Southeastern Naturalist
697
L. Wilder and J.N. Boyd
22001166 SOUTHEASTERN NATURALIST 1V5o(4l.) :1659,7 N–7o1. 34
Ecophysiological Responses of Tsuga canadensis (Eastern
Hemlock) to Projected Atmospheric CO2 and Warming
Lily Wilder1 and Jennifer N. Boyd1,*
Abstract - Tsuga canadensis (Eastern Hemlock) is a keystone tree species currently experiencing
high mortality in southeastern US forests due to Hemlock Woolly Adelgid (Adelges
tsugae; HWA) invasion. Investigating the impacts of concurrent climate change on Eastern
Hemlock is important given its potential direct effects on this species and possible interactions
of climatic factors with the continued spread of HWA. We conducted a 2-factorial
experiment in controlled-environment growth chambers to test for the main effects and
interactions of projected atmospheric CO2 concentrations and transitional-season warming
on gas-exchange traits of Eastern Hemlock saplings collected from a northern Georgia field
site. We hypothesized that elevated CO2 would increase photosynthesis and that warming
would not influence photosynthesis, given the demonstrated capacity for photosynthetic
temperature-acclimation in this species. Saplings in elevated CO2 exhibited ~30% greater
leaf-level photosynthesis (A) and ~35% greater maximum light-saturated photosynthesis
(Amax) than saplings in ambient CO2. In contrast, warming did not influence A or Amax as a
main effect. However, significant treatment interactions suggest that the response of Eastern
Hemlock to rising CO2 could be impacted by associated warming. Specifically, when saplings
were grown in elevated CO2, warming was associated with reductions in Amax, rate of
respiration in the dark, and stomatal conductance, but these variables were not influenced
by warming when combined with ambient CO2. An increase in the light-saturation estimate
with elevated CO2 across temperature-treatment levels indicates that CO2 can be a limiting
factor for Eastern Hemlock, but significant treatment interactions suggest that the capacity
for this species to utilize increased CO2 may be impacted negatively by warming.
Introduction
Tsuga canadensis (L.) Carrière (Eastern Hemlock) is a shade-tolerant conifer
typically associated with cool, moist locations throughout eastern North American
forests from southern Canada to the southern Appalachians and west to the Great
Lakes region (Godman and Lancaster 1990, McWilliams and Schmidt 2000).
Eastern Hemlock is notable within these forests for its long life-span and key ecological
role—creating distinct abiotic conditions that provide unique habitat for
other organisms (Ellison et al. 2005). Dense evergreen foliage of hemlock casts
deep shade in forests otherwise dominated by broadleaf deciduous trees and the
species is associated with acidic leaf-litter and slow nutrient-cycling (Eschtruth
et al. 2006, Orwig et al. 2002). The special abiotic environment associated with
Eastern Hemlock has been linked to numerous positive impacts on associated
biota, including plants, amphibians, reptiles, birds, and mammals (Benzinger 1994,
1University of Tennessee at Chattanooga, Department of Biology, Geology, and Environmental
Science, 615 McCallie Avenue, Chattanooga, TN 37403. *Corresponding author -
Jennifer-Boyd@utc.edu.
Manuscript Editor: Howard Neufeld
Southeastern Naturalist
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
698
DeGraaf and Rudis 1986, DeGraaf et al. 1992, Elowe 1984, Harrison et al 1989,
Holmes and Robinson 1981, Howe and Mossman 1995, Martin 1960, Strickland
and Douglas 1987).
The decline of Eastern Hemlock due to the ongoing invasion of Adelges tsugae
Annand (Hemlock Woolly Adelgid; HWA) is of growing ecological concern given
the impacts that the loss of this foundational tree species could have on native
ecosystems. In the nearly 70 years since its inadvertent introduction to Virginia
from Asia in nursery stock in the 1950s (Cheah et al. 2004), the aphid-like HWA
has spread throughout much of the range of Eastern Hemlock (Morin 2009). Once
established on an individual tree or group of trees, HWA populations can grow rapidly
and cause tree mortality within ~5 y (McClure 1990, 1991; Young et al. 1995).
HWA attacks trees of all ages and sizes, and infested trees seldom recover (Orwig
and Foster 1998). The spread of HWA into the northern range of Eastern Hemlock
occurred earlier than its relatively recent spread into the southern range, which is
likely due to differences in host abundance (Morin et al. 2009). However, in its
northern range, Eastern Hemlock tends to experience slower rates of decline in response
to HWA invasion than trees in the southern range; this difference is thought
to be influenced by the intolerance of HWA to colder winter temperatures (Gouli
et al. 2000, Skinner et al. 2003). In contrast, increased survival of HWA through
winters in the southern range could allow it to spread more quickly (Paradis et al.
2008). Investigating the potential effects of concurrent projected change in climate
on Eastern Hemlock is important given its likely direct impacts on this tree species
and the possible interactions of climatic factors with HWA.
Concentrations of CO2 in the atmosphere have increased about 40% since the
Industrial Revolution, primarily due to the anthropogenic burning of fossil fuels
(Stocker et al. 2013). Modeled projections suggest that future atmospheric CO2
concentrations could exceed 600 ppm and associated warming of up to 2.5 °C could
occur by mid-century (Stocker et al. 2013). Positive responses of tree species to
increased atmospheric CO2 via its “fertilization effect” on photosynthesis as a substrate
have been well established (Ainsworth and Long 2005, Norby et al. 1999).
However, these responses can be species-specific (Dawes et al. 2010, Körner et al.
2005) and can be influenced by resource limitations and other stresses (Karnosky
2003, Saxe et al. 1998). Conifers have been reported to exhibit more-positive
growth responses to elevated CO2 in the absence of stress than deciduous trees
(Saxe et al. 1998), and late-successional, shade-tolerant species including Eastern
Hemlock have been shown to exhibit a larger positive-growth response to increased
CO2 than less-shade–tolerant, early-successional species (e.g. Acer rubrum L. [Red
Maple] and Pinus strobus L. [White Pine]; Bazzaz et al. 1990, Kinney and Lindroth
1997), although such responses can be influenced by other abiotic factors (Bazzaz
and Miao 1993). The positive growth-response of Eastern Hemlock, in particular,
to elevated CO2 has been supported by other studies (Eller et al 2011, Godbold et
al. 1997), but no published studies to our knowledge have reported the responses of
photosynthesis to elevated CO2 in this species as a physiological process underlying
such growth responses.
