Physiological performance of three pine species provides evidence for gap partitioning.
ABSTRACT Gradients of light and moisture availability peak at different positions within canopy gaps in northern latitudes providing the opportunity for niche partitioning in and around gaps based on differences in individual species' life history attributes. This gap partitioning offers potential for increasing diversity in forests impacted by gap-creating disturbances. We examined resource availability and the physiological performance of three Pinus species with varying tolerances for shade and moisture stress across large (0.3ha) canopy gaps to investigate relationships between gap position and species performance. Light availability was lowest in southern gap edges, while water availability was lowest in northern edges, and higher at gap interior positions than edges. Pinus banksiana seedlings had higher light-saturated CO₂ assimilation rates than P. resinosa or P. strobus seedlings at interior gap positions, and outperformed P. strobus at northern gap edges, but there were no differences between species at southern edges. Both transpiration and stomatal conductance were greatest for P. banksiana in gap centers, but showed few differences between species at edges. Foliar nitrogen concentrations were highest for P. banksiana, suggesting the dominance of this species in central gap locations may be due to a combination of high photosynthetic capacity and tight stomatal control to regulate moisture stress at drier gap positions. Our results suggest P. banksiana seedlings may be competitively superior in gap positions with high light and moisture availability, but P. resinosa and P. strobus become competitive under the drier conditions and moderate shade near gap edges. These findings support the concept of gap partitioning, and suggest silvicultural systems that incorporate patch cuttings could be used to promote diverse regeneration in northern pine forests.
- SourceAvailable from: Matt Powers[Show abstract] [Hide abstract]
ABSTRACT: Overstory conditions influence understory microclimate and resource availability, leading to gradients in evaporative demand and moisture availability that influence seedling water relations. Partial canopies may either reduce seedling moisture stress by ameliorating environmental conditions, or increase moisture stress by reducing soil moisture availability. This study used stable isotope ratios of oxygen (δ¹⁸O) and carbon (δ¹³C) and mass-based foliar nitrogen concentrations to investigate changes in transpiration (E), stomatal conductance (g s) and intrinsic water use efficiency (iWUE) of pine seedlings across an overstory gradient from open canopy gap environments to closed canopy forest. Foliar δ¹⁸O increased sharply from basal areas of 0-10m² ha⁻¹ in Pinus banksiana, Pinus resinosa, and Pinus strobus seedlings, followed by a more gradual increase with further increases in basal area. Foliar δ¹³C followed a similar, but less pronounced pattern in P. banksiana and P. strobus seedlings, and had no apparent relationship with overstory basal area in P. resinosa seedlings. The slope of the δ¹⁸O:δ¹³C relationship was positive for every species. Foliar nitrogen concentrations were not correlated with overstory basal area. These results suggest seedling E declined as overstory basal area increased due to reductions in g s, while iWUE increased slightly from open gaps to partial canopy environments. Open gap environments appear to provide sufficient moisture to sustain high leaf-level gas exchange rates in the species we studied, while relatively small increases in overstory basal area apparently promote rapid declines in g s, leading to greatly reduced seedling water loss and small increases in iWUE.Forest Ecology and Management 09/2009; · 2.67 Impact Factor
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ABSTRACT: The effect of the seed cut of the uniform shelterwood silvicultural system on white pine seed production, seed characteristics, and seed viability during 2 mast seeding events was examined in operationally harvested second growth, white pine-dominated forest stands in central Ontario. Seed traps placed along transects in unmanaged and shelterwood stands in each of 3 blocks were used to monitor seed production in 2000 and 2006 (4 and 10 years after harvesting). During these 2 mast seed years between 386,000 and 2,730,600 seed ha-1 were produced among study stands. Total seed production expressed on a per hectare and unit pine basal area basis did not differ by harvest treatment or among blocks in either year. Variability in seed production among stands was primarily due to differences in stand structure, with seed production positively related to white pine basal area. Seed characteristics were largely similar between harvested and unmanaged stands and between seed years. Seed viability was relatively high in both years, with seed from shelterwoods germinating slightly slower than those from unmanaged stands. Seed quality, as estimated by laboratory germination performance, was higher in 2006 than 2000, likely due to improved seed development and maturation in the warmer, wetter growing season of 2006. Our results suggest that the seed cut of the uniform shelterwood system applied to second growth white pine stands is unlikely to adversely affect white pine seed production, seed quality, or potential for natural regeneration during mast seeding events.New Forests 05/2013; · 1.78 Impact Factor
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ABSTRACT: Itasca State Park, located in the northern central portion of Minnesota, is challenged with maintaining a pine forest covertype, yet regeneration failures may allow much of the park to succeed to northern hardwoods. Efforts to improve pine regeneration and growth have included deer exclosures and prescribed burning to reduce competing vegetation. We revisited Itasca's Mary Lake deer exclosure 66 years after establishment to compare stand development, structure, and white pine (Pinus strobus L.) regeneration and growth with that of adjacent plots that have been: (1) untreated and (2) recently repeatedly under-burned. Overstory structure and composition was similar among all three treatments, and mid- and understory structures were similar in the treatments subject to deer browse. Sapling and midstory white pine were only present in the exclosure. White pine regeneration was present in all treatments and most abundant in the burned treatment, but was restricted to the smallest height class and consistently overtopped by the shrub layer. Regeneration, sapling, and midstory layer tree densities were highest within the deer exclosure, as was white pine height growth. The untreated plot lacked young pine and will likely succeed to northern hardwoods with a shrub understory. The three-fire sequence of the burned treatment increased the abundance of white pine regeneration at this site, but may require additional measures to control competing vegetation to allow that regeneration to ascend into the sapling layer.Natural Areas Journal 04/2009; · 0.61 Impact Factor
Physiological performance of three pine species provides evidence for gap
Matthew D. Powersa,*, Kurt S. Pregitzerb, Brian J. Palikc
aSchool of Forest Resources and Environmental Science, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, United States
bDepartment of Natural Resources and Environmental Science, University of Nevada, 1000 Valley Road, Reno, NV 89512, United States
cUSDA Forest Service Northern Research Station, 1831 Hwy 169 East, Grand Rapids, MN 06514, United States
Canopy gaps provide a source of environmental heterogeneity
in forests that benefits the regeneration of many tree species.
