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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona 7 Effects of Increasing Carbon Dioxide Levels and Climate Change on Plant Growth, Evapotranspiration, and Water Resources Leon Hartwell Allen, Jr. U.S. Department of Agriculture Gainesville, Florida The atmospheric carbon dioxide concentration has risen from about 270 parts per million (ppm) before 1700 to about 355 ppm today. Climate changes, including a mean global surface temperature rise of between 2.8 and 5.2°C, have been predicted by five independent general circulation models (GCMs) for a doubling of the carbon dioxide concentration. The objectives of this paper are to examine plant responses to rising carbon dioxide levels and climatic changes and to interpret the consequences of these changes on crop water use and water resources for the United States. BACKGROUND: PLANT RESPONSES TO ENVIRONMENTAL FACTORS The main purpose of irrigation is to supply plants with adequate water for transpiration and for incorporating the element hydrogen in plant tissues through photosynthesis and subsequent biosynthesis of various tissues and organs. Transpirational flux requires several hundred times more water than photosynthesis. In a series of U.S. Department of Agriculture studies beginning in 1910 in Akron, Colorado, Briggs and Shantz (1913a,b; 1914) showed that the water requirement of plants is linearly related to the biomass production of plants. They established this linear relationship by growing plants in metal containers filled with soil. Throughout the period of growth, they monitored water use carefully by weighing and adding measured amounts of water to maintain a desirable soil water content as water lost by plant transpiration was replenished.
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona The findings of Briggs and Shantz have been confirmed repeatedly (Allison et al., 1958; Arkley, 1963; Chang, 1968; Hanks et al., 1969; Stanhill, 1960). Figure 7.1 shows the linear relationship between biomass produced and rainfall plus irrigation water used by Sart sorghum and Starr millet in Alabama, as adapted from data of Bennett et al. (1964). De Wit (1958) examined the relationships among climatic factors, yield, and water use by crops. He found the following general linear relationship to be true, especially in semiarid climates: where Y = yield component (e.g., total above-ground biomass or seed production) T = cumulative actual transpiration Tmax = maximum possible cumulative transpiration m = constant dependent on yield component and species, especially on differences among photosynthetic mechanisms Pan evaporation was used to represent Tmax, which is proportional to climatic factors, especially air vapor pressure deficit (VPD): where es = the saturation vapor pressure at a given air temperature ea = the actual vapor pressure that exists in the air. Combining these relationships, we see that yield is proportional to cumulative transpiration divided by vapor pressure deficit: where k is a constant with units millibars • g (dry matter) • g-1 (water). Like m, k depends on yield component, species, and photosynthetic mechanisms.
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona FIGURE 7.1 Linear relationship between biomass production and water use for two forage crops in 1956 and 1957 at Thorsby, Alabama. Squares: Sart sorghum. Triangles: Starr millet. SOURCE: Adapted from Bennett et al., 1964. Thus, we can see that theory predicts that yield will be proportional to cumulative transpirational water use, divided by vapor pressure deficit. There are several ways of calculating the VPD; it can be computed by aggregating seasonal daytime average VPD, or by using approximation methods based on daily maximum and minimum temperatures (Jensen, 1974). As pointed out by Tanner and Sinclair (1983), the maximum es can be computed from the daily maximum temperature, and ea can be estimated from the daily minimum temperature. Tanner and Sinclair estimated that the effective daytime es falls at a point two-thirds to three-quarters of the distance between the es computed at the daily maximum temperature and the ea computed at the daily minimum temperature. The effective daytime VPD values then must be averaged over the growing season of the crop. Regardless of the method used to compute a representative VPD, yield versus cumulative transpiration linear relationships vary with the aridity of the
