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2014 NORTHEASTERN NATURALIST 21(1):23–30
White Pine Restoration in a Mesic Forest: 3-year Results
Stacie A. Holmes1,2 and Christopher R. Webster1,*
Abstract - Pinus strobus (Eastern White Pine) was an important component of mesic and
dry-mesic northern temperate forests prior to European settlement. We evaluated the efficacy
of restoration on a degraded, mesic hardwood site with a low post-harvest residual
basal area (11.5 m2 ha-1). Three-year survival of planted pines was 57% (n = 299), with
40% mortality occurring the first year. Additional losses, however, were minimal: 2% and
1% following years two and three, respectively. Survival was associated with presence
of bare soil near the seedling (P < 0.05). High initial mortality was likely attributable to
drought. Our results suggest that White Pine establishment on more mesic hardwood sites
may be possible given sufficient soil and canopy disturbance, but that high initial mortality
may be expected under dry climatic conditions.
Introduction
Prior to selective logging and widespread slash fires in the northern Lake States
during the late 1800s, Pinus strobus L. (Eastern White Pine) was an important
component (5–35%) of the region’s forests (Abrams 2001, Bolliger et al. 2004,
Whitney 1987). While White Pine exhibited a strong association with dry-mesic,
coarse textured soils (Whitney 1986), the species was also common in mesic habitats
(Fahey et al. 2012). In the central Upper Peninsula of Michigan, for example,
Zhang et al. (2000) found that the relative densities of White Pine decreased
66–75% from the mid-1880s to the early 1990s. The continued lack of recruitment
of this species in present-day forests is often attributed to the absence of fire, lack
of suitable microsites for establishment, competition with mesic hardwoods, high
rates of Odocoileus virginianus Zimmermann (White-tailed Deer) browsing, and
mortality resulting from Cronartium ribicola J.C. Fisher ex. Rabenh. (Blister Rust)
(Abrams 2001, Frelich 2002). Recent surveys in the Great Lakes region suggest that
incidences of Blister Rust-induced mortality have become less common (Dahir and
Cummings 2001, Katovich et al. 2004) and that in many areas this disease is no
longer controlling forest composition (Geils et al. 2010).
White Pine is a restoration priority in the region and has been planted in conjunction
with various silvicultural treatments in Populus spp. (aspen), Quercus spp.
(oak), and Pinus resinosa Aiton (Red Pine) stands (e.g., Pitt et al. 2009, Powers et
al. 2009, Smidt and Puettmann 1998, Wetzel and Burgess 2001). Less work has been
done in northern hardwood stands dominated by shade-tolerant maples because of
1Ecosystem Science Center, School of Forest Resources and Environmental Science,
Michigan Technological University, 1400 Townsend Drive, Houghton, MI, 49931. 2Current
address - USDOI-Bureau of Indian Affairs, Branch of Forest Resources Planning,
13922 Denver West Parkway, Suite 350, Lakewood, CO, 80401. *Corresponding author -
cwebster@mtu.edu.
Manuscript Editor: Roland de Gouvenain
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2014 Vol. 21, No. 1
strong understory competition from Acer saccharum Marsh. (Sugar Maple) and Acer
rubrum L. (Red Maple) (Krueger 2004). Additionally, these stands are often managed
with selection and other partial harvest systems that typically leave high residual basal
areas and create only small gaps in the canopy (Tubbs 1977). Nevertheless, some
northern hardwood stands are cut more heavily in order to promote tree species diversity,
salvage dieback, or maximize short-term revenue. These more open residual
stands might provide an overlooked opportunity for White Pine restoration.
In order to examine the restoration potential of low-residual basal area and/or
degraded northern hardwood stands for White Pine, we monitored the survival and
growth of 300 randomly selected seedlings from a large operational underplanting.
The objectives of this study were: (1) to identify microsite characteristics that favor
or hamper planted seedling establishment and growth and (2) to determine the
extent to which neighborhood competition influences planted seedling success in a
northern hardwood forest.
Field-Site Description and Methods
Study area
In June of 2005, the Michigan Department of Natural Resources (MDNR) operationally
under-planted 36,550 White Pine seedlings on a 97-ha parcel located near
the tip of the Keweenaw Peninsula of Michigan (47°23'N, 87°54'W). This site was
selected for planting because of the historic presence but contemporary dearth of mesic
conifers and the low local White-tailed Deer densities due to high winter snowfall
(Doepker et al. 1995). Seedlings were hand-planted, 3-year old, bare-root nursery
stock (or “3-0” stock). In 2002, before the land was acquired by the current owner,
the property was harvested to a residual basal area of 11.5 m2 ha-1. The harvest did not
have a clearly defined silvicultural objective, but rather focused on removing highvalue
species and products prior to divestment. Most trees ≥12.7 cm were harvested.
No intentional site preparation or slash treatments were performed. Planting was
conducted through the Landowner Incentive Program Mesic Conifer Restoration Initiative.
Planters were instructed to distribute the seedlings uniformly across the tract,
which provided a wide variety of competitive and topographic environments.
