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Climate controls vegetation distribution across the globe, and some vegetation types are more vulnerable to climate change, whereas others are more resistant. Because resistance and resilience can influence ecosystem stability and determine how communities and ecosystems respond to climate change, we need to evaluate the potential for resistance as we predict future ecosystem function. In a mixed-grass prairie in the northern Great Plains, we used a large field experiment to test the effects of elevated CO2, warming, and summer irrigation on plant community structure and productivity, linking changes in both to stability in plant community composition and biomass production. We show that the independent effects of CO2 and warming on community composition and productivity depend on interannual variation in precipitation and that the effects of elevated CO2 are not limited to water saving because they differ from those of irrigation. We also show that production in this mixed-grass prairie ecosystem is not only relatively resistant to interannual variation in precipitation, but also rendered more stable under elevated CO2 conditions. This increase in production stability is the result of altered community dominance patterns: Community evenness increases as dominant species decrease in biomass under elevated CO2. In many grasslands that serve as rangelands, the economic value of the ecosystem is largely dependent on plant community composition and the relative abundance of key forage species. Thus, our results have implications for how we manage native grasslands in the face of changing climate. [ABSTRACT FROM AUTHOR]

As global changes reorganize plant communities, invasive plants may benefit. We hypothesized that elevated CO2 and warming would strongly influence invasive species success in a semi-arid grassland, as a result of both direct and water-mediated indirect effects., To test this hypothesis, we transplanted the invasive forb Linaria dalmatica into mixed-grass prairie treated with free-air CO2 enrichment and infrared warming, and followed survival, growth, and reproduction over 4 yr. We also measured leaf gas exchange and carbon isotopic composition in L. dalmatica and the dominant native C3 grass Pascopyrum smithii., CO2 enrichment increased L. dalmatica biomass 13-fold, seed production 32-fold, and clonal expansion seven-fold, while warming had little effect on L. dalmatica biomass or reproduction. Elevated CO2 decreased stomatal conductance in P. smithii, contributing to higher soil water, but not in L. dalmatica. Elevated CO2 also strongly increased L. dalmatica photosynthesis (87% versus 23% in P. smithii), as a result of both enhanced carbon supply and increased soil water., More broadly, rapid growth and less conservative water use may allow invasive species to take advantage of both carbon fertilization and water savings under elevated CO2. Water-limited ecosystems may therefore be particularly vulnerable to invasion as CO2 increases. [ABSTRACT FROM AUTHOR]

Atmospheric concentrations of methane ( CH4) and nitrous oxide ( N2O) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO2 and temperature affected CH4 and N2O fluxes in a well-drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH4 and N2O fluxes to elevated CO2 and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO2 increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO2 and warming would have opposing effects on CH4 and N2O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell-shaped relationship was observed with soil moisture with highest CH4 uptake at intermediate soil moisture. Both N2O emission and uptake occurred at our site with some years showing cumulative N2O emission and other years showing cumulative N2O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO2 and warming reduced the sink of CH4 and N2O expressed in CO2 equivalents (across 5 years by 7% and 11% for elevated CO2 and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH4 and N2O expressed in CO2-equivalents. [ABSTRACT FROM AUTHOR]

In semiarid western North American riparian ecosystems, increased drought and lower streamflows under climate change may reduce plant growth and recruitment, and favor drought-tolerant exotic species over mesic native species. We tested whether elevated atmospheric CO2 might ameliorate these effects by improving plant water-use efficiency., We examined the effects of CO2 and water availability on seedlings of two native ( Populus deltoides spp. monilifera, Salix exigua) and three exotic ( Elaeagnus angustifolia, Tamarix spp., Ulmus pumila) western North American riparian species in a CO2-controlled glasshouse, using 1-m-deep pots with different water-table decline rates., Low water availability reduced seedling biomass by 70-97%, and hindered the native species more than the exotics. Elevated CO2 increased biomass by 15%, with similar effects on natives and exotics. Elevated CO2 increased intrinsic water-use efficiency (Δ13Cleaf), but did not increase biomass more in drier treatments than wetter treatments., The moderate positive effects of elevated CO2 on riparian seedlings are unlikely to counteract the large negative effects of increased aridity projected under climate change. Our results suggest that increased aridity will reduce riparian seedling growth despite elevated CO2, and will reduce growth more for native Salix and Populus than for drought-tolerant exotic species. [ABSTRACT FROM AUTHOR]