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona climate—specifically with the temperature and vapor pressure regime under which the crop is grown. Figure 7.2 (modified from Stanhill, 1960) shows water used versus dry matter yield of pastures from the latitude of Denmark (which has a cool, humid atmosphere) to the latitude of Trinidad (which has a hot, dry atmosphere). Based on comparisons among existing climates, we can expect that transpirational water requirements of plants will increase if climates get warmer. Atmospheric carbon dioxide is known to affect plant yield. Kimball (1983) reviewed 430 observations of carbon dioxide enrichment studies conducted prior to 1982 and reported an average yield increase of 33 percent, plus or minus 6 percent, for a doubling of the carbon dioxide concentration. This value has been generally confirmed by many other studies since that time. The yield increases seem to apply for both biomass accumulation and grain yield. Thus, plants may grow larger and, considering Figure 7.1, they may use more water as the global carbon dioxide concentration increases. Transpirational water use is clearly related to ground cover (Jensen, 1974; Doorenbos and Pruitt, 1977). Daily water use soon after crops are planted on bare soil is typically only 10 to 20 percent of water use after effective ground cover is reached. Water use rises sharply as the crop's leaf area increases. Similarly, water use drops 60 to 70 percent when hay crops such as alfalfa are cut. As leaf regrowth occurs, transpiration rates recover rapidly as the ground cover of leaves is restored. Ground cover can be quantified with a leaf area index (LAI): the ratio of leaf area per unit ground area. Therefore, any carbon dioxide-induced stimulation of early growth of leaf area or increase of total leaf area growth may increase transpiration. Increased carbon dioxide concentrations are known to cause smaller stomatal apertures and hence to decrease the leaf conductance for water vapor (Morison, 1987). This is a second mechanism whereby increased carbon dioxide concentrations may affect plant transpiration. Another effect of rising carbon dioxide concentrations is the change in water-use efficiency (WUE). Water-use efficiency has a range of definitions. For whole-season processes, it is best defined as the ratio of dry matter (or seed yield) produced to the amount of water used by crops. For shorter-term whole canopy processes, it is best defined as the ratio of the photosynthetic carbon dioxide uptake rate per unit land area to the transpiration rate per unit land area. Figure 7.2 demonstrates the effect of climate on WUE.
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona FIGURE 7.2 Cumulative dry matter yield versus cumulative potential evapotranspiration (ET) of pastures under a range of climatic regimes. Open circle: Denmark. Filled circle: The Netherlands. Open triangle: England. Filled triangle: New Jersey. Open Square: Toronto, Canada. Filled square: Gilat, Israel. Open inverted triangle: Trinidad, West Indies. SOURCE: Adapted from Stanhill, 1960. Equation 3 quantifies the relationship between WUE and vapor pressure deficit. In summary, the following relationships have been established by research: Transpiration is linearly related to biomass accumulation and yield. Transpiration is also linearly related to the aridity of the climate—in other words, to the vapor pressure deficit. Thus, rising global temperatures would increase transpiration by increasing the atmospheric vapor pressure deficit.
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona Transpiration is affected by the degree of ground cover. Rising carbon dioxide concentrations will increase plant growth. More rapid leaf area development and more total leaf area could translate into more transpiration. Rising carbon dioxide concentrations will decrease leaf stomatal conductance to water vapor. This effect could reduce transpiration. Rising carbon dioxide concentrations and rising global temperatures could change WUE. The following sections of this chapter will examine more closely the effects of rising carbon dioxide concentrations and climate change on vegetation, providing qualitative and quantitative assessments of how these changes will affect photosynthesis, growth, and transpiration water requirements of crops. DIRECT EFFECTS OF CARBON DIOXIDE ON PHOTOSYNTHESIS, TRANSPIRATION, AND GROWTH OF PLANTS Atmospheric Carbon Dioxide The carbon dioxide concentration of the earth's atmosphere has varied throughout geologic time. Ice core data from Antarctica and Greenland have been obtained and, from entrapped air bubbles, used to show carbon dioxide and methane concentrations of the atmosphere throughout the past 160,000 years (Barnola et al., 1987; Lorius et al., 1990). Changes in the deuterium content within ice crystals have been used to establish temperature changes over this same time period (Jouzel et al., 1987). In general, carbon dioxide concentrations were as low as 180 to 200 parts per million (ppm) 13,000 to 30,000 years ago and 140,000 to 160,000 years ago during the coldest parts of the last two ice ages (Barnola et al., 1987). Carbon dioxide concentrations rose to about 270 ppm during the last interglacial period (116,000 to 140,000 years ago) and during the current interglacial period (beginning about 13,000 years ago). Ice core data since about 1700 A.D. and direct atmospheric sampling data since 1958 show that the carbon dioxide concentration increased to 315 ppm by 1958 and to about 355 ppm by 1990 (Keeling et al., 1989). The rate of increase of atmospheric carbon dioxide is about 0.5 percent per year, which means that the change is accelerating.
