated
sigmoid distribution in the CMR stands, as the growth of small
trees released during the removal of the midstory canopy
increased relative to the growth of the larger overstory trees.28
It should be mentioned that Rubin et al.25 have suggested that
the rotated sigmoid, as opposed to the negative exponential
261
Forestry
Figure 3 Trees ha21 averaged across TMR and CMR treatments by species group in the sapling (a), pole (b), small sawtimber (c) and large sawtimber
(d) size classes relative to the target number of trees ha21 derived from the theoretical BDq curve.
distribution, is an artefact of inadequate sampling area. Although plot size or sampling area has been shown to influence
diameter distributions12,15 with the exception of Stand C, the
sampling intensity in this study exceeded the 13 per cent threshold recommended by Janowiak et al.15 to adequately quantify
and characterize diameter distributions of managed stands.
Species composition
Although the diameter distributions by themselves may imply
successful application of the single-tree selection system in
these mixed-hardwood stands, an examination of species composition suggests otherwise, given the objective was to regenerate the species composition of the stand prior to implementing
the selection system. Stocking of oak-hickory and intolerant
species appears adequate in the sapling size class (relative to
the theoretical target), but it is evident there has been a lack
of timely recruitment into the pole and small sawtimber size
classes relative to the goal set by the BDq parameters over the
60 years of management via single-tree selection (Figure 3).
Despite intensive treatment of shade-tolerant competition in
the TMR and CMR treatments, the abundance of the tolerant
species group in the sapling and pole size classes suggests that
available growing space created by the removal of individual
262
stems during each entry is quickly occupied by tolerant species,
which in the southern Appalachians, tend to be midstory obligates (e.g. flowering dogwood, sourwood, blackgum, Ilex spp.,
etc.) as opposed to trees that can actually attain canopy (i.e.
dominant/co-dominant) status, thereby limiting recruitment of
the oak-hickory and intolerant species into larger d.b.h. classes.
Della-Bianca and Beck2 suggested the treatment of undesirable shade-tolerant species in the smaller diameter classes
may promote the regeneration and recruitment of desirable
tree species under a single-tree selection management
scheme. The results presented here do not confirm that hypothesis. Rather, no advantage was conferred to mid-tolerant and intolerant species, regardless of whether a targeted or complete
removal of competing shade-tolerant species was conducted.
Although stocking of the combined oak-hickory and intolerant
species groups is only slightly under the theoretical target in
the sapling size class in the TMR treatment and actually
exceeds the target in the CMR treatment, it took 36 years, in
the case of the TMR treatment and 22 years in the case of the
CMR treatment, for recruitment into the sapling size class to
occur (Table 6, Figure 3). This extended time frame required for
oak-hickory and intolerant species to recruit from the seedling
into sapling size classes can hardly be considered a ‘timely’
release of regeneration sources in an operational silvicultural
Long-term effects of single-tree selection cutting on structure
system in this particularly productive forest type. Even in stands
that received the more intense midstory control (i.e. CMR treatment), the oak-hickory and intolerant species groups have not
experienced any significant advantage in the sapling and pole
size classes over the stands that received the less intensive
TMR treatment. In fact, regardless of whether stands received
the TMR or CMR treatment, when the sapling and pole size
classes in managed and unmanaged stands are compared, the
oak-hickory species group does not appear to have benefited
at all from the removal of the shade-tolerant midstory. Confirming the results presented by Schuler,6 it appears the long-term
application of single-tree selection in mixed-hardwood forests
has created a condition in which stands are increasingly dominated by shade-tolerant species that are, in submesic to mesic
mixed-hardwood forests of the southern Appalachians, largely
midstory obligates.
