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ORIGINAL PAPER
Development of branch, crown, and vertical distribution leaf areamodels for contrasting hardwood species in Maine, USA
Andrew S. Nelson • Aaron R. Weiskittel •
Robert G. Wagner
Received: 19 March 2013 / Revised: 31 July 2013 / Accepted: 7 August 2013 / Published online: 21 August 2013
� Springer-Verlag Berlin Heidelberg 2013
Abstract
Key message Branch, crown vertical leaf area distri-
bution models were developed for naturally regener-
ated hardwood species and planted hybrid poplar
clones. Species-specific differences were found at all
levels of investigation.
Abstract Coexistence in mixed-species stands is strongly
influenced by species differences in leaf area production
and distribution. The majority of leaf area models in the
literature are focused on conifer species, which have sub-
stantially different crown forms than hardwood species.
Therefore, the goal of this investigation was to develop
branch, crown, and vertical leaf area distribution models
for various hardwood species that accounted for their
greater crown complexity. A nonlinear model including
branch diameter, branch tip height, and height to the start
of the foliage was the best fit for branch leaf area. Branch
leaf area ranged from 0.05 to 0.37 m2 for Populus grand-
identata and Betula populifolia for an averaged sized
branch, respectively. The best fit model for crown leaf area
was a nonlinear form accounting for stem diameter and
crown length. Crown leaf area ranged from 3.26 to 9.85 m2
for Populus tremuloides and Betula populifolia for an
averaged sized tree, respectively. Vertical leaf area distri-
bution was best fit by a right-truncated Weibull distribution
and showed a peak in the middle third of the crown for
most of species. In addition, leaf area production varied
among four hybrid poplar clones in plantations, suggesting
a strong genetic control over crown form. Overall, leaf area
varied among species at all levels of investigation, sug-
gesting that coexistence of hardwood saplings in this
investigation was strongly influenced both by inherent
species-specific leaf area production and vertical
distribution.
Keywords Saplings � Nonlinear mixed-effects
models � Hybrid poplar � Red maple � Paper birch �Gray birch � Bigtooth aspen � Trembling aspen
Introduction
In mixed-species stands, coexistence and tree performance
are believed to be driven by differential resource utilization
(Kelty 1992; Rothe and Binkley 2001; Richards and
Schmidt 2010). In particular, species coexistence is largely
influenced by variation in crown characteristics in response
to light availability (Yokozawa et al. 1996; Ishii and Asano
2010). For instance, species classified as shade intolerant
tend to have crowns with foliage spread in a relatively even
horizontal distribution (monolayer), while shade-tolerant
species tend to have multi-layered crowns that can support
greater self-shading (Horn 1971).
Plasticity in crown form allows trees to respond to
diurnal and seasonal changes in light intensity, and has
been proposed to influence successional changes in forests
of mixed-species composition (Canham et al. 1994). Light
interception by individual trees is influenced by both the
Communicated by T. Kajimoto.
A. S. Nelson (&) � A. R. Weiskittel � R. G. Wagner
School of Forest Resources, University of Maine,
5755 Nutting Hall, Orono, ME 04469-5755, USA
e-mail: [email protected]
Present Address:
A. S. Nelson
School of Forest Resources Arkansas Forest Resources Center,
University of Arkansas at Monticello, P.O. Box 3468,
Monticello, AR 71656-3468, USA
123
Trees (2014) 28:17–30
DOI 10.1007/s00468-013-0926-5
total quantity and distribution of leaf area within the crown
(Niinemets 2007, 2010). In addition, foliage distribution is
often used to investigate spatial patterns in crown physio-
logical processes, such as photosynthesis and foliar nutrient
content (Le Roux et al. 1999; Koike et al. 2001). Thus,
species differences in leaf area production and distribution
can influence performance (Niinemets 1996), especially as
stands approach peak leaf area index, when competition for
light is often substantial since a large proportion of the
available area for growth is occupied by other trees (Oliver
and Larson 1996).
Numerous investigations have studied vertical foliage
distribution of individual trees, but the majority have
focused on conifer species (Maguire and Bennett 1996;
Makela and Vanninen 2001; Garber and Maguire 2005;
Weiskittel et al. 2009). Hardwood species have received
less attention. Therefore, many of the approaches used to
investigate allocation to foliage production in conifers may
not accurately account for the complexity of sympodial
hardwood crown forms with weak apical dominance. For
instance, the branch junction with the main stem is often
used to specify the relative vertical location of foliage
within the crown for conifers (Maguire and Bennett 1996;
Temesgen et al. 2003; Garber and Maguire 2005; Wei-
skittel et al. 2009). In contrast, sympodial hardwood crown
shapes are often composed of branches with steep vertical
angles, and the branch junction with the main stem may not
reasonably describe the vertical location of the foliage.
Methods have been developed to account for steep branch
angles in branch leaf area models and vertical leaf area
distributions (Medhurst and Beadle 2001; Forrester et al.
2012), but the suitability of the methods has not been tested
across a range of species of varying shade tolerance.
In recently disturbed forest stands in Maine USA., the
species composition of naturally regenerated trees is often
complex, composed of a mixture of fast-growing shade
intolerant species, mid- and shade tolerant hardwood
species, and slower growing conifer species (Seymour
1995). There is often strong competition for light in these
young stands due to high stem densities. Mechanisms
likely influencing the eventual dominance of young trees
in highly competitive stands include the total production
and vertical distribution of leaf area to increase light
interception. Currently, differences in leaf area production
and distribution among coexisting species in highly
competitive young stands are poorly understood. There-
fore, to better understand the combined influence of
inherent species differences and potential responses to
management intensity on forest productivity, leaf area of
young hardwood trees was investigated at multiple scales
of observations, including: (1) the branch-level,(2) the
total crown-level, and (3) vertical distribution within
the crown.
Methods
Site and experimental design
The investigation was conducted at a site on the 1,540-ha
Penobscot Experimental Forest (PEF) in east-central Maine,
USA. (44�490N, 68�380W). The PEF is located in the Aca-
dian forest region of eastern North America (Halliday 1937;
Braun 1950), a transitional forest zone of hardwood forests
to the south and boreal forests to the north. The 30-year
(1951–1980) mean annual temperature at Bangor, Maine,
USA. (*16 km from the site) was 6.6 �C, with an average
low of -7.0 �C in February and average high of 20.0 �C in
July. Precipitation averages 106 cm per year, of which
48 % occurs between May and October. Annual snowfall
averages 239 cm, and the frost-free period in the region is
between 140 and 160 days per year.