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona These changes in atmospheric carbon dioxide have important implications for plants and the global carbon cycle as well as for climate. Atmospheric carbon dioxide is the raw material for terrestrial green plant photosynthesis, and thus it represents the first molecular link in the food chain of the whole earth. In later sections, we will examine the importance of carbon dioxide for photosynthesis and plant growth, as well as the importance of potential climate change on water resources for the future. Plant Photosynthetic Mechanisms Three types of photosynthetic mechanisms of terrestrial green plants have been identified: C3, C4, and CAM. Responses of these three photosynthetic mechanisms to carbon dioxide have been reviewed by Tolbert and Zelitch (1983). The biochemical pathway of photosynthetic carbon dioxide uptake was first determined for C3 plant photosynthesis. This pathway involves the use and subsequent regeneration of ribulose 1,5-biophosphate in a cyclic series of reactions, and it is frequently called the Calvin cycle. The first product of photoassimilation of carbon dioxide is 3-phosphoglyceric acid, a three-carbon sugar—hence the term C3 pathway of photosynthesis. The C4 plants begin their carbon dioxide uptake in a different process sometimes called the Hatch-Slack pathway. In mesophyll cells of leaves, these plants form a four-carbon molecule, oxalacetate, in the first step of incorporation of carbon dioxide. This four-carbon compound is changed into aspartic acid or malic acid and then transported immediately to bundle sheath cells. Here, the carbon dioxide is released and utilized in the C3 biochemical pathway. Thus, the C4 plant mechanism first traps carbon dioxide in the mesophyll cells, and then transports and concentrates the carbon dioxide in the bundle sheath cells, where it is utilized in C3 plant metabolism (Tolbert and Zelitch, 1983). Crassulacean acid metabolism, or CAM, is a mechanism whereby plants typically take up and store carbon dioxide during the night and use it in photosynthetic carbon dioxide fixation during the day, when sunlight is available. Pineapple and ''air plants,'' such as Spanish moss and orchids, have this photosynthetic mechanism. Since few agricultural crops are CAM plants, they are not important in the process of managing water resources under conditions of climate uncertainty. Since C4 plants have a mechanism for concentrating carbon dioxide in bundle sheath cells of leaves, their photosynthetic rates
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona will not respond to rising carbon dioxide levels to the same extent as C3 plants. Irrigated crop or turf plants that fit into the C4 category include maize (corn), sorghum, millet, sugar cane, and bermuda grass. Plants that fit into the C3 category include: wheat, rice, potato, soybean, sugar beet, alfalfa, cotton, tree and vine crops, and most vegetable crops and cool-season grasses. Plant Growth Responses to Carbon Dioxide Increasing atmospheric carbon dioxide levels have caused increasing photosynthetic rates, biomass growth, and seed yield for all of the globally important C3 food and feed crops (Acock and Allen, 1985; Enoch and Kimball, 1986; Warrick et al., 1986; Allen, 1990). Some plants, such as cucumber, cabbage, and perhaps tomato, have shown a tendency to first increase leaf photosynthetic rates in response to elevated carbon dioxide concentrations, and then to decrease photosynthetic rates after several days. This behavior is called "end-product inhibition of photosynthesis," and it is caused by the failure of translocation of photoassimilates to keep up with photosynthetic rates (Guinn and Mauney, 1980). A few experiments have been conducted with carbon dioxide concentration maintained across a range of 160 to 990 ppm. Figure 7.3 shows the results of one study with soybean canopy photosynthetic rates across the 90 to 900 ppm carbon dioxide concentration range. A nonlinear hyperbolic model was used to fit soybean photosynthetic rate data to carbon dioxide concentration (Allen et al., 1987). Photosynthetic rates at the various carbon dioxide concentrations were divided by the photosynthetic rate at a carbon dioxide concentration of 330 ppm to normalize the data to a common condition. Data sets of biomass yield and seed yield from four locations over three years were also fit to the model (Allen et al., 1987). Relative yields with respect to yields at 330 to 340 ppm were used. The form of the model fit to the experimental data was: where R = relative response of photosynthetic rate, biomass yield, or seed yield Rmax = asymptotic upper limit for R from baseline Rint