Researchers in tropical forests first suggested that the gradients
in resource availability running from the center of a canopy gap
into the surrounding closed-canopy forest provide an assortment
of regeneration niches, which fosters diverse species assem-
blages by partitioning the optimal performance of species with
different life history traits along different portions of the
gradient (Ricklefs, 1977; Denslow, 1980). The gap partitioning
hypothesis has also been applied to temperate forests (Poulson
and Platt, 1989; Wright et al., 1998; Sipe and Bazzaz, 1994, 1995;
Gray and Spies, 1996, 1997; Kneeshaw and Bergeron, 1999;
Holladay et al., 2006; Raymond et al., 2006), although much of
this research has focused on gap size rather than gap position
(e.g. Runkle, 1982; Runkle and Yetter, 1987; Poulson and Platt,
1989; Whitmore, 1989; McClure and Lee, 1993; Gray and Spies,
1996; Coates, 2002; Seiwa, 2007).
Differences in the direction and scale of the availability of
individual resources within canopy gaps should provide the
potential for habitat partitioning among species with varied
tolerances for shade and water stress. Light availability varies
asymmetrically in gaps located in higher latitudes such that
irradiance increases in a northern direction across gaps and the
northern portions of gaps receive the most sun (Canham, 1988;
Poulson and Platt, 1989; Wright et al., 1998; Gray et al., 2002;
Raymond et al., 2006). Air temperature, surface soil temperature
(Gray and Spies, 1997; Gray et al., 2002; Wright et al., 1998;
Raymond et al., 2006), and evaporative demand in canopy gaps
Forest Ecology and Management 256 (2008) 2127–2135
A R T I C L E I N F O
Received 24 March 2008
Received in revised form 1 August 2008
Accepted 4 August 2008
A B S T R A C T
Gradients of light and moisture availability peak at different positions within canopy gaps in northern
latitudes providing the opportunity for niche partitioning in and around gaps based on differences in
individual species’ life history attributes. This gap partitioning offers potential for increasing diversity in
forests impacted by gap-creating disturbances. We examined resource availability and the physiological
performance of three Pinus species with varying tolerances for shade and moisture stress across large
(0.3 ha) canopy gaps to investigate relationships between gap position and species performance. Light
availability was lowest in southern gap edges, while water availability was lowest in northern edges, and
higher at gap interior positions than edges. Pinus banksiana seedlings had higher light-saturated CO2
assimilation rates than P. resinosa or P. strobus seedlings at interior gap positions, and outperformed P.
strobus at northern gap edges, but there were no differences between species at southern edges. Both
transpiration and stomatal conductance were greatest for P. banksiana in gap centers, but showed few
differences between species at edges. Foliar nitrogen concentrations were highest for P. banksiana,
suggesting the dominance of this species in central gap locations may be due to a combination of high
photosynthetic capacity and tight stomatal control to regulate moisture stress at drier gap positions. Our
results suggest P. banksiana seedlings may be competitively superior in gap positions with high light and
moisture availability, but P. resinosa and P. strobus become competitive under the drier conditions and
moderate shade near gap edges. These findings support the concept of gap partitioning, and suggest
silvicultural systems that incorporate patch cuttings could be used to promote diverse regeneration in
northern pine forests.
? 2008 Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: +1 906 370 3288; fax: +1 906 487 2915.
E-mail address: firstname.lastname@example.org (M.D. Powers).
