Establishment from seed is more important for exotic than for native plant species.

Climate change has initiated movement of both native and non-native (exotic) species across the landscape. Exotic species are hypothesized to establish from seed more readily than comparable native species. We tested the hypothesis that seed limitation is more important for exotic species than native grassland species. We compared seed limitation and invasion resistance over three growing seasons between 18 native and 18 exotic species, grown in both monocultures and mixtures in a field experiment. Half of the plots received a seed mix of the contrasting treatment (i.e., exotic species were seeded into native plots, and native species were seeded into exotic plots), and half served as controls. We found that (1) establishment in this perennial grassland is seed limited, (2) establishment from seed is greater in exotic than native species, and (3) community resistance to seedling establishment was positively related to diversity of extant species, but only in native communities. Native-exotic species diversity and composition differences did not converge over time. Our results imply that native to exotic transformations occur when diversity declines in native vegetation and exotic seeds arrive from adjacent sites, suggesting that managing for high diversity will reduce transformations to exotic dominance.

Biodiversity: Net primary productivity relationships are eliminated by invasive species dominance.

Experiments often find that net primary productivity (NPP) increases with species richness when native species are considered. However, relationships may be altered by exotic (non-native) species, which are hypothesized to reduce richness but increase productivity (i.e., 'invasion-diversity-productivity paradox'). We compared richness-NPP relationships using a comparison of exotic versus native-dominated sites across the central USA, and two experiments under common environments. Aboveground NPP was measured using peak biomass clipping in all three studies, and belowground NPP was measured in one study with root ingrowth cores using root-free soil. In all studies, there was a significantly positive relationship between NPP and richness across native species-dominated sites and plots, but no relationship across exotic-dominated ones. These results indicate that relationships between NPP and richness depend on whether native or exotic species are dominant, and that exotic species are 'breaking the rules', altering richness-productivity and richness-C stock relationships after invasion.

Global stocks and capacity of mineral-associated soil organic carbon.

Soil is the largest terrestrial reservoir of organic carbon and is central for climate change mitigation and carbon-climate feedbacks. Chemical and physical associations of soil carbon with minerals play a critical role in carbon storage, but the amount and global capacity for storage in this form remain unquantified. Here, we produce spatially-resolved global estimates of mineral-associated organic carbon stocks and carbon-storage capacity by analyzing 1144 globally-distributed soil profiles. We show that current stocks total 899 Pg C to a depth of 1 m in non-permafrost mineral soils. Although this constitutes 66% and 70% of soil carbon in surface and deeper layers, respectively, it is only 42% and 21% of the mineralogical capacity. Regions under agricultural management and deeper soil layers show the largest undersaturation of mineral-associated carbon. Critically, the degree of undersaturation indicates sequestration efficiency over years to decades. We show that, across 103 carbon-accrual measurements spanning management interventions globally, soils furthest from their mineralogical capacity are more effective at accruing carbon; sequestration rates average 3-times higher in soils at one tenth of their capacity compared to soils at one half of their capacity. Our findings provide insights into the world's soils, their capacity to store carbon, and priority regions and actions for soil carbon management.

Biotic homogenization destabilizes ecosystem functioning by decreasing spatial asynchrony.

Our planet is facing significant changes of biodiversity across spatial scales. Although the negative effects of local biodiversity (α diversity) loss on ecosystem stability are well documented, the consequences of biodiversity changes at larger spatial scales, in particular biotic homogenization, that is, reduced species turnover across space (ß diversity), remain poorly known. Using data from 39 grassland biodiversity experiments, we examine the effects of ß diversity on the stability of simulated landscapes while controlling for potentially confounding biotic and abiotic factors. Our results show that higher ß diversity generates more asynchronous dynamics among local communities and thereby contributes to the stability of ecosystem productivity at larger spatial scales. We further quantify the relative contributions of α and ß diversity to ecosystem stability and find a relatively stronger effect of α diversity, possibly due to the limited spatial scale of our experiments. The stabilizing effects of both α and ß diversity lead to a positive diversity-stability relationship at the landscape scale. Our findings demonstrate the destabilizing effect of biotic homogenization and suggest that biodiversity should be conserved at multiple spatial scales to maintain the stability of ecosystem functions and services.

