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.     

Winners always win: growth of a wide range of plant species from low to future high CO2

Evolutionary adaptation to variation in resource supply has resulted in plant strategies that are based on trade‐offs in functional traits. Here, we investigate, for the first time across multiple species, whether such trade‐offs are also apparent in growth and morphology responses to past low, current ambient, and future high CO₂ concentrations. We grew freshly germinated seedlings of up to 28 C₃ species (16 forbs, 6 woody, and 6 grasses) in climate chambers at 160 ppm, 450 ppm, and 750 ppm CO₂. We determined biomass, allocation, SLA (specific leaf area), LAR (leaf area ratio), and RGR (relative growth rate), thereby doubling the available data on these plant responses to low CO₂. High CO₂ increased RGR by 8%; low CO₂ decreased RGR by 23%. Fast growers at ambient CO₂ had the greatest reduction in RGR at low CO₂ as they lost the benefits of a fast‐growth morphology (decoupling of RGR and LAR [leaf area ratio]). Despite these shifts species ranking on biomass and RGR was unaffected by CO₂, winners continued to win, regardless of CO_2. Unlike for other plant resources we found no trade‐offs in morphological and growth responses to CO₂ variation, changes in morphological traits were unrelated to changes in growth at low or high CO₂. Thus, changes in physiology may be more important than morphological changes in response to CO₂ variation.     

Effects of elevated CO2 and cutting frequency on plant community structure in a temperate grassland

Monoliths of a fertile, N limited, C3 grassland community were subjected (or not) to an atmospheric CO₂ enrichment (600 µmol mol^‐‐1) using a Mini‐FACE system, from August 1998 to June 2001 and were subjected to two contrasting cutting frequencies (3 and 6 cuts per year). We report here the effects of the CO₂ and cutting frequency factors on the plant community structure and its diversity. Species‐specific responses to elevated CO₂ and cutting frequency were observed, which resulted in significant changes in the botanical composition of the grassland monoliths. Elevated CO₂ significantly increased the proportion of dicotyledones (forbs + legumes) and reduced that of the monocotyledones (grasses). Management differentiated this response as elevated CO₂ increased the proportion of forbs when infrequently and of legumes when frequently defoliated. However, among the two dominant forbs species only one was significantly enhanced by elevated CO₂. Moreover, not all grass species responded negatively to high CO₂. At a low cutting frequency, the observed decline under ambient CO₂ in species diversity (Shannon‐Weaver index) and in forb species number was partly alleviated by elevated CO₂. This experiment shows that the botanical composition of temperate grasslands is likely to be affected by the current rise (+ 0.5% per year) in the atmospheric CO₂ concentration, and that grassland management guidelines may need to be adapted to a future high CO₂ world.     

The type of competition modulates the ecophysiological response of grassland species to elevated CO2 and drought

The effects of elevated CO₂ and drought on ecophysiological parameters in grassland species have been examined, but few studies have investigated the effect of competition on those parameters under climate change conditions. The objective of this study was to determine the effect of elevated CO₂ and drought on the response of plant water relations, gas exchange, chlorophyll a fluorescence and aboveground biomass in four grassland species, as well as to assess whether the type of competition modulates that response. Elevated CO₂ in well‐watered conditions increased aboveground biomass by augmenting CO₂ assimilation. Drought reduced biomass by reducing CO₂ assimilation rate via stomatal limitation and, when drought was more severe, also non‐stomatal limitation. When plants were grown under the combined conditions of elevated CO₂ and drought, drought limitation observed under ambient CO₂ was reduced, permitting higher CO₂ assimilation and consequently reducing the observed decrease in aboveground biomass. The response to climate change was species‐specific and dependent on the type of competition. Thus, the response to elevated CO₂ in well‐watered grasses was higher in monoculture than in mixture, while it was higher in mixture compared to monoculture for forbs. On the other hand, forbs were more affected than grasses by drought in monoculture, while in mixture the negative effect of drought was higher in grasses than in forbs, due to a lower capacity to acquire water and mineral nutrients. These differences in species‐level growth responses to CO₂ and drought may lead to changes in the composition and biodiversity of the grassland plant community in future climate conditions.     