Contents lists available at ScienceDirect
Forest Ecology and Management
journal homepage: www.elsevier.com/locate/foreco
0378-1127/$ – see front matter ? 2008 Elsevier B.V. All rights reserved.
follow the same spatial pattern as light availability. Soil moisture
center of canopy gaps and decreases towards gap edges (Wright
et al., 1998; Ritter et al., 2005; Raymond et al., 2006), presumably
due to differences in rooting density (Silver and Vogt, 1993;
Denslow et al., 1998). Thus, light availability reaches its local
maxima in gap positions with the greatest potential for moisture
stress, while water stress should be lowest in gap positions with
intermediate to low light availability. This dichotomy suggests
species that are drought tolerant and shade intolerant should
perform best in northern gap positions, while species with greater
shade tolerance or susceptibility to moisture stress should
compete better in central or southern gap positions.
Most studies of gap partitioning have focused on comparing
demographic parameters (recruitment, survival, growth, etc.)
between species or functional groups. This approach provides
valuable informationabout gap dynamics,but provideslittle direct
evidence for the causes of variability (or lack thereof) in
performance. Variations in leaf morphology and carbon assimila-
tion rates have been related to partitioning of the light gradient in
canopy gaps (Sipe and Bazzaz, 1994, 1995; Grassi and Bagnaresi,
2001), but the cumulative effects of light and moisture gradients
on gap partitioning are less clear. The light-saturated carbon
assimilation rate (Amax) is generally higher in leaves exposed to
direct sunlight (Hollinger, 1989; Ellsworth and Reich, 1993) so we
might expect seedlings in northern gap positions to grow fastest,
but moisture stress may lead to reductions in stomatal con-
ductance (gs) that reduce transpiration (E), which limits carbon
assimilation rates (Comstock and Ehleringer, 1984; Walters and
Reich, 1989; Maier and Teskey, 1992). Thus, species with high
water use efficiency (WUE, the ratio of net carbon assimilation to
transpiration) should have a competitive advantage in the high
light, low moisture environments found along the northern edges
of large gaps.
While there is strong evidence for resource gradients in gaps,
many studies have failed to find differences in species composition
or performance associated with gap position (Busing and White,
1997; Dalling et al., 1998; Wilder et al., 1999; Wright et al., 1998;
Dalling et al., 2004). Resource gradients may be too narrow to
small gaps, or if dense understory shrub layers dampen the effects
of gap formation (Denslow, 1987; Gray and Spies, 1997; Dalling
et al., 1998). Microsite conditions may also supersede the effects of
larger scale resource gradients on growth and survival through
their influence on initial establishment (Denslow, 1987; Poulson
and Platt, 1989; Gray and Spies, 1997; Wright et al., 1998; Coates,
2002). Finally, tree growth in gaps with abundant advance
regeneration may be more reflective of relative sapling size and
vigor preceding gap formation than environmental gradients after
canopy removal (Marquis, 1982; Brokaw and Busing, 2000).
Gap partitioning may also provide a mechanism for enhancing
species diversity and structural complexity in forests that have
been simplified by anthropogenic influences. Pine forests in North
America’s Great Lakes region, for instance, covered large areas
prior to European settlement, including many stands of red pine (P.
resinosa Ait.) mixed to varying degrees with jack pine (P. banksiana
Lamb.) and eastern white pine (P. strobus L. [Heinselman, 1973;
Whitney, 1986, 1987; Palik and Pregitzer, 1992; Friedman et al.,
2001]). Disturbance regimes dominated by mixtures of frequent,
low to moderate intensity surface fires and less frequent, stand
replacing fires produced both single and multi-cohort age
structures in Lake States mixed pine forests (Spurr, 1954;
Heinselman, 1973; Whitney, 1986; Butson et al., 1987; Bergeron
and Brisson, 1990). This complex disturbance regime, and the
presence of remnant multi-cohort stands composed of a mosaic of
small patches or groves of individual cohorts (Spurr, 1954;
Heinselman, 1973) suggest gap-causing fires could have played
an important role in the dynamics of some presettlement mixed
Red pine is a shade intolerant species that depends on mixed-
severity fire regimes which create a mineral seedbed and reduce
overstory competition while leaving some mature residual trees as
a seed source for regeneration (Van Wagner, 1970; Whitney,
1987). White pine is moderately tolerant of shade, and more
capable of regenerating on organic soil beneath an established
canopy thanred pine, but also relies on residual trees to regenerate
after disturbance. Both red pine and white pine develop thick bark
as they mature that can resist low to moderate intensity surface
fires, allowing mutli-cohort stand development when small flare-
ups create canopy gaps. Jack pine, on the other hand, is very
intolerant of shade and susceptible to fire as both a juvenile and
adult, but grows rapidly and develops persistent, serotinous cones
early in life. These features generally promote the development of
even-aged, jack pine dominated stands on sites characterized by
frequent, stand replacing fires, but jack pine is also capable of
forming multi-cohort mixtures with red pine on xeric sites with
patchy fuels or natural fire breaks and frequent fires (Whitney,
1986; Bergeron and Brisson, 1990). Jack pine and red pine are
considered fairly drought tolerant, while eastern white pine
seedlings are more susceptible to moisture stress (Burns et al.,
1990). These varying tolerances for shade and drought suggest the
potential for niche partitioning in Lake States mixed pine forests
that experience patchy disturbances, which may enhance or
maintain overstory diversity in these forest types.
