Elevated carbon dioxide and warming impact silicon and phenolic‐based defences differently in native and exotic grasses

Global climate change may increase invasions of exotic plant species by directly promoting the success of invasive/exotic species or by reducing the competitive abilities of native species. Changes in plant chemistry, leading to altered susceptibility to stress, could mediate these effects. Grasses are hyper‐accumulators of silicon, which play a crucial function in the alleviation of diverse biotic and abiotic stresses. It is unknown how predicted increases in atmospheric carbon dioxide (CO₂) and air temperature affect silicon accumulation in grasses, especially in relation to primary and secondary metabolites. We tested how elevated CO₂ (eCO₂) (+240 ppm) and temperature (eT) (+4°C) affected chemical composition (silicon, phenolics, carbon and nitrogen) and plant growth in eight grass species, either native or exotic to Australia. eCO₂ increased phenolic concentrations by 11%, but caused silicon accumulation to decline by 12%. Moreover, declines in silicon occurred mainly in native species (?19%), but remained largely unchanged in exotic species. Conversely, eT increased silicon accumulation in native species (+19%) but decreased silicon accumulation in exotic species (?10%). Silicon and phenolic concentrations were negatively correlated with each other, potentially reflecting a defensive trade‐off. Moreover, both defences were negatively correlated with plant mass, compatible with a growth‐defence trade‐off. Grasses responded in a species‐specific manner, suggesting that the relative susceptibility of different species may differ under future climates compared to current species rankings of resource quality. For example, the native Microlaena stipoides was less well defended under eCO₂ in terms of both phenolics and silicon, and thus could suffer greater vulnerability to herbivores. To our knowledge, this is the first demonstration of the impacts of eCO₂ and eT on silicon accumulation in grasses. We speculate that the greater plasticity in silicon uptake shown by Australian native grasses may be partly a consequence of evolving in a low nutrient and seasonally arid environment.     

Maintenance of leaf N controls the photosynthetic CO2 response of grassland species exposed to 9 years of free‐air CO2 enrichment

Determining underlying physiological patterns governing plant productivity and diversity in grasslands are critical to evaluate species responses to future environmental conditions of elevated CO₂ and nitrogen (N) deposition. In a 9‐year experiment, N was added to monocultures of seven C₃ grassland species exposed to elevated atmospheric CO₂ (560 μmol CO₂ mol^?1) to evaluate how N addition affects CO₂ responsiveness in species of contrasting functional groups. Functional groups differed in their responses to elevated CO₂ and N treatments. Forb species exhibited strong down‐regulation of leaf N_mass concentrations (?26%) and photosynthetic capacity (?28%) in response to elevated CO₂, especially at high N supply, whereas C₃ grasses did not. Hence, achieved photosynthetic performance was markedly enhanced for C₃ grasses (+68%) in elevated CO₂, but not significantly for forbs. Differences in access to soil resources between forbs and grasses may distinguish their responses to elevated CO₂ and N addition. Forbs had lesser root biomass, a lower distribution of biomass to roots, and lower specific root length than grasses. Maintenance of leaf N, possibly through increased root foraging in this nutrient‐poor grassland, was necessary to sustain stimulation of photosynthesis under long‐term elevated CO₂. Dilution of leaf N and associated photosynthetic down‐regulation in forbs under elevated [CO₂], relative to the C₃ grasses, illustrates the potential for shifts in species composition and diversity in grassland ecosystems that have significant forb and grass components.     

Salinity and sea level mediate elevated CO2 effects on C3–C4 plant interactions and tissue nitrogen in a Chesapeake Bay tidal wetland