Productivity and resistance to weed invasion in four prairie biomass feedstocks with different diversity

High‐diversity mixtures of native tallgrass prairie vegetation should be effective biomass feedstocks because of their high productivity and low input requirements. These diverse mixtures should also enhance several of the ecosystem services provided by the traditional monoculture feedstocks used for bioenergy. In this study, we compared biomass production, year‐to‐year variation in biomass production, and resistance to weed invasion in four prairie biomass feedstocks with different diversity: one species – a switchgrass monoculture; five species – a mix of C₄ grasses; 16 species – a mix of grasses, forbs, and legumes; and 32 species – a mix of grasses, forbs, legumes, and sedges. Each diversity treatment was replicated four times on three soil types for a total of 48 research plots (0.33–0.56 ha each). We measured biomass production by harvesting all plant material to ground level in ten randomly selected quadrats per plot. Weed biomass was measured as a subset of total biomass. We replicated this design over a five‐year period (2010–2014). Across soil types, the one‐, 16‐, and 32‐species treatments produced the same amount of biomass, but the one‐species treatment produced significantly more biomass than the five‐species treatment. The rank order of our four diversity treatments differed between soil types suggesting that soil type influences treatment productivity. Year‐to‐year variation in biomass production did not differ between diversity treatments. Weed biomass was higher in the one‐species treatment than the five‐, 16‐, and 32‐species treatments. The high productivity and low susceptibility to weed invasion of our 16‐ and 32‐species treatments supports the hypothesis that high‐diversity prairie mixtures would be effective biomass feedstocks in the Midwestern United States. The influence of soil type on relative feedstock performance suggests that seed mixes used for biomass should be specifically tailored to site characteristics for maximum productivity and stand success.     

Selective grazing modifies previously anticipated responses of plant community composition to elevated CO2 in a temperate grassland

Our limited understanding of terrestrial ecosystem responses to elevated CO₂ is a major constraint on predicting the impacts of climate change. A change in botanical composition has been identified as a key factor in the CO₂ response with profound implications for ecosystem services such as plant production and soil carbon storage. In temperate grasslands, there is a strong consensus that elevated CO₂ will result in a greater physiological stimulus to growth in legumes and to a lesser extent forbs, compared with C3 grasses, and the presumption this will lead in turn to a greater proportion of these functional groups in the plant community. However, this view is based on data mainly collected in experiments of three or less years in duration and not in experiments where defoliation has been by grazing animals. Grazing is, however, the most common management of grasslands and known in itself to influence botanical composition. In a long‐term Free Air Carbon Dioxide Enrichment (FACE) experiment in a temperate grassland managed with grazing animals (sheep), we found the response to elevated CO₂ in plant community composition in the first 5 years was consistent with the expectation of increased proportions of legumes and forbs. However, in the longer term, these differences diminished so that the proportions of grasses, legumes and forbs were the same under both ambient and elevated CO₂. Analysis of vegetation before and after each grazing event showed there was a sustained disproportionately greater removal (‘apparent selection’) of legumes and forbs by the grazing animals. This bias in removal was greater under elevated CO₂ than ambient CO₂. This is consistent with sustained faster growth rates of legumes and forbs under elevated CO₂ being countered by selective defoliation, and so leading to little difference in community composition.     

Floristic composition of a Swedish semi‐natural grassland during six years of elevated atmospheric CO2

Abstract. A semi‐natural grassland in Sweden was exposed to an elevated CO₂ concentration during a six‐year open‐top chamber experiment. Vegetation composition was assessed twice a year using the point‐intercept method. The field had been grazed previously, but when the experiment started this was replaced with a cutting regime with one cut (down to ground level) each year in early August. From the third to the sixth year of the study the harvested material was divided into legumes, non‐leguminous forbs and grasses, dried and weighed. Elevated CO₂ had an effect on species composition (as analysed by Principal Component Analysis) that increased over time. It also tended to increase diversity (Shannon index) in summer, but reduce it in spring. However, the effects of the weather and/or time on species composition and diversity were much more prominent than CO₂ effects. Since the weather was largely directional over time (from dry to wet), with the exception of the fifth year, it was difficult to distinguish between weather effects and changes caused by a changed management regime. In all treatments, grasses increased over time in both mass and point‐intercept measurements, whereas non‐leguminous forbs decreased in mass, but not in point‐intercept measurements. Legumes increased in the point‐intercept measurements, but not in biomass, at elevated CO₂, but not in the other treatments. Overall, we found that elevated CO₂ affected species composition; however, it was only one of many factors and a rather weak one.     

