Water and Carbon Relations of Pinus elliottii Flatwoods Subjected to Drought
Timothy A. Martin
School of Forest Resources and Conservation, University of Florida
Methods - sap flow probes
Results - Diurnal light-saturated net photosynthesis rates
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Light-saturated net photosynthesis rate (Amax) showed consistent diurnal declines throughout the study period. Peak Amax occurred early in the
morning, and steadily declined throughout the afternoon. By 15:00 EST, Amax was generally less than 50% of its early morning value. On May 25,
the morning peak Amax was 1.9 µmol m-2 s-1, less than half of the morning peak earlier in the drought.
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Lab-built, 20-mm long Granier-style heat dissipation probes (Granier
1987) were used to measure sap flow rates in eight trees ranging from
8.4 to 13.1 cm DBH (Figure 2).
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Pre-dawn leaf water potential declined to nearly -1.0 MPa by May 25,
2000. Pinus elliottii flatwoods seldom experience pre-dawn leaf water
potentials lower than -0.6 MPa (Teskey et al. 1994). On May 25,
volumetric soil moisture content in the upper 50 cm of soil was less
than 6% (data not shown).
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Results and discussion - Vcmax and Jmax
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Figure 9. Response of Pinus elliottii daily stand transpiration to average daily vapor
pressure deficit from April 11 to June 14, 2000.
Transpiration in well-coupled conifer stands is strongly controlled by vapor pressure
deficit and stomatal conductance. In this study, stand transpiration at any given VPD
level decreased as the drought progressed. This indicates declining canopy
conductance, which could result from reduced canopy leaf area, reduced stomatal
conductance, or some combination of these two factors. Given the evidence in Figure
7, it is likely that declines in stomatal conductance are dominant in this phenomenon.
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Figure 5. Trends in Vcmax and Jmax for Pinus elliottii.
Carboxylation capacity (Vcmax) and light-saturated electron transport
capacity (Jmax) exhibited small declines over the course of the study.
Measurements taken in mid-June, after several significant rainfall
events, show slight increases in Vcmax and Jmax (data not shown).
• Stomatal limitation of photosynthesis seldom declined
below 40%, and approached 100% as the drought
progressed
• Strong stomatal limitations led to diurnal declines in Amax
exceeding 50%
• Stomatal effects were manifested at the canopy level as
decreased stand transpiration under similar VPD conditions
Literature Cited
Ellsworth, D. S. 2000. Seasonal CO2 assimilation and stomatal limitations in a Pinus taeda canopy. Tree Physiology
20:435-445.
Farqhuar, G.D., S. Von Caemmerer and J.A. Berry. 1980. A biochemical model of photosynthetic CO2 assimilation in
leaves of C3 species. Planta 149:78-90.
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V(umcol max orJ -2 smax-1 )
NetPhosyntheisRate(umol -2 s -1 )
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Figure 1. Schematic illustrating how A/Ci curves were used to
calculate the gas-phase or stomatal limitation to photosynthesis
(Jones 1985). Gas phase limitation was calculated as (A2A1)/A2 where A1 is the net photosynthesis rate under ambient
conditions, and A2 is the net photosynthesis rate that would be
achieved if the gas phase limitation was eliminated (i.e. Ci =
ambient [CO2] = 370 µmol mol-1).
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• Non-stomatal components of photosynthetic capacity
(Vcmax and Jmax) showed a slight decline as the drought
progressed
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Net photosynthesis and stomatal conductance were
strongly correlated in this study, with observations over a
span of 10 months apparently following the same function.
The relationship is clearly non-linear; in other studies of
southern pine gas exchange, this relationship is often
linear (Teskey et al.1986, Ellsworth 2000).
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Figure 4. Pre-dawn leaf water potential.
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Figure 8. Relationship between light-saturated net
photosynthesis rate and light-saturated stomatal
conductance for Pinus elliottii foliage formed in 1999.
N=114
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• The water and carbon relations of Pinus elliottii flatwoods
are strongly impacted by prolonged drought
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Summary
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Results and discussion Pre-dawn leaf water potential
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Results and Discussion- Diurnal stomatal limitation to light-saturated net photosynthesis
Stomatal limitation was calculated after Jones (1985) (Figure1).
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Figure 3. Monthly precipitation (top) and cumulative precipitation
(bottom) for the study site.
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Methods - calculating gas phase limitation
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Vcmax and Jmax were calculated from A/Ci curves using the methods of
Farquhar et al. (1980), von Caemmerer and Farquhar (1981), Sharkey
(1985), Harley and Sharkey (1991) and Harley et al. (1992). Curve
fitting and parameter calculations were performed with Photosyn
Assistant software (Dundee Scientific, Dundee, Scotland).
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Figure 6. Diurnal patterns of light-saturated net photosynthesis rate in Pinus elliottii flatwoods on four days during a developing drought.
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Study Data
16-year mean
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Net photosynthesis was measured with a Li-6400 portable
photosynthesis system (Li-Cor, Lincoln, NE). Chamber conditions were
as follows: PPFD = 2000 µmol m-2 s-1; VPD = 1.5 - 2.0 kPa; Block
temperature = 25º-30ºC; [CO2] = 370 µmol mol-1; A / Ci curves
generated with chamber [CO2] = 50, 100, 200, 300, 370, 400, 600, 800,
1200, 1500 µmol mol-1;
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Precipitation (mm)
Photosynthetic parameters
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Repeated measurements were taken in the upper half of the crowns of
eight trees, on the first flush of foliage formed in 1999. Measurements
were taken in September 1999 and March, April and May 2000.