This investigation used destructively sampled trees from
the 9.2 ha silvicultural intensity and species composition
(SIComp) experiment on the PEF. The site was clearcut
harvested in 1995 with approximately 2.3 m2 ha-1 residual
basal area, which was concentrated in a few large trees
scattered across the site. Following harvest, the site was
dominated by the hardwood species trembling aspen
(Populus tremuloides Michx.), bigtooth aspen (Populus
grandidentata Michx.), red maple (Acer rubrum L.), paper
birch (Betula papyifera Marsh.), gray birch (Betula popu-
lifolia Marsh.), and the conifer species, balsam fir (Abies
balsamifera (L.) Mill.), red spruce (Picea rubens Sarg.),
white spruce (Picea glauca (Moench) Voss), and eastern
white pine (Pinus strobus L.).
The SIComp experiment was initiated in 2004 to test the
influence of various management intensities on the stand
dynamics and productivity of early successional Acadian
forest stands. Treatments range from untreated controls, to
low intensity thinning, thinning plus enrichment planting,
and high yield plantations (Nelson et al. 2012, 2013). The
untreated controls have not received any management
following the initial harvest and are dominated by densely
stocked shade-intolerant hardwood species (*12.5 thou-
sand stems ha-1). In the thinning treatments, crop trees
were selected and spaced on a 2 9 2 m grid across the
30 9 30 m treatment plots. Around each crop tree, all
woody vegetation was controlled once with manual and
chemical techniques in 2004. The thinning ? enrichment
treatments had a similar prescription to the thinning treat-
ments, but with control of woody and herbaceous vegeta-
tion for 2 consecutive years and enrichment planting of
white spruce and hybrid poplar (Populus species) crop
trees. The plantation treatments were designed as pure and
mixed plantations of white spruce and hybrid poplar. Four
hybrid poplar clones were planted as 25 cm long cuttings,
including three Populus deltoides W. Bartram ex
18 Trees (2014) 28:17–30
123
Marshall 9 P. nigra L. clones (D51, DN10, and DN70),
and one P. nigra L. 9 P. maximowiczii A. Henry clone
(NM6), obtained from the Short-Rotation Woody Crops
Program at the State University of New York’s College of
Environmental Science and Forestry. All competing veg-
etation was removed from the plantations prior to treat-
ment, and all vegetation is controlled annually until
complete crown closure. All trees were planted on a
2 9 2 m spacing.
Data collection
Five naturally regenerated hardwood species (red maple,
gray birch, paper birch, bigtooth aspen, and trembling
aspen), and the four hybrid poplar clones (D51, DN10,
DN70, and NM6) were selected for this investigation.
Trees were destructively sampled between late June and
early August 2011, when leaves had ceased their annual
expansion. For each naturally regenerated species, between
13 and 17 trees were sampled across three management
intensities (untreated control, thinning, and thin-
ning ? enrichment planting) and a range of tree diameter.
Mean diameter at breast height (DBH; see Table 1) varied
from 1.9 cm for gray birch to 7.4 cm for the NM6 hybrid
poplar clone (Table 2). Hybrid poplar clones were only
sampled in plantations due to low survival in the thin-
ning ? enrichment treatments. Five individuals per clone
were sampled across a range of DBH.
Each tree was cut at the ground line, and stem diameter
above the root collar, DBH, total height (HT), and crown
length (CL) were measured. The crown was separated into
three equidistant sections. Detailed branch measurements
for all branches included: (1) total branch length, (2) non-
foliated branch length, (3) branch diameter 5 cm from the
junction with the main stem, (4) branch angle, and (5)
distance from the base of the crown to the branch junction
(Fig. 1). Mean branch diameter ranged from 0.60 cm for
paper birch to 1.32 cm for bigtooth aspen and the NM6
hybrid poplar clone, while mean branch length ranged from
69 cm for red maple to 146 cm for the NM6 hybrid poplar
clone (Table 3). Two branches were randomly selected
from each section to develop branch leaf area (BLA)
equations. From each sample branch, *15–20 leaves were
placed in a plastic bag and stored on ice. Samples were
placed in the refrigerator for not more than 3 days prior to
scanning to avoid decay. Fresh leaves from sample bran-
ches were scanned with a LiCor LI-3100 to the nearest
0.1 mm2. Scanned samples were then dried at 65 �C for
72 h and weighed to the nearest 0.1 mg. Specific leaf area
(SLA) was calculated as projected (one-sided) leaf area
(cm2) to weight (g) ratio. SLA in the middle third of the
crown ranged from 10.22 ± 0.01 m2 kg-1 for the D51
hybrid poplar clone to 19.00 ± 0.94 m2 kg-1 for paper
birch (Table 4). The remaining foliage from each branch
was kiln-dried at 65 �C for a minimum of 1 week or until
constant weight was obtained and then weighed to the
Table 1 List of variables
used throughout the paper.
The variable, a description of
the variable, and units are
shown
Variable Description Units
Branch variables
ANGLE Branch angle from the vertical �BD Branch diameter 5 cm from junction with main stem cm
BL Total branch length m
BLA Branch leaf area m2
BT Height of branch tip from base of the crown m
FS Height of the start of the foliage along the branch from the base of the crown m
LF Length of foliage along the branch m
NFL Non-foliated branch length (branch length–foliage length) m
SLA Specific leaf area (leaf area : weight ratio) m2 kg-1
RBT Relative branch tip depth from the top of the tree –
RFS Relative foliage start depth from the top of the tree –
VERT Height of branch junction with the main stem m
Tree variables
CL Crown length m
CLA Crown leaf area m2
DBH Stem diameter at 1.37 m from the ground cm
HT Total tree height m
HRbase Maximum horizontal crown radius at base of vertical section m
HRtop Maximum horizontal crown radius at top of vertical section m
Vlen Length of the vertical crown section m
Trees (2014) 28:17–30 19
123
nearest 10 mg. Unsampled foliage and branch biomass
were collected for each tree and kiln-dried at 65 �C for a
minimum of 14 days prior to separation by component and
weighing to the nearest 10 mg.
Analysis
Branch leaf area equations
Analysis of variance (ANOVA) was used to test for dif-
ferences in SLA among the vertical crown sections and
species, accounting for management intensity, replicate
plot within intensity, and tree within replicate within
intensity as random effects. SLA differed by vertical crown
section for all of the naturally regenerated hardwood spe-
cies (p \ 0.01), while it was similar among crown sections
for all hybrid poplar clones (p [ 0.06) (Table 4). There-
fore, the mean SLA by section was used to convert dry
weight to projected leaf area for the natural hardwood
species, but a single SLA per clone was used for each
hybrid poplar clone.