1. It has been suggested that much of the elevated CO2 effect on plant productivity and N cycling in semi-arid grasslands is related to a CO2-induced increase in soil moisture, but the relative importance of moisture-mediated and direct effects of CO2 remain unclear. 2. We grew five grassland species common to the semi-arid grasslands of northern Colorado, USA, as monocultures and as mixtures of all five species in pots. We examined the effects of atmospheric CO2 concentration (ambient vs. 780 p.p.m.) and soil moisture (15 vs. 20% m/m) on plant biomass and plant N uptake. Our objective was to separate CO2 effects not related to water from water-mediated CO2 effects by frequently watering the pots, thereby eliminating most of the elevated CO2 effects on soil moisture, and including a water treatment similar in magnitude to the water-savings effect of CO2. 3. Biomass of the C3 grasses Hesperostipa comata and Pascopyrum smithii increased under elevated CO2, biomass of the C4 grass Bouteloua gracilis increased with increased soil moisture, while biomass of the forbs Artemisia frigida and Linaria dalmatica had no or mixed responses. Increased plant N uptake contributed to the increase in plant biomass with increased soil moisture while the increase in plant biomass with CO2 enrichment was mostly a result of increased N use efficiency (NUE). Species-specific responses to elevated CO2 and increased soil moisture differed between monocultures and mixtures. Both under elevated CO2 and with increased soil moisture, certain species gained N in mixtures at the expense of species that lost N, but elevated CO2 led to a different set of winners and losers than did increased water. 4. Elevated CO2 can directly increase plant productivity of semi-arid grasslands through increased NUE, while a CO2-induced increase in soil moisture stimulating net N mineralization could further enhance plant productivity through increased N uptake. Our results further indicate that the largest positive and negative effects of elevated CO2 and increased soil moisture on plant productivity occur with interspecific competition. Responses of this grassland community to elevated CO2 and water may be both contingent upon and accentuated by competition. [ABSTRACT FROM AUTHOR]

Elevated CO2 affects plant productivity, but also water availability and plant species composition in semi-arid grasslands, thereby potentially causing complex effects on CH4 consumption and N2O emission. We studied the effects of atmospheric CO2 concentration (400 vs 780 μL L−1), water content (15 vs 20% gravimetric soil moisture), and composition of semi-arid grassland species (perennial grasses Bouteloua gracilis, Hesperostipa comata, and Pascopyrum smithii; sub-shrub Artemisia frigida; invasive forb Linaria dalmatica grown in monoculture and all five species together) on CH4 consumption and N2O emission in a full factorial greenhouse experiment. We used a unique method where we measured microbial effects on CH4 consumption and N2O emission in isolation from effects of gas diffusivity. Microbially mediated CH4 consumption was significantly higher under elevated CO2 (by 20%), but was not affected by soil water content or plant species composition. Microbially mediated N2O emission was not significantly affected by elevated CO2, but was significantly higher with high water content (by 67%) and differed significantly among species. Treatment effects on CH4 consumption and N2O emission often could not be explained simply by differences in soil moisture, suggesting that treatment-induced changes in other soil and microbial properties played a role in causing these effects. [ABSTRACT FROM AUTHOR]

