Directory UMM :Data Elmu:jurnal:T:Tree Physiology:Vol14.1994:
Tree Physiology 14, 1215-1227
© 1994 Heron Publishing----Victoria, Canada
Influence of climate and fertilization on net photosynthesis of
mature slash pine
R. O. TESKEY,1 H. L. GHOLZ2 and W. P. CROPPER, JR.2
1
Daniel B. Warnell School of Forest Resources, University of Georgia, Athens, GA 30602, USA
2
Department of Forestry, University of Florida, Gainesville, FL 32611, USA
Received February 9, 1994
Summary
Net photosynthesis was measured under field conditions in 23-year-old slash pine (Pinus elliottii
Engelm. var. elliottii) trees to determine how it was affected by fertilization and climate. There was only
a small decrease in rates of net photosynthesis from late summer through winter demonstrating that
appreciable carbon gain occurs throughout the year in slash pine. Although fertilization substantially
increased leaf area and aboveground biomass, it only slightly increased the rate of net photosynthesis.
Simultaneous measurements of gas exchange in fertilized and unfertilized (control) plots allowed the
detection of a small, but statistically significant difference in average net photosynthesis of 0.14 µmol
m −2 s − 1. Irradiance, and to a lesser extent air temperature, were the environmental factors that exerted
the most control on net photosynthesis. The highest rates of net photosynthesis occurred between air
temperatures of 25 and 35 °C. Because air temperatures were within this range for 46% of all daylight
hours during the year, air temperature was not often a significant limitation. Soil and atmospheric water
deficits had less effect on photosynthesis than irradiance or air temperature. Although the depth to the
water table changed during the year from 10 to 160 cm, predawn and midday xylem pressure potentials
only changed slightly throughout the year. Predawn values ranged from −0.63 to −0.88 MPa in the control
plot and from −0.51 to −0.87 MPa in the fertilized plot and were not correlated with water table depth.
There was no correlation between xylem pressure potentials and net photosynthesis, presumably because
water uptake was adequate. Although vapor pressure deficits reached 3.5 kPa during the summer, they
had little effect on net photosynthesis. Over a vapor pressure deficit range from 1.0 to 3.0 kPa, net
photosynthesis only decreased 21%. No differences in responses to these environmental factors could be
attributed to fertilization.
Keywords: Pinus elliottii, gas exchange, irradiance, temperature, leaf area, water deficit, water table.
Introduction
The growth of many pine forests is limited by nutrient availability, and fertilizer
application often leads to both increased growth rates and productivity (Nambiar
1990, Jokela and Stearns-Smith 1993). When fertilized, pine trees develop high leaf
areas that are positively correlated with growth rate and aboveground forest productivity (Miller 1984, Linder et al. 1987, Vose and Alan 1988, Gholz et al. 1991).
Because fertilization increases the rate of net photosynthesis in many plant species
(Evans 1989), it might be assumed that in pines and other conifers, along with
increased leaf areas, there would be an associated increase in rates of net photosynthesis of individual leaves, at least in the sunlit portion of the canopy. However, for
conifers, it is not clear whether higher rates of net photosynthesis contribute to
increased carbon gain after fertilization. In some conifer stands, fertilization substan-
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TESKEY, GHOLZ AND CROPPER, JR.
tially increased photosynthetic rates (Brix 1972, 1983, Sheriff et al. 1986, Smolander
and Oker-Blom 1989), whereas in others, it had little effect (Thompson and Wheeler
1992). Additionally, it is not clear how nutritional status modifies the response of
foliage to other environmental factors. In some studies, increased nutrient concentrations in conifers increased water use efficiency (Sheriff et al. 1986), whereas other
studies found no effect (Mitchell and Hinckley 1993). Similarly, the photosynthetic
light response may (Brix 1971, Smolander and Oker-Blom 1989), or may not (Brix
and Ebell 1969), be different after fertilization.
The purpose of this study was to characterize how fertilization and climate affect
rates of net photosynthesis in slash pine (Pinus elliottii Engelm. var. elliottii) trees.
At the outset of the study, little was known about how environment or soil nutrient
availability affects rates of net photosynthesis in this species. It was expected that,
because of the low soil nutrient availability of the site, fertilization would produce a
large increase in growth. A primary objective of the study was to determine whether
this growth response was related to an increase in rates of net photosynthesis.
We also determined the photosynthetic response to seasonal changes in air temperature and water availability. Because the climate of the region is mild in winter,
with few frosts, we hypothesized that net photosynthesis would continue throughout
the winter at appreciable rates. On the other hand, based on the photosynthetic
temperature response of loblolly pine (P. taeda L.) (Drew and Ledig 1981), a species
that has an overlapping range and similar growth characteristics, we hypothesized
that the high air temperatures during the summer, which are commonly over 30 °C
in the afternoon, would inhibit net photosynthesis.
Water deficits were also expected to reduce net photosynthesis, particularly in
summer. The water table in slash pine forests in this region can fluctuate during the
year from near the surface to depths of 2 meters as a result of the sandy texture of the
soils, high potential evapotranspiration rates and the pattern of rainfall (Gaston et al.
1990). Because most fine roots of these trees are within the upper 10 cm of the soil
(Gholz et al. 1986), a decrease in the depth of the water table below the main rooting
zone was expected to produce water stress and reduce net photosynthesis.
Materials and methods
Study site
The study was established in a 60-ha stand of plantation slash pine located 20 km
northeast of Gainesville, FL (29°44′ N, 82°9′30″ W). Soils of the stand were Ultic
haplaquods (sandy, silicious, thermic), poorly drained, low in organic matter and
available nutrients, and with a water table that fluctuated between the surface and a
2 m depth over a typical year (Gaston et al. 1990). Mean annual rainfall was
1342 mm (1955--1987) and the mean annual temperature was 21.7 °C (NOAA
1989). In 1988 at the study site, the mean annual rainfall was 1125 mm and the mean
annual air temperature was 19.4 °C. The stand was established in 1965 following
clear-cutting. Open-pollinated (mixed genotype) seedlings were planted at a harvest
CLIMATE AND FERTILIZATION EFFECTS ON NET PHOTOSYNTHESIS
1217
density of 1190 trees ha −1 and stands were not thinned, fertilized or otherwise
manipulated until this study began. The mean tree height was 15.4 m, mean stem
diameter at 1.3 m was 17.3 cm, and mean stand basal area was 25.9 m2 ha −1 at the
beginning of the study (Gholz et al. 1991).