A variety of model forms and covariates were tested for
BLA equations, including branch diameter, branch length,
foliated branch length, branch angle, and vertical location
within the crown. In addition, metrics that incorporated
branch angle and branch length to estimate actual branch
vertical location in the crown were tested as potential
covariates. Absolute branch tip height (BT) from the base
of the crown was calculated as BT = VERT ? cos(AN-
GLE) 9 BL, where VERT was the distance from the base
of the crown to the branch junction, ANGLE was the
branch angle, and BL was the total branch length. Simi-
larly, the absolute start of the foliage (FS) was calculated as
FS = VERT ? cos(ANGLE) 9 NFL, where NFL was the
non-foliated branch length. BT and FS were then scaled to
Table 2 Mean ± standard deviation (range) of attributes of the
sample trees by species. The number of trees (n), diameter at breast
height (DBH; cm), total height (HT; m), crown length (CL; m), total
leaf area (CLA; m2), and crown width (CW; m) are shown for the 9
species in the investigation
Species n DBH (cm) HT (m) CL (m) CLA (m2) CW (m)
Red maple 12 3.3 ± 2.8
(0.5–10.4)
4.79 ± 2.83
(1.65–10.40)
3.62 ± 2.25
(1.17–8.15)
10.05 ± 23.55
(0.16–75.53)
1.73 ± 1.30
(0.14–5.12)
Paper birch 14 2.1 ± 2.0
(0.5–8.4)
3.81 ± 2.29
(1.55–9.55)
2.83 ± 1.91
(1.09–8.15)
4.12 ± 9.43
(0.08–35.82)
1.22 ± 0.59
(0.51–2.64)
Gray birch 14 1.9 ± 1.8
(0.6–6.9)
3.70 ± 2.56
(1.66–11.09)
2.89 ± 1.76
(1.21–6.90)
2.60 ± 4.31
(0.28–13.96)
1.20 ± 0.46
(0.43–2.06)
Bigtooth aspen 17 5.8 ± 3.2
(1.1–13.1)
7.65 ± 3.09
(1.87–13.00)
4.37 ± 2.46
(0.71–10.50)
13.63 ± 31.86
(0.02–91.46)
2.14 ± 0.98
(0.69–4.08)
Trembling aspen 14 5.9 ± 2.7
(2.6–12.0)
8.10 ± 2.50
(4.77–12.18)
4.92 ± 2.40
(1.11–9.56)
8.97 ± 13.21
(0.90–52.36)
2.24 ± 1.24
(0.94–5.71)
Hybrid poplar
D51
5 4.3 ± 2.4
(1.4–7.5)
5.48 ± 2.37
(2.75–8.80)
5.00 ± 2.26
(2.40–8.30)
6.12 ± 5.58
(0.74–14.65)
1.74 ± 0.76
(1.02–2.99)
Hybrid poplar
DN10
5 5.4 ± 3.6
(2.3–10.9)
6.75 ± 2.74
(4.15–10.85)
5.64 ± 2.83
(4.00–10.65)
9.84 ± 10.42
(1.26–26.20)
1.88 ± 0.74
(0.99–2.86)
Hybrid poplar
DN70
5 4.5 ± 3.0
(0.7–8.7)
5.37 ± 2.44
(1.86–8.70)
4.28 ± 1.95
(1.79–6.80)
6.40 ± 6.09
(0.26–15.37)
1.85 ± 0.94
(0.42–2.86)
Hybrid poplar
NM6
5 7.4 ± 4.0
(3.0–13.6)
8.26 ± 2.59
(4.65–11.90)
7.47 ± 3.44
(2.30–11.80)
21.18 ± 23.11
(3.60–60.70)
3.05 ± 1.15
(2.02–4.92)
Fig. 1 Diagram showing the branch-level measurements used to
develop branch leaf area models and fit vertical leaf area models using
equally spaced leaf area segments along each branch
20 Trees (2014) 28:17–30
123
between 0 and 1 from the top of the tree to the base of the
crown to obtain relative branch tip depth (RBT), and rel-
ative foliage start depth (RFS).
BLA equations were fit with various linear and nonlinear
mixed-effects models for each species to determine which
model form best accounted for the variation in BLA. Mixed-
effects models are useful for hierarchical data and can
account for within-group correlation (Pinheiro and Bates
2000; Zuur et al. 2009), which is common for trees due to
allometric scaling among components, where the change in
size of one component is often related to the change in size of
other components (Niklas 1994). The various combinations
of fixed effects covariates and model forms were compared
to find models where the parameters of all covariates were
significantly different than zero (p \ 0.05), R2 was maxi-
mized, and residual standard error was minimized. In addi-
tion, various hierarchical random effects, including
management intensity, plot replicate within intensity, and
tree within plot within intensity, were tested for improving
the fit of the models using likelihood ratio tests.
Crown leaf area equations
The final BLA equations for each species were used to
predict leaf area of every branch on each sample tree. The
‘‘branch summation method’’ (Kenefic and Seymour 1999;
Monserud and Marshall 1999) was then used to estimate
total crown leaf area (CLA) per tree by summing predicted
BLA for the entire tree. CLA estimated with the branch
summation method was compared with CLA estimated by
converting the entire crown foliage weight to leaf area
using the average SLA per species. Across all species,
CLA estimated with the branch summation method was
not significantly different than CLA estimated using the
entire foliage weight (p [ 0.58). Therefore, leaf area
estimated with branch summation was used to fit the CLA
equations, as it accounted for the differences in SLA by
vertical crown section. Similar to the BLA models, variousTa
ble
3M
ean
±st
and
ard
dev
iati
on
(ran
ge)
of
attr
ibu
tes
of
the
sam
ple
bra
nch
esb
ysp
ecie
s.
