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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 (δ 13 C). Seasonality also influenced plant traits. Plants from regions with relatively warm, wet summers had large seeds, large leaves, and low δ 13 C. 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., Competing Interests: None declared., (© 2020 The Authors. Evolutionary Applications published by John Wiley & Sons Ltd. This article has been contributed to by US Government employees and their work is in the public domain in the USA.)

Grassland ecosystems are comprised of plants that occupy a wide array of phenological niches and vary considerably in their ability to resist the stress of seasonal soil-water deficits. Yet, the link between plant drought resistance and phenology remains unclear in perennial grassland ecosystems. To evaluate the role of soil water availability and plant drought tolerance in driving phenology, we measured leaf hydraulic conductance (K sat ), resistance to hydraulic failure (P 50 ), leaf gas exchange, plant and soil water stable isotope ratios (δ 18 O), and several phenology metrics on ten perennial herbaceous species in mixed-grass prairie. The interaction between P 50 and δ 18 O of xylem water explained 67% of differences in phenology, with lower P 50 values associated with later season activity, but only among shallow-rooted species. In addition, stomatal control and high water-use efficiency also contributed to the late flowering and late senescence strategies of plants that had low P 50 values and relied upon shallow soil water. Alternatively, plants with deeper roots did not possess drought-tolerant leaves, but had high hydraulic efficiency, contributing to their ability to efficiently move water longer distances while maintaining leaf water potential at relatively high values. The suites of traits that characterize these contrasting strategies provide a mechanistic link between phenology and plant-water relations; thus, these traits could help predict grassland community responses to changes in water availability, both temporally and vertically within the soil profile.

Temporal variation in soil nitrogen (N) availability affects growth of grassland communities that differ in their use and reuse of N. In a 7-year-long climate change experiment in a semi-arid grassland, the temporal stability of plant biomass production varied with plant N turnover (reliance on externally acquired N relative to internally recycled N). Species with high N turnover were less stable in time compared to species with low N turnover. In contrast, N turnover at the community level was positively associated with asynchrony in biomass production, which in turn increased community temporal stability. Elevated CO 2 and summer irrigation, but not warming, enhanced community N turnover and stability, possibly because treatments promoted greater abundance of species with high N turnover. Our study highlights the importance of plant N turnover for determining the temporal stability of individual species and plant communities affected by climate change., (© 2018 John Wiley & Sons Ltd/CNRS.)

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.)

Terrestrial plant and soil respiration, or ecosystem respiration (R eco ), represents a major CO 2 flux in the global carbon cycle. However, there is disagreement in how R eco will respond to future global changes, such as elevated atmosphere CO 2 and warming. To address this, we synthesized six years (2007-2012) of R eco data from the Prairie Heating And CO 2 Enrichment (PHACE) experiment. We applied a semi-mechanistic temperature-response model to simultaneously evaluate the response of R eco to three treatment factors (elevated CO 2 , warming, and soil water manipulation) and their interactions with antecedent soil conditions [e.g., past soil water content (SWC) and temperature (SoilT)] and aboveground factors (e.g., vapor pressure deficit, photosynthetically active radiation, vegetation greenness). The model fits the observed R eco well (R 2  = 0.77). We applied the model to estimate annual (March-October) R eco , which was stimulated under elevated CO 2 in most years, likely due to the indirect effect of elevated CO 2 on SWC. When aggregated from 2007 to 2012, total six-year R eco was stimulated by elevated CO 2 singly (24%) or in combination with warming (28%). Warming had little effect on annual R eco under ambient CO 2 , but stimulated it under elevated CO 2 (32% across all years) when precipitation was high (e.g., 44% in 2009, a 'wet' year). Treatment-level differences in R eco can be partly attributed to the effects of antecedent SoilT and vegetation greenness on the apparent temperature sensitivity of R eco and to the effects of antecedent and current SWC and vegetation activity (greenness modulated by VPD) on R eco base rates. Thus, this study indicates that the incorporation of both antecedent environmental conditions and aboveground vegetation activity are critical to predicting R eco at multiple timescales (subdaily to annual) and under a future climate of elevated CO 2 and warming., (© 2015 John Wiley & Sons Ltd.)