Multiple constraints cause positive and negative feedbacks limiting grassland soil CO2 efflux under CO2 enrichment.

Terrestrial ecosystems are increasingly enriched with resources such as atmospheric CO2 that limit ecosystem processes. The consequences for ecosystem carbon cycling depend on the feedbacks from other limiting resources and plant community change, which remain poorly understood for soil CO2 efflux, JCO2, a primary carbon flux from the biosphere to the atmosphere. We applied a unique CO2 enrichment gradient (250 to 500 µL L-1) for eight years to grassland plant communities on soils from different landscape positions. We identified the trajectory of JCO2 responses and feedbacks from other resources, plant diversity [effective species richness, exp(H)], and community change (plant species turnover). We found linear increases in JCO2 on an alluvial sandy loam and a lowland clay soil, and an asymptotic increase on an upland silty clay soil. Structural equation modeling identified CO2 as the dominant limitation on JCO2 on the clay soil. In contrast with theory predicting limitation from a single limiting factor, the linear JCO2 response on the sandy loam was reinforced by positive feedbacks from aboveground net primary productivity and exp(H), while the asymptotic JCO2 response on the silty clay arose from a net negative feedback among exp(H), species turnover, and soil water potential. These findings support a multiple resource limitation view of the effects of global change drivers on grassland ecosystem carbon cycling and highlight a crucial role for positive or negative feedbacks between limiting resources and plant community structure. Incorporating these feedbacks will improve models of terrestrial carbon sequestration and ecosystem services.

Lower soil carbon stocks in exotic vs. native grasslands are driven by carbonate losses.

Global change includes invasion by exotic (nonnative) plant species and altered precipitation patterns, and these factors may affect terrestrial carbon (C) storage. We measured soil C changes in experimental mixtures of all exotic or all native grassland plant species under two levels of summer drought stress (0 and +128 mm). After 8 yr, soils were sampled in 10-cm increments to 100-cm depth to determine if soil C differed among treatments in deeper soils. Total soil C (organic + inorganic) content was significantly higher under native than exotic plantings, and differences increased with depth. Surprisingly, differences after 8 yr in C were due to carbonate and not organic C fractions, where carbonate was ~250 g C/m2 lower to 1-m soil depth under exotic than native plantings. Our results indicate that soil carbonate is an active pool and can respond to differences in plant species traits over timescales of years. Significant losses of inorganic C might be avoided by conserving native grasslands in subhumid ecosystems.

CO2 enrichment and soil type additively regulate grassland productivity.

Atmospheric CO2 enrichment usually increases the aboveground net primary productivity (ANPP) of grassland vegetation, but the magnitude of the ANPP-CO2 response differs among ecosystems. Soil properties affect ANPP via multiple mechanisms and vary over topographic to geographic gradients, but have received little attention as potential modifiers of the ANPP-CO2 response. We assessed the effects of three soil types, sandy loam, silty clay and clay, on the ANPP response of perennial C3 /C4 grassland communities to a subambient to elevated CO2 gradient over 10 yr in Texas, USA. We predicted an interactive, rather than additive, effect of CO2 and soil type on ANPP. Contrary to prediction, CO2 and soil additively influenced grassland ANPP. Increasing CO2 by 250 µl l-1 increased ANPP by 170 g m-2 across soil types. Increased clay content from 10% to 50% among soils reduced ANPP by 50 g m-2 . CO2 enrichment increased ANPP via a predominant direct effect, accompanied by a smaller indirect effect mediated by a successional shift to increased dominance of the C4 tallgrass Sorghastrum nutans. Our results indicate a large, positive influence of CO2 enrichment on grassland productivity that resulted from the direct physiological benefits of CO2 augmented by species succession, and was expressed similarly across soils of differing physical properties.

Multiple facets of biodiversity drive the diversity-stability relationship.