Soils on the planting site ranged from moderately well-drained, cobbly very fine
sandy loams to well-drained, gravelly and cobbly fine sandy loams (Tardy 2006).
Lake Superior moderates the climate of the peninsula, and the average winter and
summer temperatures are -8.2 °C and 17.4 ˚C, respectively (Tardy 2006). Mean
annual precipitation is 85.6 liquid cm, which includes 555.0 cm of mean annual
snowfall (Tardy 2006).
Monitoring
Three hundred seedlings were selected for monitoring and sampled in June 2005.
We selected seedlings by following the paths made by the planting crew and choosing
whether to sample seedlings as they were encountered based on a coin flip. We
maintained a minimum distance of 5 m between individuals to reduce interdependence
among individuals. This approach resulted in a sample population that was
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well distributed across the planting site. We uniquely identified seedlings using
numbered metal tags secured to a 20-cm-diameter wire collar. We measured height
as the total length of the outstretched stem from ground level to tip of the terminal
bud, and diameter at 10 cm above ground level or at ground level for seedlings less than 10
cm tall.
In summer 2006, we assessed microsite conditions, competition from woody
species, and light environment for each study seedling (n = 299; Table 1). Microsite
characteristics included: slope (%), aspect, and percent cover of bare soil, herbaceous
plants, and coarse woody debris (CWD) within a 1-m-radius plot centered
on the seedling (Table 1). To assess woody competition, we measured the four
nearest neighbors for three strata: (1) understory: study seedling vertical height to
2 m tall, (2) midstory: 2 to less than 10 m tall, and (3) overstory: ≥ 10 m tall. We recorded
the diameter and height of each competitor and measured the distance between
each competitor and the study seedling (Frelich et al. 1998). We quantified the
light environment (gap fraction = fraction of pixels classified as open sky) of each
study seedling with a digital, hemispherical photograph taken directly above each
seedling at a height of 71 cm and analyzed it with the software WinSCANOPY
(WinScanopy 2005). We revisited each seedling in June of 2007 and 2008 to assess
survival, and re-measured height and diameter in 2008.
Statistical analysis
A distance-dependent competition index (Hegyi 1974) was calculated for each
study seedling using the following equation:
n = 4
Hegy’s distance-weighted size ratio = Σ [Dj / Di(Distij + 1)],
j = 1
where Dj = diameter of the competitor, Di = diameter of the subject, and Distij =
distance between subject and competitor. We assessed competition by strata and
collectively across all strata.
Table 1. Mean (± 1 SD) initial dimensions and environmental attributes for planted White Pine seedlings
(n = 299). Percent bare soil, herbaceous cover, and coarse woody debris (CWD) were measured
for a 1-m-radius circle surrounding each study seedling. Gap fraction = fraction of hemispherical
photograph pixels classified as sky (WinScanopy 2005).
Category Variable Mean (± SD)
Initial Dimensions Height (cm) 20.9 (6.1)
Diameter (mm) 3.3 (1.3)
Microsite conditions Aspect 259.4 (78.5)
% slope 17.0 (7.6)
% bare soil 12.6 (23.6)
% herbaceous cover 24.7 (25.8)
% coarse woody debris 2.0 (6.0)
Light environment Gap fraction 23.4 (6.9)
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We used binary logistic and multiple linear regressions to assess the influence
of initial size and light and competitive environments (Table 1) on 3-yr survival
and height growth, respectively. We reduced full models using a backwards
stepwise procedure with a forward check (α = 0.05; Weisberg 2005), and used
log-likelihood and Hosmer-Lemshow tests to assess statistical significance of the
binary logistic regression models. We determined the proportion of variability in
height growth explained by each multiple linear regression model by calculating
adjusted R2. We included competition index variables (for each individual strata,
the understory and midstory strata combined, and all three of the strata combined)
in the final survival and growth models only if they were significant (α = 0.05)
upon addition to the base models.
We calculated Pearson correlation coefficients to assess linear dependence between
environmental variables. We performed all statistical tests in the statistical
environment R (R Development Core Team 2008). All means are presented ± 1
standard deviation.
Results
Fifty-seven percent of White Pine seedlings were alive in June 2008, three
years after planting. The highest rate of mortality was observed following the first
growing season. In June 2006, planted seedlings exhibited 40% mortality after
experiencing one growing season and one winter, but additional losses were minimal:
2% in 2007 and 1% in 2008. While not specifically surveyed, Blister Rust, a
potential issue on mesic sites in close proximity to Lake Superior, was not observed
on monitored seedlings during the course of our study. Seedling competitive environments
varied considerably. Mean Hegyi’s distance-weighted size ratios for
understory, midstory, and overstory competitors were 8.4 ± 22.1, 39.8 ± 154.2, and
164.4 ± 443.6, respectively.
Survival was positively associated with an increase in percent cover of bare soil
(P < 0.001; Fig. 1). None of the other microsite or competition variables provided
additional significant explanatory power (P ≥ 0.05).