Southeastern Naturalist
699
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
Photosynthesis also is generally and strongly affected by temperature (Berry
and Björkman 1980), and the warming associated with increased atmospheric CO2
has been associated with positive photosynthetic responses (Ainsworth and Long
2005, Dawes et al. 2010, Körner et al. 2005). However, these responses too can be
species-specific (Dawes et al. 2010, Hättenschwiler and Körner 2000, Körner et al.
2005, Tjoelker et al. 1998), often reflecting adaptations of photosynthetic enzymes
to a range of temperatures that represent species’ native environments (Berry and
Björkman 1980). For species adapted to seasonal environments, these adaptations
can include the ability to acclimate to seasonal temperature shifts (Berry and Björkman
1980).
Across typical seasons, light-saturated (to control for deciduous canopy cover)
photosynthesis of Eastern Hemlock has been reported to be relatively consistent
(Adams and Loucks 1971, Burkle and Logan 2003, Hadley 2000, Hadley and Schledbauer
2002), suggesting that this species has a high capacity for photosynthetic
temperature acclimation. Previous research conducted in growth chambers also
reported no detectable effect of temperature ranging from 15 °C to 30 °C on Eastern
Hemlock photosynthesis (Hadley 2000). Such acclimation was further evidenced by
measures of photosynthetic response to temperature of seedlings from southern Wisconsin
that revealed a shift in their photosynthetic temperature optima from 13 °C to
17 °C from late spring to mid-summer. Yet, tree-ring chronologies have revealed that
the growth of Eastern Hemlock is responsive to temperature and that such responses
may be seasonally dependent, generally exhibiting negative correlations with summer
temperatures and positive correlations with winter temperatures (see Cook and
Cole 1991, D’Arrigo et al. 2001, Farahat et al. 2016, Hart et al. 2010).
Such findings suggest that temperatures during especially warm summers may
extend beyond optimum temperatures for photosynthetic-enzyme function, but
tree-ring chronologies also have revealed the importance of moisture availability
as influenced by relationships between temperature and precipitation on growth
(see Abrams et al. 2000, Cook and Cole 1991, Cook and Jacoby 1977, D’Arrigo
et al. 2001, Hart et al. 2010, Saladyga and Maxwell 2015). During the warmest
parts of the year, water stress can can have a greater influence than temperature on
plant photosynthetic performance (Berry and Björkman 1980); thus, it is possible
that negative correlations of Eastern Hemlock to summer temperatures could be
affected by the possible influence of temperature on moisture avai lability.
Although tree-ring chronologies have described the responses of Eastern Hemlock
to past climate, limited empirical research has examined the effects of CO2 or
temperature on Eastern Hemlock growth (e.g., Adams and Louck 1971, Bazzaz et
al. 1990, Eller et al. 2011, Hadley 2000) and/or underlying physiology (e.g., Adams
and Loucks 1971, Burkle and Logan 2003, Hadley 2000, Hadley and Schledbauer
2002) within the context of contemporary climate, and no studies to date have
examined future projections of these factors in combination. To explore the main
effects and interactions of CO2 and temperature on saplings collected from a southern
Eastern Hemlock population, we conducted a 2-way factorial experiment in
controlled-environment growth chambers. We chose to focus on ecophysiological
Southeastern Naturalist
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
700
traits linked to photosynthesis as performance measures due to the difficulty of
resolving significant growth and reproductive responses of long-lived species to
climate change. However, plant growth and reproduction fundamentally entail
energetic expenses (Mooney 1972), and photosynthesis has been linked mechanistically
to tree growth within the context of climate change (Reich et al. 2015). We
focused on warming that would characterize transitional seasons (i.e., spring, fall),
in particular, because Eastern Hemlock saplings in deciduous forests are proposed
to achieve their greatest net photosynthetic rates during spring (Hadley 2000), and
saplings were collected for our research with data collection planned for fall. We
hypothesized that increased CO2 would be benefit the photosynthetic performance
of saplings given the well-established “fertilization effect” associated with this factor
(Ainsworth and Long 2005), while associated warming would not significantly
influence photosynthetic performance given the demonstrated capacity of Eastern
Hemlock to acclimate to a range of temperatures that would include warmer spring
and fall temperatures in our region (Adams and Loucks 1971, Burkle and Logan
2003, Hadley 2000, Hadley and Schledbauer 2002).
Methods
Saplings
We collected 80 Eastern Hemlock saplings ranging from 15 cm to 45 cm in
height from a forest understory located on private property near Dahlonega, GA,
in late August 2015. We loosely wrapped the plants’ root balls in wet newspaper
and transported them to the lab on the same day. The following day, we potted the
saplings separately in ~3.75-L (~1-gallon) nursery containers (Pro Cal I-300, Pro
Cal, South Gate, CA) filled with 1 part field-collected soil to 3 parts standard commercially
available potting soil (Potting Soil, Evergreen, San Jose, CA). Although
there was no observable presence of HWA on saplings, we treated all individuals
prophylactically with insecticidal soap, as well as with a broad-spectrum antifungal
treatment (Neem, Natural Guard, Bonham, TX) to kill any fungal infections
obtained in the field. We added the manufacturer’s suggested amount of slowrelease
fertilizer (Osmocote, The Scotts Company, Marysville, OH) to each pot
and watered to saturation; we watered the pots to saturation every other day for
the first few weeks, and then every 2–4 days as needed. We divided the saplings
into 4 even groups (20 individuals/group) such that each group included a broad
representation of sapling sizes and observable health conditions. We assigned each
group to a growth chamber (PGR15, Conviron, Winnipeg, MB, Canada) set to replicate
CO2, temperature, photoperiod, light intensity, and relative humidity typical
of transitional-season (i.e., spring and fall) conditions in northern Georgia for a
4-week acclimation period. We rotated pot locations within chambers twice a week
to account for micro-environmental heterogeneity.
Environmental treatments
Following the 4-week acclimation period, we programmed the chambers to replicate
all possible combinations of CO2 and temperature-treatment levels. Ambient
Southeastern Naturalist
701
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
and elevated CO2 were set at 400 ppm and 600 ppm, respectively. We set low and
high temperature at 12/20 °C and 14/22 °C (night/day), respectively. Our elevated
CO2 and high-temperature treatment levels were designed to simulate mid-century
projections of these independent variables for the balanced A1B emissions scenario
(Stocker et al. 2013). We chose to use near-future projections rather than late-century
projections in our research due to the ongoing rapid decline of Eastern Hemlock
as a result of HWA. Our experimental design included a different combination of
CO2 and temperature in each of the 4 chambers: ambient CO2/low temperature,
ambient CO2/high temperature, elevated CO2/low temperature, and elevated CO2/
high temperature. We programmed light regimes and relative humidity similarly
in all chambers to provide conditions typical of the understory habitat in Eastern
Hemlock forests; all saplings were watered every 2–
3 d with an equal amount of
water such that plants in the ambient CO2/low temperature chamber were saturated.
