Effects of Elevated CO2 on Water Use

Knapp, A.K., J.T. Fahnestock, and C.E. Owensby. 1993. Elevated atmospheric CO2 alters stomatal response to sunlight in a C4 grass. Plant Cell and Environ. 17:189-195. PDF

Knapp, A.K., E.P. Hamerlynck, and C.E. Owensby. 1993. Photosynthetic and water relations responses to elevated CO2 in the C4 grass, Andropogon gerardii. Int. J. Plant Sci. 154:459-466.

Knapp, A.K., E.P. Hamerlynck, J. M. Ham, and C.E. Owensby. 1995. Responses in stomatal conductance to elevated CO2 and open-top chambers in 12 grassland species that differ in growth form. Vegetatio 125:31-41. PDF

Knapp, A.K., M. Cocke, E.P. Hamerlynck, and C.E. Owensby. 1994. Effects of elevated CO2 on stomatal density and distribution in a C4 grass and a C3 forb under field conditions. Annals Botany 74:595-599. PDF

Knapp, A.K., E.P. Hamerlynck, J. M. Ham, and C.E. Owensby. 1995. Responses in stomatal conductance to elevated CO2 and open-top chambers in 12 grassland species that differ in growth form. Vegetatio 125:31-41. PDF

Owensby, C.E., J.M. Ham, Alan K. Knapp, D. Bremer, and L.M. Auen. 1996. Water vapor fluxes and their impact under elevated CO2 in a C4 tallgrass prairie. Global Change Biology 3:189-195. .PDF

Hamerlynck, Erik P. , Christine A. McAllister, Alan K. Knapp, Jay M. Ham and Clenton E. Owensby. 1997. Photosynthetic gas exchange and water relation responses of three tallgrass prairie species to elevated carbon dioxide and moderate drought. Int. J. Plant Science 158:608-616. .PDF

Adam, Neal R., Clenton E. Owensby, and Jay M. Ham. 2001. The Effect of CO2 Enrichment on Photosynthetic Rates and Instantaneous Water Use Efficiency of Andropogon gerardii in the Tallgrass Prairie. Photosynthesis Research 65:121-129. .PDF

Ham, J.M., Owensby, C.E., Coyne, P.I., and D.J. Bremer. 1995. Fluxes of CO2 and water vapor from a prairie ecosystem exposed to ambient and elevated atmospheric CO2. Agric. Forest Meteorol. 77:73-93.

Stomatal Conductance

Reductions in water use by plants under elevated CO2 ultimately come from a reduced stomatal conductance (gs) of water vapor from the leaf. Changes in leaf area, canopy structure, and different stomatal responses among species will modify the effects of elevated CO2 on stomatal conductance making scaling to the canopy level difficult. In order to determine the relationships of leaf-level stomatal conductance to higher levels, we have measured gs regularly for A. gerardii, and occasionally for S. nutans and various forb and woody species. In 1991 than 1992, A. gerardii gs was significantly lower (by 21-51%) in CE than in CA or A plots during early, mid, and late growing season (Knapp et al. 1993a). In 1993, in conjunction with sap flow measurements, we showed that big bluestem gs was 6.6 mm s-1 in CA and A plots and 3.2 mm s-1 in CE plots, and indiangrass gs was 5.0 mm s-1 CA and A plots and 3.1 mm s-1 in CE plots (Bremer et al. 1996). Later in 1994, Knapp et al. (1996) measured seasonal responses in stomatal conductance (gs) to elevated CO2 among 12 tallgrass prairie species that differed in growth form and rate. During June, when water availability was high, elevated CO2 resulted in decreased gs in 10 of the 12 species measured. Lower gs under elevated CO2 (ca. 50% lower) occurred in growth forms with the highest potential growth rates (C3 and C4 grasses, and C3 ruderals). In contrast, gs of two C3 shrubs under elevated and ambient CO2. During a dry period in September, lower gs at elevated CO2 was measured in only two species (a C3 ruderal and a C4 grass) and higher gs at elevated CO2 was measured in the shrubs and a C3 forb. The greater gs at elevated CO2 was attributed to enhanced leaf water potential (v leaf) resulting from increased soil water availability and/or greater root biomass. After rainfall in September, only lower gs was measured in response to elevated CO2. Thus, there was significant interspecific variability in stomatal responses to CO2 that may be related to growth form or rate and to plant water relations. That the response was not uniform throughout the season nor among species confirms the need to consider interactions between indirect effects of elevated CO2 on plant water relations and direct effects of elevated CO2 on gs, particularly for ecosystems in which water availability often limits productivity.

