Directory UMM :Data Elmu:jurnal:T:Tree Physiology:Vol16.1996:

Tree Physiology 16, 537--546
© 1996 Heron Publishing ----Victoria, Canada

Effects of carbon dioxide, fertilization, and irrigation on
photosynthetic capacity of loblolly pine trees
RAMESH MURTHY,1 PHILLIP M. DOUGHERTY,2,4 STANLEY J. ZARNOCH3 and
H. LEE ALLEN1
1

Department of Forestry, North Carolina State University, Raleigh, North Carolina 27695, USA

2

Westvaco, P.O. Box 1950, Summerville, South Carolina 29484, USA

3

USDA Forest Service, P.O. Box 2680, Ashville, North Carolina 28802, USA

4

Author to whom correspondence should be addressed

Received April 27, 1995

Summary Branches of nine-year-old loblolly pine trees
grown in a 2 × 2 factorial combination of fertilization and
irrigation were exposed for 11 months to ambient, ambient +
175, or ambient + 350 µmol mol −1 CO2. Rates of light-saturated
net photosynthesis (Amax ), maximum stomatal conductance to
water vapor (gmax ), and foliar nitrogen concentration (% dry
mass) were assessed monthly from April 1993 until September
1993 on 1992 foliage (one-year-old) and from July 1993 to
March 1994 on 1993 foliage (current-year).
Rates of Amax of foliage in the ambient + 175 CO2 treatment
and ambient + 350 were 32--47 and 83--91% greater, respectively, than that of foliage in the ambient CO2 treatment. There
was a statistically significant interaction between CO2 treatment and fertilization or irrigation treatment on Amax on only
one measurement date for each age class of foliage. Light-saturated stomatal conductance to water vapor (gmax ) was significantly affected by CO2 treatment on only four measurement
dates. Light-saturated gmax in winter was only 42% of summer
gmax even though soil water during winter was near field
capacity and evaporative demand was low. Fertilization increased foliar N concentration by 30% over the study period

when averaged across CO2 treatments. During the study period, the ambient + 350 CO2 treatment decreased average foliar
N concentration of one-year-old foliage in the control, irrigated, fertilized and irrigated + fertized plots by 5, 6.4, 9.6 and
11%, respectively, compared with one-year-old foliage in the
corresponding ambient CO2 treatments. The percent increase
in Amax due to CO2 enrichment was similar in all irrigation and
fertilization treatments and the effect persisted throughout the
11-month study period for both one-year-old and current-year
foliage.
Keywords: elevated CO 2 , foliar N concentration, loblolly pine,
net assimilation, Pinus taeda, stomatal conductance .

Introduction
Mean global concentration of atmospheric CO2 is expected to

reach 700 µmol −1 by the middle of the next century (Conway
et al. 1988). The extent that terrestrial ecosystems act as carbon
sinks to buffer the increase in atmospheric CO2 concentration
is uncertain (Tans et al. 1990). Forests, which cover over
one-third of the Earth’s land area (Kramer 1981), are currently
estimated to account for approximately 70% of the annual

terrestrial atmospheric carbon exchange (Waring and Schlesinger 1985). Thus, a detailed knowledge of the relationship
between net photosynthesis and increasing atmospheric CO2
concentration is required to estimate the potential of forests for
reducing rising atmospheric CO2 concentrations.
Studies on the effects of elevated CO2 on net photosynthesis
have produced conflicting results. Short-term studies of the
effects of elevated CO2 on crop species, horticultural plants,
and tree seedlings have shown that the rate of photosynthetic
assimilation increases by 20--200% when atmospheric CO2
concentration is doubled (Higginbotham et al. 1985, Cure and
Acock 1986, Eamus and Jarvis 1989, Drake and Leadley 1991,
Idso and Kimball 1991, Stewart and Hoddinott 1993, Lee et al.
1993, Gunderson et al. 1993, El Kohen and Mousseau 1994).
In contrast, long-term studies have shown that there is a decrease (DeLucia et al. 1985, Peet et al. 1986) or no increase in
rates of net photosynthesis with increasing duration of exposure to elevated CO2 concentration (Gaudillere and Mousseau
1989, Grulke et al. 1993). In some species, the magnitude of
the photosynthetic response to elevated CO2 increases with
increasing nutrient availability (Conroy et al. 1986a, 1986b,
Tissue et al. 1993), however, a positive photosynthetic response to elevated CO2 has also been observed in nitrogen-deficient trees of some species (Norby et al. 1986, Norby and
O’Neill 1989, Gunderson et al. 1993). In an attempt to explain