In this study, we examined the physiological performance of
three pine species that vary in shade and drought tolerances across
large (0.3 ha) silviculturalgaps. By focusing on planted seedlings in
large gaps we hoped to minimize the effects of microsite
conditions on seedling establishment and maximize potential
resource partitioning across the gap. An understory release
treatment was also included to investigate the effects of
understory competition on both resource gradients and seedling
physiological performance at different gap positions. We predicted
increasing light availability along a south–north transect through
the gaps, and increasing moisture availability from gap edges to
gap centers. We also expected patterns of Amaxto parallel those of
light availability, while E and gswere predicted to be lowest in
northern gap edges and highest around gap centers. Shade
intolerant pines with high WUE were expected to perform best
in northern gap and northern edge positions, while more shade
tolerant pines with greater susceptibility to moisture stress were
expected to perform better in southern gap positions and southern
edges. Understory release treatments were expected to increase
resource availability at all gap positions, and strengthen partition-
ing of the gap resource gradient.
2.1. Study sites and species
Our study was conducted on the Chippewa National Forest in
north-central Minnesota, USA (478350N, 938540W). Four, naturally
regenerated, 70–90-year-old red pine stands were harvested in the
winter of 2002–2003 using a variable retention harvesting system
that created 0.3 ha canopy gaps surrounded by a forest matrix that
was thinned to produce residual basal areas around 13 m2ha?1.
The gaps were circular in shape with a diameter of approximately
62 m. The surrounding canopy averaged 23 m in height, and the
forest matrix between gaps was thinned from an average of
37 m2ha?1to approximately 20 m2ha?1. Half of each stand
M.D. Powers et al./Forest Ecology and Management 256 (2008) 2127–2135
received an understory release treatment in which all shrubs and
aspen regeneration (Populus tremuloides Michx. and Populus
grandidentata Michx.) were cut with brush saws in the spring
following harvesting. The release treatment was repeated for three
additional years after harvesting. All stands were located on deep,
well-drained sands in the Hiwood-Zimmerman association of
mixed, frigid Aquic Udipsamments and mixed, frigid Lamellic
Udipsamments or the Graycalm-Menahga association of mixed,
frigid Lamellic Udipsamments and mixed, frigid Typic Udipsam-
ments. Bare root, 2–0 seedlings of jack pine, red pine, and eastern
white pine were planted at 2.7 m ? 2.7 m spacing throughout each
stand in the spring following harvesting. Seedling diameters (?one
standard error) averaged 7.6 (?0.29) mm for jack pine, 5.9
(?0.68) mm for red pine, and 6.6 (?0.31) mm for white pine at the
time of planting.
Understory vegetation in the treatment stands was dominated
by similar species in both the understory release and understory
control treatments. Maianthemum canadense (Desf.), Pteridium
aquilinum (L.) Kuhn, Vaccinium angustifolium Ait., and Oryzopersis
pungens (Poiret) Trin were the most abundant species in both
understory treatments, with total herb cover averaging 57.9%
(S.E. = 9.1%)in understoryrelease
(S.E. = 4.8%)inunderstorycontrols.Theshrublayerwasdominated
by Corylus cornuta Marsh. and Amelanchier spp. in both understory
treatments, but total shrub densities were much lower in the
understory release treatment (2005 ? 1806 seedlings/ha) than in
the understory control treatment (5714 ? 2550 seedlings/ha).
We selected two gaps for study in each of the four treatment
stands. Study gaps were selected at random, with one in the
understory release side of each stand and the second in the
understory control side. A transect consisting of five points was
marked along a north–south axis through the center of each study
gap with study points placed at gap centers, 15 m from gap center
(north gap and south gap), and 30 m from gap center (north edge
and south edge). This spacing placed edge points immediately
adjacent to the projected canopy of the forest matrix around each
gap. Crown expansion following harvesting and the irregular
nature of residual tree crowns along gap edges caused some edge
points to be partially covered by canopy. We selected the closest
jack pine, red pine, and white pine to each point for data collection.
All measurements described below were made in 2007, during the
fifth growing season after planting.
2.2. Resource availability data
Light availability was measured at each sample point as
photosynthetically active radiation (PAR) transmittance 1 m above
the ground surface using a linear PAR/LAI ceptometer (LP-80,
Decagon Devices, Inc., Pullman, WA). PAR readings were taken
between 12:00 and 15:00 h on clear days in the second week of
June. We took 30 readings at each sample point while rotating the
ceptometer in a circle centered on the point (Law et al., 1992).