Two 50 × 50 m plots, one fertilized and one unfertilized, were used. To minimize
leaching losses, fertilizer was applied every three months, beginning one year before
the study and continuing throughout the study. Annual fertilization rates were 180 kg
ha −1 urea N, 70 kg ha −1 triple superphosphate P, 50 kg ha −1 muriate of potash K,
46 kg ha −1 triple superphosphate Ca, 50 kg ha −1 dolomitic limestone Ca, 28 kg ha −1
dolomitic limestone Mg, 50 kg ha −1 elemental S and 2 kg ha −1 triple superphosphate
S. Micronutrients (Cu, Bo, Fe, Mn, Zn and Mo) were added in the first year only.
Foliage was analyzed for nutrient concentrations in February and August 1988. At
both times, fully expanded, one-year-old needles from the upper one third of the
canopy were sampled.
Measurements
Access to the canopies of six trees per plot was facilitated by scaffold-type towers.
Net photosynthesis was estimated from diurnal measurements of carbon dioxide
exchange with two portable gas analysis systems (LCA-2, Analytical Development
Company, Hoddesdon, U.K.). Each system consisted of an air pump unit, a leaf
cuvette (narrow leaf type) and a battery-operated infrared gas analyzer. Measurements were made approximately every three weeks from January to December 1988.
All six study trees in each plot were measured each hour, with the mean representing
the hourly value. From January to July, the first flush from the previous year was
sampled. After that, the first flush from the current year, by then fully expanded, was
measured. For all measurements, the cuvette was held horizontally. Shading from
nearby foliage was intentionally minimized. Gas exchange data were calculated on
a total surface area basis. Leaf area was determined from measurements of needle
length and radius, knowing that the outside surface of the needle forms a segment of
a cylinder, and the inner surfaces of the needle are flat and can be approximated by
a rectangle.
The two gas exchange systems were calibrated together and, whenever possible,
used simultaneously in the fertilized and control plots. This sampling procedure
allowed paired comparisons to be made of the rates of net photosynthesis of fertilized
and control trees under similar environmental conditions.
Photosynthetically active radiation (PAR), air temperature and relative humidity
were measured with the sensors in the leaf cuvette. Photosynthetically active radiation was measured with a photodiode, air temperature with a thermistor and relative
humidity with a capacitive sensor. In addition, air temperature was measured with a
shielded thermistor at canopy height in each plot. These data were averaged each
hour and recorded with an Omnidata EasyLogger data logger (Logan, Utah).
Xylem pressure potentials (XPP) were measured with a pressure chamber (PMS
Instruments, Corvallis, OR). Measurements were made before dawn in conjunction
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TESKEY, GHOLZ AND CROPPER, JR.
with hourly measurements of net photosynthesis. Water table depths were recorded
continuously in both plots with a Type F recorder (Leupold & Stevens Inc., Beaverton, OR).
Statistical analyses were done with SigmaStat Ver. 1.1 (Jandel Scientific, San
Rafael, CA). Tests performed were: t-tests for comparisons of mean rates of net
photosynthesis and mean foliar nutrient concentrations, paired t-tests for comparisons of net photosynthesis when measurements were made simultaneously in the
fertilized and control plots, and simple and multiple linear regressions with net
photosynthesis as the dependent variable and environmental factors as the independent variables.
Results
Mean midday rates of net photosynthesis exceeded 1.0 µmol m −2 s −1 on all sampling
dates throughout the year (Figure 1). In April and May (Julian Dates 91 to 151), net
photosynthesis (Pn) increased to about 2.0 µmol m −2 s −1 in the control trees. During
these months, Pn appeared to be elevated in the fertilized trees; however, this trend
was not consistent over time, and for the year, the mean rates of Pn in control
(1.58 µmol m −2 s −1) and fertilized (1.76 µmol m −2 s −1) foliage were not statistically
different (t-test, P = 0.068, n = 18).
In February, although an appreciable amount of fertilizer was added to the plot
during the previous year, only Mg and Ca were higher in the fertilized foliage than
in the unfertilized foliage (Table 1). In the spring, the stem wood growth rate was
substantially higher in the fertilized trees than in the control trees, but by August,
growth had slowed (Gholz et al. 1991). In August, foliar concentrations of four (N,
P, K and Ca) of the five nutrients measured were higher in the fertilized trees than in
Figure 1. Mean midday (1000 to 1400 h) net photosynthesis for fertilized (●) and control (s) trees.
CLIMATE AND FERTILIZATION EFFECTS ON NET PHOTOSYNTHESIS
1219
Table 1. Mean nutrient concentrations (%) for fully expanded one-year-old needles in the upper canopy
of slash pine trees.
Date
N
P
K
Mg
Ca
February
Fertilized
Control
0.93
0.80
0.057
0.072
0.23
0.27
0.87a1
0.63
0.210a
0.130
August
Fertilized
Control
1.16a
0.75
0.097a
0.053
0.30a
0.13
0.84
0.89
0.285a
0.217
1
a = Mean of the fertilized plot significantly greater than for the control plot (t-test, α = 0.05).
the control trees (Table 1). The foliar concentration of nitrogen increased in the
fertilized trees from 0.93% in February to 1.16% in August and was significantly
higher than in the control trees (0.75% N).
By using only the subset of measurements made simultaneously in the two plots,
it was possible to detect an effect of fertilization on the rate of Pn (Figure 2). In this
case, paired observations were examined to determine if there was any trend in rates
of Pn when environmental differences such as irradiance, air temperature and vapor
pressure deficit were minimized. Overall, there were more observations of higher Pn
in the fertilized plot than in the control plot (Figure 2). The mean difference in Pn
between fertilized and control trees was only 0.14 µmol m −2 s −1, but this difference
was statistically significant (paired t-test, P = 0.001).
For the combined data set, a relationship between Pn and irradiance was evident
Figure 2. Comparison of measurements of net photosynthesis taken simultaneously in fertilized and
control plots. Each observation represents the difference between the mean rate of net photosynthesis
(∆ Pn) in each plot (n = 6), calculated as fertilized minus control, arranged chronologically from left to
right. A positive value indicates that rates of net photosynthesis in fertilized trees were higher than in
control trees.
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TESKEY, GHOLZ AND CROPPER, JR.
