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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 Introduction 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). 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. Results - Diurnal light-saturated net photosynthesis rates Results and discussion Amax vs. gmax relationship Results and discussion Apparent stomatal control of stand transpiration 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 - Precipitation Figure 8. Relationship between light-saturated net photosynthesis rate and light-saturated stomatal conductance for Pinus elliottii foliage formed in 1999. N=114 Methods Plantation 10 -year-old Pinus elliottii, density = 2080 trees ha-1, average DBH = 9. 8 cm. Sampling Figure 6. Diurnal patterns of light-saturated net photosynthesis rate in Pinus elliottii flatwoods on four days during a developing drought. 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. 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. 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). 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. Photosynthetic parameters 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 k. Pa; Block temperature = 25º-30ºC; [CO 2] = 370 µmol mol-1; A / Ci curves generated with chamber [CO 2] = 50, 100, 200, 370, 400, 600, 800, 1200, 1500 µmol mol-1; 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). Figure 3. Monthly precipitation (top) and cumulative precipitation (bottom) for the study site. Results and discussion Pre-dawn leaf water potential Summary Results and Discussion- Diurnal stomatal limitation to light-saturated net photosynthesis • The water and carbon relations of Pinus elliottii flatwoods are strongly impacted by prolonged drought • Non-stomatal components of photosynthetic capacity (Vcmax and Jmax) showed a slight decline as the drought progressed Stomatal limitation was calculated after Jones (1985) (Figure 1). Tree sap flow 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). • 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% Figure 4. Pre-dawn leaf water potential. Methods - calculating gas phase limitation 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). • Stomatal effects were manifested at the canopy level as decreased stand transpiration under similar VPD conditions Literature Cited Results and discussion - Vcmax and Jmax Ellsworth, D. S. 2000. Seasonal CO 2 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 CO 2 assimilation in leaves of C 3 species. Planta 149: 78 -90. Figure 7. Diurnal patterns of stomatal limitations to net photosynthesis from September 1999 - May 2000 for Pinus elliottii foliage formed in 1999. 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. 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 (A 2 A 1)/A 2 where A 1 is the net photosynthesis rate under ambient conditions, and A 2 is the net photosynthesis rate that would be achieved if the gas phase limitation was eliminated (i. e. Ci = ambient [CO 2] = 370 µmol mol-1). 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. Harley, P. C. and T. D. Sharkey. 1991. An improved model of photosynthesis at high CO 2: Reversed O 2 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 CO 2. 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 C 3 plants: physics, physiology and rate limitations. Botanical Review 51: 53 -105. 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). 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.