Th
en
um
ber
of
bra
nch
es(n
),d
iam
eter
5cm
fro
mju
nct
ion
wit
hb
ole
(BD
;cm
),to
tal
bra
nch
len
gth
(BL
;m
),fo
liat
edb
ran
chle
ng
th(F
L;
cm),
ang
lefr
om
the
ver
tica
l(A
ng
le;
�),
and
bra
nch
leaf
area
(BL
A;
m2)
are
sho
wn
for
the
9sp
ecie
sin
the
inv
esti
gat
ion
Sp
ecie
sn
BD
(cm
)B
L(c
m)
FL
(cm
)A
ng
le(D
egre
es)
BL
A(m
2)
Red
map
le2
30
.68
±0
.35
(0.1
6–
1.3
8)
69
±5
2(7
–1
92
)4
6±
44
(1–
15
0)
52
±1
6(2
9–
90
)0
.23
±0
.27
(0.0
1–
1.0
4)
Pap
erb
irch
22
0.6
0±
0.4
2(0
.14
–1
.65
)7
8±
54
(14
–2
01
)7
0±
53
(4–
18
3)
44
±1
3(2
4–
80
)0
.28
±0
.41
(0.0
1–
1.6
1)
Gra
yb
irch
18
0.8
4±
0.3
8(0
.35
–2
.00
)1
04
±5
7(3
3–
27
6)
86
±5
1(2
7–
23
0)
30
±1
2(2
–4
7)
0.0
.31
±0
.33
(0.0
1–
1.1
7)
Big
too
thas
pen
29
1.3
2±
0.6
8(0
.41
–2
.93
)1
13
±7
4(1
2–
29
5)
85
±5
9(4
–2
45
)4
8±
15
(20
–7
2)
1.1
0±
2.6
0(0
.02
–1
2.6
5)
Tre
mb
lin
gas
pen
60
1.1
8±
0.7
0(0
.25
–3
.53
)1
06
±6
7(1
3–
30
6)
79
±5
7(1
–2
56
)4
6±
16
(20
–8
1)
0.4
6±
0.6
9(0
.01
–4
.24
)
Hy
bri
dp
op
lar
D5
13
00
.83
±0
.39
(0.1
6–
1.7
4)
86
±5
6(6
–2
26
)6
2±
43
(1–
16
5)
38
±1
2(2
4–
83
)0
.21
±0
.19
(0.0
1–
0.7
4)
Hy
bri
dp
op
lar
DN
10
30
0.8
9±
0.4
6(0
.23
–2
.08
)1
00
±6
2(9
–2
44
)7
3±
46
(1–
16
5)
36
±8
(21
–5
1)
0.2
2±
0.2
6(0
.01
–1
.38
)
Hy
bri
dp
op
lar
DN
70
29
0.8
3±
0.5
0(0
.20
–1
.91
)9
8±
65
(12
–2
35
)6
8±
55
(1–
23
3)
36
±1
0(1
9–
55
)0
.20
±0
.25
(0.0
1–
0.9
7)
Hy
bri
dp
op
lar
NM
62
91
.32
±0
.72
(0.5
1–
3.1
8)
14
6±
80
(60
–3
65
)1
26
±5
5(5
2–
22
9)
42
±1
2(2
7–
68
)0
.58
±0
.80
(0.0
6–
3.1
9) Table 4 Specific leaf area (mean ± standard deviation; m2 kg-1) for
the three relative equidistant vertical crown sections by species
Species Lower third Middle third Upper third
Red maple 18.63 ± 0.58 17.11 ± 0.63 14.70 ± 0.70
Paper birch 20.21 ± 0.94 19.00 ± 0.94 17.20 ± 0.96
Gray birch 19.78 ± 0.71 18.79 ± 0.71 17.59 ± 0.67
Bigtooth aspen 17.17 ± 0.49 15.74 ± 0.49 14.17 ± 0.50
Trembling aspen 15.20 ± 0.57 13.67 ± 0.57 12.30 ± 0.57
Hybrid poplar D51 10.53 ± 0.01 10.22 ± 0.01 9.91 ± 0.01
Hybrid poplar DN10 10.49 ± 0.23 10.78 ± 0.19 10.38 ± 0.23
Hybrid poplar DN70 12.13 ± 0.28 11.85 ± 0.28 11.66 ± 0.28
Hybrid poplar NM6 13.27 ± 0.61 11.86 ± 0.61 10.80 ± 0.68
Trees (2014) 28:17–30 21
123
linear and nonlinear mixed-effects model forms and tree-
level covariates (HT, DBH, root collar diameter, CL,
crown width, and crown ratio [CL/HT]) were examined to
maximize R2 and minimize residual standard error. Like-
lihood ratio tests were used to investigate whether incor-
porating the hierarchical random effects of management
intensity and plot replicate within intensity improved the fit
of the models.
Vertical leaf area distributions
Vertical leaf area distribution models for conifer species
typically use the junction of the branch with the bole to
specify vertical location of leaf area within the crown since
branches are often perpendicular to the bole (Temesgen
et al. 2003; Garber and Maguire 2005; Weiskittel et al.
2009). Comparatively, branch angles from the vertical of
hardwood trees tend to be more acute due to weak apical
dominance (Oliver and Larson 1996). Therefore, vertical
location of leaf area within the hardwood crowns was esti-
mated using the angle of the first-order branch and branch
length, similar to Medhurst and Beadle (2001) and Forrester
et al. (2012). The foliated branch length was separated into
5 cm segments, and the vertical midpoint location of each
segment was calculated as VERT ? cos(ANGLE) 9 LF,
where LF is the foliated branch length, for the first segment
and an additional 5 cm for each subsequent segment. Pre-
liminary analysis showed that separating branches into 5 cm
provided the best fit across species. For instance, as com-
pared to 10 cm segments, root mean square error (RMSE)
was reduced between 9.7 and 47.4 % for the DN70 clone
and bigtooth aspen, respectively, while mean absolute bias
(MAB) was reduced between 15.2 % for the D51 clone and
46.8 % for bigtooth aspen, respectively. The crown of each
tree was divided into 5 % sections (20 equal-sized vertical
bins) and the leaf area occurring in each section was sum-
med to obtain section totals.
Leaf area density (m2 m-3) was calculated for 15
equally spaced vertical sections per tree by summing the
5 cm leaf area estimates within each crown section. Crown
volume was estimated by first calculating the horizontal
location of each foliage section as sin(ANGLE) 9 NFL to
account for the curvature of the branches. Similar to For-
rester et al. (2012), volume of the top section of each tree
was calculated assuming the section was a cone, while the
lower sections were considered frustrums, where volume
was calculated as:
Volume ¼ p� Vlen
3
� HR2base þ HRbase � HRtop þ HR2
top
� �ð1Þ
where Vlen is the length of the vertical section (m), HRbase
is the maximum horizontal crown radius at the base of the
section (m), and HRtop is the maximum horizontal crown
radius at the top of the section (m). Leaf area density was
then calculated for each section by dividing the summed
leaf area estimates by the volume.
Three distributions previously used to model vertical
leaf area distributions (Maguire and Bennett 1996; Jerez
et al. 2005; Weiskittel et al. 2009), were fit for leaf area and
leaf area density of each tree. The three distributions were
right-truncated Weibull, Johnson’s Sb, and four-parameter
beta, defined respectively as:
pðXÞ ¼ 1
g
� �b
bXðb�1Þe�ððX=gÞb�ðc=gÞbÞ ð2Þ
pðXÞ ¼ sffiffiffiffiffiffi2pp
Xð1� XÞe �
12
wþsIn X1�Xð Þð Þð Þ2 ð3Þ
pðXÞ ¼ 1
CðcÞCðdÞ=Cðcþ dÞXc�1ð1� XÞd�1: ð4Þ
where X is the relative vertical depth of the leaf area or leaf
area density from the top of the tree, g is the Weibull scale
parameter, b is the Weibull shape parameter, c is the
Weibull truncation point, w and s are the Johnson’s Sb
shape parameters, and c and d are the beta shape parame-
ters. Parameter estimates of the three distributions were
estimated using maximum likelihood and an expectation/
maximization algorithm modified from the work of Rob-
inson (2004). Goodness of fit among the three distributions
was compared with RMSE and MAB for each species and
among the various vertical bins by species.