these conflicting reports, we have undertaken a long-term
study to assess the photosynthetic potential of mature forest
trees to increasing availability of CO2, nutrients and water.
Long-term studies to evaluate the response of mature trees
to elevated CO2 have been limited by difficulty in accessibility
and the high cost of exposing large trees to CO2 treatments.
The use of branch chambers (Teskey et al. 1991, Barton et al.

538

MURTHY ET AL.

1993) provides a suitable alternative for studying the effects of
CO2 concentration on physiological processes of trees. Although branch chambers do not permit an evaluation of the
possibility of feedback inhibition, they do permit an evaluation
of the physiological responses to CO2 over long time periods.
Limitations of the use of branch chambers in gas exposure
studies has been described in detail by Teskey and coworkers
(Teskey et al. 1991, Teskey 1995, Liu and Teskey 1995).
The objective of this study was to characterize the magnitude and duration of Amax , gmax , and foliar N response of two

age classes of foliage of nine-year-old loblolly pine trees to
CO2, water, and nutrient availability. Down-regulation of photosynthesis was not evaluated.
Materials and methods
Study site characteristics
The study site is located on the Carolina Sandhills, Scotland
County, NC, USA (34°55′ N, 79°30′ W). Annual temperature
averages 17 °C, and air temperatures in the summer (June-September) and winter (December--March) average 26 and
9 °C, respectively. Average annual rainfall is 121 cm and
periods of drought often occur in late summer and early
autumn.
The soil is a deep sand belonging to the Wakulla series,
sandy, siliceous, thermic psammentic hapludult (USDA Soil
Classification System). The soil is well drained and available
water-holding capacity in the upper 2 m of the soil profile is
18--20 cm. Pretreatment foliage N concentration in the dormant season (December--February) was 0.98% dry mass,
which is less than the critical concentration of 1.15% dry mass
established for loblolly pine (Allen 1987). The critical N concentration is based on the N concentration below which there
is a large enough growth response to fertilization to make it
economically viable to fertilize a stand.
The site was planted in March 1985 at a spacing of 2.4 ×

2.4 m with a 10 family, half-sib mix of North Carolina piedmont seedlings. In 1992, the stand was thinned to 1260 trees
per hectare. Average height and bole diameter at 1.3 m height
after thinning were 3.4 m and 4.6 cm, respectively. Associated
understory vegetation (hardwoods and grasses) was controlled
with glyphosate applied at a rate of 1.5% by volume.
Study design
The study was a split-plot design with a 2 × 2 factorial combination of fertilization and irrigation. The four whole-plot treatment combinations (control (C); irrigation only (I);
fertilization only (F); and fertilization + irrigation (FI)) were
randomly assigned to one of four treatment plots in each of
four blocks. Treatment plots were 50 × 50 m with interior
measurement plots of 30 × 30 m. Fertilization treatment consisted of an initial application of nitrogen (200 kg ha −1),
phosphorus (50 kg ha −1), and potassium (100 kg ha −1) in
March 1992. This was followed by an application of calcium
(120 kg ha −1), magnesium (50 kg ha −1) and boron (1.5 kg ha −1)
in the April--June period of 1992. Nitrogen, P, K and Mg (23,
20, 19 and 0.16 kg ha −1, respectively) were applied in April

1993, followed by K, S, and N (82, 107 and 50 kg ha −1,
respectively) in June--August 1993.
Irrigation was begun in May 1993 and continued until November 1993. Available soil water was calculated as: ((observed available soil water)/(maximum total available soil

