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Premise: The impact of elevated CO2 concentration ([CO2]) and climate warming on plant productivity in dryland ecosystems is influenced strongly by soil moisture availability. We predicted that the influence of warming on the stimulation of photosynthesis by elevated [CO2] in prairie plants would operate primarily through direct and indirect effects on soil water. Methods: We measured light‐saturated photosynthesis (Anet), stomatal conductance (gs), maximum Rubisco carboxylation rate (Vcmax), maximum electron transport capacity (Jmax) and related variables in four C3 plant species in the Prairie Heating and CO2 Enrichment (PHACE) experiment in southeastern Wyoming. Measurements were conducted over two growing seasons that differed in the amount of precipitation and soil moisture content. Results: Anet in the C3 subshrub Artemisia frigida and the C3 forb Sphaeralcea coccinea was stimulated by elevated [CO2] under ambient and warmed temperature treatments. Warming by itself reduced Anet in all species during the dry year, but stimulated photosynthesis in S. coccinea in the wet year. In contrast, Anet in the C3 grass Pascopyrum smithii was not stimulated by elevated [CO2] or warming under wet or dry conditions. Photosynthetic downregulation under elevated [CO2] in this species countered the potential stimulatory effect under improved water relations. Warming also reduced the magnitude of CO2‐induced down‐regulation in this grass, possibly by sustaining high levels of carbon utilization. Conclusions: Direct and indirect effects of elevated [CO2] and warming on soil water was an overriding factor influencing patterns of Anet in this semi‐arid temperate grassland, emphasizing the important role of water relations in driving grassland responses to global change. [ABSTRACT FROM AUTHOR]

The flux of carbon dioxide (CO2) between terrestrial ecosystems and the atmosphere may ameliorate or exacerbate climate change, depending on the relative responses of ecosystem photosynthesis and respiration to warming temperatures, rising atmospheric CO2, and altered precipitation. The combined effect of these global change factors is especially uncertain because of their potential for interactions and indirectly mediated conditions such as soil moisture. Here, we present observations of CO2 fluxes from a multi-factor experiment in semi-arid grassland that suggests a potentially strong climate – carbon cycle feedback under combined elevated [CO2] and warming. Elevated [CO2] alone, and in combination with warming, enhanced ecosystem respiration to a greater extent than photosynthesis, resulting in net C loss over four years. The effect of warming was to reduce respiration especially during years of below-average precipitation, by partially offsetting the effect of elevated [CO2] on soil moisture and C cycling. Carbon losses were explained partly by stimulated decomposition of soil organic matter with elevated [CO2]. The climate – carbon cycle feedback observed in this semiarid grassland was mediated by soil water content, which was reduced by warming and increased by elevated [CO2]. Ecosystem models should incorporate direct and indirect effects of climate change on soil water content in order to accurately predict terrestrial feedbacks and long-term storage of C in soil. [ABSTRACT FROM AUTHOR]

Global warming is predicted to induce desiccation in many world regions through increases in evaporative demand. Rising CO2 may counter that trend by improving plant water-use efficiency. However, it is not clear how important this CO2-enhanced water use efficiency might be in offsetting warming-induced desiccation because higher CO2 also leads to higher plant biomass, and therefore greater transpirational surface. Furthermore, although warming is predicted to favour warm-season, C4 grasses, rising CO2 should favour C3, or cool-season plants. Here we show in a semi-arid grassland that elevated CO2 can completely reverse the desiccating effects of moderate warming. Although enrichment of air to 600?p.p.m.v. CO2 increased soil water content (SWC), 1.5/3.0?°C day/night warming resulted in desiccation, such that combined CO2 enrichment and warming had no effect on SWC relative to control plots. As predicted, elevated CO2 favoured C3 grasses and enhanced stand productivity, whereas warming favoured C4 grasses. Combined warming and CO2 enrichment stimulated above-ground growth of C4 grasses in 2 of 3?years when soil moisture most limited plant productivity. The results indicate that in a warmer, CO2-enriched world, both SWC and productivity in semi-arid grasslands may be higher than previously expected. [ABSTRACT FROM AUTHOR]

