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Global warming is predicted to induce desiccation in many world regions through increases in evaporative demand (1-3). Rising C[O.sub.2] may counter that trend by improving plant water-use efficiency (4,5). However, it is not clear how important this C[O.sub.2]-enhanced water use efficiency might be in offsetting warming-induced desiccation because higher C[O.sub.2] also leads to higher plant biomass, and therefore greater transpirational surface (2,6,7). Furthermore, although warming is predicted to favour warm-season, [C.sub.4] grasses, rising C[O.sub.2] should favour [C.sub.3], or cool-season plants (8). Here we show in a semi-arid grassland that elevated C[O.sub.2] can completely reverse the desiccating effects of moderate warming. Although enrichment of air to 600 p.p.m.v. C[O.sub.2] increased soil water content (SWC), 1.5/3.0 °C day/night warming resulted in desiccation, such that combined C[O.sub.2] enrichment and warming had no effect on SWC relative to control plots. As predicted, elevated C[O.sub.2] favoured [C.sub.3] grasses and enhanced stand productivity, whereas warming favoured [C.sub.4] grasses. Combined warming and C[O.sub.2] enrichment stimulated above-ground growth of [C.sub.4] grasses in 2 of 3 years when soil moisture most limited plant productivity. The results indicate that in a warmer, C[O.sub.2]-enriched world, both SWC and productivity in semi-arid grasslands may be higher than previously expected., Grass-dominated, dry rangelands account for over 30% of Earth's terrestrial surface (9,10) and provide most of the forage for the world's domestic livestock. Among the most important of these include [...]

Leaf gas exchange theory indicates that carbon isotope discrimination (delta) is negatively associated with CER/g(sub s) (leaf CO2 exchange rate/conductance). Discovery of genetic variation for delta in wheat (Triticum aestivum L.) and other species has generated interest in using carbon isotope discrimination in plant breeding. However, the hypothesized negative association between CER/g(sub s) and delta has sometimes not been observed in field-grown crops. In the present study, CER/g(sub s), delta of peduncle tissue, plant productivity and yield were examined in eight winter wheat genotypes under both irrigated and nonirrigated field conditions. Water stress- and genotype-induced variations in CER/g(sub s) were consistently correlated with variations in g(sub s) than with CER. However, low CER in 'Bezostaya' wheat resulted in a consistently low genotypic ranking for CER/g(sub s). In 1988, values of CER/g(sub s) increased with water stress, while delta values dropped from 18.17 to 17.48%. These results imply a negative relationship between CER/g(sub s) and delta, consistent with leaf gas exchange theory; however, delta was both positively (1988; r = 0.83(super **) (P is less than or equal to 0.01)) and negatively (1989; r = -0.79(super *) (P is less than or equal to 0.05)) associated with CER/g(sub s) in irrigated plots. Positive correlations were obtained between delta and both biomass productivity (1988, r = 0.54(super *); 1989, r = 0.45 (P = 0.08)) and grain yield (1988, r = 0.66(super *); 1989, r = 0.55(super *)) when data were averaged across genotypes and irrigation treatments. Consistently low delta values were obtained for 'Sturdy', an early-released, drought-susceptible cultivar with low productivity. These results suggest that genotypic rankings for peduncle delta must be carefully interpreted, as low delta can be associated with low yield and stress susceptibility in wheat.

Understanding local adaptation to climate is critical for managing ecosystems in the face of climate change. While there have been many provenance studies in trees, less is known about local adaptation in herbaceous species, including the perennial grasses that dominate arid and semiarid rangeland ecosystems. We used a common garden study to quantify variation in growth and drought resistance traits in 99 populations of Elymus elymoides from a broad geographic and climatic range in the western United States. Ecotypes from drier sites produced less biomass and smaller seeds, and had traits associated with greater drought resistance: small leaves with low osmotic potential and high integrated water use efficiency (δ13C). Seasonality also influenced plant traits. Plants from regions with relatively warm, wet summers had large seeds, large leaves, and low δ13C. Irrespective of climate, we also observed trade‐offs between biomass production and drought resistance traits. Together, these results suggest that much of the phenotypic variation among E. elymoides ecotypes represents local adaptation to differences in the amount and timing of water availability. In addition, ecotypes that grow rapidly may be less able to persist under dry conditions. Land managers may be able to use this variation to improve restoration success by seeding ecotypes with multiple drought resistance traits in areas with lower precipitation. The future success of this common rangeland species will likely depend on the use of tools such as seed transfer zones to match local variation in growth and drought resistance to predicted climatic conditions. [ABSTRACT FROM AUTHOR]

Global changes that alter soil water availability may have profound effects on semiarid ecosystems. Although both elevated CO2 (eCO2) and warming can alter water availability, often in opposite ways, few studies have measured their combined influence on the amount, timing, and temporal variability of soil water. Here, we ask how free air CO2 enrichment (to 600 ppmv) and infrared warming (+ 1.5 °C day, + 3 °C night) effects on soil water vary within years and across wet-dry periods in North American mixed-grass prairie. We found that eCO2 and warming interacted to influence soil water and that those interactions varied by season. In the spring, negative effects of warming on soil water largely offset positive effects of eCO2. As the growing season progressed, however, warming reduced soil water primarily (summer) or only (autumn) in plots treated with eCO2. These interactions constrained the combined effect of eCO2 and warming on soil water, which ranged from neutral in spring to positive in autumn. Within seasons, eCO2 increased soil water under drier conditions, and warming decreased soil water under wetter conditions. By increasing soil water under dry conditions, eCO2 also reduced temporal variability in soil water. These temporal patterns explain previously observed plant responses, including reduced leaf area with warming in summer, and delayed senescence with eCO2 plus warming in autumn. They also suggest that eCO2 and warming may favor plant species that grow in autumn, including winter annuals and C3 graminoids, and species able to remain active under the dry conditions moderated by eCO2. [ABSTRACT FROM AUTHOR]