Parameter estimates of the best fit distribution were
compared among species with mixed-effects ANOVA. The
inclusion of management intensity and plot replicate within
intensity were assessed using likelihood ratio statistics. To
examine mean differences among species, predicted pop-
ulation margins (‘‘least-square means’’) of the model
parameters were estimated for all species.
Data analyses for BLA, CLA, and vertical leaf area and
leaf area density distribution models were conducted in the
R software, version 3.0 (R Core Team 2013), using the
‘‘nlme’’ and ‘‘lsmeans’’ packages (Lenth 2013; Pinheiro
et al. 2013). Starting parameters for the nonlinear BLA and
CLA models were obtained using the Model procedure in
SAS, version 9.2 (SAS 2009).
Results
Branch leaf area
The best fit equation for BLA across all the species was a
three-parameter nonlinear mixed-effects model that incor-
porated branch diameter (BD), RBT, and RFS. The final
model explained [84 % variance for the naturally
22 Trees (2014) 28:17–30
123
regenerated species, and between 65 and 97 % for the
hybrid poplar clones (Table 5). The final BLA model form
for all species was:
BLA ¼ BDa1 RBTa2�1e�ða3þuiÞðRFSa2 Þ ð5Þ
where a1–3 are fixed parameters, and ui is the random
effect of management intensity, and the other variables as
defined above. Likelihood ratio tests indicated that random
effects of plot replicate within management intensity, and
tree within replicate within management intensity were
unnecessary in the models.
Accounting for the random effects of management
intensity did not substantially increase the percentage of
variance explained for most species, except for gray birch
and bigtooth aspen where explained variance increased
from 35 to 87 %, and from 43 to 99 %, respectively. The
a1–3 parameters were positive for all species suggesting
that BLA increased with greater branch size and was
greater toward the top of the trees. Holding RFS and RBT
constant at 0.5, predicted BLA for the mean BD of
0.82 cm ranged from 0.05 m2 for bigtooth aspen to
0.37 m2 for gray birch, among the naturally regenerated
species. Similarly, BLA ranged from 0.18 m2 for the
DN10 clone to 0.26 m2 for the NM6 clone at the mean
BD across hybrid poplar clones when holding RFS and
RBT constant at 0.5.
At the mean branch diameter of 0.82 cm for the natu-
rally regenerated hardwood species, BLA peaked at 0.16,
0.38, 0.45, 0.75, and 0.93 RBT and RFS for paper birch,
gray birch, red maple, trembling aspen, and bigtooth aspen,
respectively (Fig. 2). At the mean hybrid poplar branch
diameter of 1.00 cm, BLA peaked at 0.16, 0.18, 0.27, and
0.32 RBT and RFS for the DN10, D51, DN70, and NM6
clone, respectively.
Crown leaf area
Across all species, a modified version of the equation
proposed by Maguire and Bennett (1996) best fit the CLA
data. The equation was a three-parameter nonlinear mixed-
effects model with DBH and CL as covariates. The CLA
model form was:
CLA ¼ b1DBHb2 eðb3þuiÞðDBH=CLÞ ð6Þ
where b1–3 are fixed effects parameters, ui is the random
effect of management intensity, and other variables are
defined above. The percentage of variance explained was
[96 % and residual standard error was \0.61 m2 across
species for the CLA models (Table 6). The final equation
included management intensity as a random effect for the
naturally regenerated hardwood species and clone as a
random effect for the single hybrid poplar equation (due to
low sample size for each clone). Likelihood ratio testsTa
ble
5B
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le2
.66
17
0.3
18
1\
0.0
01
1.4
63
30
.07
05
\0
.00
11
.00
71
0.1
63
3\
0.0
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0.8
56
0.8
56
0.1
16
1
Pap
erb
irch
2.1
35
30
.06
59
\0
.00
11
.15
22
0.0
13
8\
0.0
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1.0
43
40
.08
14
\0
.00
10
.99
30
.99
50
.03
43
Gra
yb
irch
2.7
09
50
.42
81
\0
.00
11
.44
48
0.1
39
1\
0.0
01
1.2
26
30
.68
46
0.0
97
0.3
51
0.8
74
0.1
41
3
Big
too
thas
pen
2.2
66
60
.38
80
\0
.00
11
.66
31
0.1
48
0\
0.0
01
1.1
68
40
.72
53
0.1
22
0.4
27
0.9
87
0.3
32
4
Tre
mb
lin
gas
pen
1.6
98
90
.09
87
\0
.00
12
.04
63
0.1
28
5\
0.0
01
0.9
31
30
.17
59
\0
.00
10
.83
70
.83
70
.29
15
Hy
bri
dp
op
lar
D5
12
.36
37
0.3
62
3\
0.0
01
1.3
88
30
.04
93
\0
.00
12
.97
04
0.3
55
4\
0.0
01
0.6
51
0.6
51
0.1
21
5
Hy
bri
dp
op
lar
DN
10
2.3
34
60
.07
74
\0
.00
11
.34
51
0.0
27
5\
0.0
01
3.0
32
90
.17
04
\0
.00
10
.96
70
.98
10
.03
93
Hy
bri
dp
op
lar
DN
70
2.6
19
50
.13
22
\0
.00
11
.53
53
0.0
47
0\
0.0
01
2.8
00
70
.38
70
\0
.00
10
.86
20
.97
40
.04
40
Hy
bri
dp
op
lar
NM
61
.97
82
0.0
97
6\
0.0
01
1.5
53
20
.06
11
\0
.00
12
.09
32
0.2
32
1\
0.0
01
0.9
41
0.9
41
0.2
08
4
Trees (2014) 28:17–30 23
123
indicated that the random effect for plot replicate within
management intensity was unnecessary.
Similar to the BLA equations, including management
intensity as a random effect did not substantially improve
the fit of the equations, where the percent of explained
variance increased between 0.0 % for red maple, paper
birch, and bigtooth aspen to 16.4 % for gray birch. Among
the species, the estimated parameters provided a wide
range of CLA estimates. For instance, predicted CLA
ranged from 3.26 m2 for trembling aspen to 9.85 m2 for
gray birch at the mean DBH of 4.2 cm and median CL of
4.1 m.