Elevated atmospheric carbon dioxide concentrations ([CO₂]) generally increase plant photosynthesis in C₃ species, but not in C₄ species, and reduce stomatal conductance in both C₃ and C₄ plants. In addition, tissue nitrogen concentration ([N]) often fails to keep pace with enhanced carbon gain under elevated CO₂, particularly in C₃ species. While these responses are well documented in many species, implications for plant growth and nutrient cycling in native ecosystems are not clear. Here we present data on 18 years of measurement of above and belowground biomass, tissue [N] and total standing crop of N for a Scirpus olneyi‐dominated (C₃ sedge) community, a Spartina patens‐dominated (C₄ grass) community and a C₃–C₄‐mixed species community exposed to ambient and elevated (ambient +340 ppm) atmospheric [CO₂] in natural salinity and sea level conditions of a Chesapeake Bay wetland. Increased biomass production (shoots plus roots) under elevated [CO₂] in the S. olneyi‐dominated community was sustained throughout the study, averaging approximately 35%, while no significant effect of elevated [CO₂] was found for total biomass in the C₄‐dominated community. We found a significant decline in C₄ biomass (correlated with rising sea level) and a concomitant increase in C₃ biomass in the mixed community. This shift from C₄ to C₃ was accelerated by the elevated [CO₂] treatment. The elevated [CO₂] stimulation of total biomass accumulation was greatest during rainy, low salinity years: the average increase above the ambient treatment during the three wettest years (1994, 1996, 2003) was 2.9 t ha⁻¹ but in the three driest years (1995, 1999, 2002), it was 1.2 t ha⁻¹. Elevated [CO₂] depressed tissue [N] in both species, but especially in the S. olneyi where the relative depression was positively correlated with salinity and negatively related with the relative enhancement of total biomass production. Thus, the greatest amount of carbon was added to the S. olneyi‐dominated community during years when shoot [N] was reduced the most, suggesting that the availability of N was not the most or even the main limitation to elevated [CO₂] stimulation of carbon accumulation in this ecosystem.     

CO2‐caused change in plant species composition rivals the shift in vegetation between mid‐grass and tallgrass prairies

Atmospheric CO₂ enrichment usually changes the relative contributions of plant species to biomass production of grasslands, but the types of species favored and mechanisms by which change is mediated differ among ecosystems. We measured changes in the contributions of C₃ perennial forbs and C₄ grasses to aboveground biomass production of tallgrass prairie assemblages grown along a field CO₂ gradient (250–500 μmol mol^?1) in central Texas USA. Vegetation was grown on three soil types and irrigated each season with water equivalent to the growing season mean of precipitation for the area. We predicted that CO₂ enrichment would increase the forb contribution to community production, and favor tall‐grasses over mid‐grasses by increasing soil water content and reducing the frequency with which soil water fell below a limitation threshold. CO₂ enrichment favored forbs over grasses on only one of three soil types, a Mollisol. The grass fraction of production increased dramatically across the CO₂ gradient on all soils. Contribution of the tall‐grass Sorghastrum nutans to production increased at elevated CO₂ on the two most coarse‐textured of the soils studied, a clay Mollisol and sandy Alfisol. The CO₂‐caused increase in Sorghastrum was accompanied by an offsetting decline in production of the mid‐grass Bouteloua curtipendula. Increased CO₂ favored the tall‐grass over mid‐grass by increasing soil water content and apparently intensifying competition for light or other resources (Mollisol) or reducing the frequency with which soil water dipped below threshold levels (Alfisol). An increase in CO₂ of 250 μmol mol^?1 above the pre‐industrial level thus led to a shift in the relative production of established species that is similar in magnitude to differences observed between mid‐grass and tallgrass prairies along a precipitation gradient in the central USA. By reducing water limitation to plants, atmospheric CO₂ enrichment may alter the composition and even structure of grassland vegetation.     

Simple plant traits explain functional group diversity decline in novel grassland communities of Texas

Previous research has found that plant diversity declines more quickly in exotic than native grassland plots, which offers a model system for testing whether diversity decline is associated with specific plant traits. In a common garden experiment in the Southern Great Plains in central Texas, USA, we studied monocultures and 9-species mixtures of either all exotic or all native grassland species. A total of 36 native and exotic species were paired by phylogeny and functional group. We used community-level measures (relative abundance in mixture) and whole-plant (height, aboveground biomass, and light capture) and leaf-level traits (area, specific leaf area, and C:N ratio) to determine whether trait differences explained native-exotic differences in functional group diversity. Increases in species’ relative abundance in mixture were correlated with high biomass, height, and light capture in both native and exotic communities. However, increasing exotic species were all C₄ grasses, whereas, increasing native species included forb, C₃ grass and C₄ grass species. Exotic C₄ grasses had traits associated with relatively high resource capture: greater leaf area, specific leaf area, height, biomass, and light capture, but similar leaf C:N ratios compared to native C₄ grasses. Leaf C:N was consistently higher for native than exotic C₃ species, implying that resource use efficiency was greater in natives than exotics. Our results suggest that functional diversity will differ between grasslands restored to native assemblages and those dominated by novel collections of exotic species, and that simple plant traits can help to explain diversity decline.     