A substantial body of evidence has demonstrated that biodiversity stabilizes ecosystem functioning over time in grassland ecosystems. However, the relative importance of different facets of biodiversity underlying the diversity-stability relationship remains unclear. Here we use data from 39 grassland biodiversity experiments and structural equation modelling to investigate the roles of species richness, phylogenetic diversity and both the diversity and community-weighted mean of functional traits representing the 'fast-slow' leaf economics spectrum in driving the diversity-stability relationship. We found that high species richness and phylogenetic diversity stabilize biomass production via enhanced asynchrony in the performance of co-occurring species. Contrary to expectations, low phylogenetic diversity enhances ecosystem stability directly, albeit weakly. While the diversity of fast-slow functional traits has a weak effect on ecosystem stability, communities dominated by slow species enhance ecosystem stability by increasing mean biomass production relative to the standard deviation of biomass over time. Our in-depth, integrative assessment of factors influencing the diversity-stability relationship demonstrates a more multicausal relationship than has been previously acknowledged.

Flowering in grassland predicted by CO2 and resource effects on species aboveground biomass.

Continuing enrichment of atmospheric CO2 may change plant community composition, in part by altering the availability of other limiting resources including soil water, nutrients, or light. The combined effects of CO2 enrichment and altered resource availability on species flowering remain poorly understood. We quantified flowering culm and ramet production and biomass allocation to flowering culms/ramets for 10 years in C4 -dominated grassland communities on contrasting soils along a CO2 concentration gradient spanning pre-industrial to expected mid-21st century levels (250-500 µl/L). CO2 enrichment explained up to 77% of the variation in flowering culm count across soils for three of the five species, and was correlated with flowering culm count on at least one soil for four of five species. In contrast, allocation to flowering culms was only weakly correlated with CO2 enrichment for two species. Flowering culm counts were strongly correlated with species aboveground biomass (AGB; R2  = .34-.74), a measure of species abundance. CO2 enrichment also increased soil moisture and decreased light levels within the canopy but did not affect soil inorganic nitrogen availability. Structural equation models fit across the soils suggested species-specific controls on flowering in two general forms: (1) CO2 effects on flowering culm count mediated by canopy light level and relative species AGB (species AGB/total AGB) or by soil moisture effects on flowering culm count; (2) effects of canopy light level or soil inorganic nitrogen on flowering and/or relative species AGB, but with no significant CO2 effect. Understanding the heterogeneity in species responses to CO2 enrichment in plant communities across soils in edaphically variable landscapes is critical to predict CO2 effects on flowering and other plant fitness components, and species potential to adapt to future environmental changes.

Accelerated development in Johnsongrass seedlings (Sorghum halepense) suppresses the growth of native grasses through size-asymmetric competition.

Invasive plant species often dominate native species in competition, augmenting other potential advantages such as release from natural enemies. Resource pre-emption may be a particularly important mechanism for establishing dominance over competitors of the same functional type. We hypothesized that competitive success of an exotic grass against native grasses is mediated by establishing an early size advantage. We tested this prediction among four perennial C4 warm-season grasses: the exotic weed Johnsongrass (Sorghum halepense), big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparius) and switchgrass (Panicum virgatum). We predicted that a) the competitive effect of Johnsongrass on target species would be proportional to their initial biomass difference, b) competitive effect and response would be negatively correlated and c) soil fertility would have little effect on competitive relationships. In a greenhouse, plants of the four species were grown from seed either alone or with one Johnsongrass neighbor at two fertilizer levels and periodically harvested. The first two hypotheses were supported: The seedling biomass of single plants at first harvest (50 days after seeding) ranked the same way as the competitive effect of Johnsongrass on target species: Johnsongrass < big bluestem < little bluestem/switchgrass, while Johnsongrass responded more strongly to competition from Johnsongrass than from native species. At final harvest, native plants growing with Johnsongrass attained between 2-5% of their single-plant non-root biomass, while Johnsongrass growing with native species attained 89% of single-plant non-root biomass. Fertilization enhanced Johnsongrass' competitive effects on native species, but added little to the already severe competitive suppression. Accelerated early growth of Johnsongrass seedlings relative to native seedlings appeared to enable subsequent resource pre-emption. Size-asymmetric competition and resource-pre-emption may be a critical mechanism by which exotic invasive species displace functionally similar native species and alter the functional dynamics of native communities.