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona FIGURE 7.3 Photosynthetic carbon dioxide uptake rate responses of a soybean crop canopy exposed to carbon dioxide concentrations ranging from 110 to 990 ppm. All data points are relative to the response obtained at 330 ppm. SOURCE: Adapted from Allen et al., 1987. C = carbon dioxide concentration (ppm) Kc = Apparent Michaelis constant (ppm) Rint = Y-axis intercept for zero C From the parameters of this equation, photosynthetic rate, biomass accumulation, and seed yield changes of soybean due to carbon dioxide concentration changes can be estimated (Allen et al.,
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona 1987). Table 7.1 shows the changes predicted across three time periods: from the last ice age (when the carbon dioxide concentration was at a minimum) to the preindustrial revolution era (about 1700), from 1700 to 1973, and from 1973 to about a century into the future. The modeled data show that there should have been large increases in productivity between the ice-age (when carbon dioxide concentration was about 200 ppm) and the beginning of the industrial revolution (when the carbon dioxide concentration was about 270 ppm). Likewise, there should have been a 12 percent increase in grain-yield productivity between 1700 and 1973, when the carbon dioxide concentration increased from about 270 to 330 ppm. Most of the recent concerns about rising atmospheric carbon dioxide concentrations have been quantified by predicting changes for a doubling of the carbon dioxide concentration, usually from 330 to 660 ppm. Table 7.1 shows that soybean seed yields and biomass yields are predicted to increase 31 percent and 41 percent, respectively, from a doubling of carbon dioxide. Experimental studies have consistently showed a lower seed yield than biomass yield for soybean when grown under a doubled carbon dioxide concentration. If the harvest index—the ratio of seed yield to above-ground biomass yield (seed plus pod walls plus stems)—were 0.50 for soybean grown under a 330 ppm carbon dioxide concentration, then the harvest index would be 0.46 if the carbon dioxide concentration were doubled. This small decrease in soybean harvest index under elevated carbon dioxide conditions has been commonly observed (Allen, 1990; Jones et al., 1984). The relative midday maximum photosynthetic rates under carbon dioxide enrichment were consistently higher than relative biomass yields, probably because the photosynthetic response to elevated carbon dioxide levels is greater under high light conditions than it is under total daily solar irradiance conditions. Transpiration Responses to Carbon Dioxide The effect of carbon dioxide concentration on water use under field conditions has been discussed for many years. In the past, elevated carbon dioxide levels have been mentioned as the ideal antitranspirant. This conclusion seems reasonable, since elevated carbon dioxide has been observed to reduce stomatal conductance in numerous experiments. Morison (1987) reviewed 80 observations in the literature and found that a doubled carbon dioxide con-
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona TABLE 7.1 Percent increases of soybean midday photosynthesis rates, biomass yield, and seed yield predicted across selected carbon dioxide concentration ranges associated with relevant benchmark points in time. CO2 Concentration Period of Time Initial Final Midday Photosynthesis Biomass Yield Seed Yield --------ppm-------- ------% increase over initial CO2----- IA–17001 200 270 38 33 24 1700–1973 270 330 19 16 12 1973–20??2 330 660 50 41 31 1 IA, the Ice Age about 13,000 to 30,000 years before present. The atmospheric carbon dioxide concentrations that prevailed during the last Ice Age, and from the end of the glacial melt until preindustrial revolution times, were 200 and 270 ppm, respectively. 2 The first world energy "crisis" occurred in 1973, when the carbon dioxide concentration was 330 ppm. This concentration is used as the basis for many carbon dioxide–doubling studies. The carbon dioxide concentration is expected to double sometime within the twenty–first century. centration will reduce stomatal conductance of most plants by about 40 plus or minus 5 percent. Kimball and Idso (1983) calculated a 34 percent reduction in transpiration in response to a doubled carbon dioxide concentration in several short-term plant growth chamber experiments, which seems consistent with the review by Morison (1987). However, Morison and Gifford (1984) also showed that doubling carbon dioxide will cause a more rapid development of leaf area for many plants and hence an equal or greater transpiration rate in the early stages of plant growth, due to a more rapid development of transpiring surfaces. Therefore, increased rates of development of transpiring leaf surface offset the reduced stomatal conductance for water vapor. Allen et al. (1985) and Allen (1990) also discussed the effect of reduction in stomatal conductance on foliage temperature. The cause-and-effect relationships can be summarized as follows: Any reduction in stomatal conductance due to increasing the carbon dioxide concentration will restrict transpiration rates per unit leaf area. A reduction in transpiration rates will result in less eva-