Effectiveness of repeated autumn and spring fires for understorey restoration in weed‐invaded temperate eucalypt woodlands

Question. Can strategic burning, targeting differing ecological characteristics of native and exotic species, facilitate restoration of native understorey in weed‐invaded temperate grassy eucalypt woodlands? Location. Gippsland Plains, eastern Victoria, Australia. Methods. In a replicated, 5‐year experimental trial, the effects of repeated spring or autumn burning were evaluated for native and exotic plants in a representative, degraded Eucalyptus tereticornis grassy woodland. Treatments aimed to reduce seed banks and modify establishment conditions of exotic annual grasses, and to exhaust vegetative reserves of exotic perennial grasses. Treatments were applied to three grassland patch types, dominated by the native grass Austrodanthonia caespitosa, ubiquitous exotic annuals, or the common exotic perennial grass Paspalum dilatatum. Results. The dominant native grass Austrodanthonia caespitosa and native forbs were resilient to repeated fires, and target exotic annuals and perennials were suppressed differentially by autumn and spring fires. Exotic annuals were also suppressed by drought, reducing the overall treatment effects but indicating important opportunities for restoration. The initially sparse exotic geophyte Romulea rosea increased in cover with fire and the impact of this species on native forbs requires further investigation. There was minimal increase in diversity of subsidiary natives with fire, probably owing to lack of propagules. Conclusions. While fire is often considered to increase ecosystem invasibility, our study showed that strategic use of fire, informed by the relative responses of available native and exotic taxa, is potentially an effective step towards restoration of weed‐invaded temperate eucalypt woodlands.     

Variation in the impact of exotic grasses on native plant composition in relation to fire across an elevation gradient in Hawaii

The impact that an exotic species can have on the composition of the community it enters is a function of its abundance, its particular species traits and characteristics of the recipient community. In this study we examined species composition in 14 sites burned in fires fuelled by non‐indigenous C4 grasses in Hawaii Volcanoes National Park, Hawaii. We considered fire intensity, time since fire, climatic zone of site, unburned grass cover, unburned native cover and identity of the most abundant exotic grass in the adjacent unburned site as potential predictor variables of the impact of fire upon native species. We found that climatic zone was the single best variable for explaining variation in native cover among burned sites and between burned and unburned pairs. Fire in the eastern coastal lowlands had a very small effect on native plant cover and often stimulated native species regeneration, whereas fire in the seasonal submontane zone consistently caused a decline in native species cover and almost no species were fire tolerant. The dominant shrub, Styphelia tameiameia, in particular was fire intolerant. The number of years since fire, fire intensity and native cover in reference sites were not significantly correlated with native species cover in burned sites. The particular species of grass that carried the fire did however, have a significant effect on native species recovery. Where the African grass Melinis minutiflora was a dominant or codominant species, fire impacts were more severe than where it was absent regardless of climate zone. Overall, the impacts of exotic grass‐fuelled fires on native species composition and cover in seasonally dry Hawaiian ecosystems was context specific. This specificity is best explained by differences between the climatic zones in which fire occurred. Elevation was the main physical variable that differed among the climatic zones and it alone could explain a large percentage of the variation in native cover among sites. Rainfall, by contrast, did not vary systematically with elevation. Elevation is associated with differences in composition of the native species assemblages. In the coastal lowlands, the native grass Heteropogon contortus, was largely responsible for positive changes in native cover after fire although other native species also increased. Like the exotic grasses, this species is a perennial C4 grass. It is lacking in the submontane zone and there are no comparable native species there and almost all native species in the submontane zone were reduced by fire. The lack of fire tolerant species in the submontane zone thus clearly contributes to the devastating impact of fire upon native cover there.     

Elevated atmospheric CO2 stimulates aboveground biomass in a fire-regenerated scrub-oak ecosystem

The effect of elevated atmospheric CO₂ concentration (C_a) on the aboveground biomass of three oak species, Quercus myrtifolia, Q. geminata, and Q. chapmanii, was estimated nondestructively using allometric relationships between stem diameter and aboveground biomass after four years of experimental treatment in a naturally fire‐regenerated scrub‐oak ecosystem. After burning a stand of scrub‐oak vegetation, re‐growing plants were exposed to either current ambient (379 µL L^?1 CO₂) or elevated (704 µL L^?1 CO₂) C_a in 16 open‐top chambers over a four‐year period, and measurements of stem diameter were carried out annually on all oak shoots within each chamber. Elevated C_a significantly increased aboveground biomass, expressed either per unit ground area or per shoot; elevated C_a had no effect on shoot density. The relative effect of elevated C_a on aboveground biomass increased each year of the study from 44% (May 96–Jan 97), to 55% (Jan 97–Jan 98), 66% (Jan 98–Jan 99), and 75% (Jan 99–Jan 00). The effect of elevated C_a was species specific: elevated C_a significantly increased aboveground biomass of the dominant species, Q. myrtifolia, and tended to increase aboveground biomass of Q. chapmanii, but had no effect on aboveground biomass of the subdominant, Q. geminata. These results show that rising atmospheric CO₂ has the potential to stimulate aboveground biomass production in ecosystems dominated by woody species, and that species‐specific growth responses could, in the long term, alter the composition of the scrub‐oak community.     

An invasive grass shows colonization advantages over native grasses under conditions of low resource availability

Increased or fluctuating resources may facilitate opportunities for invasive exotic plants to dominate. This hypothesis does not, however, explain how invasive species succeed in regions characterized by low resource conditions or how these species persist in the lulls between high resource periods. We compare the growth of three co-occurring C₄ perennial bunchgrasses under low resource conditions: an exotic grass, Eragrostis curvula (African lovegrass) and two native grasses, Themeda triandra and Eragrostis sororia. We grew each species over 12?weeks under low nutrients and three low water regimes differentiated by timing: continuous, pulsed, and mixed treatments (switched from continuous to pulsed and back to continuous). Over time, we measured germination rates, time to germination (first and second generations), height, root biomass, vegetative biomass, and reproductive biomass. Contrary to our expectations that the pulsed watering regime would favor the invader, water-supply treatments had little significant effect on plant growth. We did find inherent advantages in a suite of early colonization traits that likely favor African lovegrass over the natives including faster germination speed, earlier flowering times, faster growth rates and from 2?weeks onward it was taller. African lovegrass also showed similar growth allocation strategies to the native grasses in terms of biomass levels belowground, but produced more vegetative biomass than kangaroo grass. Overall our results suggest that even under low resource conditions invasive plant species like African lovegrass can grow similarly to native grasses, and for some key colonization traits, like germination rate, perform better than natives.     

Response of 12 weedy species to elevated CO2 in low-phosphorus-availability soil

We conducted an experiment on responses of weedy species from an orchard ecosystem to elevated CO₂ (700–800 μmol mol⁻¹) under low phosphorus (P) soil in an environment-controlled growth chamber. Twelve local weedy species, Poa annua L., Lolium perenne L., Avena fatua L., Vicia cracca L., Medicago lupulina L., Kummerowia striata (Thunb.) Schindl., Veronica didyma Ten., Plantago virginica L., Gnaphalium affine D.Don., Echinochloa crusgalli var. mitis (L.) Beauv., Eleusine indica (L.) Gaertn. and Setaria glauca (L.) P. Beauv., grouped into four functional groups (C₃ grass, C₃ forb, legume and C₄ grass), were used in the experiment. The total plant biomass, P uptake, and mycorrhizal colonization were measured. The results showed that the total biomass of the 12 weedy species tended to increase under elevated CO₂. But changes in the total biomass under elevated CO₂ significantly differed among functional groups: legumes showed the greatest increase in the total biomass of all functional groups, following the order C₃ forbs > C₄ grasses > C₃ grasses. Elevated CO₂ significantly increased mycorrhizal colonization and P uptake of legumes, C₃ forbs and C₄ grasses but did not change C₃ grasses. Positive correlations between mycorrhizal colonization and shoot P concentration, and between total P uptake and total biomass were found under elevated CO₂. The results suggested that the interspecific difference in CO₂ response at low P availability was caused by the difference in CO₂ response in mycorrhizae and P uptake. These differences among species imply that plant interaction in orchard ecosystems may change under future CO₂ enrichment.     

Abiotic constraints on the competitive ability of exotic and native grasses in a Pacific Northwest prairie

In prairie ecosystems, abiotic constraints on competition can structure plant communities; however, the extent to which competition between native and exotic plant species is constrained by environmental factors is still debated. The objective of our study was to use paired field and greenhouse experiments to evaluate the competitive dynamics between two native (Danthonia californica and Deschampsia cespitosa) and two exotic (Schedonorus arundinaceus and Lolium multiflorum) grass species under varying nutrient and moisture conditions in an upland prairie in the Willamette Valley, Oregon. We hypothesized the two invasive, exotic grasses would be more competitive under high-nutrient, moderate-moisture conditions, resulting in the displacement of native grasses from these environments. In the field, the experimental reduction of competition resulted in shorter, wider plants, but only the annual grass, Lolium multiflorum, produced more aboveground biomass when competition was reduced. In the greenhouse, the two exotic grasses produced more total biomass than the two native grasses. Competitive hierarchies were influenced by nutrient and/or moisture treatments for the two exotic grasses, but not for the two native grasses. L. multiflorum dominated competitive interactions with all other grasses across treatments. In general, S. arundinaceus dominated when in competition with native grasses, and D. cespitosa produced the most biomass in monoculture or under interspecific competition with the other native grass, D. californica. D. californica, D. cespitosa, and S. arundinaceus all produced more biomass in high-moisture, high-nutrient environments, and D. cespitosa, L. multiflorum, and S. arundinaceus allocated more biomass belowground in the low nutrient treatment. Taken together, these experiments suggest the competitive superiority of the exotic grasses, especially L. multiflorum, but, contrary to our hypothesis, the native grasses were not preferentially excluded from nutrient-rich, moderately wet environments. Laurel Pfeifer-Meister and Esther M. Cole contributed equally to this work.     

Biomass and compositional changes occur in chalk grassland turves exposed to elevated CO2 for two seasons in FACE

Artificial turves composed of 7 chalk grassland species (Festuca ovina L.; Briza media L.; Bromopsis erecta (Hudson) Fourr.; Plantago media L.; Sanguisorba minor Scop.; Anthyllis vulneraria L. and Lotus corniculatus L.) were grown from seed and exposed to two seasons of elevated (600 μmol mol^–1) and ambient (340 μmol mol^–1) CO₂ concentrations in free air CO₂ enrichment (ETH-FACE, Zurich). The turves were clipped regularly to a height of 5 cm and assessed for above ground biomass production and relative abundance based on accumulated clipped dry biomass as well as by point quadrat recording. Below ground biomass production was assessed with root in-growth bags during the second season of growth. Increases in total biomass (> 30%) were noted in elevated CO₂, but the differences did not become significant until the second season of growth. Individual species’ biomass varied in response to elevated CO₂, with significant increases in biomass in elevated CO₂ turves for both legume species, and no significant CO₂ effect on S. minor or P. media. An initial positive CO₂ effect on biomass of combined grass species was reversed by the end of the experiment with less biomass and a significantly smaller proportion of total biomass present in elevated CO₂, which was attributed primarily to changes in proportion of F. ovina. Species relative abundance was significantly affected by elevated CO₂ in the final 4 of the 6 clip events, with the legume species increasing in proportion at the expense of the other species, particularly the grasses. Root length and dry weight were both significantly increased in elevated CO₂ (77% and 89%, respectively), and these increases were greater than increases in shoot biomass (36%) from the same period. Species responses to elevated CO₂, within the model community, were not consistent with predictions made from data on individual species, leading to the conclusion that responses to elevated CO₂, at the community level, and species within the community level, are the result of direct physiological effects and indirect competitive effects. These conclusions are discussed with respect to the ecological responses of natural communities, and the chalk grassland community in particular, to elevated CO₂.     

Comparison of damage to native and exotic tallgrass prairie plants by natural enemies

We surveyed the prevalence and amount of leaf damage related to herbivory and pathogens on 12 pairs of exotic (invasive and noninvasive) and ecologically similar native plant species in tallgrass prairie to examine whether patterns of damage match predictions from the enemy release hypothesis. We also assessed whether natural enemy impacts differed in response to key environmental factors in tallgrass prairie by surveying the prevalence of rust on the dominant C₄ grass, Andropogon gerardii, and its congeneric invasive exotic C₄ grass, A. bladhii, in response to fire and nitrogen fertilization treatments. Overall, we found that the native species sustain 56.4% more overall leaf damage and 83.6% more herbivore-related leaf damage when compared to the exotic species. Moreover, we found that the invasive exotic species sustained less damage from enemies relative to their corresponding native species than the noninvasive exotic species. Finally, we found that burning and nitrogen fertilization both significantly increased the prevalence of rust fungi in the native grass, while rust fungi rarely occurred on the exotic grass. These results indicate that reduced damage from enemies may in part explain the successful naturalization of exotic species and the spread of invasive exotic species in tallgrass prairie.     

Interactive effects of diversity, nutrients and elevated CO2 on experimental plant communities

The effects of species richness and elevated CO₂ on community productivity under altered nutrient levels were studied in experimental herbaceous communities composed of species from the Midwestern United States annual community, which consists of three functional groups C₃, C₄ and N‐fixers. Aboveground and belowground biomass were measured at flowering stage and at the end of the experiment when fruits of most plants were ripe. At the low nutrient level, species richness did not have a significant effect on community productivity. However, at the high nutrient level, the community biomass decreased with decreasing species richness at both ambient and elevated CO₂ in the first harvest, and at elevated CO₂ in the second harvest. At low nutrient level, CO₂ slightly increased community biomass at medium and high species richness. At high nutrient level, CO₂ significantly increased community biomass in all species‐richness treatments in the first harvest, but a significant response was observed only in the high richness treatment in the second harvest. At the functional group level, biomass of C₃ responded positively to CO₂, and C₄ responded very negatively to CO₂. The N‐fixers responded positively to CO₂ at low and medium species richness, but negatively at high species richness, showing a CO₂×richness interaction. CO₂ increased species evenness in the communities, depending on nutrient level. Species varied in the responses of light‐saturated net photosynthesis (P_max) to elevated CO₂, even within functional groups. Our findings suggest that (1) the relationship between productivity and species diversity was dependent on nutrient levels. (2) Species diversity enhances responses of communities to elevated CO₂. (3) Harvest time can affect the results of diversity‐productivity experiments. (4) Responses of C₃, C₄, and N‐fixers to elevated CO₂ in communities did not follow the prediction based on functional groups or plants grown individually, rather it depended on species richness.     

Responses of tropical native and invader C<Subscript>4</Subscript> grasses to water stress,clipping and increased atmospheric CO<Subscript>2</Subscript> concentration