(Figure 3). Based on nonlinear regression, a rectangular hyperbola was fitted to the
data to describe the response of Pn to PAR, and to estimate the mean apparent
quantum yield and mean maximum rate of net photosynthesis:
Pn =
ΘPAR P max
,
ΘPAR + P max
(1)
where Θ is the apparent quantum yield, and Pmax is the maximum rate of net
photosynthesis at saturating irradiance. The best fit values for the parameters in
Equation 1 were: Θ = 0.0043 and Pmax = 2.2432 µmol m −2 s −1. The 95% confidence
intervals for the parameters were: Θ = 0.0039 to 0.0047 and Pmax = 2.0987 to 2.3877
µmol m −2 s −1. The relationships were not significantly different when fertilized and
control foliage were analyzed separately.
At high irradiances, Pn varied between about 1 and 3 µmol m −2 s −1. Part of this
variation was caused by differences in air temperature during the year (Figure 4).
Rates of net photosynthesis tended to increase with increasing air temperature, with
the highest rates occurring at air temperatures between 25 and 35 °C. An examination
of the air temperatures recorded hourly at the site indicated that in 46% of all daylight
hours over the year, air temperatures were within the 25 to 35 °C range. Additionally,
more than 73% of the daylight hours at high irradiance (PAR > 900 µmol m −2 s −1)
were at air temperatures between 25 and 35 °C. There was no indication that high air
temperatures, e.g., in excess of 30 °C, reduced the rate of Pn.
Water deficits did not significantly limit Pn. No statistical relationship was found
between hourly xylem pressure potential measurements and Pn. During the year,
Figure 3. Relationship between net photosynthesis and photosynthetically active radiation (PAR) for
fertilized (●) and control (s) trees. Each point represents the mean for six trees per plot, measured at
approximately hourly intervals on the days shown in Figure 1. The solid line represents the fit to a
rectangular hyperbola. See text for equation and parameters used.
CLIMATE AND FERTILIZATION EFFECTS ON NET PHOTOSYNTHESIS
1221
Figure 4. Relationship between air temperature and net photosynthesis for fertilized (●) and control (s)
trees. Data in this figure were collected under conditions of high irradiance (> PAR 900 µmol m −2 s −1)
and low vapor pressure deficit (< 2.2 kPa). Differences in the number of data points for fertilized and
control trees were due to a larger number of measurement dates in the control plot.
mean predawn XPP for trees in the control plot only varied from − 0.63 to −0.88 MPa
(Figure 5), and the range in mean predawn XPP in the fertilized plot was − 0.51 to
−0.87 MPa. Over the same time period, the water table depth fluctuated from 10 to
160 cm (Figure 5), but there was no correlation between water table depth and xylem
pressure potentials measured at predawn or midday. Perhaps because the water
supply was adequate, we only observed a weak negative correlation between vapor
pressure deficit and Pn (Figure 6).
Multiple regression was used to assess the relative effect of the measured environmental factors on hourly mean net photosynthesis (Pn). Based on the entire data set,
the best model was
P n = −0.51 + 1.187(I) − 0.659 (VPD ) + 0.022 (T),
(2)
where I is the mean response to irradiance (Equation 1 and Figure 3), VPD is the
vapor pressure deficit, and T is the air temperature (R2 = 0.78). Irradiance was highly
correlated with Pn, and it alone explained 69% of the variation in the data. Both VPD
and T were weakly related to Pn, improving the fit of the model by 7 and 2%,
respectively. Although numerous transformations of VPD and T were attempted,
none improved the fit of the model by more than 1%, so the additional complexity
they introduced into the model did not justify their use. Xylem pressure potential was
not a significant variable in the model. Fertilization, introduced into the model as a
dummy variable, was also not significant. The multiple regression was consistent
with previous analyses of the separate environmental factors, i.e., it demonstrated
that PAR was the most important factor influencing the rate of Pn, that VPD was
negatively correlated with Pn, and that the other measured factors had little or no
1222
TESKEY, GHOLZ AND CROPPER, JR.
Figure 5. Upper figure: seasonal patterns of predawn (s, u) and solar noon (●, ■) xylem pressure
potentials. Squares represent the control plot, circles represent the fertilized plot. Lower figure shows the
seasonal changes in the depth to the water table from the soil surface (0 cm) for the control ( ) and
fertilized (●) plots.
Figure 6. Relationship between net photosynthesis (Pn) and vapor pressure deficit (VPD) for fertilized
(●) and control (s) foliage. Data shown in figure were collected in high irradiance (PAR > 900 µmol
m −2 s −1) and air temperatures > 20 °C. The line though the data represents a linear regression of the form:
Pn = 2.68 − 0.35(VPD). The slope is significantly different from 0 (α = 0.05), R2 = 0.18.
CLIMATE AND FERTILIZATION EFFECTS ON NET PHOTOSYNTHESIS
1223
effect.
Seasonal differences in cumulative net photosynthesis were estimated using Equation 2 and mean hourly values of PAR, air temperature and VPD. On a relative basis,
the highest seasonal net photosynthesis was in summer (0.35), followed by spring
(0.26), autumn (0.20) and winter (0.19).
Discussion
Rates of net photosynthesis in slash pine changed with season, although the pattern
was less pronounced than in pine species from cooler regions where net photosynthesis often falls to zero in winter (Troeng and Linder 1982, Jurik et al. 1988). The
highest Pn rates were in April, May and June, with lower rates during the remainder
of the year. High Pn rates may arise in the spring, because midday light intensity tends
to increase as the solar azimuth decreases. High rates of net photosynthesis in the
spring are also correlated with growth rate, and may be related to sink demands for
carbohydrates as well as favorable environmental conditions (Maier and Teskey
1992). Although the midday rates of net photosynthesis were lower in summer than
in spring, this could not be attributed to the higher air temperatures at this time of
year. There was no appreciable decrease in rates of net photosynthesis from late
summer and fall (July--November) through winter (December--February), demonstrating that appreciable carbon gain occurs throughout the year in slash pine.
Slash pine appeared to have lower rates of net photosynthesis than other pine
species. For example, maximum rates of net photosynthesis on a total leaf area basis
were about 5.4 µmol m −2 s −1 in P. sylvestris L. (Troeng and Linder 1982), 3.6--4.9
µmol m −2 s −1 in P. radiata D. Don (Sheriff et al. 1986, Thompson and Wheeler 1992),
4.1 µmol m − 2 s −1 in P. strobus L. (Maier and Teskey 1992) and 6.0 µmol m −2 s −1 in
P. taeda (Fites and Teskey 1988). Low rates of Pn in slash pine seedlings in this study
were similar to those reported by Doley (1988) and van den Driessche (1972).