The Earth's atmosphere will continue to be enriched with carbon dioxide (CO2) over the coming century. Carbon dioxide enrichment often reduces leaf transpiration, which in water-limited ecosystems may increase soil water content, change species abundances and increase the productivity of plant communities. The effect of increased soil water on community productivity and community change may be greater in ecosystems with lower precipitation, or on coarser-textured soils, but responses are likely absent in deserts. We tested correlations among yearly increases in soil water content, community change and community plant productivity responses to CO2 enrichment in experiments in a mesic grassland with fine- to coarse-textured soils, a semi-arid grassland and a xeric shrubland. We found no correlation between CO2-caused changes in soil water content and changes in biomass of dominant plant taxa or total community aboveground biomass in either grassland type or on any soil in the mesic grassland (P > 0.60). Instead, increases in dominant taxa biomass explained up to 85 % of the increases in total community biomass under CO2 enrichment. The effect of community change on community productivity was stronger in the semi-arid grassland than in the mesic grassland, where community biomass change on one soil was not correlated with the change in either the soil water content or the dominant taxa. No sustained increases in soil water content or community productivity and no change in dominant plant taxa occurred in the xeric shrubland. Thus, community change was a crucial driver of community productivity responses to CO2 enrichment in the grasslands, but effects of soil water change on productivity were not evident in yearly responses to CO2 enrichment. Future research is necessary to isolate and clarify the mechanisms controlling the temporal and spatial variations in the linkages among soil water, community change and plant productivity responses to CO2 enrichment., (Published by Oxford University Press on behalf of the Annals of Botany Company 2015. This work is written by (a) US Government employee(s) and is in the public domain in the US.)

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.

Observations of a longer growing season through earlier plant growth in temperate to polar regions have been thought to be a response to climate warming. However, data from experimental warming studies indicate that many species that initiate leaf growth and flowering earlier also reach seed maturation and senesce earlier, shortening their active and reproductive periods. A conceptual model to explain this apparent contradiction, and an analysis of the effect of elevated CO2--which can delay annual life cycle events--on changing season length, have not been tested. Here we show that experimental warming in a temperate grassland led to a longer growing season through earlier leaf emergence by the first species to leaf, often a grass, and constant or delayed senescence by other species that were the last to senesce, supporting the conceptual model. Elevated CO2 further extended growing, but not reproductive, season length in the warmed grassland by conserving water, which enabled most species to remain active longer. Our results suggest that a longer growing season, especially in years or biomes where water is a limiting factor, is not due to warming alone, but also to higher atmospheric CO2 concentrations that extend the active period of plant annual life cycles.

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., (No claim to original US goverment works. New Phytologist © 2013 New Phytologist Trust.)

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.

Atmospheric concentrations of methane (CH4 ) and nitrous oxide (N2 O) 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 N2 O 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 N2 O 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 N2 O 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 N2 O emission and uptake occurred at our site with some years showing cumulative N2 O emission and other years showing cumulative N2 O 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 N2 O 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 N2 O expressed in CO2 -equivalents., (© 2013 Blackwell Publishing Ltd.)

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 (Δ¹³C(leaf) ), 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., (No claim to original US government works. New Phytologist © 2012 New Phytologist Trust.)

Nitrogen (N) and phosphorus (P) are essential nutrients for primary producers and decomposers in terrestrial ecosystems. Although climate change affects terrestrial N cycling with important feedbacks to plant productivity and carbon sequestration, the impacts of climate change on the relative availability of N with respect to P remain highly uncertain. In a semiarid grassland in Wyoming, USA, we studied the effects of atmospheric CO(2) enrichment (to 600 ppmv) and warming (1.5/3.0°C above ambient temperature during the day/night) on plant, microbial and available soil pools of N and P. Elevated CO(2) increased P availability to plants and microbes relative to that of N, whereas warming reduced P availability relative to N. Across years and treatments, plant N : P ratios varied between 5 and 18 and were inversely related to soil moisture. Our results indicate that soil moisture is important in controlling P supply from inorganic sources, causing reduced P relative to N availability during dry periods. Both wetter soil conditions under elevated CO(2) and drier conditions with warming can further alter N : P. Although warming may alleviate N constraints under elevated CO(2) , warming and drought can exacerbate P constraints on plant growth and microbial activity in this semiarid grassland., (© 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.)

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.

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 CO(2), and many have tap roots that are more effective than grasses for reaching deep soil water stores that can be enhanced under elevated CO(2). 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 approximately 40-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.