Vertical leaf area distribution
There were no substantial differences in the fit of vertical
leaf area distribution among the right-truncated Weibull,
Johnson’s Sb, or beta distributions (Fig. 3). On average, the
right-truncated Weibull distribution had the lowest RMSE
among species, which was 4 and 3 % lower than the beta
and Johnson’s Sb, respectively. Among the species, the
MAB of the right-truncated Weibull distribution was
lowest for gray birch, which was between 17 and 79 %
lower than paper birch and bigtooth aspen, respectively.
Overall, MAB was low across all the hardwood species,
and tended to be greatest between relative depths in the
crown of 0.8 and 1.0 (Fig. 3).
The ANOVA models for the Weibull shape and scale
parameters were used to test for significant differences
among the species, including management intensity as a
random effect. Significant differences among the species
were found for the Weibull shape (p = 0.001) and scale
(p \ 0.001) parameters (Table 7). The management
intensity random effect standard deviation was 0.184 for
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Rel
ativ
e B
ranc
h T
ip/F
olia
ge S
tart
Dep
th
Branch leaf area (m2)
red maple
paper birch
gray birch
bigtooth aspen
trembling aspen
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.1 0.2 0.3 0.4 0.5
Rel
ativ
e B
ranc
h T
ip/F
olia
ge S
tart
Dep
th
Branch leaf area (m2)
D51
DN10
DN70
NM6
Fig. 2 Vertical distribution of branch leaf area of the naturally
regenerated hardwood species and the four hybrid poplar clones.
Branch leaf area was predicted using the species-specific equations at
the mean branch diameter size of 0.82 cm for the naturally
regenerated species and 1.00 cm for the four hybrid poplar clones
and across the range of relative branch tip depth and relative foliage
start from 0 (top of the tree) to 1 (base of the crown)
Table 6 Tree-level leaf area parameter estimates, standard error of
parameters, and p-values. R2 for the fixed effects only, the R2 when
the random effect of management intensity is added to the model, and
residual standard error are shown to demonstrate the fit of the models.
Models were fit as nonlinear mixed-effects models
Species b1 b2 b3 Fit Statistics
Estimate SE p-value Estimate SE p-value Estimate SE p-value R2 fixed R2 fixed ?random
Residualstandarderror (m2)
Red maple 0.1172 0.0343 0.014 1.7104 0.1192 \0.001 1.6918 0.1902 \0.001 0.985 0.985 0.165
Paper birch 0.7569 0.0684 \0.001 2.2520 0.0424 \0.001 -0.8978 0.1277 \0.001 0.999 0.999 0.105
Gray birch 0.2076 0.1457 0.188 1.0639 0.2500 0.0021 2.3032 1.2665 0.102 0.853 0.992 0.166
Bigtooth aspen 0.5260 0.2055 0.027 2.2374 0.1766 \0.001 -1.0232 0.1767 \0.001 0.961 0.961 0.609
Tremblingaspen
0.3118 0.1094 0.021 2.0394 0.1514 \0.001 -0.5766 0.1641 0.008 0.947 0.974 0.303
Hybrid poplar 0.1959 0.0629 0.008 1.8913 0.1068 \0.001 0.4643 0.1573 0.011 0.912 0.984 0.483
24 Trees (2014) 28:17–30
123
the shape parameter model, but \0.001 for the scale
parameter, suggesting greater variability in the shape
parameter across the intensities than the scale parameter.
Relative leaf area peaked in the middle third of the
crown for all the naturally regenerated species, ranging
from a relative depth into the crown of 0.44 for paper birch
to 0.65 for trembling aspen (Fig. 4). A similar pattern
among the species was found for absolute vertical leaf area
of a mean sized tree with DBH of 3.9 cm and CL of 3.8 m,
where the peak in absolute leaf area ranged from a depth
into the crown of 1.7 m for paper birch to 2.7 m for
trembling aspen. Relative and absolute leaf area of the four
hybrid poplar clones peaked in the upper part of the middle
third of the crown, where relative depth into the crown
ranged from 0.40 for the DN10 clone to 0.54 for the NM6
clone (Fig. 5).
Similar to leaf area, the right-truncated Weibull distri-
bution was the best fit distribution for vertical leaf area
density across all of the species. Significant differences
among species were found for both the Weibull shape
(p = 0.010) and scale (p \ 0.001) parameters. Relative
leaf area density peaked in the middle third of the crown
0.0 0.1 0.2 0.3 0.4 0.5 0.6
1.0
0.8
0.6
0.4
0.2
0.0
rela
tive
dept
h in
to c
row
n BetaWeibullJohnson Sb
red maple
0.00 0.05 0.10 0.15 0.20 0.25
1.0
0.8
0.6
0.4
0.2
0.0
BetaWeibullJohnson Sb
paper birch
0.00 0.05 0.10 0.15 0.20
1.0
0.8
0.6
0.4
0.2
0.0
rela
tive
dept
h in
to c
row
n BetaWeibullJohnson Sb
gray birch
0.0 0.2 0.4 0.6 0.8 1.0
1.0
0.8
0.6
0.4
0.2
0.0
BetaWeibullJohnson Sb
bigtooth aspen
0.0 0.2 0.4 0.6 0.8
1.0
0.8
0.6
0.4
0.2
0.0
mean absolute bias (m2)
rela
tive
dept
h in
to c
row
n BetaWeibullJohnson Sb
trembling aspen
0.0 0.1 0.2 0.3 0.4 0.5
1.0
0.8
0.6
0.4
0.2
0.0
mean absolute bias (m2)
BetaWeibullJohnson Sb
hybrid poplar
Fig. 3 Mean absolute bias (m2)
for each of the 20 equidistant
vertical bins used to fit the three
distributions investigated for
goodness of fit to vertical leaf
area distribution. The three
distributions were the
4-parameter beta, Johnson’s Sb,
and right-truncated Weibull
Table 7 ANOVA table testing for species differences in Weibull
shape and scale parameters for leaf area and leaf area density. The
right-truncated Weibull distribution was the best performing distri-
bution among the three compared. The ANOVA models included the
five naturally regenerated hardwood species and the four hybrid
poplar clones
Factor Intercept Species
F-value p-value F-value p-value
Leaf Area
Weibull shape 7096.896 \0.001 6.050 \0.001
Weibull scale 6958.390 \0.001 20.848 \0.001
Leaf Area Density
Weibull shape 914.056 \0.001 2.791 0.010
Weibull scale 1059.877 \0.001 6.091 \0.001
Trees (2014) 28:17–30 25
123
for all the naturally regenerated species, except trembling
aspen. For instance, the peak in relative leaf area density
ranged from a relative depth into the crown of 0.35 for
paper birch to 0.63 for gray birch (Fig. 6). Relative leaf
area density also peaked in the middle third of the crown
for the four hybrid poplar clones, ranging from 0.35 of
relative depth into the crown for the D51 clone to 0.45 for
the NM6 clone.