Although fertilization had little effect on net photosynthesis of slash pine trees, it
has a large effect on growth. Gholz et al. (1991) reported that, by August, as a result
of the growth of new foliage in two flushes, the fertilized plot had a leaf area index
of 6.8, which was 41% higher than that of the control plot. The annual stem wood
biomass increment was also higher, increasing from 4 Mg ha −1 in the control plot to
5.7 Mg ha −1 in the fertilized plot. The initial increase in growth in response to
fertilization was too large to be accounted for by the small increase in net photosynthesis, and suggests that the carbohydrates needed for the initial increase in growth
came from the utilization of stored carbohydrates (Cropper and Gholz 1993). Later,
the higher growth rates in the fertilized plot could be sustained by the higher leaf area.
It is also possible that a shift in carbon allocation from root to shoot occurred after
fertilization, as has been demonstrated in seedlings. Brix and Ebell (1969) also noted
that fertilization had no effect on net photosynthesis in Pseudotsuga menziesii
(Mirb.) Franco trees, even though it induced a large growth response, and concluded
that the only measured factor directly associated with the increased growth of the
1224
TESKEY, GHOLZ AND CROPPER, JR.
fertilized trees was leaf area. This observation is consistent with the high correlation
between leaf area and aboveground growth after fertilization in P. taeda stands (Vose
and Allen 1988).
Evans (1989) compared published data from a variety of C3 species and found good
agreement between rates of net photosynthesis and leaf nitrogen content. Compared
with other C3 species, conifers had smaller increases in net photosynthesis per unit
increase in nitrogen concentration. A positive correlation between leaf nitrogen
content and net photosynthesis has been demonstrated for several trees including
deciduous species (e.g., Prunus persica (L.) Batsch., DeJong et al. 1989; Acer
saccharum Marsh., Reich et al. 1991), broad-leaved evergreen species (e.g., Arbutus
menziesii Pursh. and Umbellularia californica (Hook & Arn.) Nutt., Field et al. 1983;
Eucalyptus globulus Labill., Sheriff and Nambiar 1991) and evergreen conifers (e.g.,
Pinus ponderosa Dougl., DeLucia et al. 1989; P. radiata, Thompson and Wheeler
1992; P. sylvestris, Smolander and Oker-Blom 1989; Pseudotsuga menziesii,
Mitchell and Hinckley 1993). However, there are exceptions to this general response.
Van den Driessche (1972) reported that nitrogen fertilization decreased rates of net
photosynthesis in slash pine seedlings. Sheriff et al. (1986) found that nitrogen
fertilization had no effect on net photosynthesis in the foliage of P. radiata trees, even
though the nitrogen concentration was significantly higher in fertilized trees than in
unfertilized trees; however, the higher rates of net photosynthesis were associated
with an increased phosphorus content. Similarly, Reid et al. (1983) reported that the
net photosynthetic rate of P. contorta Dougl. was correlated with the concentration
of phosphorus in the foliage, but not with nitrogen. Phosphorus concentrations in the
foliage have also been correlated with rates of net photosynthesis in P. radiata trees
(Attiwill and Cromer 1982) and seedlings (Conroy et al. 1988). We found that, by
August, the foliar concentrations of both phosphorus and nitrogen had increased as
a result of fertilization (Table 1), but these increases had little effect on net photosynthesis.
In general, seedlings show a larger photosynthetic response to fertilization than
large trees (Linder and Rook 1984). It is only when relatively large differences in
nutrient concentrations in the foliage are produced by fertilization that detectable
differences in net photosynthesis can be found in older trees. For example, in
14-year-old P. radiata, a difference of 1.0% N resulted in a 20% increase in net
photosynthesis (Thompson and Wheeler 1992). However, to produce the high nitrogen content (1.9%) on the low-fertility site, large amounts of fertilizer were applied
over a long period.
The small response of net photosynthesis to vapor pressure deficit (VPD) in slash
pine has also be observed in loblolly pine (Bongarten and Teskey 1986). The natural
ranges of the two species overlap, and both are adapted to conditions of high
humidity and plentiful soil water during the growing season. Stomatal sensitivity to
increasing VPD is an obvious advantage in droughty areas and has been well
documented in various pines (e.g., P. radiata, P. contorta, and P. ponderosa) (Sandford and Jarvis 1986). If slash pine had undergone drought stress during the study, it
CLIMATE AND FERTILIZATION EFFECTS ON NET PHOTOSYNTHESIS
1225
might have been possible to detect a more sensitive VPD response. Maier and Teskey
(1992) found that P. strobus had no detectable VPD response until predawn xylem
pressure potentials were less than −1 MPa. Similar observations have been made for
other conifers including Picea sitchensis (Bong.) Carr. (Beadle et al. 1981) and
P. sylvestris (Ng and Jarvis 1978).
The high predawn xylem pressure potentials measured throughout the year and the
lack of correlation between water table depth and XPP indicated that the small
number of roots that extend deeply through the soil profile were sufficient to supply
the trees with water (cf. Teskey et al. 1985). However, it has been reported that under
dry soil conditions, all of the root system of a loblolly pine tree is needed to maintain
water uptake (Carlson et al. 1988). Apparently, in slash pine, as long as some of the
roots are in contact with the water table, the transpirational needs of the tree are met.
In summary, net photosynthetic rates of slash pine trees growing under field
conditions were limited primarily by irradiance. The second most important limitation was air temperature. The highest rates of net photosynthesis occurred at air
temperatures between 25 and 35 °C. Over the course of the year, in 73% of the hours
when irradiance was high (> 900 µmol m −2 s −1), air temperatures were within this
range. Net photosynthesis was not constrained by air temperatures in excess of 30 °C.
Similarly, water table depth and vapor pressure deficits had little effect on the rate of
Pn. Finally, although the rates of Pn were low compared with other conifer species,
as were the foliar concentrations of nutrients, two years of fertilization had only a
slight effect on Pn. The lack of a fertilizer effect on Pn was consistent with other
reports from field studies in older conifer stands. When more nutrients were made
available through fertilization, slash pine trees increased carbon gain almost exclusively by expanded leaf area rather than by increased rates of net photosynthesis.
Acknowledgments
We thank Sheryll Vogel, Denise Guerin, Kevin McKelvy and Steven Smitherman for collecting the field
data, and Amy Horne and Erika Mavity for assistance with data analysis. This project was funded by NSF
Grant BSR 8106678. The stands for this research were used with permission of the Jefferson-Smerfit
Company. Electrical power to the site was provided by the Florida Electric Power Coordinating Group.
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needles to light: observations and a hypothesis. Plant Cell Environ. 3:207--216.