Plant roots mediate the impacts of environmental change on ecosystems, yet knowledge of root responses to environmental change is limited because few experiments evaluate multiple environmental factors and their interactions. Inferences about root functions are also limited because root length dynamics are rarely measured.Using a 5‐year experiment in a mixed‐grass prairie, we report the responses of root biomass, length and life span to elevated carbon dioxide (CO2), warming, elevated CO2 and warming combined, and irrigation. Root biomass was quantified using soil cores and root length dynamics were assessed using minirhizotrons. By comparing root dynamics with published results for soil resources and above‐ground productivity, we provide mechanistic insights into how climate change might impact grassland ecosystems.In the upper soil layer, 0–15 cm depth, both irrigation and elevated CO2 alone increased total root length by twofold, but irrigation decreased root biomass and elevated CO2 had only small positive effects on root biomass. The large positive effects of irrigation and elevated CO2 alone on total root length were due to increases in both root length production and root life span. The increased total root length and life span under irrigation and elevated CO2 coincided with apparent shifts from water limitation of plant growth to nitrogen limitation. Warming alone had minimal effects on root biomass, length and life span in this shallow soil layer. Warming and elevated CO2 combined increased root biomass and total root length by c. 25%, but total root length in this treatment was lower than expected if the effects of CO2 and warming alone were additive. Treatment effects on total root length and root life span varied with soil depth and root diameter.Synthesis. Sub‐additive effects of CO2 and warming suggest studies of elevated CO2 alone might overestimate the future capacity of grassland root systems to acquire resources. In this mixed‐grass prairie, elevated CO2 with warming stimulated total root length and root life span in deeper soils, likely enhancing plant access to more stable pools of growth‐limiting resources, including water and phosphorus. Thus, these root responses help explain previous observations of higher, and more stable, above‐ground productivity in these projected climate conditions. Sub‐additive effects of CO2 and warming suggest studies of elevated CO2 alone might overestimate the future capacity of grassland root systems to acquire resources. In this mixed‐grass prairie, elevated CO2 with warming stimulated total root length and root life span in deeper soils, likely enhancing plant access to more stable pools of growth‐limiting resources, including water and phosphorus. Thus, these root responses help explain previous observations of higher, and more stable, above‐ground productivity in these projected climate conditions [ABSTRACT FROM AUTHOR]

We evaluated the possibility of elevated CO2 concentration ([CO2]) to reduce the negative effect of drought on growth and physiological parameters of cassava (Manihot esculenta Crantz). Plants were grown with 390 ppm or 750 ppm of CO2, under well-watered or under water deficit conditions. The study was conducted in a climate-controlled greenhouse using 14 L pots, for 100 days. For any value of fraction of transpirable soil water (FTSW) the carbon assimilation was always higher for plants grown under elevated [CO2]. Still, elevated [CO2] reduced the negative effect of drought on transpiration, water use efficiency, all growth measures and harvest index. Elevated [CO2] increased the dry matter of tuber roots (DMTR) of well-watered plants by 17.4%. The DMTR of plants grown under water deficit were 124.4 g and 58.9 g, respectively, for plants under elevated and ambient CO2, an increase of 112%. Thus, the CO2 effect was relatively stronger to the production of tuberous roots when cassava were subjected to water-deficit. Our results suggest that cassava tuber production might be resilient to changes in precipitation that will accompany higher atmospheric CO2 and reinforce cassava as a specie that can significantly contribute to mitigate hunger in a changing climate environment. [ABSTRACT FROM AUTHOR]

Abstract: Increasing atmospheric [CO2] 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 CO2 (eCO2) 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 CO2 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, eCO2 substantially reduced IVDMD (two most abundant grasses) and [N] (all three dominant grass species) in most years, except the two wettest years. Furthermore, eCO2 reduced IVDMD and [N] independent of warming effects. Reduced IVDMD with eCO2 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 eCO2 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 eCO2 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. [ABSTRACT FROM AUTHOR]

Nutrient-rich organic waste derived from sewage treatment facilities or livestock manure is often applied to rangelands of western North America to increase soil fertility, forage production, forage quality, and soil carbon storage. This practice can have a number of undesirable side effects, however, including plant invasion. While characteristics of both rangeland ecosystems and invasive plants suggest that organic waste application might often promote invasion, results to date are mixed, perhaps due in part to the paucity of long-term studies. Here, we describe the long-term (22 yr) effects of three types of organic waste-composted biosolids, composted cattle manure, and fresh cattle manure-on plant productivity and invasion in native mixed-grass prairie. Although composted manure and biosolids increased plant productivity and forage quality in the second year of the study, these effects did not persist. In contrast, Bromus tectorum (cheatgrass), which invaded the study site over the course of the experiment, was strongly facilitated by composted manure addition, with particularly large effects observed in years 17 and 22. These results show that nutrient-rich organic waste can favor invasive species even in the relatively invasion- resistant grasslands of the western Great Plains. They also demonstrate that effects of organic waste on invasion can become apparent quite gradually and persist for decades following the initial organic waste application. Together, results from experimental additions of organic and inorganic nutrients to native rangelands suggest that the risk of promoting invasive species is significant, and should be considered in programs that apply organic waste to rangelands of western North America. [ABSTRACT FROM AUTHOR]

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]