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona ppm, seed yield should increase by about 31 percent, which is in agreement with a 33 plus or minus 6 percent increase in plant growth and yields reported in a literature survey by Kimball (1983). Doubling the carbon dioxide concentration will cause stomatal conductance of leaves to decrease by about 40 percent. However, whole crop cumulative transpiration over the course of a season may only be reduced by 0 to 12 percent, for two reasons. First, when stomata partially close and restrict vapor from leaves, the foliage temperature rises. This raises the vapor pressure of water in the intercellular air space of leaves and increases the leaf-to-air vapor pressure difference. Thus, the reduction of stomatal conductance is substantially overcome by the driving force for evaporation. Secondly, elevated carbon dioxide levels promote growth of a greater amount of leaf area, so that a larger surface area for transpiration exists. In conclusion, a 40 percent reduction in stomatal conductance probably provides only a 10 to 15 percent reduction in transpirational water use. The greater amount of leaf area under elevated carbon dioxide conditions could eliminate this small difference in crop water use. Increases in WUE of carbon dioxide-enriched crops is due largely to sizable increases in photosynthesis, growth, and yield. Decreases in water use are small and contribute very little, if anything, to increases in WUE. Plants exposed to elevated carbon dioxide concentrations (650 ppm in comparison with 350 ppm) have shown greater response to carbon dioxide at high average daily temperatures than at low average daily temperatures during vegetative growth across the range of 12 to 35°C. This response is in agreement with single leaf photosynthetic measurements of responses to elevated carbon dioxide across a range of temperatures. One could conclude that all crops will respond more to the combination of increasing temperature with elevated carbon dioxide than to elevated carbon dioxide levels alone. However, during reproductive growth of soybean plants, the opposite trend was found. The complex responses of various kinds of plants to interactions of carbon dioxide, temperature, water supply, light, and photoperiod (day length) need further research. Increasing temperatures across the range of 28 to 35°C appears to increase the transpiration rate by about 4 to 5 percent per 1°C, as shown in both experimental and modeling studies. This is in close agreement with the rise in saturation vapor pressure of about 6 percent per 1°C.
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona Outputs from two GCMs, the GISS and GFDL models, for a doubled carbon dioxide concentration were used as examples of predictions of soybean crop yield in the southeastern United States. The GISS model predicted about 25 percent more June through August rainfall than baseline 1951 through 1980 values, and the GFDL model predicted about 40 percent less rainfall. Under rainfed conditions and considering only the effects of climate change, the soybean crop model predicted a decrease in yields of 23 and 71 percent for the GISS and GFDL scenarios, respectively, compared with baseline weather conditions. When the direct effect of carbon dioxide on plant growth was included, the predictions were plus 11 percent for the GISS model and minus 52 percent for the GFDL model. When the simulated crops were irrigated, there was little difference between the predicted yields (plus 13 to 14 percent) of the two climate change scenarios. These data illustrate the critical importance of temperature and especially rainfall in any climate change scenario. Predicted crop yields for other regions (the Great Lakes, corn belt, Great Plains, and California) in response to carbon dioxide and climate change were generally similar to those of the Southeast. Under the soybean simulations for the southeastern United States, the average requirement for irrigation increased by 33 percent and 134 percent above the baseline for the GISS and GFDL scenarios, respectively. Rising carbon dioxide levels will cause an increase of photosynthetic rates, growth, yield, and water use efficiency for C3 crop plants. Water use per unit land area will not change much unless temperatures increase. Elevated temperatures may reduce seed crop yields in most areas of production, and potential rainfall decreases could cause serious reductions in rain-fed agriculture. Under irrigated agriculture conditions, reductions of precipitation could limit the number of acres available for irrigation and could lead to serious competition for water resources among various users. Changes in precipitation amounts and distribution are the most serious climate change factors, from the standpoint of both crop productivity and water resources. However, low rainfall during the summer growing season does not necessarily mean a large reduction in overall water yield of a basin. If high rainfall should occur during the cool seasons, annual runoff, streamflow, and reservoir storage may not be impacted as much as if there were no change in seasonal patterns. Thus, water resources for nonagricultural uses may not be impacted as much as water for agriculture. Models that incorporate sufficient details of the hydro-