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Reich, P.B., M.B. Walters and D.S. Ellsworth. 1991. Leaf age and season influence the relationships
between leaf nitrogen, leaf mass per unit area, and photosynthesis in maple and oak trees. Plant Cell
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2:89--104.
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© 1994 Heron Publishing----Victoria, Canada
Influence of climate and fertilization on net photosynthesis of
mature slash pine
R. O. TESKEY,1 H. L. GHOLZ2 and W. P. CROPPER, JR.2
1
Daniel B. Warnell School of Forest Resources, University of Georgia, Athens, GA 30602, USA
2
Department of Forestry, University of Florida, Gainesville, FL 32611, USA
Received February 9, 1994
Summary
Net photosynthesis was measured under field conditions in 23-year-old slash pine (Pinus elliottii
Engelm. var. elliottii) trees to determine how it was affected by fertilization and climate. There was only
a small decrease in rates of net photosynthesis from late summer through winter demonstrating that
appreciable carbon gain occurs throughout the year in slash pine. Although fertilization substantially
increased leaf area and aboveground biomass, it only slightly increased the rate of net photosynthesis.
Simultaneous measurements of gas exchange in fertilized and unfertilized (control) plots allowed the
detection of a small, but statistically significant difference in average net photosynthesis of 0.14 µmol
m −2 s − 1. Irradiance, and to a lesser extent air temperature, were the environmental factors that exerted
the most control on net photosynthesis. The highest rates of net photosynthesis occurred between air
temperatures of 25 and 35 °C. Because air temperatures were within this range for 46% of all daylight
hours during the year, air temperature was not often a significant limitation. Soil and atmospheric water
deficits had less effect on photosynthesis than irradiance or air temperature. Although the depth to the
water table changed during the year from 10 to 160 cm, predawn and midday xylem pressure potentials
only changed slightly throughout the year. Predawn values ranged from −0.63 to −0.88 MPa in the control
plot and from −0.51 to −0.87 MPa in the fertilized plot and were not correlated with water table depth.
There was no correlation between xylem pressure potentials and net photosynthesis, presumably because
water uptake was adequate. Although vapor pressure deficits reached 3.5 kPa during the summer, they
had little effect on net photosynthesis. Over a vapor pressure deficit range from 1.0 to 3.0 kPa, net
photosynthesis only decreased 21%. No differences in responses to these environmental factors could be
attributed to fertilization.
Keywords: Pinus elliottii, gas exchange, irradiance, temperature, leaf area, water deficit, water table.
Introduction
The growth of many pine forests is limited by nutrient availability, and fertilizer
application often leads to both increased growth rates and productivity (Nambiar
1990, Jokela and Stearns-Smith 1993). When fertilized, pine trees develop high leaf
areas that are positively correlated with growth rate and aboveground forest productivity (Miller 1984, Linder et al. 1987, Vose and Alan 1988, Gholz et al. 1991).
Because fertilization increases the rate of net photosynthesis in many plant species
(Evans 1989), it might be assumed that in pines and other conifers, along with
increased leaf areas, there would be an associated increase in rates of net photosynthesis of individual leaves, at least in the sunlit portion of the canopy. However, for
conifers, it is not clear whether higher rates of net photosynthesis contribute to
increased carbon gain after fertilization. In some conifer stands, fertilization substan-
1216
TESKEY, GHOLZ AND CROPPER, JR.
tially increased photosynthetic rates (Brix 1972, 1983, Sheriff et al. 1986, Smolander
and Oker-Blom 1989), whereas in others, it had little effect (Thompson and Wheeler
1992). Additionally, it is not clear how nutritional status modifies the response of
foliage to other environmental factors. In some studies, increased nutrient concentrations in conifers increased water use efficiency (Sheriff et al. 1986), whereas other
studies found no effect (Mitchell and Hinckley 1993). Similarly, the photosynthetic
light response may (Brix 1971, Smolander and Oker-Blom 1989), or may not (Brix
and Ebell 1969), be different after fertilization.
The purpose of this study was to characterize how fertilization and climate affect
rates of net photosynthesis in slash pine (Pinus elliottii Engelm. var. elliottii) trees.
At the outset of the study, little was known about how environment or soil nutrient
availability affects rates of net photosynthesis in this species. It was expected that,
because of the low soil nutrient availability of the site, fertilization would produce a
large increase in growth. A primary objective of the study was to determine whether
this growth response was related to an increase in rates of net photosynthesis.
We also determined the photosynthetic response to seasonal changes in air temperature and water availability. Because the climate of the region is mild in winter,
with few frosts, we hypothesized that net photosynthesis would continue throughout
the winter at appreciable rates. On the other hand, based on the photosynthetic
temperature response of loblolly pine (P. taeda L.) (Drew and Ledig 1981), a species
that has an overlapping range and similar growth characteristics, we hypothesized
that the high air temperatures during the summer, which are commonly over 30 °C
in the afternoon, would inhibit net photosynthesis.
Water deficits were also expected to reduce net photosynthesis, particularly in
summer. The water table in slash pine forests in this region can fluctuate during the
year from near the surface to depths of 2 meters as a result of the sandy texture of the
soils, high potential evapotranspiration rates and the pattern of rainfall (Gaston et al.
1990). Because most fine roots of these trees are within the upper 10 cm of the soil
(Gholz et al. 1986), a decrease in the depth of the water table below the main rooting
zone was expected to produce water stress and reduce net photosynthesis.
Materials and methods
Study site
The study was established in a 60-ha stand of plantation slash pine located 20 km
northeast of Gainesville, FL (29°44′ N, 82°9′30″ W). Soils of the stand were Ultic
haplaquods (sandy, silicious, thermic), poorly drained, low in organic matter and
available nutrients, and with a water table that fluctuated between the surface and a
2 m depth over a typical year (Gaston et al. 1990). Mean annual rainfall was
1342 mm (1955--1987) and the mean annual temperature was 21.7 °C (NOAA
1989). In 1988 at the study site, the mean annual rainfall was 1125 mm and the mean
annual air temperature was 19.4 °C. The stand was established in 1965 following
clear-cutting. Open-pollinated (mixed genotype) seedlings were planted at a harvest
CLIMATE AND FERTILIZATION EFFECTS ON NET PHOTOSYNTHESIS
1217
density of 1190 trees ha −1 and stands were not thinned, fertilized or otherwise
manipulated until this study began. The mean tree height was 15.4 m, mean stem
diameter at 1.3 m was 17.3 cm, and mean stand basal area was 25.9 m2 ha −1 at the
beginning of the study (Gholz et al. 1991).