The saplings remained in their respective treatment combinations for an additional
6 weeks to acclimate prior to gas-exchange measurements. However, we rotated
pots within chambers twice weekly to control for potential spatial differences in
microclimate, and randomly reassigned the treatment levels assigned to each chamber
each week and moved all saplings accordingly to help minimize any chamber
effects and alleviate issues of pseudoreplication (Gibson 2014).
Physiological performance
Following the 6-week treatment-acclimation period, we measured leaf-level
gas exchange during late mornings and early afternoons on a 3-cm length of the
uppermost-branch tip of all saplings per treatment combination with a portable gasexchange
analyzer (LI-COR 6400, Lincoln, NE) equipped with a CO2 module. The
mortality of 6 saplings during the acclimation periods resulted in a slightly unbalanced
sample design of 17–20 individuals per treatment combination. We generated
a photosynthetic response to light level (an A/PAR curve) for each sapling by measuring
the steady-state responses of photosynthesis to external photosynthetically
active radiation (PAR) provided by blue–red light-emitting diodes mounted above
the leaf cuvette at 12 setpoints from 0 μmol to 1600 μmol photons m-2 s-1. Eastern
Hemlock is not a broadleaf species; thus, we estimated leaf area in accordance
the protocol described for this species by Nelson et al. (2014). With the exception
of PAR, conditions inside the leaf cuvette were set to replicate conditions inside
the growth chamber in which each measured sapling was housed, including CO2,
maximum midday temperature regulated with thermoelectric coolers, and relative
humidity. We collected light-response measurements over the course of 1 month,
typically with 3–5 saplings measured per day. We moved between chambers to
select saplings for measurements each day to prevent inadvertent influence of time
on measured variables.
Data analyses
We analyzed A/PAR curves to calculate net photosynthesis (A) and needle transpiration
(E) at ambient PAR, the maximum rate of photosynthesis at saturating
PAR (Amax), maximum apparent quantum yield (ɸ), the light-compensation point
Southeastern Naturalist
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
702
(LCP), and the light-saturation estimate associated with 90% (LSE) of Amax. We
conducted analyses of light-response variables by fitting measured A/PAR curves to
the nonrectangular model described by Prioul and Chartier (1977) with the Excel
Solver utility (Version 14.5.7., 2011, Microsoft, Redmond, WA). We determined
the rate of respiration in dark (RD) as the intercept of the lower part of the A/PAR
curves with the y-axis. We averaged all leaf-level gas-exchange variables across
treatments. Means of select measured variables were used to model average A/PAR
curves for each treatment-level combination with the Prioul and Chartier (1977)
model. We employed a 2-way analysis of variance (ANOVA) model to assess the
main effects and interaction of our CO2 and temperature treatments on all measured
gas-exchange variables (SPSS for Windows, Rel. 7.5.1, 2016, SPSS, Inc.,
Chicago, IL). We considered mean differences significant if P ≤ 0.05. In the case of
a significant interaction, we compared means of treatment combinations with leastsignificant
difference (LSD) post-hoc analysis at the 0.05 probability level.
Results
Average A/PAR curves modeled using mean photosynthetic light-response characteristics
illustrated differences among CO2 and temperature treatment groups in
photosynthetic responses to external PAR (Fig. 1). Saplings growing in elevated
CO2 and low temperature exhibited the greatest overall photosynthetic response to
increasing PAR, while saplings growing in the ambient CO2 and low-temperature
treatment exhibited the most-minimal response to elevated PAR. Elevated CO2 was
associated with a ~30% increase in mean A (F1, 70 = 5.273, P = 0.025) and a ~35%
increase in mean Amax (F1, 70 = 7.527, P = 0.008) of saplings (Table 1). However,
measures of respiratory-carbon loss and the ratio of net photosynthesis to dark
respiration (A/RD) were not significantly influenced by CO2 (Table 1). Specifically,
there were no significant differences in RD (F1, 70 = 0.027, P = 0.870), or A/RD
(F1, 70 = 0.277, P = 0.635) between our CO2 treatment levels (Table 1). Similarly,
CO2 did not significantly influence E (F1, 70 = 1.093, P = 0.299) or stomatal conductance
(gs) (F1, 70 = 0.548, P = 0.462; Table 1).
In contrast to the influence of CO2 on photosynthesis, temperature did not significantly
influence mean A (F1, 70 = 0.007, P = 0.935) or Amax (F1, 70 = 0.47, P =
0.830) as a main effect. There also was no significant effect of temperature on mean
RD (F1, 70 = 0.001, P = 0.970). As a result, A/RD also was not influenced significantly
by temperature (F1, 70 = 0.322, P = 0.572). Similarly, there was no significant influence
of temperature on E (F1, 70 = 1.116, P = 0.294) or gs (F1 ,70 = 0.848, P = 0.360;
Table 1).
Although temperature did not influence significantly basic measures of photosynthesis
and respiration as a main effect, the interaction of temperature and CO2
significantly influenced both Amax (F1, 70 = 5.303, P = 0.024) and RD (F1, 70 = 7.625,
P = 0.007). Specifically, Amax was not influenced by CO2 when saplings were exposed
to warming, (P = 0.762), but Amax increased with elevated CO2 in the absence
of warming (P ≤ 0.001) (Fig. 2a). Similarly, RD was not influenced by CO2 when
saplings were exposed to warming (P = 0.078); but in the absence of warming, RD
Southeastern Naturalist
703
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
increased with elevated CO2 (P = 0.037), although the increase was not statistically
significant. In addition, although not significant, RD exhibited a trend in response to
warming when CO2 was elevated (P = 0.052), but an increasing trend in response
to warming in ambient CO2 (P = 0.057; Fig. 2b). Leaf gs also was significantly
influenced by the interaction of temperature and CO2 (F1, 70 = 3.854, P = 0.050).