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona logic cycle, as well as vegetation and energy balance factors, should be developed to provide a more informed physical basis for managing water resources. This information could be used in assessing long-range ecological, food supply, economic, and societal consequences of water management decisions under conditions of climate uncertainty. ACKNOWLEDGMENTS This work was supported in part by the U.S. Department of Energy Interagency Agreements DE-AI05-88ER69014 and DE-AI01-81ER60001 with the U.S. Department of Agriculture, Agricultural Research Service. This work was conducted in cooperation with the University of Florida at Gainesville. Florida Agricultural Experiment Station Journal Series number R-01423. REFERENCES Acock, B., and L. H. Allen, Jr. 1985. Crop responses to elevated carbon dioxide concentration. Pp. 53-97 in B. R. Strain and J. D. Cure, eds., Direct Effects of Increasing Carbon Dioxide on Vegetation. Report DOE/ER-0238. Washington, D.C.: U.S. Department of Energy, Carbon Dioxide Research Division. Adams, R. M., C. Rosenzweig, R. M. Peart, J. T. Ritchie, B. A. McCarl, J. D. Glyer, R. B. Curry, J. W. Jones, K. J. Boote, and L. H. Allen, Jr. 1990. Global climate change and U.S. agriculture. Nature 345:219-224. Allen, L. H., Jr. 1990. Plant responses to rising carbon dioxide and potential interactions with air pollutants. J. Environ. Qual. 19:15-34. Allen, L. H., Jr., K. J. Boote, J. W. Jones, P. H. Jones, R. R. Valle, B. Acock, H. H. Rogers, and R. C. Dahlman. 1987. Response of vegetation to rising carbon dioxide: Photosynthesis, biomass, and seed yield of soybean. Global Biogeochemical Cycles 1:1-14. Allen, L. H., Jr., P. Jones, and J. W. Jones. 1985. Rising atmospheric CO2 and evapotranspiration. Pp. 13-27 in Advances in Evapotranspiration. ASAE Pub. 14-85. St. Joseph, Michigan: American Society of Agricultural Engineers. Allen, L. H., Jr., W. G. Knisel, and P. Yates. 1982. Evapotranspiration, rainfall, and water yield in south Florida research watersheds. Soil Crop Sci. Soc. Fla. Proc. 41:127-139.
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona Allen, R. G., and F. N. Gichuki. 1989. Effects of projected CO2-induced climatic changes on irrigation water requirements in the Great Plains States (Texas, Oklahoma, Kansas, and Nebraska). Appendix C (Agriculture), Vol. 1, Chapter 6 in J. B. Smith and D. A. Tirpak, eds., The Potential Effects of Global Climate Change on the United States. Report EPA-230-05-89-053. Washington, D.C.: U.S. Environmental Protection Agency. Allen, R. G., F. N. Gichuki, and C. Rosenzweig. 1991. CO2-induced climatic changes on irrigation water requirements. J. Water Resources Planning and Management 117:157-178. Allen, S. G., S. B. Idso, B. A. Kimball, and M. G. Anderson. 1988. Interactive effects of CO2 and environment on photosynthesis of Azolla . Agric. For. Meteorol. 42:209-217. Allen, S. G., S. B. Idso, and B. A. Kimball. 1990a. Interactive effects of CO2 and environment on net photosynthesis of water lily. Agric., Ecosystems, and Environ. 30:81-88. Allen, S. G., S. B. Idso, B. A. Kimball, J. T. Baker, L. H. Allen, Jr., J. R. Mauney, J. W. Radin, and M. G. Anderson. 1990b. Effects of Air Temperature on Atmospheric CO2-Plant Growth Relationships--TR048 . Report DOE/ER-0450T. Washington, D.C.: U.S. Department of Energy and U.S. Department of Agriculture. Allison, F.E., E. M. Roller, and W. A. Raney. 1958. Relationship between evapotranspiration and yield of crops grown in lysimeters receiving natural rainfall. Agron. J. 50:506-511. Arkley, R. J. 1963. Relationship between plant growth and transpiration. Hilgardia 34:559-584. Aston, A. R. 1984. The effect of doubling atmospheric CO2 on streamflow: a simulation. Jour. Hydrol. 67:273-280. Baker, J. T., L. H. Allen, Jr., K. J. Boote, P. Jones, and J. W. Jones. 1989. Response of soybean to air temperature and carbon dioxide concentration. Crop Sci. 29:98-105. Barnola, J. M., D. Raynaud, Y. S. Korotkevich, and C. Lorius. 1987. Vostok ice core provides 160,000-year record of atmospheric CO2. Nature 329:408-414. Bennett, O. I., B. D. Doss, D. A. Ashley, V. J. Kilmer, and E. C. Richardson. 1964. Effects of soil moisture regime on yield, nutrient content, and evapotranspiration for three annual forage species. Agron. J. 56:195-198. Berry, J., and O. Björkman. 1980. Photosynthetic response and adaptation to temperature in higher plants. Ann. Rev. Plant Physiol. 31:491-543. Bowes, G., and W. L. Ogren. 1972. Oxygen inhibition and other properties of soybean ribulose-1, 5-diphosphate carboxylase. J. Biol. Chem. 247:2171-2176.
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Representative terms from entire chapter: