Two Years of Carbon Dioxide Enrichment on the Shortgrass Steppe of Colorado J.A. Morgan, D.R. LeCain, A.R. Mosier, D.G. Milchunas, W.J. Parton and D.S. Ojima USDA/ARS and Colorado State University, Fort Collins, CO, USA INTRODUCTION In North America, the Great Plains represents the largest of the native grasslands, stretching south to north from Mexico into Canada and east to west from the Front Range of the Rocky Mountains to the eastern deciduous forest. Along the western edge of the Great Plains lies the shortgrass steppe, an important grassland of which 50% today remains intact. Growth chamber studies have indicated a doubling of CO2 concentrations over current ambient levels will enhance growth of the native shortgrass steppe grasses by approximately 20% (Hunt et al. 1996). Growth enhancements from CO2 have been attributed to improved water status and increased photosynthesis in both C3 and C4 grasses (Morgan et al., 1994; Read et al., 1997). The objective of this experiment was to use open-top chambers (OTCs) to double CO2 concentrations over native shortgrass steppe to evaluate the effect within an intact grassland on plant production, physiology, soil microbiology, and trace gas exchange (CH4, N2O, NO). Herein we report on the first two year's findings for plant responses. Materials and Methods Experimental Site: Shortgrass Steppe of Colorado 42% Bouteloua gracilis (C4) 26% Stipa comata (C3) 21% Pascopyrum smithii(C3) 11% other species (27) Treatments: Control Plots (non-chambered) Ambient CO2 OTC (360 ppm) Elevated CO2 OTC (720 ppm) Measurements: Above-ground phytomass Neutron probe soil water content Pressure chamber leaf water potential Steady-state leaf gas exchange Fig. 2. Precipitation and soil moisture. Growing season precipitation was high in the first two years of this study, with 562 and 422 mm in 1997 and 1998, compared with a long-term average of 320 mm. In 1997, after a significant mid-summer drought, an unusual amount of precipitation occurred at the site. This was followed by a separation in soil moisture due to CO2 treatment, with ambient plots drying out faster than elevated plots. This soil water effect was maintained over the winter and continued until the later part of the 1998 season. Soil moisture in the unchambered plots was higher than the ambient plots indicating a desiccating effect of the chambers. LITERATURE CITED Hunt, HW, ET Elliott, JK Detling, JA Morgan and D-X Chen. 1996. Responses of a C3 and a C4 perennial grass to elevated CO2 and temperature under different water regimes. Global Change Biology 2:35-47. Morgan, JA, WG Knight, LM Dudley and HW Hunt. 1994. Enhanced root system C-sink activity, water relations and aspects of nutrient acquisition in mycotrophic Bouteloua gracilis subjected to CO2 enrichment. Plant and Soil 165:139-146. Read, JJ, JA Morgan, NJ Chatterton and PA Harrison. 1997. Gas exchange and carbohydrate and nitrogen concentrations in leaves of Pascopyrum smithii (C3) and Bouteloua gracilis (C4) at different carbon dioxide concentrations and temperatures. Ann. Bot. 79:197-206. ACKNOWLEDGEMENTS. This research was funded by USDA/ARS and funding received from the Terrestrial Ecology and Global Change Program (IBN-9524068), an inter-agency granting program of the United States Government. We thank Dean Ackerman, Scott Andre, Mary Ashby, Susan Crookall, Dennis Mueller, Mary Smith, Larry Tisue, Jeff Thomas and Barry Weaver for their technical contributions to the project, and without whom it would not have been possible. Results Fig. 1. Above-ground phytomass. In 1997 production at peak standing crop was 27% higher in elevated (E) vs. ambient (A) chambers. In 1998 elevated CO2 improved production by 43% compared to ambient plots, while ambient plots had 29% more phytomass than unchambered (U) plots, probably due to warmer temperatures in the spring (chambers averaged 2o C warmer than outside). Improved growth occurred in C3 and C4 grasses and forbs. Fig. 3. Plant water status. Leaf water potentials largely reflected treatment differences in soil water content, and were often higher in elevated CO2 plots compared to ambient plots in both C3 grasses (P. smithii and S. comata) and the C4 grass B. gracilis. Fig. 4. Photosynthesis. Measuring CO2 exchange rate (CER) over a range of CO2 concentrations shows that neither B.gracilis (C4) nor P. smithii (C3) were saturated at 360 ppm. CO2 exchange