Not only has single-tree selection affected the smaller diameter classes, but a change in the abundance of species in the
larger, sawtimber size classes is also apparent. For example,
the vast majority of the trees ha21 and basal area in the sawtimber size classes of unmanaged stands were in the oak-hickory
species group while, in managed stands, the small and large
sawtimber size classes displayed no differences in trees ha21
and basal area among the oak-hickory, intolerant and tolerant
species groups, resulting in an evening out of species composition in the larger diameter classes. Although this finding is important, it is not necessarily unexpected as the small and large
sawtimber size classes are dominated by the oak-hickory and intolerant species groups. In fact, between 1946, when this study
was initiated and 1984, which was the second-to-last entry, no
tolerant species were cut from the ≥60 cm d.b.h. classes
because of the lack of tolerant species in those larger size
classes.2 Consequently, if this system was implemented at a
much broader scale, the reduction in the abundance of mature
(i.e. sawtimber size classes) oak-hickory species, in particular,
could have adverse effects on biodiversity and wildlife habitat
quality.31
Overall, the successful application of single-tree selection in
broadleaved forests, even those forests with a shade-tolerant
canopy tree species [e.g. beech (Fagus sylvatica L.)], has proven
difficult. In Europe, where there is a long history of managing
forests using the selection system, the likelihood of successfully
implementing single-tree selection (or single-stem plentering)
in broadleaved forests is low relative to conifer-dominated
stands, with the loss of apical dominance and ability to
respond to release, lateral crown expansion in lieu of height
growth and overall crown shape cited as possible reasons for its
failure.32,33
Results from European silvicultural trials suggest regenerating
oak (Q. robur L. and Q. petraea Liebl.) in stands that contain
shade-tolerant competitors is possible under continuous
canopy cover (as in single-tree selection); however, the likelihood
of success increases when the following conditions are met: (i)
light does not fall below 15 –20 per cent of full sunlight; (ii)
oaks (and other mid-tolerant species) are present as advanced
growth; (iii) browsing by wildlife is controlled; and (iv) the
control or elimination of shade-tolerant competition in the regeneration layer is undertaken.34 As those recommendations
pertain to this study, (i) light was not measured; however, the
presence of shade-intolerant species in the sapling layer
(Figure 3) suggests light levels met or exceeded the 15 –20 per
cent threshold; (ii) small and medium sized (,1.2 m in height)
oak and hickory advance reproduction was present in the regeneration layer at densities averaging 7100 seedlings ha21 (data
not presented); (iii) although browse pressure was likely high at
the time this study was established,22 current browse pressure
in the Bent Creek Watershed is low and (iv), control of shadetolerant species in the 5 through 20 cm d.b.h. classes (but not
of advance reproduction ,2.5 cm d.b.h.) was performed during
a single (TMR stands) or twice applied (CMR stands) herbicide
treatment. Despite meeting the conditions put forth by von
Lüpke34 successful regeneration of oak and hickory species, in
particular, has not occurred, with multiple competing shadetolerant species present in this study (Table 1), as opposed to
the single shade-tolerant beech species in Europe, a likely
reason for the lack of successful regeneration over the 60-year
period.
The only successful application of single-tree selection in oak
forests in the US occurred on the Pioneer Forest, which, relative to
our study site, is a xeric oak-dominated forest in the Ozark Highlands of Missouri. There, oaks experience little competition from
shade-tolerant species and advance oak reproduction, which is
required for successful oak regeneration and recruitment into
larger size classes, accumulates even under undisturbed forest
conditions.35 Despite the success of using single-tree selection
to create and maintain an uneven-aged forest structure on the
Pioneer Forest, the likely success of creating and recruiting new
oak dominated cohorts is uncertain given that stocking continues to increase.1,36 In fact, Larsen et al.36 suggest overstory
densities as low as 6.5 m2 ha21 may be required to sustain, in
the long-term, oak recruitment and the uneven-aged structure
currently observed on the Pioneer Forest. If this low level of
residual overstory density is required to sustain an uneven-aged
structure in dry, mixed-oak forests, where competition from
shade-tolerant species is comparably low, it seems reasonable
to assume the residual density used in this study, which was
13.8 m2 ha21, may be too high to continuously and reliably
regenerate and recruit oak-hickory and intolerant species.
Conclusions
As applied in this study, single-tree selection cutting has not promoted conditions conducive to the timely regeneration and
recruitment of shade-intolerant species and more importantly,
mid-tolerant oak and hickory species. Even after the adaptive
management efforts were undertaken in 1972 and 1986 to increase the likelihood of successful oak regeneration, competition
from shade-tolerant species continuous to limit regeneration
and recruitment of these desirable species. These findings
suggest the BDq method of controlling stocking under the singletree selection system is ill-suited to ensuring that timely and sufficient regeneration of the shade-intolerant and mid-tolerant
species occurs.
At the time this study was initiated, the concept of managing
for a constant q and the balanced, uneven-aged stand was being
introduced in North America.7,8 Today, there is growing acceptance that successful application of a selection system must
take into account numerous factors, including disturbance
regimes, stand dynamics and regeneration ecology of all
263
Forestry
species, not simply those of management concern, in any given
ecosystem.37,38 Because this study can only attest to the efficacy
of the BDq approach to single-tree selection in mixed-hardwood
forests, the results from this study should not be used to conclude that uneven-aged management and this forest type are incompatible. Other methods of regulating stocking, such as
management of growing space among cohorts26,38 – 40 and/or
the creation of irregular multi-aged stands27 that may be more
appropriate in forest types where intolerant and mid-tolerant
species are the species of management concern, remain untested. Observational data from a group selection study on Bent
Creek Experimental Forest suggests an irregular group shelterwood or femelschlag system41 may be particularly effective, in
terms of regenerating shade-intolerant and mid-tolerant
species as well as creating an uneven-aged structure and
should be a focus of future silvicultural research in this diverse
and complex forest type.
Acknowledgements
Numerous principal investigators and dedicated technicians are
responsible for the integrity and longevity of this study. Comments
from Stan Zarnoch, Laura Kenefic, Skip Smith and the editor of Forestry
along with two anonymous reviewers greatly improved this manuscript.
Funding
This study was initiated and continuously funded by the United States
Department of Agriculture, Forest Service, Southern Research Station,
Upland Hardwood Ecology and Management Research Work Unit
stationed at Bent Creek Experimental Forest.
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