The invasion of African grasses into Neotropical savannas has altered savanna composition, structure and function. The projected increase in atmospheric CO₂ concentration has the potential to further alter the competitive relationship between native and invader grasses. The objective of this study was to quantify the responses of two populations of a widespread native C₄ grass (Trachypogon plumosus) and two African C₄ grass invaders (Hyparrhenia rufa and Melinis minutiflora) to high CO₂ concentration interacting with two primary savanna stressors: drought and herbivory. Elevated CO₂ increased the competitive potential of invader grasses in several ways. Germination and seedling size was promoted in introduced grasses. Under high CO₂, the relative growth rate of young introduced grasses was twice that of native grass (0.58 g g⁻¹ week⁻¹ vs 0.25 g g⁻¹ week⁻¹). This initial growth advantage was maintained throughout the course of the study. Well-watered and unstressed African grasses also responded more to high CO₂ than did the native grass (biomass increases of 21–47% compared with decreases of 13–51%). Observed higher water and nitrogen use efficiency of invader grasses may aid their establishment and competitive strength in unfertile sites, specially if the climate becomes drier. In addition, high CO₂ promoted lower leaf N content more in the invader grasses. The more intensive land use, predicted to occur in this region, may interact with high CO₂ to fincreasesavor the African grasses, as they generally recovered faster after simulated herbivory. The superiority of invader grasses under high CO₂ suggests further in their competitive strength and a potential increased rate of displacement of the native savannas in the future by grasslands dominated by introduced African species.     

The invasive grass,Melinis minutiflora,inhibits tree regeneration in a Neotropical savanna

Abstract Exotic grasses are becoming increasingly abundant in Neotropical savannas, with Melinis minutiflora Beauv. being particularly invasive. To better understand the consequences for the native flora, we performed a field study to test the effect of this species on the establishment, survival and growth of seedlings of seven tree species native to the savannas and forests of the Cerrado region of Brazil. Seeds of the tree species were sown in 40 study plots, of which 20 were sites dominated by M. minutiflora, and 20 were dominated by native grasses. The exotic grass had no discernable effect on initial seedling emergence, as defined by the number of seedlings present at the end of the first growing season. Subsequent seedling survival in plots dominated by M. minutiflora was less than half that of plots dominated by native species. Consequently, at the end of the third growing season, invaded plots had only 44% as many seedlings as plots with native grasses. Above‐ground grass biomass of invaded plots was more than twice that of uninvaded plots, while seedling survival was negatively correlated with grass biomass, suggesting that competition for light may explain the low seedling survival where M. minutiflora is dominant. Soils of invaded plots had higher mean Ca, Mg and Zn, but these variables did not account for the higher grass biomass or the lower seedling survival in invaded plots. The results indicate that this exotic grass is having substantial effects on the dynamics of the tree community, with likely consequences for ecosystem structure and function.     

CO<Subscript>2</Subscript> enrichment increases element concentrations in grass mixtures by changing species abundances

Atmospheric carbon dioxide (CO₂) enrichment may increase plant growth more than the uptake of chemical elements from soil. Increased CO₂ also may alter element levels in biomass from multi-species vegetation by changing plant species abundances. We measured concentrations of ten elements in aboveground tissues of three C₄ grasses that had been exposed for 2–3 growing seasons to a continuous gradient in CO₂ from 250 to 500 μmol mol⁻¹. The grasses, Bouteloua curtipendula, Schizachyrium scoparium, and Sorghastrum nutans, are competitive dominants in assemblages of tallgrass prairie vegetation growing on each of three soil types along a field CO₂ gradient in central Texas, USA. Our objective was to determine whether CO₂ influences element concentrations in grass mixtures by changing concentrations in individual species or shifting species abundances. Increased CO₂ had little effect on element concentrations in grasses compared to differences observed among grass species and soils. Increasing CO₂ from the pre-Industrial to elevated levels reduced the phosphorus concentration in grasses grown on a clay and sandy loam soil. Concentrations of most other elements did not respond to CO₂ treatment. Cover of the mid-grass Bouteloua declined at higher CO₂ levels as cover of the taller grass Sorghastrum increased. Concentrations of several elements were lower in Bouteloua than Sorghastrum; hence, this exchange of species at higher CO₂ increased element concentrations in grass assemblages. Potential consequences include an improvement in the nutritional quality of plants for herbivores. Results highlight the underappreciated impact that CO₂ enrichment may have on ecosystem functioning by changing plant composition.     

Long‐term response of a Mojave Desert winter annual plant community to a whole‐ecosystem atmospheric CO2 manipulation (FACE)

Desert annuals are a critically important component of desert communities and may be particularly responsive to increasing atmospheric (CO₂) because of their high potential growth rates and flexible phenology. During the 10‐year life of the Nevada Desert FACE (free‐air CO₂ enrichment) Facility, we evaluated the productivity, reproductive allocation, and community structure of annuals in response to long‐term elevated (CO₂) exposure. The dominant forb and grass species exhibited accelerated phenology, increased size, and higher reproduction at elevated (CO₂) in a wet El Niño year near the beginning of the experiment. However, a multiyear dry cycle resulted in no increases in productivity or reproductive allocation for the remainder of the experiment. At the community level, early indications of increased dominance of the invasive Bromus rubens at elevated (CO₂) gave way to an absence of Bromus in the community during a drought cycle, with a resurgence late in the experiment in response to higher rainfall and a corresponding high density of Bromus in a final soil seed bank analysis, particularly at elevated (CO₂). This long‐term experiment resulted in two primary conclusions: (i) elevated (CO₂) does not increase productivity of annuals in most years; and (ii) relative stimulation of invasive grasses will likely depend on future precipitation, with a wetter climate favoring invasive grasses but currently predicted greater aridity favoring native dicots.     

Responses of a C3 and a C4 perennial grass to elevated CO2 and temperature under different water regimes

An experiment was carried out to determine the effects of elevated CO₂, elevated temperatures, and altered water regimes in native shortgrass steppe. Intact soil cores dominated by Bouteloua gracilis, a C₄ perennial grass, or Pascopyrum smithii, a C₃ perennial grass, were placed in growth chambers with 350 or 700 μL L^?1 atmospheric CO₂, and under either normal or elevated temperatures. The normal regime mimicked field patterns of diurnal and seasonal temperatures, and the high-temperature regime was 4 °C warmer. Water was supplied at three different levels in a seasonal pattern similar to that observed in the field. Total biomass after two growing seasons was 19% greater under elevated CO₂, with no significant difference between the C₃ and C₄ grass. The effect of elevated CO₂ on biomass was greatest at the intermediate water level. The positive effect of elevated CO₂ on shoot biomass was greater at normal temperatures in B. gracilis, and greater at elevated temperatures in P. smithii. Neither root-to-shoot ratio nor production of seed heads was affected by elevated CO₂. Plant tissue N and soil inorganic N concentrations were lower under elevated Co₂, but no more so in the C₃ than the C₄ plant. Elevated CO₂ appeared to increase plant N limitation, but there was no strong evidence for an increase in N limitation or a decrease in the size of the CO₂ effect from the first to the second growing season. Autumn samples of large roots plus crowns, the perennial organs, had 11% greater total N under elevated CO₂, in spite of greater N limitation.     

Cheatgrass is favored by warming but not CO2 enrichment in a semi‐arid grassland

Elevated CO₂ and warming may alter terrestrial ecosystems by promoting invasive plants with strong community and ecosystem impacts. Invasive plant responses to elevated CO₂ and warming are difficult to predict, however, because of the many mechanisms involved, including modification of phenology, physiology, and cycling of nitrogen and water. Understanding the relative and interactive importance of these processes requires multifactor experiments under realistic field conditions. Here, we test how free‐air CO₂ enrichment (to 600 ppmv) and infrared warming (+1.5 °C day/3 °C night) influence a functionally and phenologically distinct invasive plant in semi‐arid mixed‐grass prairie. Bromus tectorum (cheatgrass), a fast‐growing Eurasian winter annual grass, increases fire frequency and reduces biological diversity across millions of hectares in western North America. Across 2 years, we found that warming more than tripled B. tectorum biomass and seed production, due to a combination of increased recruitment and increased growth. These results were observed with and without competition from native species, under wet and dry conditions (corresponding with tenfold differences in B. tectorum biomass), and despite the fact that warming reduced soil water. In contrast, elevated CO₂ had little effect on B. tectorum invasion or soil water, while reducing soil and plant nitrogen (N). We conclude that (1) warming may expand B. tectorum's phenological niche, allowing it to more successfully colonize the extensive, invasion‐resistant northern mixed‐grass prairie, and (2) in ecosystems where elevated CO₂ decreases N availability, CO₂ may have limited effects on B. tectorum and other nitrophilic invasive species.