Globally consistent influences of seasonal precipitation limit grassland biomass response to elevated CO2.

Rising atmospheric carbon dioxide concentration should stimulate biomass production directly via biochemical stimulation of carbon assimilation, and indirectly via water savings caused by increased plant water-use efficiency. Because of these water savings, the CO2 fertilization effect (CFE) should be stronger at drier sites, yet large differences among experiments in grassland biomass response to elevated CO2 appear to be unrelated to annual precipitation, preventing useful generalizations. Here, we show that, as predicted, the impact of elevated CO2 on biomass production in 19 globally distributed temperate grassland experiments reduces as mean precipitation in seasons other than spring increases, but that it rises unexpectedly as mean spring precipitation increases. Moreover, because sites with high spring precipitation also tend to have high precipitation at other times, these effects of spring and non-spring precipitation on the CO2 response offset each other, constraining the response of ecosystem productivity to rising CO2. This explains why previous analyses were unable to discern a reliable trend between site dryness and the CFE. Thus, the CFE in temperate grasslands worldwide will be constrained by their natural rainfall seasonality such that the stimulation of biomass by rising CO2 could be substantially less than anticipated.

Ambient changes exceed treatment effects on plant species abundance in global change experiments.

The responses of species to environmental changes will determine future community composition and ecosystem function. Many syntheses of global change experiments examine the magnitude of treatment effect sizes, but we lack an understanding of how plant responses to treatments compare to ongoing changes in the unmanipulated (ambient or background) system. We used a database of long-term global change studies manipulating CO2 , nutrients, water, and temperature to answer three questions: (a) How do changes in plant species abundance in ambient plots relate to those in treated plots? (b) How does the magnitude of ambient change in species-level abundance over time relate to responsiveness to global change treatments? (c) Does the direction of species-level responses to global change treatments differ from the direction of ambient change? We estimated temporal trends in plant abundance for 791 plant species in ambient and treated plots across 16 long-term global change experiments yielding 2,116 experiment-species-treatment combinations. Surprisingly, for most species (57%) the magnitude of ambient change was greater than the magnitude of treatment effects. However, the direction of ambient change, whether a species was increasing or decreasing in abundance under ambient conditions, had no bearing on the direction of treatment effects. Although ambient communities are inherently dynamic, there is now widespread evidence that anthropogenic drivers are directionally altering plant communities in many ecosystems. Thus, global change treatment effects must be interpreted in the context of plant species trajectories that are likely driven by ongoing environmental changes.

The use of biogeochemical models to evaluate mitigation of greenhouse gas emissions from managed grasslands.

Simulation models quantify the impacts on carbon (C) and nitrogen (N) cycling in grassland systems caused by changes in management practices. To support agricultural policies, it is however important to contrast the responses of alternative models, which can differ greatly in their treatment of key processes and in their response to management. We applied eight biogeochemical models at five grassland sites (in France, New Zealand, Switzerland, United Kingdom and United States) to compare the sensitivity of modelled C and N fluxes to changes in the density of grazing animals (from 100% to 50% of the original livestock densities), also in combination with decreasing N fertilization levels (reduced to zero from the initial levels). Simulated multi-model median values indicated that input reduction would lead to an increase in the C sink strength (negative net ecosystem C exchange) in intensive grazing systems: -64 ±â€¯74 g C m-2 yr-1 (animal density reduction) and -81 ±â€¯74 g C m-2 yr-1 (N and animal density reduction), against the baseline of -30.5 ±â€¯69.5 g C m-2 yr-1 (LSU [livestock units] ≥ 0.76 ha-1 yr-1). Simulations also indicated a strong effect of N fertilizer reduction on N fluxes, e.g. N2O-N emissions decreased from 0.34 ±â€¯0.22 (baseline) to 0.1 ±â€¯0.05 g N m-2 yr-1 (no N fertilization). Simulated decline in grazing intensity had only limited impact on the N balance. The simulated pattern of enteric methane emissions was dominated by high model-to-model variability. The reduction in simulated offtake (animal intake + cut biomass) led to a doubling in net primary production per animal (increased by 11.6 ±â€¯8.1 t C LSU-1 yr-1 across sites). The highest N2O-N intensities (N2O-N/offtake) were simulated at mown and extensively grazed arid sites. We show the possibility of using grassland models to determine sound mitigation practices while quantifying the uncertainties associated with the simulated outputs.

