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gcb415 451..466 Elevated CO2 enhances water relations and productivity and affects gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe. J A C K A . M O R G A N * , D A N I E L R . L E C A I N * , A R V I N R . M O S I E R ² and D A N I E L G . M I L C H U N A S ³ *USDA-ARS, Rangeland Resources Research Unit Crops Research Lab, 1701 Centre Ave., Fort Collins, CO 80526, USA; ²USDA-ARS, Soil, Plant Nutrient Research Unit, Federal Bldg., 301 S. Howes Street, Room 407, PO Box E, Fort Collins, CO80522, USA and ³Dept. of Rangeland Ecosystem Science and Natural Resources Ecology Lab, Colorado State University, Fort Collins, CO80523, USA Abstract Six open-top chambers were installed on the shortgrass steppe in north-eastern Colorado, USA from late March until mid-October in 1997 and 1998 to evaluate how this grassland will be affected by rising atmospheric CO2. Three chambers were main- tained at current CO2 concentration (ambient treatment), three at twice ambient CO2, or approximately 720 mmol mol±1 (elevated treatment), and three nonchambered plots served as controls. Above-ground phytomass was measured in summer and autumn during each growing season, soil water was monitored weekly, and leaf photo- synthesis, conductance and water potential were measured periodically on important C3 and C4 grasses. Mid-season and seasonal above-ground productivity were enhanced from 26 to 47% at elevated CO2, with no differences in the relative responses of C3/C4 grasses or forbs. Annual above-ground phytomass accrual was greater on plots which were defoliated once in mid-summer compared to plots which were not defoliated during the growing season, but there was no interactive effect of defoliation and CO2 on growth. Leaf photosynthesis was often greater in Pascopyrum smithii (C3) and Bouteloua gracilis (C4) plants in the elevated chambers, due in large part to higher soil water contents and leaf water potentials. Persistent downward photosynthetic acclima- tion in P. smithii leaves prevented large photosynthetic enhancement for elevated CO2-grown plants. Shoot N concentrations tended to be lower in grasses under ele- vated CO2, but only Stipa comata (C3) plants exhibited signi®cant reductions in N under elevated compared to ambient CO2 chambers. Despite chamber warming of 2.6 °C and apparent drier chamber conditions compared to unchambered controls, above-ground production in all chambers was always greater than in unchambered plots. Collectively, these results suggest increased productivity of the shortgrass steppe in future warmer, CO2 enriched environments. Keywords: Bouteloua gracilis, Pascopyrum smithii, global change, photosynthesis, acclimation, grazing, defoliation Received 14 June 2000; revised version received 8 October and accepted 18 October 2000 Introduction Atmospheric CO2 concentrations have increased from approximately 280 mmol mol±1 in the late 19th century to over 360 mmol mol±1 today, and are projected to double over present ambient concentrations by the mid- to late- 21st century (Alcamo et al. 1996). Numerous studies have been conducted to determine how CO2 will affect crops as well as natural ecosystems and their dominant species. The vast majority of these studies have been conducted with C3 plant species since C3 photosynthesis is unsaturated at present atmospheric CO2 concentrations (Stitt 1991), and most evidence suggests that growth of C3 Correspondence: fax: 970-482-2909, e-mail: morgan@ lamar.colostate.edu Global Change Biology (2001) 7, 451±466 # 2001 Blackwell Science Ltd 451 plants responds more to CO2 enrichment than C4 species. C4 photosynthesis is believed to be CO2-saturated at present atmospheric CO2 concentrations due to a highly ef®cient CO2 pump that elevates CO2 in the bundle sheath cells (Bowes 1993). However, signi®cant CO2- induced growth enhancements have been observed for many C4 species, and their sensitivity to CO2 is often similar to that observed in C3 plants (Poorter 1993; Hunt et al. 1996; Poorter et al. 1996; Wand et al. 1999; Ghannoum et al. 2000). Increasing CO2 causes stomates of most species to close, resulting in increased water use ef®ciency (Drake et al. 1996; Kirkham et al. 1991; Nie et al. 1992). This water- relations enhancement from CO2 helps explain the growth enhancement of C4 plants, especially in water- limited systems (Owensby et al. 1996, 1999; Ghannoum et al. 2000). Improved plant water status from CO2 enrichment may also stimulate C4 (and C3) plant growth by enhancing leaf expansion (Wand 1999). Although C4 photosynthesis is nearly saturated at present atmo- spheric CO2 concentrations, recent studies indicate that carbon ®xation is sometimes enhanced in C4 grasses with a doubling of CO2 concentration, especially under well- illuminated conditions (Sionit & Patterson 1984; Morgan et al. 1994a; Ghannoum et al. 1997; Ziska & Bunce 1997; LeCain & Morgan 1998; Wand 1999; Wand et al. 1999). Therefore, it appears that CO2 enhances growth in C4 species primarily through improved water relations, and secondarily through photosynthetic enhancement. While productivity and species composition of native grasslands are in¯uenced by animal grazing (Milchunas & Lauenroth 1993), few ®eld studies have explicitly evaluated the interaction of defoliation with CO2 enrich- ment (Hebeisen et al. 1997; Owensby et al. 1999). The growth enhancing effect of elevated CO2 is typically greatest in young plants, due to the high photosynthetic ef®ciency of young leaves, and also to the presence of strong sinks for assimilates (Baxter et al. 