water))100. The target of the irrigation treatment was to irrigate to field capacity when 40% of maximum total available
soil water was depleted in the upper 50 cm of the soil profile.
Water was applied by a head sprinkler system with nozzles
located below the canopy. Soil water of the irrigated plots was
measured by time domain reflectrometry (TDR) every two
days during the growing season to determine the need for
irrigation. In addition, soil water content at depths of 10, 25,
and 50 cm was measured every two weeks in all plots.
Subplot treatments were ambient, ambient + 175 and ambient + 350 µmol mol −1 CO2. The CO2 treatments were randomly
assigned to a single tree in each plot (16 trees in total) using
the branch chamber technology developed by Teskey et al.
(1991). Three branches were selected from the mid crown
(1989 or 1990 whorl) of each tree and exposed to ambient,
ambient + 175, or ambient + 350 µmol mol −1 CO2. Exposures
started in March 1993 and continued throughout the study
period for 24 h daily. A computer-based control system was
used to measure and control CO2 concentrations in each of the
48 branch chambers. The computer was linked to a data logger
process controller (Keithley 500A, Keithley Inc., Data Systems, OH) that regulated the opening and closing of solenoids
to switch sample air coming from the chambers to an infrared

gas analyzer (LI 6262, Li-Cor, Inc., Lincoln, NE) that sequentially measured the CO2 and water vapor concentrations of the
sample air stream in each of the exposure chambers. All 48
chambers were cycled through within 30 min. Carbon dioxide
was dispensed at a fixed addition rate to ambient CO2 irrespective of the ambient CO2 concentration. Blowers, regulated to
achieve a minimum of six exchanges of chamber air per minute, were used to mix CO2 with ambient air and deliver the
mixture through each exposure chamber. Temperature within
the chamber was not controlled. A photosynthetic photon flux
density (PPFD) sensor (G1118, Hamamatsu Corp., Bridgewater, NJ) and a copper-constantan thermocouple located within
each chamber were used to measure PPFD and chamber air
temperature, respectively. Output from all PPFD and temperature sensors was measured every 6 s, averaged over each hour
and recorded by four data loggers (CR-7, Campbell Scientific
Inc., Logan, UT). Ambient weather conditions were monitored
from an on-site weather station.
Plant measurements
Light-saturated net photosynthesis (Amax ) and maximum stomatal conductance to water vapor (gmax ) were measured once
per month on current-year (1993) and one-year-old foliage
(1992) on all 48 branches exposed to CO2 treatments. A portable infrared gas analyzer (ADC-LCA3, Analytical Development Corporation, Hoddesdson, U.K.) equipped with a
Parkinson leaf chamber (PLC-3) was used for all Amax and
gmax measurements. The ADC unit was calibrated daily before
and after measurements using primary standard calibration gas

PHOTOSYNTHETIC CAPACITY OF LOBLOLLY PINE

539

Figure 1a--d. Average weekly (a)
chamber temperature, (b) photosynthetic photon flux density, (c)
carbon dioxide concentration
during the photoperiod (average
from 48 chambers), and (d) percent available soil water in the
upper 50 cm of the soil profile
determined for control (circles),
irrigated (triangles), fertilized
(squares) and fertilized + irrigated (diamond) plots, respectively, for the 11-month study
period.

of CO2 in air. Monthly checks of ADC humidity sensors were
also made. All Amax and gmax measurements were made at the
CO2 concentration to which the branches were exposed in the
branch chambers. Measurements of Amax and gmax were taken

in the early morning (0300--0900 h) to minimize water stress
and to maintain cuvette temperatures between 25 and 30 °C in
summer and between 15 and 20 °C in winter and early spring
(cf. Strain et al. 1976). Three fascicles (nine needles) from the
first flush of two age classes of foliage (1992 and 1993) were
enclosed in the cuvette for each gas exchange measurement.
All Amax and gmax measurements were made under light-saturating conditions (> 1600 µmol m −2 s −1) at a relative humidity
of approximately 40%. Humidity was regulated by columns of
‘‘Drierite’’ and FeSO4.7H2O attached to the ADC unit. Vapor
pressure deficit at a relative humidity of 40% ranged from 1.02
to 1.404 kPa at 15--20 °C and from 1.902 to 2.544 kPa at
25--30 °C. To ensure that the foliage was equilibrated to a high
PPFD when gas exchange measurements were made, branches
were first exposed for 45--60 min to artifical light provided by
tungsten-halogen lamps (Osram Corp., NY, USA) mounted
within each exposure chamber. A similar lamp was mounted
permanently on the ADC leaf cuvette and used for all gas
exchange measurements. After measurements of Amax and
gmax were completed, the needles were destructively sampled
for leaf area and foliar N concentrations. Leaf area was calculated from average needle radius and needle length (Fites and

Teskey 1988). Each set of nine needles was then oven dried at
65 °C and analyzed for nitrogen (N) by the combustion method
with an N auto analyzer (Carlo-Erba NA 1500, Carlo-Erba
Strumentatzione, Milan, Italy). Foliar N concentrations were
expressed as percentages of dry mass.
Gas exchange measurements on one-year-old foliage
(formed in 1992) were taken from April to September 1993.