Open-canopy readings were taken in large (>1 ha) clearings
immediately before and after PAR sampling in each gap, and the
average of these values was used to compute PAR transmittance.
Light measurements for each gap pair were completed on the same
Predawn xylem water potentials (Cpd) were measured on the
three beaked hazel (C. cornuta Marsh.) stems nearest to each
sample point in early June, July, and August as an estimate of soil
Hazel was chosen for Cpd measurements because it was a
ubiquitous shrub that could be found within 1 m of every sample
point. Primary shoots were excised from the main stem with a
sharp knife, and immediately placed into a pressure chamber (PMS
Instrument Company, Corvallis, OR). The average of the three Cpd
readings taken at eachpoint was used instatistical analyses. Water
potential measurements for each gap pair were completed on the
2.3. Growth and physiology data
Seedling diameter and height were measured in early May,
before leaf-out of deciduous species or shoot extension of the
pines. Diameter was measured 2.5 cm above the mineral soil
surface with digital calipers, and height was measured with a
meter stick or tape. Net carbon assimilation, transpiration, and
stomatal conductance were measured on 1-year-old needles on
the same days asCpdin June, July, and August using a portable gas-
exchange system equipped with a CO2supply and a 6 cm2cuvette
(LI-6400, Li-Cor Biosciences, Lincoln, NE, USA). Two fascicles (four
needles) of jack pine and red pine foliage or one fascicle (five
needles) of white pine foliage were excised from the middle third
ofthe canopyand immediatelyplacedinthe LI-6400cuvette under
a red/blue light source providing a photosynthetic photon flux
density of 1500 mmol m?2s?1. This level of irradiance meets or
exceeds the light saturation points reported for each study species,
so carbon assimilation rates are reported as light saturated (Amax).
WUE was calculated from gas exchange data as Amax/E. Gas
exchange measurements were made on clear or mostly sunny
The diameter of each needle was measured at two points using
a digital caliper immediately after gas exchange measurements
were completed, and the average of these two measurements was
used to compute the all-sided leaf area enclosed in the LI-6400
cuvette. Leaf area was calculated based on the assumption that
needles represent segments of a cylinder (Johnson, 1984) using the
caliper-radius method described by Svenson and Davies (1992).
The all-sided surface area of each needle in a sample was summed
to provide a measure of total leaf area in the cuvette, and gas
exchange data were recalculated on an all-sided leaf area basis.
Foliage from each tree was collected, dried and ground to a fine
powder for determination of mass-based foliar nitrogen content
(Nmass, Fisons NA Elemental Analyzer).
2.4. Statistical analysis
PAR transmittance and season mean Cpddata were analyzed
usinga split-plotanalysis of
randomized complete block design (RCBD). Understory treat-
ment was used as the whole-plot factor, gap location as the
split-plot factor, and stand as the blocking variable. Preliminary
tests indicated analyses using raw PAR transmittance data did
not satisfy ANOVA normality assumptions, so an arcsin square-
root transformation was applied and the tests were rerun.
Monthly Cpd data were analyzed using repeated-measures,
split-plot ANOVA. Whole-plot, split-plot, and blocking variables
were the same as described for the season mean Cpdtest, but
month of measurement was added as a repeated-measure.
Means comparisons were made using orthogonal contrasts to
test our initial hypotheses. These contrasts included south edge
vs. all other locations, south gap vs. gap center, north gap and
north edge, center vs. north gap and north edge, and north gap
vs. north edge for PAR transmittance and edges vs. gap positions,
north edge vs. south edge, center vs. north gap and south gap,
and north gap vs. south gap for Cpd.
Seedling diameter, height, Nmass, and season mean gas
exchange data (computed as the average of values from the three
measurement periods) were analyzed using a split-split-plot
ANOVA with a RCBD. Understory treatment was used as the
M.D. Powers et al./Forest Ecology and Management 256 (2008) 2127–2135
whole-plot factor, with gap location as the split-plot factor, species
as the split-split-plot factor, and stand as the blocking variable.