rate was increased about 72% in P. smithii and 20% in B. gracilis at 720 vs. 360 ppm (arrows) within each CO2 treatment. The degree of photosynthetic acclimation that occurred in P. smithii was striking, with elevated CO2 grown plants having much lower rates than ambient CO2 grown plants. Bouteloua gracilis also displayed a small degree of photosynthetic acclimation to high CO2. Acclimation was so significant on some dates that when we compare CER measured at the growth CO2 concentration (asterisks) there is little photosynthetic advantage in plants grown at elevated CO2. On other dates photosynthesis was higher under elevated CO2. Bouteloua gracilis C O 2 e x c h a n g e r a te ( µ m o l m -2 s -1 ) 0 10 20 30 40 Pascopyrum smithii Intercellular CO2 (µmol mol -1 ) 0 200 400 600 800 1000 0 10 20 30 40 Ambient CO2 Elevated CO2 ** ** measured at treatment CO2 conc.* A b o v e g ro u n d p h y to m a s s ( g m -2 ) 0 50 100 150 200 U A E U A E U A E Forbs C 3 grasses C 4 grasses a b b c b a 1996 1997 1998 CONCLUSIONS 1. Doubling CO2 from 360 to 720 ppm enhanced aboveground production of shortgrass steppe vegetation an average of 35%. 2. No difference was detected in the growth response of C3 vs. C4 grasses to elevated CO2. 3. Warmer spring temperatures in the chambered plots (about 2o C) increased productivity by about 22% over the unchambered plots. 4. CO2 enrichment reduced soil water usage, and improved leaf water potentials in C3 and C4 shortgrass steppe plants. Improved water status was a major factor in the increased productivity in elevated CO2 plots. 5. In the C3 grass, P. smithii, photosynthetic acclimation under elevated CO2 consistently reduced leaf photosynthetic rate relative to leaves from ambient chambers. Photosynthetic acclimation was also evident at times in the C4 grass B. gracilis, although to a lesser degree than in P. smithii. Acclimation greatly reduced the photosynthetic advantage at elevated CO2, especially in the C3 grass. S o il m o is tu re ( m m m -1 ) 80 120 160 200 Day of the year 1997 100 150 200 250 P re c ip it a ti o n ( m m ) 0 10 20 30 40 50 Ambient Elevated Unchambered Day of the year 1998 100 150 200 250 300 Ambient Elevated Unchambered L e a f w a te r p o te n ti a l (M P a ) -5 -4 -3 -2 -1 -5 -4 -3 -2 -1 120 140 160 180 200 220 240 260 280 -5 -4 -3 -2 -1 Ambient CO2 Elevated CO2 Unchambered Bouteloua gracilis Pascopyrum smithii Stipa comata Day of the year 1997 Bouteloua gracilis Pascopyrum smithii 120 140 160 180 200 220 240 260 280 300 Stipa comata Day of the year 1998 ABSTRACT This study assessed how doubling the CO2 concentration over present levels affects the growth and physiology of shortgrass steppe vegetation in eastern Colorado. In March, 1997, six open-top chambers (OTCs) were installed on native shortgrass steppe in NE Colorado, USA. Three grass species make up about 88% of the above-ground biomass of this ecosystem; Bouteloua gracilis (C4), Pascopyrum smithii (C3) and Stipa comata (C3). More than 20 other grass and forb species also occur here. CO2 was injected into three OTCs to raise the concentration to 720 ppm, approximately twice that in the three ambient chambers. Three non-chambered plots were established to evaluate chamber effects. The air temperature in the chambers averaged 2o C warmer than outside. During 1997 and 1998 significant chamber and CO2 effects were detected. Averaging over the two years, above-ground production in the ambient chambers was 22% greater than that in unchambered plots, probably due to warmer spring temperatures in the chambers. Production under elevated CO2 averaged 35% greater than that in ambient OTCs. Significant growth increases occurred for both C3 and C4 grasses and forbs in 1998. These CO2 -induced growth increases were primarily due to improved water status. Soil water content was often higher in elevated CO2 chambers. Leaf water potentials were generally higher in plants grown at elevated CO2 compared to ambient chambers. Leaf intercellular CO2 photosynthesis response curves indicated neither P. smithii nor B. gracilis leaves were saturated with CO2 at 360 ppm. Photosynthetic capacity of both species was reduced in plants grown at elevated CO2, although this response was much stronger in the the C3 species, P. smithii. Results suggest that future CO2 enrichment will lead to growth enhancements in both C3 and C4 grasses of the shortgrass steppe.