Needles began senescing in September and by October had
either senesced or were too fragile for additional gas exchange
measurements. Gas exchange measurements on current-year
foliage were begun in July, after the needles had achieved
about 70% of their full growth and continued until March
1994. Measurements on current-year foliage were not made in
November 1993 because of instrument malfunction.
Statistical analysis
Gas exchange and foliar N concentration data were analyzed
according to standard split-plot methodology on a monthly
basis (Steel and Torrie 1980). The Statistical Analysis System
(SAS Institute, Cary, NC) software was used for all statistical
analyses. All analyses were conducted at the 0.05 probability
level.
Results
Environmental characteristics
Average daytime temperature within the branch chambers
ranged from 5.47 to 34.67 °C during the study (Figure 1a).
Chamber temperatures were 1, 2, 3, 4, and 5 °C above ambient
temperature for 44, 33, 13, 3.8 and 0.5% of all measurements
taken. Chamber temperatures were 1 °C below ambient for
only 0.5% of the total readings. During the daytime, daily
average PPFD within the branch chambers was 505 µmol m −2
s −1. Branch chambers located at mid crown received an average of 50% of the PPFD measured above the canopy (Figure
1b). During the daytime, average daily CO2 concentrations for
the three CO2 treatments were 385, 555, and 731 µmol mol −1
with standard deviations of 14.2, 27.2, and 41.3, respectively
(Figure 1c).

540

MURTHY ET AL.

Irrigation maintained available soil water in the upper 50 cm
of the soil profile of the irrigated plots above 60% of the
maximum available soil water (Figure 1d). Available soil water
in the upper 50 cm of the profile of the non-irrigated plots
decreased to less than 50% of the maximum available soil
water during June and July. In almost all cases, the plots treated
with fertilizer only (F) exhibited the largest depletions in
available soil water.
Stomatal conductance
One-year-old foliage Average light-saturated stomatal conductance (gmax ) of one-year-old foliage was 0.07 mol m −2 s −1
in April and increased to 0.14 mol m −2 s −1 by May (Figure 2).
Depressions in gmax occurred in June, coinciding with low
available soil water. Mean gmax values over the study period
were 0.12, 0.13, and 0.12 mol m −2 s −1 for the ambient, ambient
+ 175 and ambient + 350 CO2 treatments, respectively, and
0.13, 0.13, 0.09, and 0.12 mol m −2 s −1 for the C, I, F, and FI
treatments, respectively.
The CO2 treatments significantly affected gmax only in July
and August, whereas there was a significant fertilizer effect
and a significant F × I interaction in September. Irrigation did
not significantly affect gmax on any measurement date. None of
the other two- or three-way interactions were significant (Table 1).

Figure 2. Trends in mean g max for one-year-old foliage for control
(circles), irrigated (triangles), fertilized (squares) and fertilized + irrigated (diamonds) plots. Each point is an average of four block measurements.

Current-year foliage Light-saturated stomatal conductance
(gmax ) of current-year foliage increased with foliage age, reaching an average maximum of 0.22 mol m −2 s −1 in October 1993
when foliage was fully developed (Figure 3). Light-saturated
gmax decreased with the onset of winter to a minimum of 0.04
mol m −2 s −1 in February 1994 (Figure 3). Neither maximum
nor minimum gmax observed for current-year foliage appeared
to coincide with soil water status. Fertilization and irrigation
significantly affected gmax in July and CO2 significantly af-