Stomatal Response to Sun/Shade Events

Stomates open and close in response to sun/shade events (Knapp et al. 1993b), and the length of time that it takes for the stomate to close affects water loss. In those species for which stomatal response time to the shade event is slow, water loss is greater than in those species with faster response times. Under elevated CO2, stomatal response to sunlight may impart additional water savings in A. gerardii, the dominant C4 grass in tallgrass prairie (Knapp et al. 1993b). In A, CA, and CE plots, A. gerardii was subjected to fluctuations in sunlight similar to that resulting from intermittent clouds or within canopy shading (full sun > 1500 umol m-2 s-1 vs. shade 350 umol m-2 s-1) and stomatal conductance measured. Time constants describing stomatal responses were significantly lower (29-33%) at elevated CO2. Using a statistically-based simulation model (Knapp 1992,1993), the effect of different stomatal response times between ambient and elevated CO2 on transpiration were simulated. That simulation indicated that water loss was reduced by 6.5% due to more rapid stomatal responses in elevated CO2 treatments compared to ambient. Leaf xylem pressure potential increased during periods of sunlight variability, indicating that more rapid stomatal responses at elevated CO2 enhanced plant water status. It is important to note that CO2-induced alterations in the kinetics of stomatal responses to variable sunlight will likely amplify direct effects of elevated CO2 and increase WUE in all ecosystems.

Stomatal Density, Distribution, and Size

Many authors have theorized that, as atmospheric CO2 increases, stomatal density and/or size will decrease due to the reduced requirement for gas exchange. Those studies that have measured stomatal density have not shown a consistent decline in density with increased atmospheric CO2 and have largely not been conducted in intact ecosystems (Woodward 1987; Korner 1988; Woodward and Bazzaz 1988; Ferris and Taylor 1994). Knapp et al. (1994) measured stomatal density, abaxial and adaxial distribution, and guard cell length of two common tallgrass prairie species, A. gerardii, and the C3 forb, Salvia pitcheri, in A, CA, and CE plots throughout the 1993 growing season. After full canopy development, stomatal density on abaxial and adaxial surfaces and guard cell length were determined. High rainfall amounts during the 1993 growing season minimized water stress in these plants (leaf xylem pressure potential was usually > -1-5 MPa in A. gerardii) and also minimized differences in water status among treatments. In A. gerardii, stomatal density was significantly higher (190 + 7 mm-2; mean ± s.e.) in plants in A plots compared to plants in CA plots (161 ± 5 mm-2). Thus, there was a significant 'chamber effect' on stomatal density. At elevated levels of CO2, stomatal density was even lower (P < 0-05; 121 ± 5 mm-2). Most stomata were on abaxial leaf surfaces in A. gerardii, but the ratio of adaxial to abaxial stomatal density was greater at elevated levels of CO2 than at ambient. In S. pitcheri, stomatal density was also significantly higher for plants in A plots (235 ± 10 mm-2) compared to plants in CA plots (140 ± 6 mm-2). However, stomatal density was greater in CE plots (218 ± 12 mm-2) compared to plants in CA plots. The ratio of stomata on adaxial vs. abaxial surfaces did not vary significantly in this herb. Guard cell lengths were not significantly affected by any treatment for either species. These results indicate that stomatal density responses to elevated CO2 are species specific.

Sap Flow

Measurement of transpiration of the entire plant both improves estimates of elevated CO2 effects on water use by plants and offers the opportunity to measure differences among species at the whole plant level. Heat balance sap flow gauges (Senock and Ham 1993, 1995) were used to measure transpiration in ironweed (Vernonia baldwini var. interior (Small) Schub.), a C3 forb, and on individual grass culms of A. gerardii and S. nutans, both C4 grasses, in CE, CA and A plots in 1993 (Bremer et al. 1996). Because of frequent rainfall during 1993, all data were collected under well-watered conditions. Comparisons of plants in the CE and CA plots showed that sap flow was reduced by 33% in ironweed, 18% in big bluestem, and 22% in indiangrass under CO2 enrichment. Soil water was consistently highest under elevated CO2, reflecting the lower transpiration with that treatment. During sap flow measurements, whole-plant stomatal resistance to water vapor flux in big bluestem was 103 s m-1 in CA plots and 194 s m-1 in CE plots. Whole plant transpiration was considerably lower under elevated CO2 than in ambient CO2, but the difference between gs for CE and CA was even greater, indicating the need to measure transpiration at least at the whole plant level or measure gs for leaves throughout the plant canopy in order to adjust for differences in radiation and vapor pressure deficit.