1995); these attributes may also enhance CO2-induced photosynthetic and re-growth responses of young leaves in recently defoliated canopies. However, repeated and/or severe defoliations may eliminate yield response to CO2 in the present growing season (Hebeisen et al. 1997) or weaken plants and reduce regrowth in subsequent growing seasons (Trlica et al. 1977; Menke & Trlica 1983), which presumably could affect their long-term responsiveness to CO2. In water-limited grasslands, the effects of elevated CO2 on regrowth often involve interactions with water. Owensby et al. (1999) observed that in C4-dominated tallgrass prairie, CO2 enhanced regrowth when late- season water stress occurred, presumably because of the enhanced water use ef®ciency. In a subsequent year when late-season conditions were wetter, no such CO2 enhancement of regrowth was observed. Defoliation may temporarily reduce canopy-level transpiration, thereby improving water use ef®ciency (Milchunas et al. 1995) and possibly interacting with the effect of elevated CO2 on water relations. The shortgrass steppe is a semiarid grassland along the western edge of the Great Plains of the United States, stretching from south-eastern New Mexico and western Texas north to the Colorado-Wyoming border at 41 °N latitude (Lauenroth & Milchunas 1991). Vegetation of this region is dominated by warm-season, C4 grasses (Bouteloua and Buchloe spp.), but contains an abundance of cool-season, C3 grasses (e.g. Pascopyrum and Stipa spp.), as well as a variety of C3 forbs and woody vegetation; C4 forbs and woody vegetation are un- common. Previous CO2 enrichment studies in North American grasslands have been conducted on tallgrass prairie in Kansas (Kirkham et al. 1991; Nie et al. 1992; Owensby et al. 1993b, 1996) and annual grassland in California (Chiariello & Field 1996). Only one has considered how defoliation might affect the response of grasses to CO2 (Owensby et al. 1999). This study was undertaken to evaluate how doubling the CO2 concentration in¯uences growth of important C3 and C4 species in the shortgrass steppe of eastern Colorado, USA. Based on previous controlled-environ- ment work with these species (Hunt et al. 1996; Morgan et al. 1998) we hypothesized similar and substantial growth enhancement with elevated CO2. Two defoliation regimes were initiated to simulate grazing and to determine how CO2 enrichment interacts with defolia- tion. We hypothesized that the growth-enhancing effects of elevated CO2 would be greater in summer-defoliated plots compared to plots which remained un-defoliated during the growing season. Measurements of photo- synthesis and water potential were obtained at the leaf level to evaluate how long-term exposure to high CO2 affected basic plant physiological traits in individual C3 and C4 grasses. An important goal was to determine whether photosynthetic acclimation measured pre- viously in growth chamber studies of these grasses (Morgan et al. 1994a; Read et al. 1997; LeCain & Morgan 1998) also occurred in the ®eld. Materials and methods The study site is at the USDA-ARS Central Plains Experimental Range (CPER), lat. 40°40¢ N, long. 104°45¢ W, in the shortgrass steppe region of north-eastern Colorado (Lauenroth & Milchunas 1991), about 56 km north-east of Fort Collins, CO. Long-term (55 year) mean annual precipitation averaged 320 mm, with the majority occurring during May, June and July. Mean air tempera- tures are 15.6 °C in summer and 0.6 °C in winter with 452 J A C K A . M O R G A N et al. # 2001 Blackwell Science Ltd, Global Change Biology, 7, 451±466 maximum July temperatures averaging 30.6 °C. Basal cover is 25±35% of which up to 90% is Bouteloua gracilis (H.B.K.) Lag. (blue grama), a warm season, C4 grass. In some areas the cool season, C3 grasses Pascopyrum smithii (Rydb.) A. Love (western wheatgrass) and Stipa comata Trin and Rupr. (needle-and-thread grass) are also a major vegetation component. The soil at the experimental site is a Remmit ®ne sandy loam (Ustollic camborthids). This sandy soil holds 18% water at ®eld capacity, and 4% at the permanent wilting point. The effect of elevated CO2 on this native ecosystem was investigated using open top chambers (4.5 m diameter, enclosing 15.5 m2). The experiment was estab- lished on a six ha native rangeland pasture with a mixture of C3 and C4 grass species. Prior to 1996 the ®eld had been grazed by cattle at a light to moderate intensity (about 30% annual forage removal). A portion of the pasture was initially divided into three blocks based on uniformity of vegetation, and three 15.5 m2 circular plots per block were randomly chosen as experimental plots. From late March until mid-October in 1997 (March 20 and October 18) and 1998 (March 24 and October 13), open top chambers were placed on two plots in each of the three blocks (six total). One chamber was randomly assigned an ambient CO2 treatment (360 6 20 mmol mol±1), the other an elevated CO2 treatment (720 6 20 mmol mol±1). Carbon dioxide fumigation pro- ceeded as soon as the chambers were placed on the plots, and continued until they were removed in the autumn. Each block also had an unchambered plot of equal ground area, which was used to monitor the effect of the chamber. Chambers were constructed with six 3.8 m high by 2.5 m wide walls made with a Unistrut galvanized steel tubing frame (Unistrut Corp. Wayn, MI, USA) covered with clear, Lexan* (Regal Plastics, Littleton, CO, USA) panels. The top was covered with a Unistrut and Lexan frustrum, reducing the opening to 0.75 m diameter. Chambers were aspirated with outside air by large fans