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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 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. Methods Plantation 10-year-old Pinus elliottii, density = 2080 trees ha-1, average DBH = 9.8 cm. Sampling 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. 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 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; 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). Stomatal limitation was calculated after Jones (1985) (Figure1). 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). 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. 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. 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 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. Summary 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 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

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