Predicting net C balance under future global change scenarios requires a comprehensive understanding of how ecosystem photosynthesis (gross primary production; GPP) and respiration (Re) respond to elevated atmospheric [CO2] and altered water availability. We measured net ecosystem exchange of CO2 (NEE), GPP and Re under ambient and elevated [CO2] in a northern mixed-grass prairie (Wyoming, USA) during dry intervals and in response to simulated precipitation pulse events. Elevated [CO2] resulted in higher rates of both GPP and Re across the 2006 growing season, and the balance of these two fluxes (NEE) accounted for cumulative growing season C uptake (−14.4 ± 8.3 g C m−2). Despite lower GPP and Re, experimental plots under ambient [CO2] had greater cumulative uptake (−36.2 ± 8.2 g C m−2) than plots under elevated [CO2]. Non-irrigated control plots received 50% of average precipitation during the drought of 2006, and had near-zero NEE (1.9 ± 6.4 g C m−2) for the growing season. Elevated [CO2] extended the magnitude and duration of pulse-related increases in GPP, resulting in a significant [CO2] treatment by pulse day interaction, demonstrating the potential for elevated [CO2] to increase the capacity of this ecosystem to respond to late-season precipitation. However, stimulation of Re throughout the growing season under elevated [CO2] reduced net C uptake compared to plots under ambient [CO2]. These results indicate that although elevated [CO2] stimulates gross rates of ecosystem C fluxes, it does not necessarily enhance net C uptake, and that C cycle responses in semi-arid grasslands are likely to be more sensitive to changes in precipitation than atmospheric [CO2]. [ABSTRACT FROM AUTHOR]

Climate models predict, and empirical evidence confirms, that more extreme precipitation regimes are occurring in tandem with warmer atmospheric temperatures. These more extreme rainfall patterns are characterized by increased event size separated by longer within season drought periods and represent novel climatic conditions whose consequences for different ecosystem types are largely unknown. Here, we present results from an experiment in which more extreme rainfall patterns were imposed in three native grassland sites in the Central Plains Region of North America, USA. Along this 600 km precipitation–productivity gradient, there was strong sensitivity of temperate grasslands to more extreme growing season rainfall regimes, with responses of aboveground net primary productivity (ANPP) contingent on mean soil water levels for different grassland types. At the mesic end of the gradient (tallgrass prairie), longer dry intervals between events led to extended periods of below-average soil water content, increased plant water stress and reduced ANPP by 18%. The opposite response occurred at the dry end (semiarid steppe), where a shift to fewer, but larger, events increased periods of above-average soil water content, reduced seasonal plant water stress and resulted in a 30% increase in ANPP. At an intermediate mixed grass prairie site with high plant species richness, ANPP was most sensitive to more extreme rainfall regimes (70% increase). These results highlight the inherent complexity in predicting how terrestrial ecosystems will respond to forecast novel climate conditions as well as the difficulties in extending inferences from single site experiments across biomes. Even with no change in annual precipitation amount, ANPP responses in a relatively uniform physiographic region differed in both magnitude and direction in response to within season changes in rainfall event size/frequency. [ABSTRACT FROM AUTHOR]

Water availability is the primary constraint to aboveground net primary productivity (ANPP) in many terrestrial biomes, and it is an ecosystem driver that will be strongly altered by future climate change. Global circulation models predict a shift in precipitation patterns to growing season rainfall events that are larger in size but fewer in number. This “repackaging” of rainfall into large events with long intervening dry intervals could be particularly important in semi-arid grasslands because it is in marked contrast to the frequent but small events that have historically defined this ecosystem. We investigated the effect of more extreme rainfall patterns on ANPP via the use of rainout shelters and paired this experimental manipulation with an investigation of long-term data for ANPP and precipitation. Experimental plots ( n = 15) received the long-term (30-year) mean growing season precipitation quantity; however, this amount was distributed as 12, six, or four events applied manually according to seasonal patterns for May–September. The long-term mean (1940–2005) number of rain events in this shortgrass steppe was 14 events, with a minimum of nine events in years of average precipitation. Thus, our experimental treatments pushed this system beyond its recent historical range of variability. Plots receiving fewer, but larger rain events had the highest rates of ANPP (184 ± 38 g m−2), compared to plots receiving more frequent rainfall (105 ± 24 g m−2). ANPP in all experimental plots was greater than long-term mean ANPP for this system (97 g m−2), which may be explained in part by the more even distribution of applied rain events. Soil moisture data indicated that larger events led to greater soil water content and likely permitted moisture penetration to deeper in the soil profile. These results indicate that semi-arid grasslands are capable of responding immediately and substantially to forecast shifts to more extreme precipitation patterns. [ABSTRACT FROM AUTHOR]