Increasing atmospheric CO2 has potentially significant impacts on the dynamics of water use and conservation in semi-arid rangelands. In this study we used large (15.5 m2) open top chambers to investigate effects of twice ambient CO2 concentration (720 μL L-1) on plant and soil water relations of semi-arid shortgrass steppe (SGS) of northeastern Colorado from 1997 to 2001. Seasonal average soil moisture throughout the soil profile (0–15, 15–45, 45–75, 75–105 cm) was increased under elevated CO2 compared to ambient CO2 for much of the study period. When averaged across years, the greatest relative increase (elevated vs. ambient) in soil moisture occurred in the 75–105 cm depth increment (16.4%). Averaged over the study period, leaf water potential (Ψleaf) was enhanced 24–30% under elevated CO2 in the major warm- and cool-season grass species of the SGS (Bouteloua gracilis, C4, 28.5%; Pascopyrum smithii, C3, 24.7%; Stipa comata, C3, 30.4%), and the degree of responsiveness in Ψleaf to elevated CO2 did not differ between C3 and C4 plant functional types, but did differ between C3 species. Water-use efficiency (WUE; g aboveground biomass harvested/ kg water consumed) was 43% higher in elevated (6.10) than ambient (4.27) CO2 plots over the study period. Results suggest that a future, elevated CO2 environment may result not only in increased plant productivity due to improved WUE, but also lead to increased water drainage and deep soil moisture storage in this semi-arid grassland ecosystem. This, along with the ability of the major grass species to maintain a favorable water status under elevated CO2, should result in the SGS being less susceptible to prolonged periods of drought. However, increases in deep soil water may eventually favor deeper-rooted over shallow-rooted species. [ABSTRACT FROM AUTHOR]

The article presents a study which aims to explore the CO2 response of the shortgrass steppe used for livestock grazing in the North American Great Plains. The study uses three with ambient air and three with air CO2 chambers as well as three unchambered controls to evaluate responses to rising atmospheric CO2. It also suggests the enhancement of the rising atmospheric CO2 production of lower quality forage along with a species composition shift.

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 FROM AUTHOR]

Plants with the C4 photosynthetic pathway have predominantly one of three decarboxylation enzymes in their bundle sheath cells. Within the grass family (Poaceae) bundle sheath leakiness to CO2 is purported to be lowest in the nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME, EC 1.1.1.40) group, highest in the NAD-ME (EC 1.1.1.39) group and intermediate in the phosphoenolpyruvate carboxykinase (PCK, EC 4.1.1.32) group. We investigated the hypothesis that growth and photosynthesis of NAD-ME C4 grasses would respond more to elevated CO2 treatment than NADP-ME grasses. Plants were grown in 8-1 pots in growth chambers with ample water and fertilizer for 39 days at a continuous CO2 concentration of either 350 or 700 µl l-1. NAD-ME species included Bouteloua gracilis Lag. ex Steud (Blue grama), Buchloe dactyloides (Nutt.) Engelm. (Buffalo grass) and Panicum virgatum L. (Switchgrass) and the NADP-ME species were Andropogon gerardii Vittman (Big bluestem), Schizachyrium scoparium (Michx.) Nash (Little bluestem), and Sorghastrum nutans (L.) Nash (Indian grass). Contrary to our hypothesis, growth of the NADP-ME grasses was generally greater under elevated CO2 (significant for A. gerardii and S. nutans), while none of the NAD-ME grasses had a significant growth response. Increased leaf total non-structural carbohydrate (TNC) was associated with greater growth responses of NADP-ME grasses. Decreased leaf nitrogen in NADP-ME species grown at elevated CO2 was found to be an artifact of TNC dilution. Assimilation (A) vs intercellular CO2 (Ci) curves revealed that leaf photosynthesis was not saturated at 350 µl l-1 CO2 in any of these C4 grasses. Assimilation of elevated CO2-grown A. gerardii was higher than in plants grown in ambient CO2. In contrast, B. gracilis grown in elevated CO2 displayed lower A, a trait more commonly reported in C3 plants. Photosynthetic acclimation in B. gracilis was not related to leaf TNC or nitrogen concentrations, but A:Ci curves suggest a reduction in activity of both phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39). Some adaptation of stomatal functioning was also seen in B. gracilis and A. gerardii leaves grown in elevated CO2. Our study shows that C4 grasses have the capacity for increased growth and photosynthesis under elevated CO2 even when water and nutrients are non-limiting. While it was the NADP-ME species which had significant responses in the present study, we have previously reported significant growth increases in elevated CO2 for B. gracilis. [ABSTRACT FROM AUTHOR]