Xylem Pressure Potential

Xylem pressure potential (y) reflects the water status of a plant at a given point in time and integrates the soil water and environmental impacts on plant water status. Therefore y offers a better estimate of the effects of elevated CO2 on water status of plants growing in competition with others in the ecosystem. Owensby et al. (1993) measured the diurnal course of y on July 29, 1991. Both midday y, between 1200 and 1300 hrs. CST, and pre-dawn y were estimated at approximately weekly intervals in CE, CA, and A plots from May 30, to September 25, 1991, a dry year. Midday y indicated less moisture stress for A. gerardii plants in CE plots compared to those in CA or A plots. The seasonal mean midday A. gerardii y was -1.89 MPa ± 0.03 (se) for the CE plots, -2.18 MPa ± 0.03 for the CA plots, and -2.38 MPa ± 0.04 for the A plots. The diurnal y on 29 July 1991 of A. gerardii was less negative in CE plots than in CA and A plots from 1200 to 1400 hrs. CST, and A. gerardii y was less negative at 2000 hr in CE and CA plots than in A plots. Predawn A. gerardii y did not differ among treatments. Knapp et al. (1993a) measured midday y of A. gerardii plants in the CE, CA, and A plots throughout the 1991 and 1992 growing seasons. Midday y was significantly higher (less negative) throughout the season in plants grown at elevated CO2 during both years. When averaged over the growing season, y was 0.48 to 0.70 MPa lower in 1991 than 1992. Elevated CO2 improves plant water status of A. gerardii plants growing in a natural stand compared to ambient CO2 levels.

Ecosystem-Level Water Vapor Fluxes

Whole chamber water vapor fluxes at the ecosystem level offer the best opportunity to scale estimates of the impact of elevated CO2 on water use to the landscape level. In 1993, we measured whole-chamber water vapor fluxes and net carbon exchange (NCE) in CE and CA plots using the method of Ham et al. (1993). Continuous data were collected over a 34-day period when the canopy was near peak biomass (LAI 4 to 5) and soil water was not limiting. Results showed that elevated CO2 reduced evapotranspiration by 22% and also increased NCE compared to ambient CO2 (Ham et al., 1994).

Impacts of Improved Water Status under Elevated CO2

The impact of the reduced stomatal conductance, the reduced transpiration (sap flow measurements), and the improved water status of the plant (xylem pressure potential) under elevated CO2 are reflected by the lower ET at the ecosystem level. Daily ET was 22% lower with CO2 enrichment compared to ambient, sap flow was 18-33% lower, and canopy resistance to water vapor flux was 24 s m-1 greater with CO2 enrichment than at ambient. Not surprisingly, greater NCE at the ecosystem level under elevated CO2 was primarily caused by continued photosynthesis in the CE plots when water stress stopped leaf gas exchange in the CA plots. Greater NCE and lower ET resulted in higher daytime water use efficiency (WUE) in elevated CO2 than in ambient (9.84 vs. 7.26 g CO2 kg-1 H20). Additionally, whole-chamber data collected on days with high evaporative demand showed that ecosystem quantum yield, (umol CO2 umol PAR-1) in the CE plots remained high in the afternoon period, but decreased in the CA plots (e.g., CA - 0.021 umol umol-1, CE - 0.029 umol umol-1). These data tend to confirm the leaf-level measurements of Owensby et al. (1993) and Knapp et al. (1993a) that showed more favorable leaf water potentials under elevated CO2. Lower sap flow and greater canopy resistance to water vapor transport in the CE plots than in the CA plots provide further evidence that CO2 strongly influenced the hydrology and plant water relations of the ecosystem. Data collected at the leaf, whole-plant, and ecosystem scale all suggest that C4 plant communities exposed to elevated CO2 will maintain a more favorable water status when subjected to periodic moisture stress. However, if water availability is sufficient to not limit growth, there will not be an improved biomass production under elevated CO2.

The improved water status of the ecosystem under elevated CO2, when water limits ecosystem production, explains the increased above- and belowground biomass production in dry years reported by Owensby et al. (1993, 1996) (Fig. 1&2). Compared to ambient CO2 levels, elevated CO2 increased production of C4 grass species, but not of C3 grass species. Belowground biomass production, estimated by root ingrowth bags, responded similarly to that of the aboveground, but the relative increase was greater than that aboveground during dry years. Relative amounts of C4 grasses did not change from 1989 to 1996, but P. pratensis (C3) declined, and C3 forbs increased in the stand exposed to elevated CO2 compared to ambient. It is likely that the reduction in C3 grasses was partly due to the lack of grazing, which allowed the taller C4 grasses to quickly overtop the shorter C3 species, but, since the C3 grass populations in the ambient CO2 treatments remained relatively high, the primary force behind the decline of the C3 grasses was probably the increased biomass and leaf area in CO2-enriched plots. There was also drought in two of the initial three years in the study, which favored the C4 species. The taller C3 forbs increased under elevated CO2, supporting the hypothesis that canopy response (i.e. competition for light) associated with CO2 enrichment affected interspecific competition. The major limit to C3 grasses in the northern Kansas Flint Hills is nitrogen, while C4 grasses are limited more by water, and the relatively greater impact of elevated CO2 on water relations made it unlikely that the C3 grasses would competitively displace the C4 grasses component.