Table 1. Monthly split-plot analysis (P-values) for stomatal conductance (g max ) measurements of one-year-old and current-year foliage.
Source

df

April

May

June

July

Aug

Sept

One-year-old foliage
Fert (F)
Irr (I)
F×I
CO2
F × CO2
I × CO2
F × I × CO2

1
1
1
2
2
2
2

0.087
0.467
0.089
0.060
0.817
0.698
0.229

0.318
0.121
0.513
0.059
0.817
0.698
0.229

0.065
0.063
0.983
0.884
0.094
0.737
0.098

0.086
0.905
0.609
0.021
0.734
0.493
0.490

0.692
0.946
0.692
0.022
0.624
0.543
0.408

0.028
0.202
0.050
0.636
0.256
0.607
0.318

Source

df

July

Aug

Sept

Oct

Dec

Jan

Feb

Current-year foliage
Fert (F)
Irr (I)
F×I
CO2
F × CO2
I × CO2
F × I × CO2

1
1
1
2
2
2
2

0.053
0.013
0.497
0.579
0.821
0.904
0.655

0.327
0.785
0.083
0.789
0.223
0.382
0.652

0.523
0.521
0.547
0.255
0.562
0.804
0.490

0.275
0.457
0.602
0.038
0.176
0.394
0.924

0.854
0.851
0.811
0.410
0.928
0.079
0.094

0.329
0.297
0.903
0.351
0.411
0.931
0.829

0.371
0.231
0.635
0.339
0.486
0.304
0.957

PHOTOSYNTHETIC CAPACITY OF LOBLOLLY PINE

Figure 3. Trends in mean g max for current-year foliage for control
(circles), irrigated (triangles), fertilized (squares) and fertilized + irrigated (diamonds) plots. Each point is an average of four block measurements.

fected gmax in October. None of the two- or three-way interactions were significant (Table 1).

541

Figure 4. Trends in mean Amax for one-year-old foliage for control
(circles), irrigated (triangles), fertilized (squares) and fertilized + irrigated (diamonds) plots. Each point is an average of four block measurements.

Light-saturated net photosynthesis

significant two- or three-way interactions were detected for
fertilizer, irrigation, or CO2 except for an I × CO2 interaction
in April (Table 2).

One-year-old foliage In all treatment combinations, Amax of
one-year-old foliage increased to a seasonal maximum in May
and then declined more than 40% by September just before
senescence (Figure 4).
Fertilization significantly increased Amax in one-year-old
foliage in April, May, June, and August. Over the four months,
Amax averaged 7.6 µmol m −2 s −1 for the fertilized plots and 6.8
µmol m −2 s −1 for the unfertilized plots. Irrigation significantly
increased mean Amax in April, May, and June. Over the three
months, Amax averaged 8.5 µmol m −2 s −1 for the irrigated plots
and 7.25 µmol m −2 s −1 for the unirrigated plots. Mean Amax was
significantly different for each of the CO2 treatments for all
months. Irrespective of the fertilizer and irrigation treatments,
Amax in the + 350 CO2 treatment was 83 to 91% greater than in
the ambient CO2 treatment. Average Amax values for the study
period were 9.2, 7.1 and 4.8 µmol m −2 s −1 for the + 350 CO2,
+ 175 CO2, and ambient CO2 treatments, respectively. No

Current-year foliage In current-year developing foliage,
Amax peaked in July, when foliage was 70% of its final length
(Figure 5). Light-saturated net photosynthesis remained high
throughout the summer, declined during the winter, and increased again in the late winter--early spring period following
a trend similar to that exhibited by gmax for current-year foliage
(Figure 3).
Fertilization significantly increased mean Amax in August,
September, February, and March (Table 2). Over these four
months, Amax averaged 6.6 µmol m −2 s −1 for the fertilized plots
and 5.5 µmol m −2 s −1 for the unfertilized plots, respectively.
Irrigation significantly increased Amax in July, August and
September, a period when soil water was low. Over these three
months, Amax averaged 7.84 µmol m −2 s −1 for the irrigated plots
and 6.93 µmol m −2 s −1 for the unirrigated plots. The elevated
CO2 treatments had a significant effect on Amax throughout the
study period. Averaged across the fertilizer and irrigation treat-

542

MURTHY ET AL.

Table 2. Monthly split-plot analysis (P-values) for A max measurements of one-year-old and current-year foliage.
Source

df

April

May

June

July

Aug

Sept.