Elevated CO2 and warming effects on grassland plant mortality are determined by the timing of rainfall.

Background and aims: Global warming is expected to increase the mortality rate of established plants in water-limited systems because of its effect on evapotranspiration. The rising CO 2 concentration ([CO 2 ]), however, should have the opposite effect because it reduces plant transpiration, delaying the onset of drought. This potential for elevated [CO 2 ] (eCO 2 ) to modify the warming effect on mortality should be related to prevailing moisture conditions. This study aimed to determine the impacts of warming by 2 °C and eCO 2 (550 µmol mol -1 ) on plant mortality in an Australian temperate grassland over a 6-year period and to test how interannual variation in rainfall influenced treatment effects. Methods: Analyses were based on results from a field experiment, TasFACE, in which grassland plots were exposed to a combination of eCO 2 by free air CO 2 enrichment (FACE) and warming by infrared heaters. Using an annual census of established plants and detailed estimates of recruitment, annual mortality of all established plants was calculated. The influence of rainfall amount and timing on the relative impact of treatments on mortality in each year was analysed using multiple regression techniques. Key Results: Warming and eCO 2 effects had an interactive influence on mortality which varied strongly from year to year and this variation was determined by temporal rainfall patterns. Warming tended to increase density-adjusted mortality and eCO 2 moderated that effect, but to a greater extent in years with fewer dry periods. Conclusions: These results show that eCO 2 reduced the negative effect of warming but this influence varied strongly with rainfall timing. Importantly, indices involving the amount of rainfall were not required to explain interannual variation in mortality or treatment effects on mortality. Therefore, predictions of global warming effects on plant mortality will be reliant not only on other climate change factors, but also on the temporal distribution of rainfall.

Constraints to nitrogen acquisition of terrestrial plants under elevated CO2.

A key part of the uncertainty in terrestrial feedbacks on climate change is related to how and to what extent nitrogen (N) availability constrains the stimulation of terrestrial productivity by elevated CO2 (eCO2 ), and whether or not this constraint will become stronger over time. We explored the ecosystem-scale relationship between responses of plant productivity and N acquisition to eCO2 in free-air CO2 enrichment (FACE) experiments in grassland, cropland and forest ecosystems and found that: (i) in all three ecosystem types, this relationship was positive, linear and strong (r(2) = 0.68), but exhibited a negative intercept such that plant N acquisition was decreased by 10% when eCO2 caused neutral or modest changes in productivity. As the ecosystems were markedly N limited, plants with minimal productivity responses to eCO2 likely acquired less N than ambient CO2 -grown counterparts because access was decreased, and not because demand was lower. (ii) Plant N concentration was lower under eCO2 , and this decrease was independent of the presence or magnitude of eCO2 -induced productivity enhancement, refuting the long-held hypothesis that this effect results from growth dilution. (iii) Effects of eCO2 on productivity and N acquisition did not diminish over time, while the typical eCO2 -induced decrease in plant N concentration did. Our results suggest that, at the decennial timescale covered by FACE studies, N limitation of eCO2 -induced terrestrial productivity enhancement is associated with negative effects of eCO2 on plant N acquisition rather than with growth dilution of plant N or processes leading to progressive N limitation.

Bacteria on leaves: a previously unrecognised source of N2O in grazed pastures.