Mean (± SD) height growth over the study period was 8.13 (± 5.36) cm yr-1 for
3-year survivors (n = 169). For surviving individuals that exhibited positive height,
the only environmental factor that was significantly associated with growth was
gap fraction (Fig. 2). This variable, however, only explained about 8% of the variance
in growth. As with mortality, competition with advance regeneration of mesic
hardwoods was not significantly associated with observed growth rates (P > 0.05).
Higher growth rates were associated with declining canopy cover.
A Pearson correlation matrix revealed significant cross correlations between
environmental variables describing light availability and disturbance: gap fraction
vs. percent cover of bare soil (r = 0.361, P less than 0.001) and % slope (r = -0.281, P less than
0.001); percent cover of herbs vs. percent cover of bare soil (r = -0.236, P less than 0.001)
and % slope (r = -0.263, P less than 0.001). Consequently, it is difficult to isolate the independent
effects of light and disturbance on understory vegetation. Strata-specific
competition index values were significantly cross-correlated (r ≥ 0.482, P less than 0.001)
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and the understory index was correlated with herbaceous percent cover (r = 0.184,
P = 0.001). No other significant correlations were observed.
Discussion
Early growth and survival of planted White Pine on this mesic site appears to
be linked to open, disturbed understory conditions. Survival was significantly associated
with percent bare soil, and height growth was positively associated with
decreased canopy cover. White Pine requires at least 20% full sunlight for seedling
survival (Wendel and Smith 1990), and height and diameter growth are both linked
to light availability and reduced understory competition (Saunders and Puettmann
1999, Smidt and Puettmann 1998). Our results suggest that suitable planting sites
may be found in recently harvested northern hardwood stands with low residual
basal areas.
Survival rates observed in our study (≈60%) were lower than those generally
observed in experimental studies. For example, Pitt et al. (2009) observed 91–99%
Figure 1. Relationship
between planted White
Pine survival and bare
soil (P(Survival) = 1/(1
+ e-(-0.01 + 0.03x)), where x =
% bare soil) after three
growing seasons. Model
fit: Hosmer-Lemeshow:
χ2 =1.86, df = 3, P =
0.603; Log-Likelihood:
χ2 = -195.25, df = 1, P less than
0.001.
Figure 2. Relationship
between light environment
and height growth
(ΔH) for 3-0 planted
White Pine (ΔH = 10.47
+ 0.60(gap fraction); df
= 2, 83; F = 13.33, P less than
0.001; R2 = 0.08). Only
surviving individuals
with positive height
growth (ΔH ≥ 0) were
included in this analysis.
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2014 Vol. 21, No. 1
6-year survival across a range of treatments for 1-0 containerized White Pine in
Ontario, Canada, and Noland et al. (2001) reported 77–91% 5-year survival for
both bare root and containerized nursery stock. Our higher mortality rates are likely
the result of inadequate soil moisture during the first growing season. During the
summer of 2005, the western Upper Peninsula of Michigan experienced a drought.
The Palmer drought severity index ranged from moderate (-2.05) in July to severe
(-3.1) in September (National Climatic Data Center 2011). Coincidentally, drought
may have reduced the competitive advantage of shade-tolerant hardwoods and
other competing vegetation, as we found a surprising lack of association between
the level of competition with the mostly shade-tolerant hardwood understory and
survival and growth of planted White Pine.
Longer-term survival rates for planted White Pine on mesic sites in the region,
however, warrant caution (Fahey and Lorimer 2013, Kern et al. 2012). For example,
across 12 sites examined by Fahey and Lorimer (2013), 7–15 years post-planting
survival of planted White Pines averaged only 13.6%. Similarly, Kern et al. (2012)
observed survival rates averaging less than 30% across a range of treatments following
12 growing seasons. In contrast to our findings, periodic inventories by Kern
et al. (2012) suggested lower initial mortality but more consistent rates of annual
mortality. Future monitoring at our study site should shed light on the periodicity
of mortality and performance of survivors.
In conclusion, post-harvest underplanting may hold promise for establishing
White Pine in northern hardwood stands, especially those stands that lack a seed
source. Targeted soil disturbance, release treatments, and protection from browsing
may enhance survival and growth assuming favorable climatic conditions (Krueger
2004). Second-growth hardwood stands have traditionally been avoided for pine
restoration due to the dominance of shade-tolerant hardwood species in the understory
(Kelty and Entcheva 1993). Nevertheless, recently harvested and/or degraded
mesic hardwood stands may provide an overlooked opportunity for White Pine
restoration, especially under prevailing drought conditions in the region.
Acknowledgments
We would like to thank landowner Gina Nicholas and the Michigan Department of
Natural Resources, especially Terry McFadden and Robert Doepker, for making this study
possible. We also thank Brandon Bal, Melissa Jarvi, Blair Tweedale, Jill Witt, Doug Holmes,
Aaron Wuori, Steve Miceli, Andy Quinn, and Adam Komar for assisting with fieldwork
and data entry, and two anonymous reviewers for their comments on the draft manuscript.
Financial support for this research was provided by the McIntire–Stennis Cooperative Forestry
Research Program, the Keweenaw Community Forest Company, and the School of
Forest Resources and Environmental Science, Michigan Technological University.
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