One-year-old foliage
Fert (F)
Irr (I)
F×I
CO2
F × CO2
I × CO2
F × I × CO2

1
1
1
2
2
2
2

0.009
0.001
0.361
0.001
0.508
0.045
0.093

0.001
0.031
0.317
0.001
0.439
0.302
0.669

0.048
0.001
0.685
0.001
0.218
0.289
0.283

0.230
0.068
0.989
0.001
0.795
0.617
0.349

0.031
0.395
0.429
0.001
0.517
0.873
0.442

0.189
0.653
0.158
0.001
0.093
0.817
0.898

Source

df

July

Aug

Sept

Oct

Dec

Jan

Feb

Mar

Current-year foliage
Fert (F)
Irr (I)
F×I
CO2
F × CO2
I × CO2
F × I × CO2

1
1
1
2
2
2
2

0.243
0.005
0.792
0.001
0.094
0.887
0.808

0.002
0.024
0.282
0.001
0.290
0.031
0.147

0.002
0.039
0.986
0.001
0.653
0.933
0.307

0.235
0.566
0.497
0.002
0.822
0.857
0.273

0.274
0.431
0.609
0.001
0.498
0.223
0.064

0.435
0.497
0.625
0.001
0.475
0.351
0.604

0.003
0.188
0.154
0.001
0.114
0.068
0.833

0.002
0.879
0.258
0.001
0.454
0.081
0.546

ments, Amax values were 10.6, 7.7, and 6.6 µmol m −2 s −1, for
foliage exposed to + 350, + 175 and ambient concentrations of
CO2, respectively. The effect of elevated CO2 on Amax persisted
even when mean Amax reached a minimum in January 1994.
Averaged over the irrigation and fertilizer treatments in January, Amax values were 3.72, 2.75 and 1.51 µmol m −2 s −1 in the
+ 350 CO2, + 175 CO2, and ambient CO2 treatments, respectively. There were no significant interactions between CO2 and
fertilizer or irrigation, except for an I × CO2 interaction in
August (Table 2).
Foliar N concentration

Figure 5. Trends in mean Amax for current-year foliage for control
(circles), irrigated (triangles), fertilized (squares) and fertilized + irrigated (diamonds) plots. Each point is an average of four block measurements.

One-year-old foliage Over the study period, fertilizer increased foliar N concentrations of one-year-old foliage by
25--34% in all three CO2 treatments (Figure 6). Foliar N
concentrations steadily decreased from May to September
1993, coinciding with a decrease in Amax (Figure 4).
Fertilization significantly increased foliar N concentration
for all months, whereas there was no significant effect of
irrigation on foliar N (Table 3). Foliar N concentration averaged over the study period and CO2 treatments was 1.03% for
the fertilized plot and 0.77% for the unfertilized plots. Foliar
N concentration of foliage growing in the + 350 CO2 treatment
was significantly lower than that of foliage growing in the
ambient CO2 treatment for all months except September, immediately before abscission. Foliar N concentrations averaged
over the study period and across the C, I, F and FI treatments,
were 0.85 and 0.93% for the + 350 CO2 and ambient CO2
treatments, respectively. There were no significant two- or
three-way interactions, except for an F × I interaction in May,
an F × CO2 interaction in June, and an F × I × CO2 interaction

PHOTOSYNTHETIC CAPACITY OF LOBLOLLY PINE

Figure 6. Trends in mean foliar N concentration (% dry mass basis) for
one-year-old foliage for control (circles), irrigated (triangles), fertilized (squares) and fertilized + irrigated (diamonds) plots. Each point
is an average of four block measurements.

543

Figure 7. Trends in mean foliar N concentration (% dry mass) for
current-year foliage for control (circles), irrigated (triangles), fertilized (squares) and fertilized + irrigated (diamonds) plots. Each point
is an average of four block measurements.