Nitrous oxide (N2O) emissions from grazed pastures are a product of microbial transformations of nitrogen and the prevailing view is that these only occur in the soil. Here we show this is not the case. We have found ammonia-oxidising bacteria (AOB) are present on plant leaves where they produce N2O just as in soil. AOB (Nitrosospira sp. predominantly) on the pasture grass Lolium perenne converted 0.02-0.42% (mean 0.12%) of the oxidised ammonia to N2O. As we have found AOB to be ubiquitous on grasses sampled from urine patches, we propose a 'plant' source of N2O may be a feature of grazed grassland.

Seasonal not annual rainfall determines grassland biomass response to carbon dioxide.

The rising atmospheric concentration of carbon dioxide (CO2) should stimulate ecosystem productivity, but to what extent is highly uncertain, particularly when combined with changing temperature and precipitation. Ecosystem response to CO2 is complicated by biogeochemical feedbacks but must be understood if carbon storage and associated dampening of climate warming are to be predicted. Feedbacks through the hydrological cycle are particularly important and the physiology is well known; elevated CO2 reduces stomatal conductance and increases plant water use efficiency (the amount of water required to produce a unit of plant dry matter). The CO2 response should consequently be strongest when water is limiting; although this has been shown in some experiments, it is absent from many. Here we show that large annual variation in the stimulation of above-ground biomass by elevated CO2 in a mixed C3/C4 temperate grassland can be predicted accurately using seasonal rainfall totals; summer rainfall had a positive effect but autumn and spring rainfall had negative effects on the CO2 response. Thus, the elevated CO2 effect mainly depended upon the balance between summer and autumn/spring rainfall. This is partly because high rainfall during cool, moist seasons leads to nitrogen limitation, reducing or even preventing biomass stimulation by elevated CO2. Importantly, the prediction held whether plots were warmed by 2 °C or left unwarmed, and was similar for C3 plants and total biomass, allowing us to make a powerful generalization about ecosystem responses to elevated CO2. This new insight is particularly valuable because climate projections predict large changes in the timing of rainfall, even where annual totals remain static. Our findings will help resolve apparent differences in the outcomes of CO2 experiments and improve the formulation and interpretation of models that are insensitive to differences in the seasonal effects of rainfall on the CO2 response.

Testing simulations of intra- and inter-annual variation in the plant production response to elevated CO(2) against measurements from an 11-year FACE experiment on grazed pasture.

Ecosystem models play a crucial role in understanding and evaluating the combined impacts of rising atmospheric CO2 concentration and changing climate on terrestrial ecosystems. However, we are not aware of any studies where the capacity of models to simulate intra- and inter-annual variation in responses to elevated CO2 has been tested against long-term experimental data. Here we tested how well the ecosystem model APSIM/AgPasture was able to simulate the results from a free air carbon dioxide enrichment (FACE) experiment on grazed pasture. At this FACE site, during 11 years of CO2 enrichment, a wide range in annual plant production response to CO2 (-6 to +28%) was observed. As well as running the full model, which includes three plant CO2 response functions (plant photosynthesis, nitrogen (N) demand and stomatal conductance), we also tested the influence of these three functions on model predictions. Model/data comparisons showed that: (i) overall the model over-predicted the mean annual plant production response to CO2 (18.5% cf 13.1%) largely because years with small or negative responses to CO2 were not well simulated; (ii) in general seasonal and inter-annual variation in plant production responses to elevated CO2 were well represented by the model; (iii) the observed CO2 enhancement in overall mean legume content was well simulated but year-to-year variation in legume content was poorly captured by the model; (iv) the best fit of the model to the data required all three CO2 response functions to be invoked; (v) using actual legume content and reduced N fixation rate under elevated CO2 in the model provided the best fit to the experimental data. We conclude that in temperate grasslands the N dynamics (particularly the legume content and N fixation activity) play a critical role in pasture production responses to elevated CO2 , and are processes for model improvement.

Selective grazing modifies previously anticipated responses of plant community composition to elevated CO(2) 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.

Food security and climate change: on the potential to adapt global crop production by active selection to rising atmospheric carbon dioxide.

