• Energy Research

AbstractProjections of future climate are highly sensitive to uncertainties regarding carbon (C) uptake and storage by terrestrial ecosystems. The Eucalyptus Free‐Air CO2 Enrichment (EucFACE) experiment was established to study the effects of elevated atmospheric CO2 concentrations (eCO2) on a native mature eucalypt woodland with low fertility soils in southeast Australia. In contrast to other FACE experiments, the concentration of CO2 at EucFACE was increased gradually in steps above ambient (+0, 30, 60, 90, 120, and 150 ppm CO2 above ambient of ~400 ppm), with each step lasting approximately 5 weeks. This provided a unique opportunity to study the short‐term (weeks to months) response of C cycle flux components to eCO2 across a range of CO2 concentrations in an intact ecosystem. Soil CO2 efflux (i.e., soil respiration or Rsoil) increased in response to initial enrichment (e.g., +30 and +60 ppm CO2) but did not continue to increase as the CO2 enrichment was stepped up to higher concentrations. Light‐saturated photosynthesis of canopy leaves (Asat) also showed similar stimulation by elevated CO2 at +60 ppm as at +150 ppm CO2. The lack of significant effects of eCO2 on soil moisture, microbial biomass, or activity suggests that the increase in Rsoil likely reflected increased root and rhizosphere respiration rather than increased microbial decomposition of soil organic matter. This rapid increase in Rsoil suggests that under eCO2, additional photosynthate was produced, transported belowground, and respired. The consequences of this increased belowground activity and whether it is sustained through time in mature ecosystems under eCO2 are a priority for future research.

Human actions are causing declines in plant biodiversity, increases in atmospheric CO2 concentrations and increases in nitrogen deposition; however, the interactive effects of these factors on ecosystem processes are unknown. Reduced biodiversity has raised numerous concerns, including the possibility that ecosystem functioning may be affected negatively, which might be particularly important in the face of other global changes. Here we present results of a grassland field experiment in Minnesota, USA, that tests the hypothesis that plant diversity and composition influence the enhancement of biomass and carbon acquisition in ecosystems subjected to elevated atmospheric CO2 concentrations and nitrogen deposition. The study experimentally controlled plant diversity (1, 4, 9 or 16 species), soil nitrogen (unamended versus deposition of 4 g of nitrogen per m2 per yr) and atmospheric CO2 concentrations using free-air CO2 enrichment (ambient, 368 micromol mol-1, versus elevated, 560 micromol mol-1). We found that the enhanced biomass accumulation in response to elevated levels of CO2 or nitrogen, or their combination, is less in species-poor than in species-rich assemblages.

SummaryAtmospheric carbon dioxide concentration ([CO2]) is increasing, which increases leaf‐scale photosynthesis and intrinsic water‐use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO2] increase and thus climate change. However, ecosystem CO2 responses are complex or confounded by concurrent changes in multiple agents of global change and evidence for a [CO2]‐driven terrestrial carbon sink can appear contradictory. Here we synthesize theory and broad, multidisciplinary evidence for the effects of increasing [CO2] (iCO2) on the global terrestrial carbon sink. Evidence suggests a substantial increase in global photosynthesis since pre‐industrial times. Established theory, supported by experiments, indicates that iCO2 is likely responsible for about half of the increase. Global carbon budgeting, atmospheric data, and forest inventories indicate a historical carbon sink, and these apparent iCO2 responses are high in comparison to experiments and predictions from theory. Plant mortality and soil carbon iCO2 responses are highly uncertain. In conclusion, a range of evidence supports a positive terrestrial carbon sink in response to iCO2, albeit with uncertain magnitude and strong suggestion of a role for additional agents of global change.

Summary  To evaluate whether functional groups have a similar response to global change, the responses to CO2 concentration and N availability of grassland species from several functional groups are reported here.  Sixteen perennial grassland species from four trait‐based functional groups (C3 grasses, C4 grasses, non‐leguminous forbs, legumes) were grown in field monocultures under ambient or elevated (560 µmol mol−1) CO2 using free‐air CO2 enrichment (FACE), in low N (unamended field soil) or high N (field soil +4 g N m−2 years−1) treatments.  There were no CO2 × N interactions. Functional groups responded differently to CO2 and N in terms of biomass, tissue N concentration and soil solution N. Under elevated CO2, forbs, legumes and C3 grasses increased total biomass by 31%, 18%, and 9%, respectively, whereas biomass was reduced in C4‐grass monocultures. Two of the four legume species increased biomass and total plant N pools under elevated CO2, probably due to stimulated N‐fixation. Only one species markedly shifted the proportional distribution of below‐ vs aboveground biomass in response to CO2 or N.  Although functional groups varied in responses to CO2 and N, there was also substantial variation in responses among species within groups. These results suggest that current trait‐based functional classifications might be useful, but not sufficient, for understanding plant and ecosystem responses to elevated CO2 and N availability.

AbstractPredicted responses of transpiration to elevated atmospheric CO2 concentration (eCO2) are highly variable amongst process‐based models. To better understand and constrain this variability amongst models, we conducted an intercomparison of 11 ecosystem models applied to data from two forest free‐air CO2 enrichment (FACE) experiments at Duke University and Oak Ridge National Laboratory. We analysed model structures to identify the key underlying assumptions causing differences in model predictions of transpiration and canopy water use efficiency. We then compared the models against data to identify model assumptions that are incorrect or are large sources of uncertainty. We found that model‐to‐model and model‐to‐observations differences resulted from four key sets of assumptions, namely (i) the nature of the stomatal response to elevated CO2 (coupling between photosynthesis and stomata was supported by the data); (ii) the roles of the leaf and atmospheric boundary layer (models which assumed multiple conductance terms in series predicted more decoupled fluxes than observed at the broadleaf site); (iii) the treatment of canopy interception (large intermodel variability, 2–15%); and (iv) the impact of soil moisture stress (process uncertainty in how models limit carbon and water fluxes during moisture stress). Overall, model predictions of the CO2 effect on WUE were reasonable (intermodel μ = approximately 28% ± 10%) compared to the observations (μ = approximately 30% ± 13%) at the well‐coupled coniferous site (Duke), but poor (intermodel μ = approximately 24% ± 6%; observations μ = approximately 38% ± 7%) at the broadleaf site (Oak Ridge). The study yields a framework for analysing and interpreting model predictions of transpiration responses to eCO2, and highlights key improvements to these types of models.

The importance of phosphorus (P) in regulating ecosystem responses to climate change has fostered P-cycle implementation in land surface models, but their CO 2 effects predictions have not been evaluated against measurements. Here, we perform a data-driven model evaluation where simulations of eight widely used P-enabled models were confronted with observations from a long-term free-air CO 2 enrichment experiment in a mature, P-limited Eucalyptus forest. We show that most models predicted the correct sign and magnitude of the CO 2 effect on ecosystem carbon (C) sequestration, but they generally overestimated the effects on plant C uptake and growth. We identify leaf-to-canopy scaling of photosynthesis, plant tissue stoichiometry, plant belowground C allocation, and the subsequent consequences for plant-microbial interaction as key areas in which models of ecosystem C-P interaction can be improved. Together, this data-model intercomparison reveals data-driven insights into the performance and functionality of P-enabled models and adds to the existing evidence that the global CO 2 -driven carbon sink is overestimated by models.