Two 50 × 50 m plots, one fertilized and one unfertilized, were used. To minimize
leaching losses, fertilizer was applied every three months, beginning one year before
the study and continuing throughout the study. Annual fertilization rates were 180 kg
ha −1 urea N, 70 kg ha −1 triple superphosphate P, 50 kg ha −1 muriate of potash K,
46 kg ha −1 triple superphosphate Ca, 50 kg ha −1 dolomitic limestone Ca, 28 kg ha −1
dolomitic limestone Mg, 50 kg ha −1 elemental S and 2 kg ha −1 triple superphosphate
S. Micronutrients (Cu, Bo, Fe, Mn, Zn and Mo) were added in the first year only.
Foliage was analyzed for nutrient concentrations in February and August 1988. At
both times, fully expanded, one-year-old needles from the upper one third of the
canopy were sampled.
Measurements
Access to the canopies of six trees per plot was facilitated by scaffold-type towers.
Net photosynthesis was estimated from diurnal measurements of carbon dioxide
exchange with two portable gas analysis systems (LCA-2, Analytical Development
Company, Hoddesdon, U.K.). Each system consisted of an air pump unit, a leaf
cuvette (narrow leaf type) and a battery-operated infrared gas analyzer. Measurements were made approximately every three weeks from January to December 1988.
All six study trees in each plot were measured each hour, with the mean representing
the hourly value. From January to July, the first flush from the previous year was
sampled. After that, the first flush from the current year, by then fully expanded, was
measured. For all measurements, the cuvette was held horizontally. Shading from
nearby foliage was intentionally minimized. Gas exchange data were calculated on
a total surface area basis. Leaf area was determined from measurements of needle
length and radius, knowing that the outside surface of the needle forms a segment of
a cylinder, and the inner surfaces of the needle are flat and can be approximated by
a rectangle.
The two gas exchange systems were calibrated together and, whenever possible,
used simultaneously in the fertilized and control plots. This sampling procedure
allowed paired comparisons to be made of the rates of net photosynthesis of fertilized
and control trees under similar environmental conditions.
Photosynthetically active radiation (PAR), air temperature and relative humidity
were measured with the sensors in the leaf cuvette. Photosynthetically active radiation was measured with a photodiode, air temperature with a thermistor and relative
humidity with a capacitive sensor. In addition, air temperature was measured with a
shielded thermistor at canopy height in each plot. These data were averaged each
hour and recorded with an Omnidata EasyLogger data logger (Logan, Utah).
Xylem pressure potentials (XPP) were measured with a pressure chamber (PMS
Instruments, Corvallis, OR). Measurements were made before dawn in conjunction
1218
TESKEY, GHOLZ AND CROPPER, JR.
with hourly measurements of net photosynthesis. Water table depths were recorded
continuously in both plots with a Type F recorder (Leupold & Stevens Inc., Beaverton, OR).
Statistical analyses were done with SigmaStat Ver. 1.1 (Jandel Scientific, San
Rafael, CA). Tests performed were: t-tests for comparisons of mean rates of net
photosynthesis and mean foliar nutrient concentrations, paired t-tests for comparisons of net photosynthesis when measurements were made simultaneously in the
fertilized and control plots, and simple and multiple linear regressions with net
photosynthesis as the dependent variable and environmental factors as the independent variables.
Results
Mean midday rates of net photosynthesis exceeded 1.0 µmol m −2 s −1 on all sampling
dates throughout the year (Figure 1). In April and May (Julian Dates 91 to 151), net
photosynthesis (Pn) increased to about 2.0 µmol m −2 s −1 in the control trees. During
these months, Pn appeared to be elevated in the fertilized trees; however, this trend
was not consistent over time, and for the year, the mean rates of Pn in control
(1.58 µmol m −2 s −1) and fertilized (1.76 µmol m −2 s −1) foliage were not statistically
different (t-test, P = 0.068, n = 18).
In February, although an appreciable amount of fertilizer was added to the plot
during the previous year, only Mg and Ca were higher in the fertilized foliage than
in the unfertilized foliage (Table 1). In the spring, the stem wood growth rate was
substantially higher in the fertilized trees than in the control trees, but by August,
growth had slowed (Gholz et al. 1991). In August, foliar concentrations of four (N,
P, K and Ca) of the five nutrients measured were higher in the fertilized trees than in
Figure 1. Mean midday (1000 to 1400 h) net photosynthesis for fertilized (●) and control (s) trees.
CLIMATE AND FERTILIZATION EFFECTS ON NET PHOTOSYNTHESIS
1219
Table 1. Mean nutrient concentrations (%) for fully expanded one-year-old needles in the upper canopy
of slash pine trees.
Date
N
P
K
Mg
Ca
February
Fertilized
Control
0.93
0.80
0.057
0.072
0.23
0.27
0.87a1
0.63
0.210a
0.130
August
Fertilized
Control
1.16a
0.75
0.097a
0.053
0.30a
0.13
0.84
0.89
0.285a
0.217
1
a = Mean of the fertilized plot significantly greater than for the control plot (t-test, α = 0.05).
the control trees (Table 1). The foliar concentration of nitrogen increased in the
fertilized trees from 0.93% in February to 1.16% in August and was significantly
higher than in the control trees (0.75% N).
By using only the subset of measurements made simultaneously in the two plots,
it was possible to detect an effect of fertilization on the rate of Pn (Figure 2). In this
case, paired observations were examined to determine if there was any trend in rates
of Pn when environmental differences such as irradiance, air temperature and vapor
pressure deficit were minimized. Overall, there were more observations of higher Pn
in the fertilized plot than in the control plot (Figure 2). The mean difference in Pn
between fertilized and control trees was only 0.14 µmol m −2 s −1, but this difference
was statistically significant (paired t-test, P = 0.001).
For the combined data set, a relationship between Pn and irradiance was evident
Figure 2. Comparison of measurements of net photosynthesis taken simultaneously in fertilized and
control plots. Each observation represents the difference between the mean rate of net photosynthesis
(∆ Pn) in each plot (n = 6), calculated as fertilized minus control, arranged chronologically from left to
right. A positive value indicates that rates of net photosynthesis in fertilized trees were higher than in
control trees.
1220
TESKEY, GHOLZ AND CROPPER, JR.