Rising atmospheric CO2 is reducing the protein concentration of a floral pollen source essential for North American bees.

At present, there is substantive evidence that the nutritional content of agriculturally important food crops will decrease in response to rising levels of atmospheric carbon dioxide, Ca However, whether Ca-induced declines in nutritional quality are also occurring for pollinator food sources is unknown. Flowering late in the season, goldenrod (Solidago spp.) pollen is a widely available autumnal food source commonly acknowledged by apiarists to be essential to native bee (e.g. Bombus spp.) and honeybee (Apis mellifera) health and winter survival. Using floral collections obtained from the Smithsonian Natural History Museum, we quantified Ca-induced temporal changes in pollen protein concentration of Canada goldenrod (Solidago canadensis), the most wide spread Solidago taxon, from hundreds of samples collected throughout the USA and southern Canada over the period 1842-2014 (i.e. a Ca from approx. 280 to 398 ppm). In addition, we conducted a 2 year in situtrial of S. Canadensis populations grown along a continuous Ca gradient from approximately 280 to 500 ppm. The historical data indicated a strong significant correlation between recent increases in Ca and reductions in pollen protein concentration (r(2)= 0.81). Experimental data confirmed this decrease in pollen protein concentration, and indicated that it would be ongoing as Ca continues to rise in the near term, i.e. to 500 ppm (r(2)= 0.88). While additional data are needed to quantify the subsequent effects of reduced protein concentration for Canada goldenrod on bee health and population stability, these results are the first to indicate that increasing Ca can reduce protein content of a floral pollen source widely used by North American bees.

Plant diversity effects on grassland productivity are robust to both nutrient enrichment and drought.

Global change drivers are rapidly altering resource availability and biodiversity. While there is consensus that greater biodiversity increases the functioning of ecosystems, the extent to which biodiversity buffers ecosystem productivity in response to changes in resource availability remains unclear. We use data from 16 grassland experiments across North America and Europe that manipulated plant species richness and one of two essential resources-soil nutrients or water-to assess the direction and strength of the interaction between plant diversity and resource alteration on above-ground productivity and net biodiversity, complementarity, and selection effects. Despite strong increases in productivity with nutrient addition and decreases in productivity with drought, we found that resource alterations did not alter biodiversity-ecosystem functioning relationships. Our results suggest that these relationships are largely determined by increases in complementarity effects along plant species richness gradients. Although nutrient addition reduced complementarity effects at high diversity, this appears to be due to high biomass in monocultures under nutrient enrichment. Our results indicate that diversity and the complementarity of species are important regulators of grassland ecosystem productivity, regardless of changes in other drivers of ecosystem function.

A CO2 Concentration Gradient Facility for Testing CO2 Enrichment and Soil Effects on Grassland Ecosystem Function.

Continuing increases in atmospheric carbon dioxide concentrations (CA) mandate techniques for examining impacts on terrestrial ecosystems. Most experiments examine only two or a few levels of CA concentration and a single soil type, but if CA can be varied as a gradient from subambient to superambient concentrations on multiple soils, we can discern whether past ecosystem responses may continue linearly in the future and whether responses may vary across the landscape. The Lysimeter Carbon Dioxide Gradient Facility applies a 250 to 500 µl L-1 CA gradient to Blackland prairie plant communities established on lysimeters containing clay, silty clay, and sandy soils. The gradient is created as photosynthesis by vegetation enclosed in in temperature-controlled chambers progressively depletes carbon dioxide from air flowing directionally through the chambers. Maintaining proper air flow rate, adequate photosynthetic capacity, and temperature control are critical to overcome the main limitations of the system, which are declining photosynthetic rates and increased water stress during summer. The facility is an economical alternative to other techniques of CA enrichment, successfully discerns the shape of ecosystem responses to subambient to superambient CA enrichment, and can be adapted to test for interactions of carbon dioxide with other greenhouse gases such as methane or ozone.

Biodiversity increases the resistance of ecosystem productivity to climate extremes.