Plants with the C4 photosynthetic pathway have predominantly one of three decarboxylation enzymes in their bundle sheath cells. Within the grass family (Poaceae) bundle sheath leakiness to CO2 is purported to be lowest in the nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME, EC 1.1.1.40) group, highest in the NAD-ME (EC 1.1.1.39) group and intermediate in the phosphoenolpyruvate carboxykinase (PCK, EC 4.1.1.32) group. We investigated the hypothesis that growth and photosynthesis of NAD-ME C4 grasses would respond more to elevated CO2 treatment than NADP-ME grasses. Plants were grown in 8-1 pots in growth chambers with ample water and fertilizer for 39 days at a continuous CO2 concentration of either 350 or 700 µl l-1. NAD-ME species included Bouteloua gracilis Lag. ex Steud (Blue grama), Buchloe dactyloides (Nutt.) Engelm. (Buffalo grass) and Panicum virgatum L. (Switchgrass) and the NADP-ME species were Andropogon gerardii Vittman (Big bluestem), Schizachyrium scoparium (Michx.) Nash (Little bluestem), and Sorghastrum nutans (L.) Nash (Indian grass). Contrary to our hypothesis, growth of the NADP-ME grasses was generally greater under elevated CO2 (significant for A. gerardii and S. nutans), while none of the NAD-ME grasses had a significant growth response. Increased leaf total non-structural carbohydrate (TNC) was associated with greater growth responses of NADP-ME grasses. Decreased leaf nitrogen in NADP-ME species grown at elevated CO2 was found to be an artifact of TNC dilution. Assimilation (A) vs intercellular CO2 (Ci) curves revealed that leaf photosynthesis was not saturated at 350 µl l-1 CO2 in any of these C4 grasses. Assimilation of elevated CO2-grown A. gerardii was higher than in plants grown in ambient CO2. In contrast, B. gracilis grown in elevated CO2 displayed lower A, a trait more commonly reported in C3 plants. Photosynthetic acclimation in B. gracilis was not related to leaf TNC or nitrogen concentrations, but A:Ci curves suggest a reduction in activity of both phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39). Some adaptation of stomatal functioning was also seen in B. gracilis and A. gerardii leaves grown in elevated CO2. Our study shows that C4 grasses have the capacity for increased growth and photosynthesis under elevated CO2 even when water and nutrients are non-limiting. While it was the NADP-ME species which had significant responses in the present study, we have previously reported significant growth increases in elevated CO2 for B. gracilis. [ABSTRACT FROM AUTHOR]

Increasing atmospheric [CO 2 ] and temperature are expected to affect the productivity, species composition, biogeochemistry, and therefore the quantity and quality of forage available to herbivores in rangeland ecosystems. Both elevated CO 2 (eCO 2 ) and warming affect plant tissue chemistry through multiple direct and indirect pathways, such that the cumulative outcomes of these effects are difficult to predict. Here, we report on a 7-yr study examining effects of CO 2 enrichment (to 600 ppm) and infrared warming (+1.5°C day/3°C night) under realistic field conditions on forage quality and quantity in a semiarid, mixedgrass prairie. For the three dominant forage grasses, warming effects on in vitro dry matter digestibility (IVDMD) and tissue [N] were detected only in certain years, varied from negative to positive, and were relatively minor. In contrast, eCO 2 substantially reduced IVDMD (two most abundant grasses) and [N] (all three dominant grass species) in most years, except the two wettest years. Furthermore, eCO 2 reduced IVDMD and [N] independent of warming effects. Reduced IVDMD with eCO 2 was related both to reduced [N] and increased acid detergent fiber (ADF) content of grass tissues. For the six most abundant forage species (representing 96% of total forage production), combined warming and eCO 2 increased forage production by 38% and reduced forage [N] by 13% relative to ambient climate. Although the absolute magnitude of the decline in IVDMD and [N] due to combined warming and eCO 2 may seem small (e.g., from 63.3 to 61.1% IVDMD and 1.25 to 1.04% [N] for Pascopyrum smithii), such shifts could have substantial consequences for the rate at which ruminants gain weight during the primary growing season in the largest remaining rangeland ecosystem in North America. With forage production increases, declining forage quality could potentially be mitigated by adaptively increasing stocking rates, and through management such as prescribed burning, fertilization at low rates, and legume interseeding to enhance forage quality., (© 2018 by the Ecological Society of America.)