Differences in rhizosphere carbon-partitioning among plant species of different families

Interspecific variations in carbon (C) allocation and partitioning in the rhizosphere were investigated on 12 Mediterranean species belonging to different family groups (grasses, legumes, non-legume forbs) and having different life cycles. Plants grown individually in artificial soil, in a greenhouse and inoculated with rhizosphere microflora were labelled with ¹⁴CO₂ for 3 h at the vegetative stage. Rhizosphere respiration was measured during 6 days after which labelled C partitioning between shoots, roots, soil, root washing solution and respiration was estimated. The percentage of assimilated ¹⁴C allocated below ground differed significantly between species (41 – 76%) but no significant difference was found between grasses, legumes and non-legume forbs. When expressed as percentage of below-ground ¹⁴C, rhizosphere respiration was significantly smaller for non-legume forbs (42%) than for grasses (46%) and legumes (51%). Consequently more ¹⁴C was incorporated into root biomass in the former. Half-life of ¹⁴CO₂ evolution through respiration ranged from 23 h in legumes to 27 h for non-legume forbs and 37 h for grasses. This suggested differences in microbial activities due to quantities and quality of root exuded C. Rhizosphere respiration was positively correlated with the amount of ¹⁴C in the solution used to wash the roots on one hand, and root N concentration on the other hand. This led to a functional hierarchy between plant family groups of the overall rhizosphere activity. It went from non-legume forbs being the less active (except Crepis sancta)in terms of respiration and exudation, to grasses and then legumes, the most active but also the richest in nitrogen.     

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.     

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.     

Strong photosynthetic acclimation and enhanced water‐use efficiency in grassland functional groups persist over 21 years of CO2 enrichment,independent of nitrogen supply

Uncertainty about long‐term leaf‐level responses to atmospheric CO₂ rise is a major knowledge gap that exists because of limited empirical data. Thus, it remains unclear how responses of leaf gas exchange to elevated CO₂ (eCO₂) vary among plant species and functional groups, or across different levels of nutrient supply, and whether they persist over time for long‐lived perennials. Here, we report the effects of eCO₂ on rates of net photosynthesis and stomatal conductance in 14 perennial grassland species from four functional groups over two decades in a Minnesota Free‐Air CO₂ Enrichment experiment, BioCON. Monocultures of species belonging to C₃ grasses, C₄ grasses, forbs, and legumes were exposed to two levels of CO₂ and nitrogen supply in factorial combinations over 21 years. eCO₂ increased photosynthesis by 12.9% on average in C₃ species, substantially less than model predictions of instantaneous responses based on physiological theory and results of other studies, even those spanning multiple years. Acclimation of photosynthesis to eCO₂ was observed beginning in the first year and did not strengthen through time. Yet, contrary to expectations, the response of photosynthesis to eCO₂ was not enhanced by increased nitrogen supply. Differences in responses among herbaceous plant functional groups were modest, with legumes responding the most and C₄ grasses the least as expected, but did not further diverge over time. Leaf‐level water‐use efficiency increased by 50% under eCO₂ primarily because of reduced stomatal conductance. Our results imply that enhanced nitrogen supply will not necessarily diminish photosynthetic acclimation to eCO₂ in nitrogen‐limited systems, and that significant and consistent declines in stomatal conductance and increases in water‐use efficiency under eCO₂ may allow plants to better withstand drought.     