Agricultural production is under increasing pressure by global anthropogenic changes, including rising population, diversion of cereals to biofuels, increased protein demands and climatic extremes. Because of the immediate and dynamic nature of these changes, adaptation measures are urgently needed to ensure both the stability and continued increase of the global food supply. Although potential adaption options often consider regional or sectoral variations of existing risk management (e.g. earlier planting dates, choice of crop), there may be a global-centric strategy for increasing productivity. In spite of the recognition that atmospheric carbon dioxide (CO(2)) is an essential plant resource that has increased globally by approximately 25 per cent since 1959, efforts to increase the biological conversion of atmospheric CO(2) to stimulate seed yield through crop selection is not generally recognized as an effective adaptation measure. In this review, we challenge that viewpoint through an assessment of existing studies on CO(2) and intraspecific variability to illustrate the potential biological basis for differential plant response among crop lines and demonstrate that while technical hurdles remain, active selection and breeding for CO(2) responsiveness among cereal varieties may provide one of the simplest and direct strategies for increasing global yields and maintaining food security with anthropogenic change.

Influence of warming on soil water potential controls seedling mortality in perennial but not annual species in a temperate grassland.

In a water-limited system, the following hypotheses are proposed: warming will increase seedling mortality; elevated atmospheric CO2 will reduce seedling mortality by reducing transpiration, thereby increasing soil water availability; and longevity (i.e. whether a species is annual or perennial) will affect the response of a species to global changes. Here, these three hypotheses are tested by assessing the impact of elevated CO2 (550 micromol mol(-1) and warming (+2 degrees C) on seedling emergence, survivorship and establishment in an Australian temperate grassland from autumn 2004 to autumn 2007. Warming impacts on seedling survivorship were dependent upon species longevity. Warming reduced seedling survivorship of perennials through its effects on soil water potential but the seedling survivorship of annuals was reduced to a greater extent than could be accounted for by treatment effects on soil water potential. Elevated CO2 did not significantly affect seedling survivorship in annuals or perennials. These results show that warming will alter recruitment of perennial species by changing soil water potential but will reduce recruitment of annual species independent of any effects on soil moisture. The results also show that exposure to elevated CO2 does not make seedlings more resistant to dry soils.

Flowering phenology in a species-rich temperate grassland is sensitive to warming but not elevated CO2.

* Flowering is a critical stage in plant life cycles, and changes might alter processes at the species, community and ecosystem levels. Therefore, likely flowering-time responses to global change drivers are needed for predictions of global change impacts on natural and managed ecosystems. * Here, the impact of elevated atmospheric CO2 concentration ([CO2]) (550 micromol mol(-1)) and warming (+2 masculineC) is reported on flowering times in a native, species-rich, temperate grassland in Tasmania, Australia in both 2004 and 2005. * Elevated [CO2] did not affect average time of first flowering in either year, only affecting three out of 23 species. Warming reduced time to first flowering by an average of 19.1 d in 2004, acting on most species, but did not significantly alter flowering time in 2005, which might be related to the timing of rainfall. Elevated [CO2] and warming treatments did not interact on flowering time. * These results show elevated [CO2] did not alter average flowering time or duration in this grassland; neither did it alter the response to warming. Therefore, flowering phenology appears insensitive to increasing [CO2] in this ecosystem, although the response to warming varies between years but can be strong.

Warming and free-air CO2 enrichment alter demographics in four co-occurring grassland species.