Although we focused on season mean gas exchange data, monthly
data was also analyzed using repeated-measures, split-split-plot
ANOVA. Natural logarithm transformations were applied to
diameter, height, E, and gsdata to normalize the ANOVA error
residuals and homogenize error variances. ANOVA model assump-
tions were evaluated using normal probability plots of residuals,
plots of residuals against predicted values, and the Kolmogoroff–
Smirnoff test. Significant main effects and interactions were
investigated using Tukey’s HSD to make comparisons among
means. Means from the three study species were also analyzed
within each gap location using Tukey’s HSD. We chose Tukey’s test
because individual pairwise tests that control experiment-wise
error rates arevalid evenwhentheANOVA’s Ftest isnot significant
(Day and Quinn, 1989). All statistical tests were performed at the
a = 0.05 significance level using SAS version 9.0 (SAS Institute,
3.1. Resource availability
PAR transmittance was affected only by position along the gap
transect (P < 0.001), and not understory treatment or the
interaction between understory treatment and position. PAR
at southern edges, but did not vary between other positions
(Fig. 1). Predawn water potential was also impacted by position
along the gap transect (P < 0.001), but not by understory
treatment or the interaction between understory treatment and
position. Water potentials were higher within gaps than at gap
edges, and greateratsouthern edgesthan northern edges,butCpd
did not vary within gaps. Water potentials declined during the
growing season (P < 0.001, Fig. 2), and the change between
measurement periods varied with transect locations (P < 0.001),
but not understory treatment or the interaction between
understory treatment and transect location. In June and July,
within gaps than at gap edges, higher at southern edges than
northern edges, and greater at gap centers than northern and
southern gap positions.
3.2. Seedling physiology
Photosynthesis varied among species (P < 0.001) and there
was a significant understory treatment ? species interaction
(P = 0.0114), but no other factors or interactions were significant.
On average, jack pine had higher Amaxthan red pine or white pine,
but there were no differences between red pine and white pine
(Fig. 3). This main effect relationship was also true within the
understory release treatment, but jack pine only had significantly
higher Amaxthan red pine in the understory control treatment
(Fig. 4). Jack pine had the highest Amaxat all positions within gaps,
but was only greater than white pine at northern edges, and there
were no significant differences between species in southern edges
(Fig. 5). Red pine and white pine had similar Amaxin all positions
along the gap transect. Light-saturated photosynthetic rates
declined from the June to August sampling periods (P < 0.001,
Fig. 6), but the change was not affected by understory treatment,
position, species, or any interaction among the three.
Transpiration varied with both position along the gap transect
(P = 0.047) and species (P < 0.001), but not understory treatment
or the interaction terms. Gap centers had higher E than northern
edges, but there were no other differences associated with
position. Jack pine had higher E than red or white pine, but red
pine and white pine were not significantly different (Fig. 3). When
position was considered, Jack pine had higher E than red pine in
gap centers, and higher E than white pine in gap centers and
northern edges (Fig. 5). Red pine and white pine had similar E in all
positions. Mean season E declined during the summer (Fig. 6), but
Fig. 1. Light availability and predawn xylem water potentials (Cpd) at five locations and in two understory treatments across 0.3 ha canopy gaps. Dotted bars indicated
understory release treatments. Error bars represent one standard error.
Fig. 2. Predawn xylem water potentials (Cpd) at five locations across 0.3 ha canopy
gaps measured for three periods during the growing season. Error bars represent
one standard error.
M.D. Powers et al./Forest Ecology and Management 256 (2008) 2127–2135
understory treatment, position, species, and their interactions did
not affect the rate of decline.
transect (P = 0.036) and species (P < 0.001), but were not affected
by understory treatment or any interactions. Stomatal conduc-
tance was higher at southern gap positions than northern edges,
but did differ among other positions. Jack pine had greater gsthan
red pine or white pine, but there was no difference between red
pine and white pine (Fig. 3). Comparing species within individual
positions along the gap transect, jack pine had greater gsthan red
was greater than white pine at gap centers and northern edges
(Fig. 5). Stomatal conductance did not differ among species at
northern gap positions or southern edges. Stomatal conductance
declined through the growing season (P < 0.001, Fig. 6), but the
change was not affected by understory treatment, position,
species, or any of their interactions.
gap transect, understory treatments, or species associated with
within any gap position. Water-use efficiency did not change
during the growing season (Fig. 6) and WUE was not affected by
understory treatment, gap position, species, or any interaction
among these factors across the three measurement periods.
Foliar nitrogen content was influenced species (P < 0.001), but
not by position along the gap transect, understory treatment, or
any interaction between these three variables. Jack pine seedlings
had the highest Nmass, followed by white pine, then red pine
Fig. 3. Light-saturated carbon assimilation rates (Amax), transpiration (E), stomatal conductance to water vapor (gs), water use efficiency (WUE), mass-based foliar nitrogen
content (Nmass), height, and diameter of pine seedlings at five locations, in three species, and in two understory treatments across 0.3 ha canopy gaps. Dotted bars indicate
understory release treatments. Different letters indicate significantly different means and error bars represent one standard error. Means comparisons for E, gs, height, and
diameter were made use log-transformed data.
M.D. Powers et al./Forest Ecology and Management 256 (2008) 2127–2135
(Fig. 3). Jack pine had higher Nmassthan red pine at all positions
within gaps, while jack pine and white pine were only significantly
different within northern edges (Fig. 5). Red pine and white pine
were not significantly different within any gap position.