It remains unclear whether biodiversity buffers ecosystems against climate extremes, which are becoming increasingly frequent worldwide. Early results suggested that the ecosystem productivity of diverse grassland plant communities was more resistant, changing less during drought, and more resilient, recovering more quickly after drought, than that of depauperate communities. However, subsequent experimental tests produced mixed results. Here we use data from 46 experiments that manipulated grassland plant diversity to test whether biodiversity provides resistance during and resilience after climate events. We show that biodiversity increased ecosystem resistance for a broad range of climate events, including wet or dry, moderate or extreme, and brief or prolonged events. Across all studies and climate events, the productivity of low-diversity communities with one or two species changed by approximately 50% during climate events, whereas that of high-diversity communities with 16-32 species was more resistant, changing by only approximately 25%. By a year after each climate event, ecosystem productivity had often fully recovered, or overshot, normal levels of productivity in both high- and low-diversity communities, leading to no detectable dependence of ecosystem resilience on biodiversity. Our results suggest that biodiversity mainly stabilizes ecosystem productivity, and productivity-dependent ecosystem services, by increasing resistance to climate events. Anthropogenic environmental changes that drive biodiversity loss thus seem likely to decrease ecosystem stability, and restoration of biodiversity to increase it, mainly by changing the resistance of ecosystem productivity to climate events.

Dominant plant taxa predict plant productivity responses to CO2 enrichment across precipitation and soil gradients.

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.

Plant community change mediates the response of foliar δ(15)N to CO 2 enrichment in mesic grasslands.

Rising atmospheric CO2 concentration may change the isotopic signature of plant N by altering plant and microbial processes involved in the N cycle. CO2 may increase leaf δ(15)N by increasing plant community productivity, C input to soil, and, ultimately, microbial mineralization of old, (15)N-enriched organic matter. We predicted that CO2 would increase aboveground productivity (ANPP; g biomass m(-2)) and foliar δ(15)N values of two grassland communities in Texas, USA: (1) a pasture dominated by a C4 exotic grass, and (2) assemblages of tallgrass prairie species, the latter grown on clay, sandy loam, and silty clay soils. Grasslands were exposed in separate experiments to a pre-industrial to elevated CO2 gradient for 4 years. CO2 stimulated ANPP of pasture and of prairie assemblages on each of the three soils, but increased leaf δ(15)N only for prairie plants on a silty clay. δ(15)N increased linearly as mineral-associated soil C declined on the silty clay. Mineral-associated C declined as ANPP increased. Structural equation modeling indicted that CO2 increased ANPP partly by favoring a tallgrass (Sorghastrum nutans) over a mid-grass species (Bouteloua curtipendula). CO2 may have increased foliar δ(15)N on the silty clay by reducing fractionation during N uptake and assimilation. However, we interpret the soil-specific, δ(15)N-CO2 response as resulting from increased ANPP that stimulated mineralization from recalcitrant organic matter. By contrast, CO2 favored a forb species (Solanum dimidiatum) with higher δ(15)N than the dominant grass (Bothriochloa ischaemum) in pasture. CO2 enrichment changed grassland δ(15)N by shifting species relative abundances.

Plant invasions differentially affected by diversity and dominant species in native- and exotic-dominated grasslands.

Plant invasions are an increasingly serious global concern, especially as the climate changes. Here, we explored how plant invasions differed between native- and novel exotic-dominated grasslands with experimental addition of summer precipitation in Texas in 2009. Exotic species greened up earlier than natives by an average of 18 days. This was associated with a lower invasion rate early in the growing season compared to native communities. However, invasion rate did not differ significantly between native and exotic communities across all sampling times. The predictors of invasion rate differed between native and exotic communities, with invasion being negatively influenced by species richness in natives and by dominant species in exotics. Interestingly, plant invasions matched the bimodal pattern of precipitation in Temple, Texas, and did not respond to the pulse of precipitation during the summer. Our results suggest that we will need to take different approaches in understanding of invasion between native and exotic grasslands. Moreover, with anticipated increasing variability in precipitation under global climate change, plant invasions may be constrained in their response if the precipitation pulses fall outside the normal growing period of invaders.