Table 3. Monthly split-plot analysis (P-values) for foliar N concentrations of one-year-old and current-year foliage.
Source

df

April

May

June

July

Aug

Sept

One-year-old foliage
Fert (F)
Irr (I)
F×I
CO2
F × CO2
I × CO2
F × I × CO2

1
1
1
2
2
2
2

--------

0.001
0.992
0.047
0.001
0.228
0.198
0.768

0.003
0.382
0.315
0.001
0.006
0.858
0.663

0.003
0.619
0.575
0.002
0.212
0.763
0.333

0.001
0.478
0.506
0.002
0.129
0.596
0.048

0.006
0.861
0.448
0.172
0.120
0.889
0.696

Source

df

July

Aug

Sept

Oct

Dec

Jan

Feb

Mar

Current-year foliage
Fert (F)
Irr(I)
F×I
CO2
F × CO2
I × CO2
F × I × CO2

1
1
1
2
2
2
2

0.003
0.381
0.273
0.001
0.168
0.772
0.115

0.002
0.733
0.924
0.205
0.453
0.053
0.337

0.006
0.634
0.293
0.001
0.001
0.014
0.528

0.037
0.562
0.739
0.101
0.063
0.526
0.870

0.002
0.099
0.843
0.677
0.768
0.829
0.944

0.001
0.983
0.272
0.006
0.096
0.174
0.578

0.001
0.668
0.878
0.001
0.017
0.043
0.392

0.001
0.738
0.446
0.001
0.070
0.952
0.908

544

MURTHY ET AL.

in August (Table 3).
Current-year foliage Nitrogen concentration of current-year
foliage reached a maximum by July (Figure 7). Fertilization
significantly increased foliar N concentrations (1.20 versus
0.95%) for all months, whereas irrigation had no significant
effect on foliar N (Table 3). Carbon dioxide treatments significantly decreased foliar N concentration in all months except
August, October, and December. Foliage in the ambient CO2
treatment had the highest mean N concentration of 1.12%
followed by foliage in the + 175 (1.05%) and + 350 CO2
(1.03%) treatments, respectively. There were significant F ×
CO2 interactions in September and February, and significant I
× CO2 interactions in August, September and February. No
significant three-way interaction was observed (Table 3).

Discussion
When expressed as a percentage of Amax of foliage grown in
ambient CO2, Amax of foliage grown in the + 175 or + 350 CO2
treatment showed similar increases irrespective of whether the
trees were grown in a fertilized (F), irrigated (I), fertilized +
irrigated (FI) or control (C) plot. Thus, an increase in CO2
concentration from ambient to + 350 µmol mol −1 resulted in
an average increase in Amax of 93, 85, 91 and 83%, respectively
for trees in the C, I, F and FI plots, respectively. Positive
responses of Amax to elevated CO2 have been reported in nitrogen-deficient trees (Norby and O’Neill 1989, Gunderson et al.
1993, Teskey 1995) and in water-stressed trees (Huber et al.
1984, Johnsen 1993). In addition, many other studies in which
nutrients or water were not enhanced report increases in net
photosynthesis in response to elevated CO2 (e.g., Norby et al.
1986 in hardwood seedlings, Fetcher et al. 1988, Stewart and
Hoddinott 1993, Lee et al. 1993 in various conifers, Idso and
Kimball 1992 in sour orange trees and Downton et al. 1987 in
Valencia orange). However, in other studies it has been shown
that the increase in Amax in response to elevated CO2 is dependent on nutrient availability (Conroy et al. 1986a, Cure et al.
1988, Tissue et al. 1993, Thomas et al. 1994), or water availability (Miao et al. 1992).
Fertilization alone was responsible for a 20 and 24% increase in Amax of foliage grown in the + 350 CO2 and ambient
CO2 treatments, respectively. This increase is comparable to
reports of a 15--20% increase in photosynthetic capacity of
Pinus sylvestris foliage in response to irrigation and fertilization (Linder and Axelsson 1982, Zhang 1993).
In addition to increasing Amax , fertilization also increases
stand leaf area. In the same study that our Amax measurements
were made in, Allen et al. (1996) determined that fertilization
increased leaf area index (LAI) by 78% over that of the control
plots. The combined effects of fertilizer on LAI and Amax
determine the total effect of fertilization on stand carbon gain.
Zhang (1993) reported that the increases in LAI and photosynthetic rate accounted for 70 and 30%, respectively, of the
increase in carbon gain due to fertilization. Because the effects
of fertilization and elevated CO2 on Amax are additive, the
elevated-CO2-induced increase in whole-tree or stand carbon
gain potential should be greatly enhanced in a high nitrogen