Specifically, gs was not influenced by CO2 when saplings were exposed to warming
(P = 0.403), but gs under conditions of elevated CO2 was higher in the absence of
warming (P = 0.050). In elevated CO2, gs decreased in response to warming (P =
0.045), but was not affected by warming in ambient CO2 (P = 0.464) (Fig. 2c). In
contrast, there were no significant interactions of CO2 and temperature on A (F1, 70 =
0.007, P = 0.562), A/RD (F1, 70 = 3.567, P = 0.063), or E (F1, 70 = 2.323, P = 0.132).
Elevated CO2 was associated with a ~30% increase in LSE (F1, 70 = 4.126, P =
0.046; Table 1). However, other measures of photosynthetic light responses were
not significantly influenced by CO2 (Table 1). Specifically, there were no significant
differences in mean ɸ (F1, 70 = 1.141, P = 0.289) or LCP (F1, 70 = 0.243, P = 0.623)
Figure 1. Curves depicting mean leaf-level photosynthetic response of Eastern Hemlock
saplings to photosynthetically active radiation (PAR). Curves were generated using the
light-response model developed by Prioul and Chartier (1977). Solid lines and dashed lines
represent saplings grown in ambient CO2 (400 ppm) and elevated CO2 (600 ppm), respectively.
Filled and open circles represent saplings grown in ambient (20 °C) and elevated
temperature (22 °C), respectively. Error bars represent ± 1 SE of the mean.
Southeastern Naturalist
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
704
between our CO2 treatment levels (Table 1). As with basic measures of photosynthesis
and respiration, temperature had no significant influence as a main effect on
any measured photosynthetic light responses, including LSE (F1, 70 = 0.475, P =
0.493), ɸ (F1, 70 = 0.405, P = 0.526), and LCP (F1, 70 = 0.363, P = 0.549). Although
temperature did not significantly influence any of our measured photosynthetic
light-response variables, the interaction of temperature and CO2 significantly influenced
LSE (F1, 70 = 5.690, P = 0.020). Specifically, LSE was reduced by warming
(P = 0.033) when saplings were grown in elevated CO2, but LSE was not influenced
by warming when saplings were grown in ambient CO2 (P = 0.235; Fig. 2d). In
contrast, there were no significant interactions of CO2 and temperature on ɸ (F1, 70 =
0.462, P = 0.499) or LCP (F1, 70= 0.097, P = 0.757).
Discussion
Potential impacts of climate change on Eastern Hemlock
Overall, our findings support our hypothesis that increased CO2 enhances Eastern
Hemlock photosynthesis but that warming as a main effect does not (Table 1;
Table 1. The main effects of CO2 and temperature on leaf-level gas-exchange characteristics of Eastern
Hemlock saplings, including net photosynthesis (A), maximum rate of photosynthesis at saturating
light level (Amax), dark respiration (RD), A/RD, transpiration (E), stomatal conductance (gs), maximum
apparent quantum yield (ɸ), light compensation point (LCP), and light saturation estimate (LSE).
Saplings were grown in 2 CO2 treatment levels (ambient: 400 ppm, elevated, 600 ppm) crossed with
2 temperature levels (ambient: 20 °C, elevated 22 °C) in controlled-environment growth chambers.
Values presented are means ± 1 SE; values followed by the same superscript letter are not statistically
different at the P ≤ 0.05 level of significance. Ambient CO2, n = 35; elevated CO2, n = 35; ambient
temperature n = 38; elevated temperature, n = 32.
Independent variable Dependent variable Ambient Elevated
CO2
A (μmol CO2 m-2 s-1) 2.9 ± 0.18A 3.7 ± 0.30B
Amax (μmol CO2 m-2 s-1) 5.5 ± 0.34A 7.4 ± 0.59B
RD (μmol CO2 m-2 s-1) 1.1 ± 0.15A 1.2 ± 0.16A
A/RD 5.7 ± 1.14A 6.6 ± 1.64A
E (mmol H2O m-2 s-1) 0.22 ± 0.026A 0.35 ± 0.117A
gs (mmol H2O m-2 s-1) 12.7 ± 1.75A 13.8 ± 1.76A
ɸ (mol CO2 mol-1 photons) 0.046 ± 0.0139A 0.030 ± 0.0046A
LCP (μmol photons m-2 s-1) 8.9 ± 1.76A 10.4 ± 2.19A
LSE90 (μmol photons m-2 s-1) 569.3 ± 45.17A 750.4 ± 73.91B
Temperature
A (μmol CO2 m-2 s-1) 3.3 ± 0.27A 3.3 ± 0.24A
Amax (μmol CO2 m-2 s-1) 6.5 ± 0.57A 6.4 ± 0.41A
RD (μmol CO2 m-2 s-1) 1.1 ± 0.17A 1.1 ± 0.15A
A/RD 5.6 ± 1.0A 6.7 ± 1.76A
E (mmol H2O m-2 s-1) 0.34 ± 0.114A 0.22 ± 0.028A
gs (mmol H2O m-2 s-1) 14.1 ± 1.73A 11.6 ± 1.78A
ɸ (mol CO2 mol-1 photons) 0.034 ± 0.0127A 0.043 ± 0.0070A
LCP (μmol photons m-2 s-1) 8.8 ± 1.67A 10.6 ± 2.29A
LSE90 (μmol photons m-2 s-1) 685.4 ± 67.82A 631.4 ± 56.94A
Southeastern Naturalist
705
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
Figure 2. (a) mean
leaf-level maximum
rate of photosynthesis
at saturating
light (Amax), (b) the
rate of respiration
in dark (RD), (c) rate
of stomatal conductance
(gs), and (d)
the light-saturation
estimate associated
with 90% of
the maximum rate
of photosynthesis
at saturating light
(LSE) of fieldcollected
Eastern
Hemlock saplings
collected grown in
controlled-environment
growth chambers
under interacting
ambient (400
ppm) and elevated
(600 ppm) CO2 and
ambient (20 °C) and
elevated (22 °C)
temperature treatment
levels. Values
shown below the
same letter are not
significantly different
at the P ≤
0.05 level of significance.
Error bars
represent ± 1 SE of
the mean.