Water Use Efficiency

Hammerlynck et al. (1997) exposed undisturbed tallgrass prairie to ambient and elevated (twice ambient) levels of atmospheric CO2 and experimental dry periods. Seasonal and diurnal midday leaf water potential, net photosynthesis (Anet, and stomatal conductance (gs) responses of three tallgrass prairie growth forms, a C4 grass, A. gerardii, a broad-leaved woody C3 shrub, Symphoricarpos orbiculatus, and a C3 perennial forb, Salvia pitcheri, were assessed. Leaf water potential in A. gerardii and S. orbiculatus was higher under elevated CO2, regardless of soil moisture, while Leaf water potential in S. pitcheri responded only to drought. Elevated CO2, always stimulated Anet in the C3 species, while Anet of A. gerardii increased only under dry conditions. However, Anet under elevated CO2 in the C3 species declined with drought, but not in the C4 grass. Under wet conditions, gs was reduced in elevated CO2 for all species. During dry periods, gs at elevated CO2 was sometimes higher than in ambient CO2. Their results support claims that elevated CO2 will stimulate tallgrass prairie productivity during dry periods, and possibly reduce temporal and spatial variability in productivity in these grasslands. In water-stressed grasslands, it appears that the C4 grasses may have a competitive advantage over C3 species under elevated CO2 due to improved water relations.

Elevated CO2 in the tallgrass prairie increases water use efficiency, and in ecosystems in which water availability is a primary limiting resource, it will increase primary production until some other resource limits growth.

Literature Cited

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Ham, J.M., C.E. Owensby, and P.I. Coyne. 1993. Technique for measuring air flow and CO2 flux in large open-top chambers. J. Environ. Qual. 22:759-766

Ham, J.M., Owensby, C.E., Coyne, P.I., and D.J. Bremer. 1995. Fluxes of CO2 and water vapor from a prairie ecosystem exposed to ambient and elevated atmospheric CO2. Agric. Forest Meteorol. 77:73-93.

Knapp, A.K., E.P. Hamerlynck, and C.E. Owensby. 1993a. Photosynthetic and water relations responses to elevated CO2 in the C4 grass, Andropogon gerardii. Int. J. Plant Sci. 154:459-466.

Knapp, A.K., J.T. Fahnestock, and C.E. Owensby. 1993b. Elevated atmospheric CO2 alters stomatal response to sunlight in a C4 grass. Plant Cell and Environ. 17:189-195.

Knapp, A.K., M. Cocke, E.P. Hamerlynck, and C.E. Owensby. 1994. Effects of elevated CO2 on stomatal density and distribution in a C4 grass and a C3 forb under field conditions. Annals Botany 74:595-599.

Knapp, A.K., E.P. Hamerlynck, J. M. Ham, and C.E. Owensby. 1996. Responses in stomatal conductance to elevated CO2 and open-top chambers in 12 grassland species that differ in growth form. Vegetatio (in press)

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Owensby, C.E., P.I. Coyne, J.M. Ham, L.M. Auen, and A.K. Knapp. 1993. Biomass production in a tallgrass prairie ecosystem exposed to ambient and elevated levels of CO2. Ecological Appl. 3:644-653.

Owensby, C.E., J.M. Ham, A.K. Knapp, C.W. Rice, P.I. Coyne, and L.M. Auen. 1996. Ecosystem-Level responses of Tallgrass Prairie to elevated CO2. In Carbon Dioxide and Terrestrial Ecosystems. Koch, G., and H. Mooney, eds. Physiological Ecology Series. pp.175-193. Academic Press. New York.

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Figure 1. Mean total aboveground peak biomass (g m-2) for native tallgrass prairie exposed to twice-ambient and ambient CO2 concentrations for the indicated years. Means within species or species groups with a common letter do not differ [Duncan's Multiple Range Test, P< 0.10].

Figure 2. Root biomass (g m-2) in ingrowth bags to a 15 cm depth in tallgrass prairie exposed to twice ambient and ambient CO2 concentrations for the indicated years. Data are means of four bags per plot in 1990 and eight bags per plot in the other years. Means with a common letter within year do not differ [Duncan's Multiple Range Test, P< 0.10] .

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