Fungal Community Responses to Past and Future Atmospheric CO2 Differ by Soil Type.

Soils sequester and release substantial atmospheric carbon, but the contribution of fungal communities to soil carbon balance under rising CO2 is not well understood. Soil properties likely mediate these fungal responses but are rarely explored in CO2 experiments. We studied soil fungal communities in a grassland ecosystem exposed to a preindustrial-to-future CO2 gradient (250 to 500 ppm) in a black clay soil and a sandy loam soil. Sanger sequencing and pyrosequencing of the rRNA gene cluster revealed that fungal community composition and its response to CO2 differed significantly between soils. Fungal species richness and relative abundance of Chytridiomycota (chytrids) increased linearly with CO2 in the black clay (P < 0.04, R(2) > 0.7), whereas the relative abundance of Glomeromycota (arbuscular mycorrhizal fungi) increased linearly with elevated CO2 in the sandy loam (P = 0.02, R(2) = 0.63). Across both soils, decomposition rate was positively correlated with chytrid relative abundance (r = 0.57) and, in the black clay soil, fungal species richness. Decomposition rate was more strongly correlated with microbial biomass (r = 0.88) than with fungal variables. Increased labile carbon availability with elevated CO2 may explain the greater fungal species richness and Chytridiomycota abundance in the black clay soil, whereas increased phosphorus limitation may explain the increase in Glomeromycota at elevated CO2 in the sandy loam. Our results demonstrate that soil type plays a key role in soil fungal responses to rising atmospheric CO2.

Biodiversity, photosynthetic mode, and ecosystem services differ between native and novel ecosystems.

Human activities have caused non-native plant species with novel ecological interactions to persist on landscapes, and it remains controversial whether these species alter multiple aspects of communities and ecosystems. We tested whether native and exotic grasslands differ in species diversity, ecosystem services, and an important aspect of functional diversity (C3:C4 proportions) by sampling 42 sites along a latitudinal gradient and conducting a controlled experiment. Exotic-dominated grasslands had drastically lower plant diversity and slightly higher tissue N concentrations and forage quality compared to native-dominated sites. Exotic sites were strongly dominated by C4 species at southern and C3 species at northern latitudes with a sharp transition at 36-38°, whereas native sites contained C3:C4 mixtures. Large differences in C3:C4 proportions and temporal niche partitioning were found between native and exotic mixtures in the experiment, implying that differences in C3:C4 proportions along the latitudinal gradient are caused partially by species themselves. Our results indicate that the replacement of native- by exotic-dominated grasslands has created a management tradeoff (high diversity versus high levels of certain ecosystem services) and that models of global change impacts and C3/C4 distribution should consider effects of exotic species.

Impacts of climate change drivers on C4 grassland productivity: scaling driver effects through the plant community.

Climate change drivers affect plant community productivity via three pathways: (i) direct effects of drivers on plants; (ii) the response of species abundances to drivers (community response); and (iii) the feedback effect of community change on productivity (community effect). The contribution of each pathway to driver-productivity relationships depends on functional traits of dominant species. We used data from three experiments in Texas, USA, to assess the role of community dynamics in the aboveground net primary productivity (ANPP) response of C4 grasslands to two climate drivers applied singly: atmospheric CO2 enrichment and augmented summer precipitation. The ANPP-driver response differed among experiments because community responses and effects differed. ANPP increased by 80-120g m(-2) per 100 µl l(-1) rise in CO2 in separate experiments with pasture and tallgrass prairie assemblages. Augmenting ambient precipitation by 128mm during one summer month each year increased ANPP more in native than in exotic communities in a third experiment. The community effect accounted for 21-38% of the ANPP CO2 response in the prairie experiment but little of the response in the pasture experiment. The community response to CO2 was linked to species traits associated with greater soil water from reduced transpiration (e.g. greater height). Community effects on the ANPP CO2 response and the greater ANPP response of native than exotic communities to augmented precipitation depended on species differences in transpiration efficiency. These results indicate that feedbacks from community change influenced ANPP-driver responses. However, the species traits that regulated community effects on ANPP differed from the traits that determined how communities responded to drivers.