Southeastern Naturalist
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
706
Figs. 1, 2). We also found that increased CO2 was associated with an increase in
light-saturated Amax, suggesting that CO2 currently could be a limiting factor for
this species in relatively high-light environments. In contrast, respiratory-carbon
loss (RD) was not affected by CO2 or temperature. When CO2 and temperature are
considered as single factors, these findings suggest that Eastern Hemlock may benefit
from projected climate change. However, CO2 and temperature are projected to
rise concurrently, and our results also reveal that CO2 and warming could counteract
the effects of each other. In particular, Amax increased with elevated CO2 when
samplings were grown in current temperatures, but warming negated the positive
effect of elevated CO2 on Amax (Fig. 2a). This offset may be explained by concurrent
reductions in stomatal conductance in response to warming for saplings grown in
elevated CO2 (Fig. 2c), which could reduce carbon uptake. An associated decrease
in RD (Fig. 2b) may have balanced reductions in carbon gained, resulting in overall
similarities in A/RD. Reductions in leaf respiration have been reported as a common
response to elevated CO2 (Wullschleger et al. 1994), and could result in greater
carbon-use efficiency given a concurrent increase in photosynthesis as is typical under
elevated CO2 (Ziska and Bunce 1998). Although decreased respiration rates are
not a common response to increasing temperature, the increase in respiration typically
associated with warming can be lost to acclimation resulting from enzymatic
processes and biochemical changes (Amthor 1997, Curtis and Wang 1998, Drake et
al. 1997, Gifford 1994, Gonzalez-Meler et al. 2004, Larigauderie and Körner 1995).
Decreases in leaf respiration and stomatal conductance are common responses to
water stress (Atkin et al. 2005, Chaves et al. 2002), which can be exacerbated by
warming. Although not significant, the declining trend of RD of Eastern Hemlock
with warming in elevated CO2 could be influenced by a possible combination of
inhibition of respiration in response to elevated CO2 and potential water-stress
associated with our warming treatment. While we did not measure soil-moisture
availability, we concede that restrictions of rooting volume imposed by pots could
have exacerbated water stress (see Poorter et al. 2012) in our experiment, especially
for saplings grown with warming.
Our ecophysiological findings could provide a mechanistic explanation for
previous research that demonstrated a positive influence of increased CO2 on
Eastern Hemlock growth when grown in temperatures representing warm, summer
field conditions in controlled-environment growth chambers (Bazzaz et al.
1990). Although studies based on tree-ring chronologies have reported that Eastern
Hemlock growth is negatively affected by warming (e.g., Cook and Cole 1991,
D’Arrigo et al. 2001, Hart et al. 2010, Farahat et al. 2015), such responses could
also be caused by reduced soil-moisture availability associated with high summer
temperatures (see Abrams et al. 2000, Cook and Cole 1991, Cook and Jacoby 1977,
D’Arrigo et al. 2001, Hart et al. 2010, Saladyga and Maxwell 2015). Analyses
of Eastern Hemlock decline and delayed recovery during the mid-Holocene also
have demonstrated its sensitivity to soil moisture (Marsicek et al. 2013, Oswald
and Foster 2012). We suggest that the transitional-season warming treatment used
in our experiment may not have exceeded the tolerable temperature range of this
Southeastern Naturalist
707
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
species and/or caused enough moisture stress to negatively affect its photosynthetic
performance as a main effect. Further investigations of the responses of Hemlock
to climate-change scenarios for other seasons could benefit our understanding of
its responses to climate change, and such investigations also could consider further
projections of future climate change. We chose to assess the effects of near-future
(i.e., mid-century versus late century) projections of CO2 and temperature given the
rapid mortality of Eastern Hemlock in our region due to the ongoing HWA invasion,
but climate-change predictions for the year 2100 assert that CO2 and temperature
will continue to rise (Stocker et al. 2013). If Eastern Hemlock survives to 2100 and
beyond in the face of HWA invasion, we suggest that a continued fertilization effect
of CO2 could further enhance the photosynthetic performance of this species;
however, it is possible that associated warming could exceed its temperature range
and/or increase water stress in the absence of increased precipitation.
Although our experiment explored the climate-change responses of Eastern
Hemlock saplings from its southern range, the wide geographic distribution of this
species could make it important to consider potential local adaptations across its
range in response to climate change. Particularly, because northern and southern
populations of this species are genetically distinct (Potter et al. 2012), they may
be characterized by strong local adaptations to climate. Studies have shown an
increased sensitivity to warming in populations of tree species near their highesttemperature
range limits relative to other locations (Reich et al. 2015), yet our
ecophysiological findings suggest that the sensitivity of saplings collected from
their southern limits to spring warming is minimal. In contrast, other plant species
have been characterized by significantly higher sensitivity to warming in their
northern versus southern ranges, suggesting that populations near warm range
limits may be better adapted to warming (Peñuelas et al. 2004). In addition to the
potential for local adaptations along a latitudinal temperature gradient to influence
responses to climate change, previous research has suggested that temperature- and
moisture-sensitive ecotypes of this species can exist among microsites distributed
across regions (Kessell 1979). In the southeastern US, intermediate ecotypes also
may exist (Kessell 1979), suggesting that climate-change responses could vary
across populations even at relatively small spatial scales, a possibility that warrants
further investigation.
Implications for Eastern Hemlock within the context of HWA
The impact of climate change on insect damage of trees has become apparent
in many forest systems, with warming and associated drought often implicated as
detrimental (Kurz et al. 2008, Logan et al. 2003). Such outcomes are influenced
ultimately by a combination of the responses of trees to climate change, as well as
the indirect impacts of climate change on insect interactions (Dukes et al. 2009).
In part, such interactions between climate and insect activity are thought to have
contributed to the decline of Eastern Hemlock during the mid-Holocene (Foster
et al. 2006). Though insects are unlikely to be affected directly by increased CO2,
insects can be affected indirectly by associated nutritional changes in their plant
Southeastern Naturalist
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
708
hosts (Robinson et al. 2012). In Eastern Hemlock, increased CO2 has been associated
with a greater foliar carbon-to-nitrogen ratio (Eller et al. 2011), a change in
biochemistry that could represent diminished nutrient quality for the HWA and
influence its feeding pressure and/or fitness (see Bezemer and Jones 1998, Stiling
and Cornelissen 2007). Warming also could indirectly influence Eastern Hemlock
through its impacts on HWA feeding activity and/or fitness becaus insect physiological
and behavioral processes can be highly temperature dependent (see Irlich
et al. 2009). Although our research increases understanding of the response of Eastern
Hemlock to rising CO2 and temperature as climate change factors, the extent
to which responses to climate change will impact this species within the context
of ongoing HWA infestation remains unknown. We did not include an HWA treatment
in our experiment, but our investigation of the combined effects of increased
CO2 and warming establishes an important baseline of the direct responses of this
species to concurrent climate change factors. Further research investigating the
impacts of such factors on HWA fitness and resultant indirect impacts on Eastern
Hemlock would improve the future prognosis of this species in the context of concurrent
climate change and HWA invasion.