Discussion
In this investigation, leaf area was modeled across a range
of branch sizes, tree sizes, and vertical location within the
crown for various hardwood species growing in early
successional stands. Substantial differences in leaf area
were found among the species at all levels of investigation.
Results from this investigation provided evidence that
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.2 0.4 0.6 0.8
Rel
ativ
e de
pth
into
the
crow
n
Relative leaf area
red maple
paper birch
gray birch
bigtooth aspen
trembling aspen
0
1
2
3
4
0.0 0.5 1.0 1.5 2.0
Dep
th in
to th
e cr
own
(m)
Leaf area (m2)
red maple
paper birch
gray birch
bigtooth aspen
trembling aspen
Fig. 4 Relative and absolute vertical leaf area for five naturally regenerated hardwood species fit with the right-truncated Weibull distribution.
The Weibull shape and scale parameters are least-square means estimates from ANOVA models testing for differences among species
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.1 0.2 0.3 0.4
Rel
ativ
e de
pth
into
the
crow
n
Relative leaf area
D51
DN10
DN70
NM6
0
1
2
3
4
5
6
0.0 0.5 1.0 1.5 2.0
Dep
th in
to th
e cr
own
(m)
Leaf area (m2)
D51
DN10
DN70
NM6
Fig. 5 Relative and absolute vertical leaf area for each of the four clones of hybrid poplar fit with the right-truncated Weibull distribution. The
Weibull shape and scale parameters were the least-square means estimated from the ANOVA model for each parameter
26 Trees (2014) 28:17–30
123
autecological differences in leaf area production and ver-
tical distribution influence the development of young,
mixed-species stands in the Acadian forest region of North
America.
Branch-level leaf area
Across species, BLA parameters were positive, and similar
to many conifer species, BLA increased exponentially with
branch size. Numerous investigations have found greater
BLA increased with increasing greater branch size when
vertical location is specified as the junction of the branch
with the main bole (Gillespie et al. 1994; Xu and Har-
rington 1998; Porte et al. 2000; Garber and Maguire 2005).
In addition, BLA showed a curvilinear relationship with
branch tip height, where BLA peaked in the upper third of
the crown for paper birch, middle third for gray birch and
red maple, and lower third for bigtooth aspen and trem-
bling aspen (Fig. 2). This investigation builds on the work
of Medhurst and Beadle (2001) and Forrester et al. (2012)
by specifying the vertical location of BLA within the
crown by incorporating the position of the branch tip height
and start of the foliage. Hardwood branches are often not
perpendicular to the bole (Harper 2008; Zellers et al. 2012)
due to weak apical dominance. Thus, the combination of
RBT and RFS provided a better metric of the location of
leaf area within the crown for hardwood species than using
the location of the branch junction with the main bole. In
conifer species, the quantity of foliage typically increases
from the top of the tree toward to middle of the crown and
then decreases near the crown base (Kantola and Makela
2004), but the complex crown forms of hardwood species
often results in greater foliage in the upper part of the
crown (Niinemets 1996). Therefore, if vertical location of
the branch junction with the bole was used to predict
hardwood BLA, the models would assume that BLA
peaked toward the base of the crown. Incorporating RBT
and RFS accounted for the more acute angles from the
vertical and actual location of BLA.
Most of the species in this investigation are considered
intolerant of shade. Therefore, it is not surprising that the
models estimated increasing BLA when branches extended
toward the top of the crown since crown forms were likely
influenced by the high stem densities within the stands. The
high levels of competition require greater leaf area in the
upper crown to reduce self-shading and shading from
neighboring trees. Compared to many shade-tolerant
conifer species, shade intolerants often shift leaf area to the
top of the crown to reduce the probability of density-
dependent mortality.
Crown-level leaf area
CLA was also found to vary substantially among the spe-
cies, across the range of tree sizes sampled. For instance, at
the mean DBH and CL among all naturally regenerated
trees, predicted CLA ranged from 3.26 m2 for trembling
aspen to 9.85 m2 for gray birch. The substantial differences
among the species may be due to inherent differences in
partitioning of growth to leaf area production. The
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.2 0.4 0.6 0.8
Rel
ativ
e de
pth
into
the
crow
n
Relative leaf area density
red maple
paper birch
gray birch
bigtooth aspen
trembling aspen
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.1 0.2 0.3 0.4
Relative leaf area density
D51
DN10
DN70
NM6
Fig. 6 Vertical distributions of relative leaf area density (m2 m-3)
for the five naturally regenerated species and the four planted clones
of hybrid poplar. Distributions were fit with right-truncated Weibull
distributions with shape and scale parameter estimates obtained from
an ANOVA model testing for species differences
Trees (2014) 28:17–30 27
123
proportion of biomass partitioned to various components
often varies by species and is often correlated with their
ability to tolerate shade (Niinemets 2006). For instance,
species with strong shade avoidance strategies tend to
allocate less biomass to foliage and more to woody struc-
tures since they often cannot maintain positive carbon
balances in shaded conditions (Niinemets 1998). This is
one possible reason for the differences in CLA found
between red maple and the aspen species, since red maple
is considered moderately tolerant of shade (Walters and
Yawney 1990), and both aspen species are considered
intolerant of shade (Laidly 1990; Perala 1990). For
instance, red maple CLA was predicted to be 67 and 136 %
greater than bigtooth aspen and trembling aspen, respec-
tively, for the average size tree.
Paper birch and gray birch CLA were substantially
greater than the aspen species for a given tree size, even
though both birch species are also considered intolerant of
shade. Differences between these two genera may be
explained by not only inherent differences in crown char-
acteristics, but also from the management history at the
site. The median DBH of trembling aspen and bigtooth
aspen were 5.1 and 5.6 cm, respectively, when compared
to paper birch (1.4 cm) and gray birch (1.3 cm). Thus, the
aspen trees in this investigation likely were part of the
original cohort of trees that regenerated following the
harvest in 1995. The small diameter of birch trees suggests
that many of the trees likely regenerated following treat-
ment application in 2004, when stand densities were sub-
stantially lower due to thinning. Therefore, the lower CLA
of the aspen species may be due to a combination of lower
biomass allocation to foliage, and stand conditions at the
start of the experiment, when stem densities of shade-
intolerant hardwood species were high (Nelson et al. 2013).
Inherent autecological crown characteristics among the
genera are also likely influencing the differences, since the
prediction of CLA in the untreated control for birch was
67 % greater than aspen for the averaged sized tree.