Food selection in <Emphasis Type="Italic">Microtus arvalis</Emphasis>: the role of plant functional traits

We offered captive common voles (Microtus arvalis) a choice of 11 plant species (representing four ecological groups) growing in vivaria. Selection was evaluated by measuring (1) the biomass of each plant species consumed and (2) functional and life-history plant traits. The legume Trifolium pratense, known for its high nutrient level, and well accessible rosette forbs creating the highest biomass at the soil ground level, were mostly preferred. Voles avoided mainly grasses and the creeping forb Thymus pulegioides. The experiment showed that foraging was strongly plant species-specific. We assessed whether plant functional traits explain selective foraging in common voles. To explore this, we reanalyzed Holišová’s (1959) data about common vole stomach contents and plant trait databases. Regression tree analysis indicated that plant guild and life span were the best predictors of dietary selection, with a probability exceeding 0.5 that voles would eat more grasses and/or legumes than forbs. These results do not correspond with the feeding trial. We suggest that the voles usually consume grasses in the field because grasses are abundant and readily available, but prefer protein-rich forbs when possible.     

Seed production and seed quality in a calcareous grassland in elevated CO2

In diverse plant communities the relative contribution of species to community biomass may change considerably in response to elevated CO₂. Along with species‐specific biomass responses, reproduction is likely to change as well with increasing CO₂ and might further accelerate shifts in species composition. Here, we ask if, after 5 years of CO₂ exposure, seed production and seed quality in natural nutrient‐poor calcareous grassland are affected by elevated CO₂ (650 μ L L^?1 vs 360 μ L L^?1) and how this might affect long‐term community dynamics. The effect of elevated CO₂ on the number of flowering shoots (+ 24%, P < 0.01) and seeds (+ 29%, P = 0.06) at the community level was similar to above ground biomass responses in this year, suggesting that the overall allocation to sexual reproduction remained unchanged. Compared among functional groups of species we found a 42% increase in seed number (P < 0.01) of graminoids, a 33% increase (P = 0.07) in forbs, and no significant change in legumes (? 38%, n.s.) under elevated CO₂. Large responses particularly of two graminoid species and smaller responses of many forb species summed up to the significant or marginally significant increase in seed number of graminoids and forbs, respectively. In several species the increase in seed number resulted both from an increase in flowering shoots and an increase in inflorescence size. In most species, seeds tended to be heavier (+ 12%, P < 0.01), and N‐concentration of seeds was significantly reduced in eight out of 13 species. The fraction of germinating seeds did not differ between seeds produced in ambient and elevated CO₂, but time to germination was significantly shortened in two species and prolonged in one species when seeds had been produced in elevated CO₂. Results suggest that species specific increases in seed number and changes in seed quality will exert substantial cumulative effects on community composition in the long run.     

Effects of Soil Nutrient Heterogeneity and Earthworms on Aboveground Biomass of Experimental Plant Communities

Soil nutrients are commonly heterogeneously distributed and earthworms are one of the most common soil organisms. While effects of both soil nutrient heterogeneity and earthworms have been well studied, their interactive effect on plant community productivity has rarely been tested. In a greenhouse experiment, we constructed experimental plant communities by sowing seed mixtures of four grasses, two legumes and two forbs in either a heterogeneous soil consisting of low and high nutrient soil patches or a homogeneous soil where the low and high nutrient soil patches were evenly mixed. The earthworm Eisenia fetida was either added to these soils or not. Aboveground biomass of the whole communities, grasses and legumes did not differ between the homogeneous and heterogeneous soils or between the soils with and without earthworms. However, soil nutrient heterogeneity reduced aboveground biomass of forbs, and such an effect did not interact with earthworms. In response to soil heterogeneity and earthworms, biomass ratio of the three functional groups showed similar patterns as that of their biomass. At the patch level, aboveground biomass of the whole community, grasses and legumes were greater in the high than in the low nutrient soil patches within the heterogeneous soil. A similar pattern was found for the forbs, but this was only true in the absence of earthworms. Our results suggest that soil nutrient heterogeneity and earthworms may not influence aboveground biomass of plant communities, despite the fact that they may modify the growth of certain plant functional groups within the community.     