3.3. Seedling size
Seedling height was affected by position along the gap transect
(P = 0.021) and species (P < 0.001), but not understory treatment
or any interaction among the three. Seedlings in gap centers were
taller than those in southern edges, but there were no other
significant differences between positions (Fig. 3). Jack pine
seedlings were taller than the other species, and white pine were
taller than red pine. Jack pine seedlings were taller than red pine
within each individual position, and taller than white pine within
Fig. 4. Light-saturated carbon assimilation rates (Amax) of Pinus banksiana (black
bars), P. resninosa (gray bars), and P. strobus (white bars) seedlings in canopy gaps
treated with an understory release treatment or receiving no treatment (control).
Dotted bars indicate understory release treatments. Different letters indicate
significantly different means within an understory treatment. Error bars represent
one standard error.
Fig. 5. Differences in light-saturated carbon assimilation rates (Amax), transpiration (E), stomatal conductance to water vapor (gs), water use efficiency (WUE), height,
diameter, and mass-based nitrogen content (Nmass) of Pinus banksiana (black bars), P. resinosa (gray bars), and P. strobus (white bars) seedlings in five locations across 0.3 ha
canopy gaps. Different letters indicate significantly different means within each gap location and error bars represent one standard error. Means comparisons for E, gs, height,
and diameter data were performed on log-transformed data.
of pine seedlings growing in 0.3 ha canopy gaps. Error bars represent one standard error.
M.D. Powers et al./Forest Ecology and Management 256 (2008) 2127–2135
tallerthanred pinewhencomparedwithinnorthernedgesand gap
centers, but similar within all other positions.
The main effects of treatments on seedling diameter were
similar to those observed for seedling height, but within-position
trends were different. Diameter was affected by position along the
gaptransect(P = 0.046) andspecies(P < 0.001),butnot understory
treatment or any interactions. The diameter of seedlings in gap
centers was greater than those in southern edges, but no other pair
of positions was significantly different (Fig. 3). On average, jack
pine seedlings were larger in diameter than the other species, and
white pine were larger than red pine. Jack pine seedlings were
larger in diameter than red pine at all positions except southern
edges, but only larger than white pine within north and south gap
positions (Fig. 5). Red pine and white pine had similar diameters at
most positions along the gap transect, but white pine were larger
than red pine in gap centers.
The data generally support our hypotheses regarding light and
water availability. As expected, light availability was lowest at
southern edge locations, but we did not find evidence of any
increases from southern gap positions to more northern positions.
Many of the studies reporting peak light availability in the
group (<0.1 ha) gaps surrounded by undisturbed, closed-canopy
forest (e.g. Canham, 1988; Poulson and Platt, 1989; Wright et al.,
1998; Raymond et al., 2006). Large gap sizes and diffuse radiation
entering from the thinned forest matrix surrounding our gaps may
have combined to provide higher PAR availability at more
southerly gap locations than would occur in smaller canopy
showing the expected peak at central gap locations, and higher
moisture availability at southern edges than northern edges due
presumably to the increased evaporative demand associated with
high light levels in northern edges.
Leaf-level physiological indices indicate the resource gradients
we observed could contribute to niche partitioning. Jack pine’s
carbon assimilation rates were considerably higher than red pine
and white pine at gap interior locations, suggesting jack pine may
outperform the other species in the central portions of large canopy
gaps. Jack pine was not as competitive at edge locations, and our
results suggest both red and white pine could be competitive at the
pine’s intolerance for shade likely contributed to its reduced
competitiveness at gap edges. We must note, however, that this
conclusion is based on qualitative comparisons of results from
from a single test ranking species across all locations simulta-
neously. Thus, our resultsshould be interpreted cautiously with the
recognition that we found only mild evidence of physiological
partitioning across our gap transects.
These data only represent leaf-level measures of assimilation,
and previous studies of gap or resource partitioning have found
differences in leaf morphology and shoot architecture complicate
inferences about growth potential drawn from leaf-level assimila-
tion measurements (Sipe and Bazzaz, 1994; Grassi and Bagnaresi,
2001). Our height and diameter data, however, suggest above-
ground growth patterns mostly followed the leaf-level assimila-
tion trends for each species. Trends in seedling diameter among
species were similar to the trends in diameter at the time of initial
planting when averaged across all gap positions, but diverged from
initial planting size trends within individual gap positions. Jack
pine was generally largest in diameter at gap interior locations,
where this species also had the highest assimilation rates. There
were fewer differences in diameter among species at edge
locations, where assimilation rates were more similar. Jack pine
was taller than red pine at all transect locations, but red pine
generally exhibits slow height growth as a seedling (Rudlolph,
1990) so the observed differences in height of our study seedlings
are not necessarily indicative of future growth potential. Red pines
were also the smallest seedlings in diameter at the time of
planting, but initial size may not always be related to performance
across canopy gaps (Sipe and Bazzaz, 1995).