LNetPhosyniRateight-Saur(uemdol -2s -1)
LNetPhosyniRateight-Saur(uemdol -2s -1)
Alachua county, Florida
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10-year-old Pinus elliottii, density = 2080 trees ha-1, average DBH = 9.8
cm.
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Methods
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Results and discussion Apparent stomatal control of stand transpiration
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Results and discussion - Precipitation
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The objective of this study was to characterize tree physiological
responses to these presumably severe water deficits, and to determine
the existence and mechanism of any limitations to carbon gain resulting
from those water deficits.
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Figure 2. 20 mm long Granier-style heat dissipation sap flow
probes installed in a Pinus elliottii stem (left). Insulation and
protection for the probes were provided by styrofoam spheres
(center), reflective plastic bubble wrap, and polyethylene sheets
(right).
Results and discussion Amax vs. gmax relationship
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Pine flatwoods are the most extensive type of terrestrial ecosystem in
Florida, occupying about 50% of the state’s land area. Flatwoods
characteristically are located in low-lying areas, have level topography
and relatively poorly-drained, acidic, sandy soil. This research centers
on a 10-year-old Pinus elliottii (slash pine) plantation growing on a
flatwoods site 20 km northeast of Gainesville, Florida. This site
normally receives over 1300 mm of rain annually, evenly distributed
throughout the year. Starting in the fall of 1998, the region entered a
series of droughts that subjected vegetation to early growing season
(January-May) precipitation almost 60% below normal. Previous
research has suggested that water limitations seldom if ever limit carbon
gain in these systems (Teskey et al. 1994).
NetPhosyntheisRateLig(uhmt-Soalurted -2 s -1 )
Introduction
Figure 7. Diurnal patterns of stomatal limitations to net photosynthesis from September 1999 - May 2000 for Pinus elliottii foliage formed in 1999.
Granier, A. 1987. Mesure du flux de sève brute dans le tronc du Douglas par une nouvelle méthode thermique. Annales
des Sciences Forestieres 44:1-14.
Short term, diurnal declines in Amax (Figure 6) were primarily attributable to stomatal limitations. Stomata strongly controlled photosynthesis rates
in this study, with stomatal limitations increasing from about 0.4 early in the morning, to almost 1.0 in the afternoon, late in the drought cycle.
Ellsworth (2000) found similar levels of stomatal limitation in a mature Pinus taeda canopy during periods of drought stress. By comparison, Teskey
et al. (1986) found that stomatal limitations in Pinus taeda seedlings were relatively small, remaining below 0.3 in almost all cases, and not
exceeding 0.39 in drought-stressed plants. Other studies have found similarly small stomatal limitations in drought stressed seedlings (Ni and
Pallardy 1992, Stewart et al. 1994). These observations reinforce the difficulty of extrapolating results from studies conducted on seedlings under
artificial drought treatments to mature trees under naturally-occurring drought.
Harley, P.C. and T.D. Sharkey. 1991. An improved model of photosynthesis at high CO2: Reversed O2 sensitivity
explained by lack of glycerate re-entry into the chloroplast. Photosynthesis Research 27:169-178.
Harley, P.C., R.B. Thomas, J.F. Reynolds and B.R. Strain. 1992. Modelling photosynthesis of cotton grown in elevated
CO2. Plant, Cell and Environment 15:271-282.
Jones, H.G. 1985. Partitioning stomatal and non-stomatal limitations to photosynthesis. Plant, Cell and Environment
8:95-104.
Ni, B.-R. and S.G. Pallardy. 1992. Stomatal and nonstomatal limitations to net photosynthesis in seedlings of woody
angiosperms. Plant Physiology 99:1502-1508.
Sharkey, T.D. 1984. Photosynthesis of intact leaves of C3 plants: physics, physiology and rate limitations. Botanical
Review 51:53-105.
Stewart, J.D., A. Z. El Abidine and P.Y. Bernier. 1994. Stomatal and mesophyll limitations of photosynthesis in black
spruce seedlings during multiple cycles of drought. Tree Physiology 15:57-64.
Teskey, R.O., J.A. Fites, L.J. Samuelson and B.C. Bongarten. 1986. Stomatal and nonstomatal limitations to net
photosynthesis in Pinus taeda L. under different environmental conditions. Tree Physiology 2:131-142.
Teskey, R.O., H.L. Gholz and W.P. Cropper, Jr. 1994. Influence of climate and fertilization on net photosynthesis of
mature slash pine. Tree Physiology 14:1215-1227.
Von Caemmerer, S. and G.D. Farquhar. 1981. Some relationships between the biochemistry of photosynthesis and the
gas exchange rates of leaves. Planta 153:376-387.
Acknowledgements
Drs. Ken Clark and Henry Gholz supplied
meteorological and inventory data and
valuable discussions. Dr. Nathan Phillips
provided schematics and advice for
construction of sap flow probes. David
Nolletti and Sean Gallagher helped with data
collection. Funding was provided by UF's
Institute of Food and Agricultural Sciences,
the Forest Biology Research Cooperative and
a grant from DOE/NIGEC. Rayonier
provided access to the study site.