Host selectivity,haustorial anatomy and impact of the invasive parasite Parentucellia viscosa on floodplain vegetative communities in Japan

The parasitic plant Parentucellia viscosa has been introduced recently to the Japanese floodplain. Because of its parasitic nature and high fecundity, P. viscosa may well become a major plant invader with a significant impact on floodplain ecosystems. Thus, a knowledge of the host selectivity of P. viscosa will not only provide basic biological information, but will also contribute to the understanding of floodplain vegetation conservation. We evaluated the host selectivity by comparing the observed numbers of haustoria with those expected from the relative below‐ground biomass. In addition, we examined the haustorial anatomy to determine whether haustoria are functional, and compared the above‐ground biomass of three functional plant groups (grasses, legumes and nonleguminous forbs) in intact and parasite removal quadrats. We found that haustoria were nonrandomly distributed amongst host species, suggesting host preference for Poaceae and Fabaceae. In addition, haustoria attached to certain forbs did not penetrate into the stele. The above‐ground biomass of grasses and legumes was increased significantly by the removal of P. viscosa, but the biomass of forbs did not change significantly. These results suggest that host preference depresses the performance of Poaceae and Fabaceae, thus affecting the competitive relationships among plants, meaning that P. viscosa may pose a serious threat to indigenous endangered legumes and grasses. © 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 170 , 69–78.     

Aboveground Biomass Removal by Burning and Raking Increases Diversity in a Reconstructed Prairie

Prescribed spring burning often contributes to a predominance of C₄ grasses and low forb abundance and is impractical at many sites, especially near development. We tested raking after mowing as an alternative to prescribed burning in a reconstructed Minnesota prairie. We also tested mowing without raking as a possible means of maintaining prairie communities. Frequency, flowering stem abundance, and cover were measured for all plant species and native functional groups (C₄ grasses, C₃ graminoids, forbs, legumes, and annual or biennial forbs). Mowing alone did not differ from the control in its effect on any functional groups of plants. Round‐headed bush clover (Lespedeza capitata), a legume, and Black‐eyed Susan (Rudbeckia hirta), a biennial, increased in frequency with treatments that removed biomass (i.e., fire or raking), but they did not have significantly more flowering stems. Thus, new plants established well from seed, whereas the vitality of mature plants did not change. Raking had similar effects to burning on most functional groups, although flowering stems of C₄ grasses were significantly more abundant after fire than after raking. Burning reduced some C₃ forbs and grasses and favored the dominance of C₄ grasses. Therefore, raking after mowing in the spring provides an alternative to prescribed burning that has many of the same positive aspects as fire but does not promote aggressive C₄ grasses to the same extent.     

Root responses to nitrogen pulse frequency under different nitrogen amounts

Responses of morphology and biomass allocation of roots to frequency of nitrogen (N) pulse potentially influence the fitness of plants, but such responses may be determined by root size. We grew 12 plant species of three functional groups (grasses, forbs, and legumes) under two N pulse frequencies (high vs. low supply frequency) and two N amounts (high vs. low supply amount). Compared to low-amount N supply, high-amount N supply stimulated biomass accumulation and root growth by either increasing the thickness and length of roots or decreasing the root mass fraction. Compared to low-frequency N supply, high-frequency N supply improved biomass accumulation and root growth in forbs or grasses, but not in legumes. Furthermore, the magnitude of the response to N frequency was significantly negatively correlated with root size at the species scale, but this was only true when the N amount was high. We conclude that root responses to N frequency are related to plant functional types, and non-legume species is more sensitive to N frequency than legume species. Our results also suggest that root size is a determinant of root responses to N frequency when N supply amount is high.     

Tallgrass prairie restoration: implications of increased atmospheric nitrogen deposition when site preparation minimizes adventive grasses

Site preparation designed to exhaust the soil seedbank of adventive species can improve the success of tallgrass prairie restoration. Despite these efforts, increased rates of atmospheric nitrogen (N) deposition over the next century could potentially promote the growth of nitrophilic, adventive species in tallgrass restoration projects. We used a field experiment to examine how N addition affected species composition and plant productivity over the first 3 years of a tallgrass prairie restoration that was preceded by the planting of glyphosate‐resistant crops and multiple applications of glyphosate to exhaust the pre‐existing seedbank. We predicted that N addition would increase the percent cover of adventive plant species not included in the original seeding. Contrary to our prediction, only the cover of native species increased with N addition; native non‐leguminous forbs increased substantially, with Conyza canadensis (a weedy native species not part of the restoration seed mix) exploiting the combination of high N and bare ground in the first year, and non‐leguminous forbs (in particular Monarda fistulosa) and native C₃ grasses, all of which were seeded, increasing with N addition by the third year. Native legumes was the only functional group that exhibited lower cover in N addition plots than in control plots. There was no significant response by native C₄ grasses to N addition, and adventive grasses remained mostly absent from the plots. Overall, our results suggest that site pre‐treatment with herbicide may continue to be effective in minimizing adventive grasses in restored tallgrass prairie, despite future increases in atmospheric N deposition.     