addition regime.
Irrigation caused only a 5% increase in Amax in both the +
350 and ambient CO2 treatments. This result does not reflect
the extent that water may be limiting daily or annual carbon
gain. It is only an estimate of the effect of water on limiting
potential Amax . We conclude that, if the amounts of presently
available nutrients and water at the site do not change, the
response of Amax to a doubling of CO2 will greatly surpass the
response of Amax to either fertilization or irrigation; both in the
magnitude of the Amax response and the consistency of the
response. Twice ambient CO2 produced a significant Amax
response every month, whereas fertilizer and irrigation did so
only in certain months. Furthermore, even though Amax and
foliar N concentration of one-year-old foliage gradually declined over the study period for all treatment combinations, the
enhancing effect of elevated CO2 on Amax of loblolly pine was
maintained over the entire lifespan of the foliage. Similar
results were obtained for yellow-poplar and white oak seedlings (Gunderson et al. (1993).
We did not observe a consistent decrease in gmax in the
elevated CO2 treatments or in the F, I and FI treatments. Similar
findings have been reported by Gunderson et al. (1993), Samuelson and Seiler (1994), and Teskey (1995), but contrasting
findings have been reported by DeLucia et al. (1985), Surano
et al. (1986) and Fetcher et al. (1988). We obtained preliminary
evidence that the value of gmax in winter may be important in
determining the potential response to elevated CO2. Maximum
gmax in winter was only 42% of gmax observed in the summer
months, even though the winter measurements were made
under light-saturated conditions at a chamber temperature of
15 to 20 °C and soil was near field capacity and evaporative
demand was low. DeLucia (1986) has shown that stomatal
conductance in Engelmann spruce seedlings declines with
decreasing soil temperature. Results reported by Brissette and
Chambers (1992) for shortleaf pine also indicate decreases in
root growth, xylem pressure potential and gmax as water temperature is reduced from 20 to 15 °C. During the winter period
when gmax was low, Amax was at a minimum; however, the
relative increase in Amax of current-year foliage in the elevated
CO2 treatment over that in the ambient CO2 treatment was
maintained despite the 58% decrease in gmax from summer
(July) to winter (March). These results suggest that elevated
CO2 has the potential to increase winter season carbon gain of
loblolly pine. In addition, if low soil temperatures are responsible for the observed low values of gmax in winter, an increase
in winter temperatures may enhance winter gas exchange
capacity of loblolly pine. The extent to which winter carbon
gain in conifers is increased will be an important factor in
determining the success of conifers in competing with hardwoods, which have inherently higher Amax but shorter leaf area
duration.
We have drawn five conclusions from the study. (1) Both
one-year-old and current-year foliage have the potential to
increase Amax in response to increased CO2 concentrations. (2)
In both age classes of foliage, the percent increase in Amax due
to increased CO2 concentration persists in a similar magnitude,
(a) over the life of the foliage, (b) over a wide range of nutrient

PHOTOSYNTHETIC CAPACITY OF LOBLOLLY PINE

and water availabilities, and (c) even when gmax is low in
winter. (3) In both age classes of foliage, the addition of
fertilizer increases Amax and the effects of elevated CO2 and
fertilization on Amax are additive. (4) In both current-year and
one-year-old foliage, enhanced CO2 does not consistently reduce gmax . (5) Winter gmax is low even when measurement
conditions are optimum.
Our findings have several ecophysiological implications.
First, the large positive response of Amax to increased CO2
concentration over a wide range of site resource treatments
suggests that significant enhancement in carbon gain may
occur over most of the current range of loblolly pine under
elevated CO2 conditions. Second, increased winter temperatures and CO2 concentrations would both favor a northward
extension in the range of loblolly pine. Kramer and Kozlowski
(1979) concluded that low temperature effects on stomatal
conductance and gas exchange presented a major limitation to
the northward distribution of loblolly pine. Our results suggest
that increased CO2 enhances Amax even when low winter temperatures cause a more than 50% reduction in gmax . Third,
because carbohydrate production potential will increase in
response to elevated CO2, and nitrogen is currently a major
factor limiting productivity across much of the range of loblolly pine, it can be expected that the potential gains from
fertilization will be increased. Fourth, because elevated CO2
concentration increases winter carbon gain, the coniferous
habit of loblolly pine may give the species an advantage over
its deciduous hardwood competitors.

Acknowledgment
This project was funded by the USDA Forest Service Southern Global
Change Program.

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