(Figure 3). Based on nonlinear regression, a rectangular hyperbola was fitted to the
data to describe the response of Pn to PAR, and to estimate the mean apparent
quantum yield and mean maximum rate of net photosynthesis:
Pn =
ΘPAR P max
,
ΘPAR + P max
(1)
where Θ is the apparent quantum yield, and Pmax is the maximum rate of net
photosynthesis at saturating irradiance. The best fit values for the parameters in
Equation 1 were: Θ = 0.0043 and Pmax = 2.2432 µmol m −2 s −1. The 95% confidence
intervals for the parameters were: Θ = 0.0039 to 0.0047 and Pmax = 2.0987 to 2.3877
µmol m −2 s −1. The relationships were not significantly different when fertilized and
control foliage were analyzed separately.
At high irradiances, Pn varied between about 1 and 3 µmol m −2 s −1. Part of this
variation was caused by differences in air temperature during the year (Figure 4).
Rates of net photosynthesis tended to increase with increasing air temperature, with
the highest rates occurring at air temperatures between 25 and 35 °C. An examination
of the air temperatures recorded hourly at the site indicated that in 46% of all daylight
hours over the year, air temperatures were within the 25 to 35 °C range. Additionally,
more than 73% of the daylight hours at high irradiance (PAR > 900 µmol m −2 s −1)
were at air temperatures between 25 and 35 °C. There was no indication that high air
temperatures, e.g., in excess of 30 °C, reduced the rate of Pn.
Water deficits did not significantly limit Pn. No statistical relationship was found
between hourly xylem pressure potential measurements and Pn. During the year,
Figure 3. Relationship between net photosynthesis and photosynthetically active radiation (PAR) for
fertilized (●) and control (s) trees. Each point represents the mean for six trees per plot, measured at
approximately hourly intervals on the days shown in Figure 1. The solid line represents the fit to a
rectangular hyperbola. See text for equation and parameters used.
CLIMATE AND FERTILIZATION EFFECTS ON NET PHOTOSYNTHESIS
1221
Figure 4. Relationship between air temperature and net photosynthesis for fertilized (●) and control (s)
trees. Data in this figure were collected under conditions of high irradiance (> PAR 900 µmol m −2 s −1)
and low vapor pressure deficit (< 2.2 kPa). Differences in the number of data points for fertilized and
control trees were due to a larger number of measurement dates in the control plot.
mean predawn XPP for trees in the control plot only varied from − 0.63 to −0.88 MPa
(Figure 5), and the range in mean predawn XPP in the fertilized plot was − 0.51 to
−0.87 MPa. Over the same time period, the water table depth fluctuated from 10 to
160 cm (Figure 5), but there was no correlation between water table depth and xylem
pressure potentials measured at predawn or midday. Perhaps because the water
supply was adequate, we only observed a weak negative correlation between vapor
pressure deficit and Pn (Figure 6).
Multiple regression was used to assess the relative effect of the measured environmental factors on hourly mean net photosynthesis (Pn). Based on the entire data set,
the best model was
P n = −0.51 + 1.187(I) − 0.659 (VPD ) + 0.022 (T),
(2)
where I is the mean response to irradiance (Equation 1 and Figure 3), VPD is the
vapor pressure deficit, and T is the air temperature (R2 = 0.78). Irradiance was highly
correlated with Pn, and it alone explained 69% of the variation in the data. Both VPD
and T were weakly related to Pn, improving the fit of the model by 7 and 2%,
respectively. Although numerous transformations of VPD and T were attempted,
none improved the fit of the model by more than 1%, so the additional complexity
they introduced into the model did not justify their use. Xylem pressure potential was
not a significant variable in the model. Fertilization, introduced into the model as a
dummy variable, was also not significant. The multiple regression was consistent
with previous analyses of the separate environmental factors, i.e., it demonstrated
that PAR was the most important factor influencing the rate of Pn, that VPD was
negatively correlated with Pn, and that the other measured factors had little or no
1222
TESKEY, GHOLZ AND CROPPER, JR.
Figure 5. Upper figure: seasonal patterns of predawn (s, u) and solar noon (●, ■) xylem pressure
potentials. Squares represent the control plot, circles represent the fertilized plot. Lower figure shows the
seasonal changes in the depth to the water table from the soil surface (0 cm) for the control ( ) and
fertilized (●) plots.
Figure 6. Relationship between net photosynthesis (Pn) and vapor pressure deficit (VPD) for fertilized
(●) and control (s) foliage. Data shown in figure were collected in high irradiance (PAR > 900 µmol
m −2 s −1) and air temperatures > 20 °C. The line though the data represents a linear regression of the form:
Pn = 2.68 − 0.35(VPD). The slope is significantly different from 0 (α = 0.05), R2 = 0.18.
CLIMATE AND FERTILIZATION EFFECTS ON NET PHOTOSYNTHESIS
1223
effect.
Seasonal differences in cumulative net photosynthesis were estimated using Equation 2 and mean hourly values of PAR, air temperature and VPD. On a relative basis,
the highest seasonal net photosynthesis was in summer (0.35), followed by spring
(0.26), autumn (0.20) and winter (0.19).
Discussion
Rates of net photosynthesis in slash pine changed with season, although the pattern
was less pronounced than in pine species from cooler regions where net photosynthesis often falls to zero in winter (Troeng and Linder 1982, Jurik et al. 1988). The
highest Pn rates were in April, May and June, with lower rates during the remainder
of the year. High Pn rates may arise in the spring, because midday light intensity tends
to increase as the solar azimuth decreases. High rates of net photosynthesis in the
spring are also correlated with growth rate, and may be related to sink demands for
carbohydrates as well as favorable environmental conditions (Maier and Teskey
1992). Although the midday rates of net photosynthesis were lower in summer than
in spring, this could not be attributed to the higher air temperatures at this time of
year. There was no appreciable decrease in rates of net photosynthesis from late
summer and fall (July--November) through winter (December--February), demonstrating that appreciable carbon gain occurs throughout the year in slash pine.
Slash pine appeared to have lower rates of net photosynthesis than other pine
species. For example, maximum rates of net photosynthesis on a total leaf area basis
were about 5.4 µmol m −2 s −1 in P. sylvestris L. (Troeng and Linder 1982), 3.6--4.9
µmol m −2 s −1 in P. radiata D. Don (Sheriff et al. 1986, Thompson and Wheeler 1992),
4.1 µmol m − 2 s −1 in P. strobus L. (Maier and Teskey 1992) and 6.0 µmol m −2 s −1 in
P. taeda (Fites and Teskey 1988). Low rates of Pn in slash pine seedlings in this study
were similar to those reported by Doley (1988) and van den Driessche (1972).