Meta‐analysis reveals profound responses of plant traits to glacial CO2 levels

A general understanding of the links between atmospheric CO₂ concentration and the functioning of the terrestrial biosphere requires not only an understanding of plant trait responses to the ongoing transition to higher CO₂ but also the legacy effects of past low CO₂. An interesting question is whether the transition from current to higher CO₂ can be thought of as a continuation of the past trajectory of low to current CO₂ levels. Determining this trajectory requires quantifying the effect sizes of plant response to low CO₂. We performed a meta‐analysis of low CO₂ growth experiments on 34 studies with 54 species. We quantified how plant traits vary at reduced CO₂ levels and whether C₃ versus C₄ and woody versus herbaceous plant species respond differently. At low CO₂, plant functioning changed drastically: on average across all species, a 50% reduction in current atmospheric CO₂ reduced net photosynthesis by 38%; increased stomatal conductance by 60% and decreased intrinsic water use efficiency by 48%. Total plant dry biomass decreased by 47%, while specific leaf area increased by 17%. Plant types responded similarly: the only significant differences being no increase in SLA for C₄ species and a 16% smaller decrease in biomass for woody C₃ species at glacial CO₂. Quantitative comparison of low CO₂ effect sizes to those from high CO₂ studies showed that the magnitude of response of stomatal conductance, water use efficiency and SLA to increased CO₂ can be thought of as continued shifts along the same line. However, net photosynthesis and dry weight responses to low CO₂ were greater in magnitude than to high CO₂. Understanding the causes for this discrepancy can lead to a general understanding of the links between atmospheric CO₂ and plant responses with relevance for both the past and the future.     

Low phosphorus supply constrains plant responses to elevated CO2: A meta‐analysis

Phosphorus (P) is an essential macro‐nutrient required for plant metabolism and growth. Low P availability could potentially limit plant responses to elevated carbon dioxide (eCO₂), but consensus has yet to be reached on the extent of this limitation. Here, based on data from experiments that manipulated both CO₂ and P for young individuals of woody and non‐woody species, we present a meta‐analysis of P limitation impacts on plant growth, physiological, and morphological response to eCO₂. We show that low P availability attenuated plant photosynthetic response to eCO₂ by approximately one‐quarter, leading to a reduced, but still positive photosynthetic response to eCO₂ compared to those under high P availability. Furthermore, low P limited plant aboveground, belowground, and total biomass responses to eCO₂, by 14.7%, 14.3%, and 12.4%, respectively, equivalent to an approximate halving of the eCO₂ responses observed under high P availability. In comparison, low P availability did not significantly alter the eCO₂‐induced changes in plant tissue nutrient concentration, suggesting tissue nutrient flexibility is an important mechanism allowing biomass response to eCO₂ under low P availability. Low P significantly reduced the eCO₂‐induced increase in leaf area by 14.3%, mirroring the aboveground biomass response, but low P did not affect the eCO₂‐induced increase in root length. Woody plants exhibited stronger attenuation effect of low P on aboveground biomass response to eCO₂ than non‐woody plants, while plants with different mycorrhizal associations showed similar responses to low P and eCO₂ interaction. This meta‐analysis highlights crucial data gaps in capturing plant responses to eCO₂ and low P availability. Field‐based experiments with longer‐term exposure of both CO₂ and P manipulations are critically needed to provide ecosystem‐scale understanding. Taken together, our results provide a quantitative baseline to constrain model‐based hypotheses of plant responses to eCO₂ under P limitation, thereby improving projections of future global change impacts.