Water relations may partly explain the carbon assimilation
patterns observed across the gap transect. Jack pine had greater
stomatal conductance and transpiration than red pine and white
pine at gap centers where soil moisture availability was high, but
there were fewer distinctions between species at north gap, south
gap, and edge locations, where moisture availability was lower.
the gap transect more tightly than the other species, allowing for
profligate water use in environments where moisture was less
limiting. Greater conductance would allow the high assimilation
rates observed for jack pine in central gap locations, assuming
increased assimilation rates at relatively high conductance. Jack
pine seedlings did have higher Nmassthan the other species when
averaged across gap locations, and nitrogen content is closely tied
to photosynthetic potential (Field and Mooney, 1986; Reich et al.,
1991, 1992; Ellsworth and Reich, 1993). High photosynthetic
capacity in jack pine may promote high conductance at locations
where light availability is high and water is less limiting. Red pine,
on the other hand, maintains relatively high levels of stomatal
conductance across a wider range of internal moisture stress
(Pereira and Kozlowski, 1977; Schulte and Marshall, 1983), which
may explain why this species was able to support assimilation
rates similar to jack pine in the dry, sunny, northern edge positions
despite the difference in foliar nitrogen concentrations between
the two species. The finding that white pine, which can be
susceptible to moisture stress as a seedling, also displayed
competitive assimilation rates and growth in dry edge locations
might be surprising, but white pine is also the most shade tolerant
of the species studied so it would be expected to fare better in
comparisons within shady southern edge environments.
The data do not support our hypotheses regarding WUE and the
trend in WUE across the gap transect, or partitioning based on
individual species differences in WUE. Understory release treat-
ments had no significant impacts on resource availability, and only
jack pine showed any differences in physiological performance
between understory treatments. A variety of studies suggest
understory release treatments that follow overstory removal
increase resource availability (Harrington and Edwards, 1999;
Beckage et al., 2000; Wetzel and Burgess, 2001; Boucher et al.,
analysis provided less power for detecting the whole-plot
understory treatment effects than gap location or species effects,
so further studies that avoid the randomization restrictions and
associated error structure of split-plot designs might be better
suited for exploring understory treatment effects in gaps.
In general, our findings are consistent with our understanding
intolerant species that limits moisture stress through strong
stomatal regulation was dominant in the open, high resource
environment of the gap interior. Red pine and white pine, which
are somewhat more tolerant of shade, and show less dramatic
stomatal response to moisture stress were able to compete better
at edge locations where dry conditions or shade limited jack pine’s
photosynthesis and growth. It is difficult to estimate what
M.D. Powers et al./Forest Ecology and Management 256 (2008) 2127–2135
proportion of the gap area would favor any given species since we
only characterized environmental conditions, physiology, and
growth in five discreet locations. However, even if the edge
conditions that seem to benefit red pine and white pine extend
only 5–7.5 m into the gap (one third to one half the distance
between our study points), that would represent approximately
30–40% of the total gap area.
While these results are likely representative of shade intolerant
to mid-tolerant conifers growing on xeric sites, they may not be
applicable to other forests on more mesic soils. Sugar maple (Acer
saccharum Marsh.) seedlings, for instance, show greater drought
stress characterized by low Cpdin open environments where high
vapor pressure deficits limit gsand carbon assimilation (Ellsworth
and Reich, 1992). Surface drying in northern and center gap
locations has also been linked to poor seedling performance in
northwestern North American conifer forests (Wright et al., 1998).
Although red pine dominated forests in the Lake States have
traditionally been managed using even-aged techniques, there has
development in these forests (Palik and Zasada, 2003; Gilmore and
Palik, 2005). Our results suggest silvicultural systems that incorpo-
rate large gaps may be a viable method for achieving these goals, at
least in the absence of pine shoot-blight diseases which can restrict
multi-cohort management opportunities (Gilmore and Palik, 2005).
Gap-based approaches forincreasing diversity have been suggested
for a variety of other forest types across North America (Coates and
Burton, 1997; Gray and Spies, 1996; Wright et al., 1998; Raymond
et al., 2006), although direct evidence of gap partitioning has been
limited in some of these studies. Our results suggest gap-based
systems may promote overstory diversity and multi-cohort
structural development, even in forests characterized by shade
availability and moisture stress.
Our findings add to a large body of literature reporting
gradients in resource availability across canopy gaps. Our study
species also showed some evidence of niche partitioning across
these gradients, and differences in performance based on carbon
assimilation rates seemed to be regulated by both light availability
and moisture stress. This represents an important extension to
traditional interpretations of the gap partitioning hypothesis,
which rely primarily on gradients in light availability to explain
patterns of species dominance and diversity across the gap-intact
canopy continuum. Our findings also suggest silvicultural systems
that incorporate patch cuttings could promote species diversity in
the potential for niche partitioning across large openings.
We thank Thomas Drummer, Linda Nagel, and Christopher
Webster for their advice and comments throughout the develop-
ment of this project. Julia Robinson assisted with sample
preparation. Funding for this research was provided by the USDA
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