Acknowledgments
We are very grateful to Donna Shearer of the non-profit organization Save Georgia’s
Hemlocks for providing the saplings used in this project. We also appreciate the support of
the University of Tennessee at Chattanooga Departmental Honors Program and Drs. Hope
Klug and Yukie Kajita for their advisement and review of an earlier draft of this manuscript.
We thank 2 anonymous peer reviewers who provided comments on an earlier draft of this
publication. The growth chambers used in this research were supported by NSF award
#1337540.
Literature Cited
Abrams, M.D., S. van de Gevel, R.C. Dobson, and C.A. Copenheaver. 2000. The dendroecology
and climatic impacts of old-growth White Pine and hemlock on the extreme
slopes of the Berkshire Hills, Massachusetts, USA. Canadian Journal of Botany
78:851–861.
Adams, M.S., and O.L. Loucks. 1971. Summer air temperatures as a factor affecting net
photosynthesis and distribution of Eastern Hemlock (Tsuga canadensis L. [Carriere]) in
southwestern Wisconsin. American Midland Naturalist 85:1–10.
Ainsworth, E.A., and S.P. Long. 2005. What have we learned from 15 years of free-air CO2
enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy
properties, and plant production to rising CO2. New Phytologist 165:351–371.
Amthor, J.S. 1997. Plant respiratory responses to elevated carbon dioxide partial pressure.
Pp. 35–77, In L.H. Allen, M.B. Kirkham, D.M. Olszyk, and C.E. Whitman
(Eds.). Advances in Carbon Dioxide-Effects Research. American Society of Agronomy,
Madison, WI.
Atkin, O.K., D. Bruhn, V.M. Hurry, and M.G. Tjoelker. 2005. The hot and the cold: Unraveling
the variable response of plant respiration to temperature. Functional Plant Biology
32:87–105.
Southeastern Naturalist
709
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
Bazzaz, F.A., and S.L. Maio. 1993. Successional status, seed size, and response of tree
seedlings to CO2, light, and nutrients. Ecology 74:104–112.
Bazzaz, F.A., J.S. Coleman, and S.R. Morse. 1990. Growth responses of seven major
co-occurring tree species of the northeastern United States to elevated CO2. Canadian
Journal of Forest Research 20:479–484.
Benzinger, J. 1994. Hemlock decline and breeding birds: 1. Hemlock ecology. Records of
New Jersey Birds 20:2–12.
Berry, J., and O. Björkman. 1980. Photosynthetic response and adaptation to temperature
in higher plants. Annual Review of Plant Physiology 31:491–543.
Bezemer, T.M., and T.H. Jones. 1998. Plant–insect–herbivore interactions in elevated atmospheric
CO2: Quantitative analyses and guild effects. Oikos 82:212–222.
Burkle, L.A., and B.A. Logan. 2003. Seasonal acclimation of photosynthesis in Eastern
Hemlock and Partridgeberry in different light environments. Northeastern Naturalist
10:1–16.
Chaves, M.M., J.S. Pereira, J. Maroco, M.L. Rodrigues, C.P.P. Ricardo, M.L. Osório, I.
Carvalho, T. Faria, and C. Pinheiro. 2002. How plants cope with water stress in the field?
Photosynthesis and growth. Annals of Botany 89:907–916.
Cheah C., M.E. Montgomery, S. Salom, and B.L. Parker. 2004. Biological control of Hemlock
Woolly Adelgid. FHTET-2004-04. US Department of Agriculture, Forest Service,
Washington, DC.
Cook, E.R., and J. Cole. 1991. On predicting the response of forests in eastern North
America to future climatic change. Climatic Change 19:271–282.
Cook, E.R., and G.C. Jacoby. 1997. Tree-ring–drought relationships in the Hudson Valley,
New York. Science 198:399–401.
Curtis, P.S., and X. Wang. 1998. A meta-analysis of elevated CO2 effects on woody-plant
mass, form, and physiology. Oecologia 113:299–313.
D’Arrigo, R.D., W.S.F. Schuster, D.M. Lawrence, E.R. Cook, M. Wiljanen, R.D. Thetford.
2001. Climate-growth relationships of Eastern Hemlock and Chestnut Oak from
Black Rock Forest in the highlands of southeastern New York. Tree-Ring Research
57:183–190.
Dawes, M.A., S. Hättenschwiler, P. Bebi, F. Hagedorn, I.T. Handa, C. Körner, and C. Rixen.
2010. Species-specific tree-growth responses to 9 years of CO2 enrichment at the alpine
treeline. Journal of Ecology 99:383–394.
DeGraaf, R.M., and D.D. Rudis. 1986. New England Wildlife: Habitat, Natural History,
and Distribution. US Department of Agriculture, Forest Service, Northeastern Forest
Experiment Station, Newtown Square, PA. 491 pp.
DeGraaf, R.M., M. Yamasaki, W.B. Leak, and J.W. Lanier. 1992. New England Wildlife:
Management of Forested Habitats. US Department of Agriculture, Forest Service,
Northeastern Forest Experiment Station, Newtown Square, PA. 271 pp.
Drake, B.G., and M.A. Gonzàlez-Meler, and S.P. Long. 1997. More-efficient plants: A
consequence of rising atmospheric CO2? Annual Review of Plant Physiology and Plant
Molecular Biology 48 609–639.
Dukes, J.S., J. Pontius, D. Orwig, J.R. Garnas, V.L.Rodgers, N. Brazee, B. Cooke, K.A.
Theoharides, E. Stange, R. Harrington, J. Ehrenfeld, J. Gurevitch, M. Lerdau, K. Stinson,
R. Wick, and M. Ayres. 2009. Responses of insect pests, pathogens, and invasive
plant species to climate change in the forests of northeastern North America: What can
we predict? Canadian Journal of Forest Research 39:231–248.
Eller, A.S.D., K.L. McGuire, and J.P. Sparks. 2011. Responses of Sugar Maple and hemlock
seedlings to elevated carbon dioxide under altered above- and belowground nitrogen
sources. Tree Physiology 00:1–11.
