Global warming is predicted to induce desiccation in many world regions through increases in evaporative demand. Rising CO(2) may counter that trend by improving plant water-use efficiency. However, it is not clear how important this CO(2)-enhanced water use efficiency might be in offsetting warming-induced desiccation because higher CO(2) also leads to higher plant biomass, and therefore greater transpirational surface. Furthermore, although warming is predicted to favour warm-season, C(4) grasses, rising CO(2) should favour C(3), or cool-season plants. Here we show in a semi-arid grassland that elevated CO(2) can completely reverse the desiccating effects of moderate warming. Although enrichment of air to 600 p.p.m.v. CO(2) increased soil water content (SWC), 1.5/3.0 °C day/night warming resulted in desiccation, such that combined CO(2) enrichment and warming had no effect on SWC relative to control plots. As predicted, elevated CO(2) favoured C(3) grasses and enhanced stand productivity, whereas warming favoured C(4) grasses. Combined warming and CO(2) enrichment stimulated above-ground growth of C(4) grasses in 2 of 3 years when soil moisture most limited plant productivity. The results indicate that in a warmer, CO(2)-enriched world, both SWC and productivity in semi-arid grasslands may be higher than previously expected.

Climatic changes are altering Earth's hydrological cycle, resulting in altered precipitation amounts, increased interannual variability of precipitation, and more frequent extreme precipitation events. These trends will likely continue into the future, having substantial impacts on net primary productivity (NPP) and associated ecosystem services such as food production and carbon sequestration. Frequently, experimental manipulations of precipitation have linked altered precipitation regimes to changes in NPP. Yet, findings have been diverse and substantial uncertainty still surrounds generalities describing patterns of ecosystem sensitivity to altered precipitation. Additionally, we do not know whether previously observed correlations between NPP and precipitation remain accurate when precipitation changes become extreme. We synthesized results from 83 case studies of experimental precipitation manipulations in grasslands worldwide. We used meta-analytical techniques to search for generalities and asymmetries of aboveground NPP (ANPP) and belowground NPP (BNPP) responses to both the direction and magnitude of precipitation change. Sensitivity (i.e., productivity response standardized by the amount of precipitation change) of BNPP was similar under precipitation additions and reductions, but ANPP was more sensitive to precipitation additions than reductions; this was especially evident in drier ecosystems. Additionally, overall relationships between the magnitude of productivity responses and the magnitude of precipitation change were saturating in form. The saturating form of this relationship was likely driven by ANPP responses to very extreme precipitation increases, although there were limited studies imposing extreme precipitation change, and there was considerable variation among experiments. This highlights the importance of incorporating gradients of manipulations, ranging from extreme drought to extreme precipitation increases into future climate change experiments. Additionally, policy and land management decisions related to global change scenarios should consider how ANPP and BNPP responses may differ, and that ecosystem responses to extreme events might not be predicted from relationships found under moderate environmental changes.

Summary 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 maintained at current CO2 concentration (ambient treatment), three at twice ambient CO2, or approximately 720 μmol 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 photosynthesis, 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 acclimation in P. smithii leaves prevented large photosynthetic enhancement for elevated CO2‐grown plants. Shoot N concentrations tended to be lower in grasses under elevated CO2, but only Stipa comata (C3) plants exhibited significant 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.

Abstract. The impact of increasing atmospheric CO 2 concentrations has been studied in a number of field experiments, but little information exists on the response of semiarid rangelands to CO 2 , or on the consequences for forage quality. This study was initiated to study the CO 2 response of the shortgrass steppe, an important semiarid grassland on the western edge of the North American Great Plains, used extensively for livestock grazing. The experiment was conducted for five years on native vegetation at the USDA-ARS Central Plains Experimental Range in northeastern Colorado, USA. Three perennial grasses dominate the study site, Bouteloua gracilis, a C 4 grass, and two C 3 grasses, Pascopyrum smithii and Stipa comata. The three species comprise 88% of the aboveground phytomass. To evaluate responses to rising atmospheric CO 2 , we utilized six open-top chambers, three with ambient air and three with air CO 2 enriched to 720 mol/mol, as well as three unchambered controls. We found that elevated CO 2 enhanced production of the shortgrass steppe throughout the study, with 41% greater aboveground phytomass harvested annually in elevated compared to ambient plots. The CO 2 -induced production response was driven by a single species, S. comata, and was due in part to greater seedling recruitment. The result was species movement toward a composition more typical of the mixed-grass prairie. Growth under elevated CO 2 reduced the digestibility of all three dominant grass species. Digestibility was also lowest in the only species to exhibit a CO 2 -induced production enhancement, S. comata. The results suggest that rising atmospheric CO 2 may enhance production of lower quality forage and a species composition shift toward a greater C 3 component.

A hypothesis has been advanced that the incursion of woody plants into world grasslands over the past two centuries has been driven in part by increasing carbon dioxide concentration, [CO 2], in Earth's atmosphere. Unlike the warm season forage grasses they are displacing, woody plants have a photosynthetic metabolism and carbon allocation patterns that are responsive to CO2, and many have tap roots that are more effective than grasses for reaching deep soil water stores that can be enhanced under elevated CO2. However, this commonly cited hypothesis has little direct support from manipulative experimentation and competes with more traditional theories of shrub encroachment involving climate change, management, and fire. Here, we show that, although doubling [CO 2] over the Colorado shortgrass steppe had little impact on plant species diversity, it resulted in an increasingly dissimilar plant community over the 5-year experiment compared with plots maintained at present-day [CO 2]. Growth at the doubled [CO 2] resulted in an Ϸ40-fold increase in aboveground biomass and a 20-fold increase in plant cover of Artemisia frigida Willd, a common subshrub of some North American and Asian grasslands. This CO 2-induced enhancement of plant growth, among the highest yet reported, provides evidence from a native grassland suggesting that rising atmospheric [CO 2] may be contributing to the shrubland expansions of the past 200 years. Encroachment of shrubs into grasslands is an important problem facing rangeland managers and ranchers; this process replaces grasses, the preferred forage of domestic livestock, with species that are unsuitable for domestic livestock grazing.have increased from Ϸ280 volumetric ppm in preindustrial times to Ϸ380 ppm today and are projected to exceed 600 ppm by the end of this century, it is perhaps more important to point out that CO 2 levels are higher today than they have been for at least 650,000 years (1). Furthermore, levels of atmospheric CO 2 for the past half million years have tended to stay closer to the lowest glacial levels of Ϸ180 ppm compared with the Ϸ280-300 ppm of interglacial periods. These recent abrupt changes in atmospheric CO 2 have tremendous implications for the adaptation and evolution of relatively modern ecosystems, such as C 4 grasslands, that have evolved under relatively low atmospheric [CO 2 ] by today's standards. This report focuses on the responses of vegetation in a Colorado semiarid grassland to growth at variable [CO 2 ], but our report has implications for other rangelands around the world.Rangelands comprise Ͼ40% of Earth's terrestrial surface (2). Although these lands are characteristically water-limited and unsuitable for intensive agriculture, they support one of the world's most extensive agricultural practices, domestic livestock grazing (3-5). Rangelands are important not only for the plant and animal products they provide but also as regions in which distinct pastoral cultures and societies have developed. Rising atmospheric [CO 2 ] and predicted glob...