Although fertilization had little effect on net photosynthesis of slash pine trees, it
has a large effect on growth. Gholz et al. (1991) reported that, by August, as a result
of the growth of new foliage in two flushes, the fertilized plot had a leaf area index
of 6.8, which was 41% higher than that of the control plot. The annual stem wood
biomass increment was also higher, increasing from 4 Mg ha −1 in the control plot to
5.7 Mg ha −1 in the fertilized plot. The initial increase in growth in response to
fertilization was too large to be accounted for by the small increase in net photosynthesis, and suggests that the carbohydrates needed for the initial increase in growth
came from the utilization of stored carbohydrates (Cropper and Gholz 1993). Later,
the higher growth rates in the fertilized plot could be sustained by the higher leaf area.
It is also possible that a shift in carbon allocation from root to shoot occurred after
fertilization, as has been demonstrated in seedlings. Brix and Ebell (1969) also noted
that fertilization had no effect on net photosynthesis in Pseudotsuga menziesii
(Mirb.) Franco trees, even though it induced a large growth response, and concluded
that the only measured factor directly associated with the increased growth of the
1224
TESKEY, GHOLZ AND CROPPER, JR.
fertilized trees was leaf area. This observation is consistent with the high correlation
between leaf area and aboveground growth after fertilization in P. taeda stands (Vose
and Allen 1988).
Evans (1989) compared published data from a variety of C3 species and found good
agreement between rates of net photosynthesis and leaf nitrogen content. Compared
with other C3 species, conifers had smaller increases in net photosynthesis per unit
increase in nitrogen concentration. A positive correlation between leaf nitrogen
content and net photosynthesis has been demonstrated for several trees including
deciduous species (e.g., Prunus persica (L.) Batsch., DeJong et al. 1989; Acer
saccharum Marsh., Reich et al. 1991), broad-leaved evergreen species (e.g., Arbutus
menziesii Pursh. and Umbellularia californica (Hook & Arn.) Nutt., Field et al. 1983;
Eucalyptus globulus Labill., Sheriff and Nambiar 1991) and evergreen conifers (e.g.,
Pinus ponderosa Dougl., DeLucia et al. 1989; P. radiata, Thompson and Wheeler
1992; P. sylvestris, Smolander and Oker-Blom 1989; Pseudotsuga menziesii,
Mitchell and Hinckley 1993). However, there are exceptions to this general response.
Van den Driessche (1972) reported that nitrogen fertilization decreased rates of net
photosynthesis in slash pine seedlings. Sheriff et al. (1986) found that nitrogen
fertilization had no effect on net photosynthesis in the foliage of P. radiata trees, even
though the nitrogen concentration was significantly higher in fertilized trees than in
unfertilized trees; however, the higher rates of net photosynthesis were associated
with an increased phosphorus content. Similarly, Reid et al. (1983) reported that the
net photosynthetic rate of P. contorta Dougl. was correlated with the concentration
of phosphorus in the foliage, but not with nitrogen. Phosphorus concentrations in the
foliage have also been correlated with rates of net photosynthesis in P. radiata trees
(Attiwill and Cromer 1982) and seedlings (Conroy et al. 1988). We found that, by
August, the foliar concentrations of both phosphorus and nitrogen had increased as
a result of fertilization (Table 1), but these increases had little effect on net photosynthesis.
In general, seedlings show a larger photosynthetic response to fertilization than
large trees (Linder and Rook 1984). It is only when relatively large differences in
nutrient concentrations in the foliage are produced by fertilization that detectable
differences in net photosynthesis can be found in older trees. For example, in
14-year-old P. radiata, a difference of 1.0% N resulted in a 20% increase in net
photosynthesis (Thompson and Wheeler 1992). However, to produce the high nitrogen content (1.9%) on the low-fertility site, large amounts of fertilizer were applied
over a long period.
The small response of net photosynthesis to vapor pressure deficit (VPD) in slash
pine has also be observed in loblolly pine (Bongarten and Teskey 1986). The natural
ranges of the two species overlap, and both are adapted to conditions of high
humidity and plentiful soil water during the growing season. Stomatal sensitivity to
increasing VPD is an obvious advantage in droughty areas and has been well
documented in various pines (e.g., P. radiata, P. contorta, and P. ponderosa) (Sandford and Jarvis 1986). If slash pine had undergone drought stress during the study, it
CLIMATE AND FERTILIZATION EFFECTS ON NET PHOTOSYNTHESIS
1225
might have been possible to detect a more sensitive VPD response. Maier and Teskey
(1992) found that P. strobus had no detectable VPD response until predawn xylem
pressure potentials were less than −1 MPa. Similar observations have been made for
other conifers including Picea sitchensis (Bong.) Carr. (Beadle et al. 1981) and
P. sylvestris (Ng and Jarvis 1978).
The high predawn xylem pressure potentials measured throughout the year and the
lack of correlation between water table depth and XPP indicated that the small
number of roots that extend deeply through the soil profile were sufficient to supply
the trees with water (cf. Teskey et al. 1985). However, it has been reported that under
dry soil conditions, all of the root system of a loblolly pine tree is needed to maintain
water uptake (Carlson et al. 1988). Apparently, in slash pine, as long as some of the
roots are in contact with the water table, the transpirational needs of the tree are met.
In summary, net photosynthetic rates of slash pine trees growing under field
conditions were limited primarily by irradiance. The second most important limitation was air temperature. The highest rates of net photosynthesis occurred at air
temperatures between 25 and 35 °C. Over the course of the year, in 73% of the hours
when irradiance was high (> 900 µmol m −2 s −1), air temperatures were within this
range. Net photosynthesis was not constrained by air temperatures in excess of 30 °C.
Similarly, water table depth and vapor pressure deficits had little effect on the rate of
Pn. Finally, although the rates of Pn were low compared with other conifer species,
as were the foliar concentrations of nutrients, two years of fertilization had only a
slight effect on Pn. The lack of a fertilizer effect on Pn was consistent with other
reports from field studies in older conifer stands. When more nutrients were made
available through fertilization, slash pine trees increased carbon gain almost exclusively by expanded leaf area rather than by increased rates of net photosynthesis.
Acknowledgments
We thank Sheryll Vogel, Denise Guerin, Kevin McKelvy and Steven Smitherman for collecting the field
data, and Amy Horne and Erika Mavity for assistance with data analysis. This project was funded by NSF
Grant BSR 8106678. The stands for this research were used with permission of the Jefferson-Smerfit
Company. Electrical power to the site was provided by the Florida Electric Power Coordinating Group.
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