Southeastern Naturalist
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
710
Ellison, A.M., M.S. Banks, B.D. Clinton, E.A. Colburn, K. Elliot, C.R. Ford, D.R. Foster,
B.D. Kloeppel, J.D. Knoepp, G.M. Lovett, J. Mohan, D.A. Orwig, N.L. Rodenhouse,
W.V. Sobczak, K.A. Stinson, J.K. Stone, C.M. Swan, J. Thompson, B. Von Holle, and
J.R. Webster. 2005. Loss of foundation species: Consequences for the structure and dynamics
of forested ecosystems. Frontiers in Ecology and the Environment 3:479–486.
Elowe, K.D. 1984. Home range, movements, and habitat preferences of Black Bears (Ursus
americanus) in western Massachusetts. M.Sc. Thesis. University of Massachusetts,
Amherst, MA.
Eschtruth, A.K., N.L. Cleavitt, J.J. Battles, R.A. Evans, and T.J. Fahey. 2006. Vegetation
dynamics in declining Eastern Hemlock stands: 9 years of forest response to Hemlock
Woolly Adelgid infestation. Canadian Journal of Forest Research 36:1435–1450.
Farahat, E., H.W. Linderholm, and M.J. Lechowicz. 2016. Influence of dust deposition and
climate on the radial growth of Tsuga canadensis near its northern range-limit. European
Journal of Forest Research 135:69–76.
Foster, D.R., W.W. Oswald, E.K. Faison, E.D. Doughty, and B.C.S. Hansen. 2006. A climatic
driver for abrupt mid-Holocene vegetation dynamics and the hemlock decline in
New England. Ecology 87:2959–2966.
Gibson, D.J. 2014. Methods in Comparative Plant Ecology. Oxford University Press, UK,
320 pp.
Gifford, R.M. 1994. The global carbon cycle: A view point on the missing sink. Australian
Journal of Plant Physiology 21:1–15.
Godbold, D.L., G.M. Berntson, and F.A. Bazzaz. 1997. Growth and mycorrhizal colonization
of three North American tree species under elevated atmospheric CO2. New Phytologist
137:433–440.
Godman, R.M., and K. Lancaster. 1990. Tsuga canadensis. Pp. 604–612, In R.M. Burns
and B.H. Honkala (Eds.). Silvics of North America. Volume 1. Conifers. Agricultural
Handbook 654, US Department of Agriculture Forest Service, Washington, DC.
Gonzàlez-Meler, M.A., L. Taneva, and R.J. Trueman. 2004. Plant respiration and elevated
atmospheric CO2 concentration: Cellular respiration and global significance. Annals of
Botany 94:647–656.
Gouli, V., B.L. Parker, and M. Skinner. 2000. Haemocytes of the Hemlock Woolly Adelgid
Adelges tsugae Annand (Hom., Adelgidae) and changes after exposure to low temperatures.
Journal of Applied Entomology 124:201–206.
Hadley, J.L. 2000. Understory microclimate and photosynthetic response of saplings in an
old-growth Eastern Hemlock. Forest Ecoscience 7:66–72.
Hadley, J.L., and J.L. Shedlbauer. 2002. Carbon exchange of an old-growth Eastern Hemlock
(Tsuga canadensis) forest in central New England. Tree Physiology 22:1079–1092.
Harrison D.J., J.A. Bissonette, and J.A. Sherburne. 1989. Spatial relationships between
Coyotes and Red Foxes in eastern Maine. Journal of Wildlife Management 53:181–185.
Hart, J.L., S.L. van de Gevel, J. Sakulich, and H.D. Grissino-Mayer. 2010. Influence of
climate and disturbance on the growth of Tsuga candensis at its southern limit in eastern
North America. Trees 24:621–633.
Hättenschwiler, S., and C. Körner. 2000. Tree seedling responses to in situ CO2-enrichment
differ among species and depend on understory light availability. Global Change Biology
6:213–226.
Holmes R.T., and S.K. Robinson. 1981. Tree species preferences of foraging insectivorous
birds in a northern hardwoods forest. Oecologia 48:31–35.
Southeastern Naturalist
711
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
Howe, R.W., and M. Mossman. 1995. The significance of hemlock for breeding birds in the
western Great Lakes region. Pp. 125–139, In G. Mroz and J. Martin (Eds.). Hemlock
Ecology and Management. Department of Forestry, University of Wisconsin-Madison,
Madison, WI. 200 pp.
Irlich, U.M., J.S. Terblanche, T.M. Blackburn, and S.L. Chown. 2009. Insect rate–temperature
relationships: Environmental variation and the metabolic theory of ecology.
American Naturalist 174:819–835.
Karnosky, D.F. 2003. Impacts of elevated atmospheric CO2 on forest trees and forest ecosystems:
Knowledge gaps. Environment International 29:161–169.
Kessell, S.R. 1979. Adaptation and dimorphism in Eastern Hemlock, Tsuga canadensis (L.)
Carr. American Naturalist 113:333–350.
Kinney, K.K., and R.L. Lindroth. 1997. Responses of three deciduous tree species to atmospheric
CO2 and soil NO3-availability. Canadian Journal of Forest Resources 27:1–10.
Körner, C., R. Asshoff, O. Bignucolo, S. Hättenschwiler, S.G. Keel, S. Paláez-Riedl, S.
Pepin, R.W. Siegwolf, and G. Zotz. 2005. Carbon flux and growth in mature deciduous
forest trees exposed to elevated CO2. Science 309:1360–1362.
Kurz, W.A., C.C. Dymond, G. Stinson, G.J. Rampley, E.T. Neilson, A.L. Carroll AL, T.
Ebata, and L. Safranyik. 2008. Mountain Pine Beetle and forest-carbon feedback to
climate change. Nature 452:987–990.
Larigauderie, A., and C. Körner. 1995. Acclimation of leaf dark-respiration to temperature
in alpine and lowland plant species. Annals of Botany 76:245–252.
Logan, J.A., J. Régnière, and J. Powell. 2003. Assessing the impacts of global warming on
forest-pest dynamics. Frontiers in Ecology and the Environment 1:130–137.
Marsicek, J.P., B. Shuman, S. Brewer, D.R. Foster, and W.W. Oswald. 2013. Moisture and
temperature changes associated with the mid-Holocene Tsuga decline in the northeastern
United States. Quaternary Science Reviews 80:129–142.
Martin, N.D. 1960. An analysis of bird populations in relation to forest succession in Algonquin
Provincial Park, Ontario. Ecology 41:126–140.
McClure, M.S. 1990. Role of wind, birds, deer, and humans in the dispersal of Hemlock
Woolly Adelgid (Homoptera: Adelgidae). Environmental Entomology 19:36–43.