We investigated the influence of long-term (56 years) grazing on organic and inorganic carbon (C) and nitrogen (N) contents of the plant-soil system (to 90 cm depth) in shortgrass steppe of northeastern Colorado. Grazing treatments included continuous season-long (May-October) grazing by yearling heifers at heavy (60-75% utilization) and light (20-35% utilization) stocking rates, and nongrazed exclosures. The heavy stocking rate resulted in a plant community that was dominated (75% of biomass production) by the C4 grass blue grama (Bouteloua gracilis), whereas excluding livestock grazing increased the production of C3 grasses and prickly pear cactus (Opuntia polycantha). Soil organic C (SOC) and organic N were not significantly different between the light grazing and nongrazed treatments, whereas the heavy grazing treatment was 7.5 Mg ha(-1) higher in SOC than the nongrazed treatment. Lower ratios of net mineralized N to total organic N in both grazed compared to nongrazed treatments suggest that long-term grazing decreased the readily mineralizable fraction of soil organic matter. Heavy grazing affected soil inorganic C (SIC) more than the SOC. The heavy grazing treatment was 23.8 Mg ha(-1) higher in total soil C (0-90 cm) than the nongrazed treatment, with 68% (16.3 Mg ha(-1)) attributable to higher SIC, and 32% (7.5 Mg ha(-1)) to higher SOC. These results emphasize the importance in semiarid and arid ecosystems of including inorganic C in assessments of the mass and distribution of plant-soil C and in evaluations of the impacts of grazing management on C sequestration.

To model the effect of increasing atmospheric CO2 on semi-arid grasslands, the gas exchange responses of leaves to seasonal changes in soil water, and how they are modified by CO2, must be understood for C3 and C4 species that grow in the same area. In this study, open-top chambers were used to investigate the photosynthetic and stomatal responses of Pascopyrum smithii (C3) and Bouteloua gracilis (C4) grown at 360 (ambient CO2) and 720 micro mol mol-1 CO2 (elevated CO2) in a semi-arid shortgrass steppe. Assimilation rate (A) and stomatal conductance (gs) at the treatment CO2 concentrations and at a range of intercellular CO2 concentrations and leaf water potentials (psileaf) were measured over 4 years with variable soil water content caused by season and CO2 treatment. Carboxylation efficiency of ribulose bisphosphate carboxylase/oxygenase (Vc,max), and ribulose bisphosphate regeneration capacity (Jmax) were reduced in P. smithii grown in elevated CO2, to the degree that A was similar in elevated and ambient CO2 (when soil moisture was adequate). Photosynthetic capacity was not reduced in B. gracilis under elevated CO2, but A was nearly saturated at ambient CO2. There were no stomatal adaptations independent of photosynthetic acclimation. Although photosynthetic capacity was reduced in P. smithii growing in elevated CO2, reduced gs and transpiration improved soil water content and psileaf in the elevated CO2 chambers, thereby improving A of both species during dry periods. These results suggest that photosynthetic responses of C3 and C4 grasses in this semi-arid ecosystem will be driven primarily by the effect of elevated CO2 on plant and soil water relations.

The eastern Colorado shortgrass steppe is dominated by the C 4 grass, Bouteloua gracilis, but contains a mixture of C 3 grasses as well, including Pascopyrum smithii. Although the ecology of this region has been extensively studied, there is little information on how increasing atmospheric CO 2 will affect it. This growth chamber study investigated gas exchange, water relations, growth, and biomass and carbohydrate partitioning in B. gracilis and P. smithii grown under present ambient and elevated CO 2 concentrations of 350 μl l -1 and 700 μl l -1 , respectively, and two deficit irrigation regimes. The experiment was conducted in soil-packed columns planted to either species over a 2-month period under summer-like conditions and with no fertilizer additions. Our objective was to better understand how these species and the functional groups they represent will respond in future CO 2 -enriched environments. Leaf CO 2 assimilation (A n ), transpiration use efficiency (TUE, or A n /transpiration), plant growth, and whole-plant water use efficiency (WUE, or plant biomass production/water evapotranspired) of both species were greater at elevated CO 2 , although responses were more pronounced for P. smithii. Elevated CO 2 enhanced photosynthesis, TUE, and growth in both species through higher soil water content (SWC) and leaf water potentials (Ψ) and stimulation of photosynthesis. Consumptive water use was greater and TUE less for P. smithii than B. gracilis during early growth when soil water was more available. Declining SWC with time was associated with a steadily increased sequestering of total non-structural carbohydrates (TNCs), storage carbohydrates (primarily fructans for P. smithii) and biomass in belowground organs of P. smithii, but not B. gracilis. The root:shoot ratio of P. smithii also increased at elevated CO 2 , while the root:shoot ratio of B. gracilis was unresponsive to CO 2 . These partitioning responses may be the consequence of different ontogenetic strategies of a cool-season and warm-season grass entering a warm, dry summer period; the cool-season P. smithii responds by sequestering TNCs belowground in preparation for summer dormancy, while resource partitioning of the warm-season B. gracilis remains unaltered. One consequence of greater partitioning of resources into P. smithii belowground organs in the present study was maintenance of higher Ψ and A n rates. This, along with differences in photosynthetic pathway, may have accounted for the greater responsiveness of P. smithii to CO 2 enrichment compared to B. gracilis.