Species differ in their responses to global changes such as rising CO(2) and temperature, meaning that global changes are likely to change the structure of plant communities. Such alterations in community composition must be underlain by changes in the population dynamics of component species. Here, the impact of elevated CO(2) (550 micromol mol(-1)) and warming (+2 degrees C) on the population growth of four plant species important in Australian temperate grasslands is reported. Data collected from the Tasmanian free-air CO(2) enrichment (TasFACE) experiment between 2003 and 2006 were analysed using population matrix models. Population growth of Themeda triandra, a perennial C(4) grass, was largely unaffected by either factor but population growth of Austrodanthonia caespitosa, a perennial C(3) grass, was reduced substantially in elevated CO(2) plots. Warming and elevated CO(2) had antagonistic effects on population growth of two invasive weeds, Hypochaeris radicata and Leontodon taraxacoides, with warming causing population decline. Analysis of life cycle stages showed that seed production, seedling emergence and establishment were important factors in the responses of the species to global changes. These results show that the demographic approach is very useful in understanding the variable responses of plants to global changes and in elucidating the life cycle stages that are most responsive.

A comment on scaling methane emissions from vegetation and grazing ruminants in New Zealand.

Keppler et al. (2006, Nature 439, 187-191) showed that plants produce methane (CH4) in aerobic environments, leading Lowe (2006, Nature 439, 148-149) to postulate that in countries such as New Zealand, where grazed pastures have replaced forests, the forests could have produced as much CH4 as the ruminants currently grazing these areas. Estimating CH4 emissions from up to 85 million ruminants in New Zealand is challenging and, for completeness, the capacity of forest and pastoral soils to oxidise CH4 should be included. On average, the CH4 emission rate of grazing ruminants is estimated to be 9.6 ± 2.6 g m-2 year-1 (±standard deviation), six times the corresponding estimate for an indigenous forest canopy (1.6 ± 1.1 g m-2 year-1). The forest's soil is estimated to oxidise 0.9 ± 0.2 g m-2 year-1 more CH4 than representative soils beneath grazed pasture. Taking into account plant and animal sources and the soil's oxidative capacity, the net CH4 emission rates of forest and grazed ecosystems are 0.6 ± 1.1 and 9.8 ± 2.6 g m-2 year-1, respectively.

Initial observations of increased requirements for light-energy dissipation in ryegrass (Lolium perenne) when source / sink ratios become high at a naturally grazed free air CO2 enrichment (FACE) site.

Although photosynthetic response to long-term elevated CO2 has been extensively studied, little attention has yet been directed at coordinated adjustments between the use of absorbed light for CO2 fixation, and the dissipation of potentially harmful excess light. In this study, we have performed an initial analysis of photosynthetic light use and excess light dissipation in response to grazing-induced variation in the source / sink ratio in ryegrass (Lolium perenne L.) after 6 years' exposure to Free Air CO2 Enrichment (FACE). Before grazing, when the source / sink ratio was relatively large, significant down-regulation of photosynthetic capacity (Amax) was observed in the FACE leaves compared with control leaves at the same stage of maturity. The decrease in Amax partly offset the direct stimulation of elevated CO2 on light-saturated photosynthesis, and was accompanied by a reduction in photochemical electron flow that was accompanied by a large increase in susceptibility to photoinhibition. This was indicated by large increases in both non-photochemical quenching (NPQ) and the de-epoxidised state of xanthophyll cycle (DEPS), and also by changes in the photochemical reflectance index (PRI). However, no significant increase in the xanthophyll pool size in FACE leaves was observed, despite the apparent large increase in requirements for photodissipation in FACE leaves. After grazing, when the source / sink ratio was relatively small, the CO2 fixation rates in both the FACE and control leaves were, as expected, significantly higher compared with those before grazing, and there was no down-regulation of photosynthetic capacity in the leaves under FACE conditions. In addition, the extent of photodissipation in the FACE and control leaves was not significantly different. Overall, the profile of leaf physiological and biochemical responses supports the hypothesis that the effect of long-term elevated CO2 can be significantly influenced by short-term variation in the source / sink ratio. As the xanthophyll pool size does not change significantly, this poses the question of whether the increased photodissipative demand observed here under even moderately elevated CO2 concentrations may lead to increased plant susceptibility to photoinhibition, and thus to an increased risk of damage to plant function, under conditions of low sink demand. This question clearly deserves further study.