McClure, M.S. 1991. Density-dependent feedback and population cycles in Adelges tsugae
(Homoptera: Adelgidae) on Tsuga canadensis. Environmental Entomology 20:258–264.
McWilliams, W.H., and T.L. Schmidt. 2000. Composition, structure, and sustainability
of hemlock ecosystems in eastern North America. Pp. 5–10, In K.A. McManus, K.S.
Shields, and D.R. Souto (Eds). Proceedings: Symposium on Sustainable Management of
Hemlock Ecosystems in Eastern North America. General Technical Report NE-267, US
Department of Agriculture, Forest Service, Northeastern Experiment Station, Newtown
Square, PA.
Mooney, H.A. 1972. The carbon balance of plants. Annual Reviews of Ecology and Systematics
3:315–346.
Morin, R.S., A.M. Liebhold, and K.W. Gottschalk. 2009. Anisotropic spread of Hemlock
Woolly Adelgid in the eastern United States. Biological Invasions 11:2341–2350.
Nelson, L.A., D.N. Dillaway, and L.K. Rieske. 2014. Effect of an exotic herbivore, Adelges
tsugae, on photosynthesis of a highly susceptible Tsuga host, with notes on conspecifics.
Arthropod–Plant Interactions 8:9–15.
Norby, R.J., S.D. Wullschleger, C.A. Gunderson, D.W. Johnson, and R. Ceulemans. 1999.
Tree responses to rising CO2 in field experiments: Implications for the future forest.
New Phytologist 22:683–714.
Southeastern Naturalist
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
712
Orwig, D.A., and D.R. Foster. 1998. Forest response to the introduced Hemlock Woolly
Adelgid in southern New England, USA. Journal of the Torrey Botanical Society
125:60–73.
Orwig, D.A., D.R. Foster, and D.L. Mausel. 2002. Landscape patterns of hemlock decline in
New England due to the introduced Hemlock Woolly Adelgid. Journal of Biogeography
29:1475–1487.
Oswald, W.W., and D.R. Foster. 2012. Middle-Holocene dynamics of Tsuga canadensis
(Eastern Hemlock) in northern New England, USA. The Holocene 22:71–78.
Paradis, A., J. Elkinton, K. Hayhoe, and J. Buanoaccorsi. 2008. Role of winter temperature
and climate change on the survival and future range expansion of the Hemlock Woolly
Adelgid (Adelges tsugae) in Eastern North America. Mitigation and Adaptation Strategies
for Global Change 13:541–554.
Peñuelas, J., C. Gordon, L. Llorens, T. Nielson, A. Tietema, C. Beier, P. Bruna, B. Emmett,
M. Estiarte, and A. Gorissen. 2004. Nonintrustive field experiments to show different
plant responses to warming and drought among sites, seasons, and species in a north–
south European gradient. Ecosystems 7:598–612.
Poorter, H.A., J. Bühler, D. van Dusschoten, J. Climent, and J.A. Postma. 2012. Pot size
matters: A meta-analysis of the effects of rooting volume on plant growth. Functional
Plant Biology 39:839–850.
Potter, K.M., R.M. Jetton, W.S. Dvorak, V.D. Hipkins, R. Rhea, and W.A. Whittier. 2012.
Widespread inbreeding and unexpected geographic patterns of genetic variation in Eastern
Hemlock (Tsuga canadensis), an imperiled North American conifer. Conservation
Genetics 13:475–498.
Prioul, J.L., and P. Chartier. 1977. Partitioning of transfer and carboxylation components of
intracellular resistance to photosynthetic CO2 fixation. A critical analysis of the methods
used. Annals of Botany 41:789–800.
Reich, P.B., K.M. Sendall, K. Rice, R.L. Rich, A. Stefanski, S.E. Hobbie, and R.A. Montgomery.
2015. Geographic range predicts photosynthetic and growth responses to warming
in co-occurring tree species. Nature Climate Change 5:148–152.
Robinson, E.A., G.D. Ryan, and J.A. Newman. 2012. A meta-analytical review of the effects
of elevated CO2 on plant–arthropod interactions highlights the importance of interacting
environmental and biological variables. New Phytologist 194:321–336.
Saladyga, T., and R.S. Maxwell. 2015. Temporal variability in climate response of Eastern
Hemlock in the Central Appalachian Region. Southeastern Geographer 55:143–163.
Saxe, H., D.S. Ellsworth, and J. Heath. 1998. Tree and forest functioning in an enriched CO2
atmosphere. New Phytologist 139:395–436.
Skinner, M., B.L. Parker, S. Gouli, and T. Ashikaga. 2003. Regional responses of Hemlock
Woolly Adelgid (Homoptera: Adelgidae) to low temperatures. Environmental Entomology
32:523–28.
Stiling, P., and T. Cornelissen. 2007. How does elevated carbon dioxide (CO2) affect plant–
herbivore interactions? A field experiment and meta-analysis of CO2-mediated changes
on plant chemistry and herbivore performance. Global Change Biology 13:1823–1842.
Stocker, T.F., D. Qin, G.K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y.
Xia, V. Bex, and P.M. Midgley (Eds.). 2013. The Physical Science Basis. Contribution
of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change. Cambridge University Press, Cambridge, UK.
Southeastern Naturalist
713
L. Wilder and J.N. Boyd
2016 Vol. 15, No. 4
Strickland, M.A., and C.W. Douglas, 1987. Marten. Pp. 531–546, In M.J. Novak, A. Baker,
M.E. Obbard, and R. Malloch (Eds.). Wild Furbearer Management and Conservation in
North America. Ontario Ministry of Natural Resources, Toronto, ON, Canada.
Tjoelker, M.G., J. Oleksyn, and P.B. Reich. 1998. Seedlings of five boreal tree species differ
in acclimation of net photosynthesis to elevated CO2 and temperature. Tree Physiology
18:715–726.
Wullschleger, S.D., L.H. Ziska, and J.A. Bunce. 1994. Respiratory responses of higher
plants to atmospheric CO2 enrichment. Physiologia Plantarum 90:221–229.
Young, R.F., K.S. Shields, and G.P. Berlyn. 1995. Hemlock Woolly Adelgid (Homoptera:
Adelgidae): Stylet-bundle insertion and feeding sites. Annals of the Entomological Society
of America 88:827–835.
Ziska, L.H., and J.A. Bunce. 1998. The influence of increasing growth temperature and CO2
concentration on the ratio of respiration to photosynthesis in soybean seedlings. Global
Change Biology 4:637–643.