The rapid growth rates of hybrid poplar in plantations
have been shown to be related to large quantities of leaf
area, and fast and continuous foliage production throughout
the growing season (Rhodenbaugh and Pallardy 1993;
Pellis et al. 2004); traits that are often selected for in
genetic trials. Hybrid poplar CLA was found to vary sub-
stantially among the four clones in this investigation
ranging from 3.35 m2 for DN70 to 5.30 m2 for NM6 for an
average sized tree. It was hypothesized that the three
P. deltoides 9 P. nigra (DN) clones would have similar
CLA for a given tree size due to the same planting density
and similar crosses of Populus species, while NM6 would
have greatest CLA since this clone had the greatest
aboveground net primary productivity at the site (Nelson
et al. 2012). Among the DN clones, CLA of D51 was 17 and
36 % greater than DN10 and DN70, respectively. Other
studies have found substantial difference in biomass allo-
cation to foliage among DN clones in control treatments and
in response to increased carbon dioxide (CO2) and ozone
(O3) exposure (Dickson et al. 1998), suggesting inherent
difference in carbon allocation patterns. Compared to the
results found for CLA, total aboveground biomass pro-
duction of DN70 was greater than the D51 and DN10 clone
at the site (Nelson et al. 2012), suggesting that the DN70
clone had greater allocation to woody biomass than foliage.
Vertical leaf area distribution
We hypothesized that vertical leaf area distribution would
either be constant across the length of the crown or show a
peak in the upper third of the crown due to weak apical
dominance and sympodial crown forms of hardwood sap-
lings, similar to previous research (Niinemets 1996).
However, the results showed that the patterns of vertical
leaf area differed by species, expressed both as relative and
absolute leaf area (Fig. 4). For instance, relative leaf area
was almost evenly distributed along the vertical crown
length for gray birch, but peaked at 0.65 from the top of the
crown for trembling aspen. Comparatively, the distribution
of red maple and paper birch relative leaf areas peaked at
0.51 and 0.49 from the top of the tree, respectively. The
distribution of absolute leaf area was similar for red maple
and paper birch with the greatest amount of leaf area being
2 m from the top of the mean sized tree. The vertical
distribution of leaf area has also been shown to peak in the
middle of the crown across a range of shade tolerances in
conifer species (Maguire and Bennett 1996; Garber and
Maguire 2005; Jerez et al. 2005; Weiskittel et al. 2009) and
shade-intolerant hardwood species (Forrester et al. 2012;
Alcorn et al. 2013) suggesting a common pattern across
species and shade tolerance classes.
Many of the stands where the trees were sampled had
relatively open canopies because of their young age and the
intermediate thinning treatments applied to reduce stand
densities and increase residual tree growth rates. In stands
with more open canopy conditions that have been thinned,
a greater proportion of leaf area and leaf area density is
allocated to lower portions of the crown when compared to
closed canopy stands where allocation tends to be greater
in upper portions of the crown (Garber and Maguire 2005;
Forrester et al. 2012). As stands approach crown closure,
there is less light penetration through the canopy resulting
in reduced light interception by branches deeper into the
crown. The light compensation point can vary by species
and shade tolerance (Lambers et al. 2008), resulting in
differential changes in vertical leaf area distribution. In the
relatively open canopy stands in this investigation, leaf
area and leaf area density peaked toward the middle of the
28 Trees (2014) 28:17–30
123
crown. It is possible that vertical distribution will change as
trees mature and stands approach crown closure. Many of
the species in this investigation are intolerant of shade and
these type of species tend to alter vertical leaf area distri-
butions more than shade-tolerant species in response to
stand density (Garber and Maguire 2005). For instance,
Garber and Maguire (2005) found that leaf area was allo-
cated higher in the crown of Pinus contorta Douglas ex
Loudon and Pinus ponderosa Lawson and C. Lawson, two
shade-intolerant species, with more narrow spacing, while
the distribution of leaf area of the shade-tolerant species
Abies grandis (Douglas ex D. Don) Lindl. did not vary with
spacing.
The depth into the crown where leaf area peaked was
similar among the hybrid poplar clones, with slight varia-
tion among the DN clones (Fig. 5); and was contrary to the
initial hypothesis that the depths would be the same due to
the same species crosses. For instance, peak relative LA
occurred between 0.40 for DN10 to 0.54 for NM6. Similar
results have been found for a Populus tristis Fis-
ch. 9 Populus balsamifera L. clone, where over 80 % of
the LA in the crown was located between 0.13 and 0.50
from the top of the tree (Isebrands and Nelson 1982). These
results suggest that leaf area in plantation hybrid poplar
tends to be distributed more toward to upper and middle
portions of the crown, where self-shading is reduced and
net photosynthesis rates are typically greater (Calfapietra
et al. 2005).
Conclusion
Species differences in branch, crown, and vertical distri-
bution of leaf area have been well-documented for conifer
species, but have been less studied for hardwood species.
Hardwood species often have greater crown complexity
and many of the methods developed to model conifer leaf
area may not be appropriate. We developed a set of branch,
crown, and vertical distribution leaf area models for dif-
ferent hardwood species that included refined estimates of
vertical location by accounting for branch angle. Our
results revealed substantial differences in leaf area at
multiple levels of investigation across a range of hardwood
species that naturally regenerated following clearcut har-
vesting. For instance, aspen species had substantially less
leaf area per unit tree size when compared to birch species
and red maple. Although the aspen species are considered
intolerant and red maple moderately tolerant of shade, the
shade-intolerant crowns of these species are often mono-
layered allowing less light penetration to the understory
and lower branches when compared to multi-layered
crowns typical of the more shade-tolerant species. In
contrast, the birch species exhibited an opposite pattern of
both crown leaf area and vertical leaf area distribution,
suggesting that disturbance history and time of establish-
ment can also strongly influence patterns of leaf area
development. At the other extreme, hybrid poplar in
plantations provided an example of the strong genetic
effects influencing crown form and leaf area production.
Overall, leaf area varied among species at all levels of
investigation, and it was found that vertical leaf area and
leaf area density distributions peaked toward the middle of
the crown in these young stands. Results suggest that
coexistence of hardwood saplings in this investigation were
likely influenced by inherent species-specific leaf area
production and distribution.
Acknowledgments This work was funded by the University of
Maine Cooperative Forestry Research Unit; Northeastern States
Research Cooperative, Theme 3; and the Henry W. Saunders’ Chair,
School of Forest Resources, University of Maine. We thank Derek
Brockmann for helping to collect and process leaf area samples. We
also want to thank the two anonymous reviewers and the Commu-
nicating Editor for their comments that greatly improved the
manuscript.
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