Assessing uncertainties in crop and pasture ensemble model simulations of productivity and N2 O emissions.

Simulation models are extensively used to predict agricultural productivity and greenhouse gas emissions. However, the uncertainties of (reduced) model ensemble simulations have not been assessed systematically for variables affecting food security and climate change mitigation, within multi-species agricultural contexts. We report an international model comparison and benchmarking exercise, showing the potential of multi-model ensembles to predict productivity and nitrous oxide (N2 O) emissions for wheat, maize, rice and temperate grasslands. Using a multi-stage modelling protocol, from blind simulations (stage 1) to partial (stages 2-4) and full calibration (stage 5), 24 process-based biogeochemical models were assessed individually or as an ensemble against long-term experimental data from four temperate grassland and five arable crop rotation sites spanning four continents. Comparisons were performed by reference to the experimental uncertainties of observed yields and N2 O emissions. Results showed that across sites and crop/grassland types, 23%-40% of the uncalibrated individual models were within two standard deviations (SD) of observed yields, while 42 (rice) to 96% (grasslands) of the models were within 1 SD of observed N2 O emissions. At stage 1, ensembles formed by the three lowest prediction model errors predicted both yields and N2 O emissions within experimental uncertainties for 44% and 33% of the crop and grassland growth cycles, respectively. Partial model calibration (stages 2-4) markedly reduced prediction errors of the full model ensemble E-median for crop grain yields (from 36% at stage 1 down to 4% on average) and grassland productivity (from 44% to 27%) and to a lesser and more variable extent for N2 O emissions. Yield-scaled N2 O emissions (N2 O emissions divided by crop yields) were ranked accurately by three-model ensembles across crop species and field sites. The potential of using process-based model ensembles to predict jointly productivity and N2 O emissions at field scale is discussed.

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.

Climate change is likely to increase the development rate of anthelmintic resistance in equine cyathostomins in New Zealand.

Climate change is likely to influence livestock production by increasing the prevalence of diseases, including parasites. The traditional practice of controlling nematodes in livestock by the application of anthelmintics is, however, increasingly compromised by the development of resistance to these drugs in parasite populations. This study used a previously developed simulation model of the entire equine cyathostomin lifecycle to investigate the effect a changing climate would have on the development of anthelmintic resistance. Climate data from six General Circulation Models based on four different Representative Concentration Pathways was available for three New Zealand locations. These projections were used to estimate the time resistance will take to develop in the middle (2040-49) and by the end (2090-99) of the century in relation to current (2006-15) conditions under two treatment scenarios of either two or six yearly whole-herd anthelmintic treatments. To facilitate comparison, a scenario without any treatments was included as a baseline. In addition, the size of the infective and parasitic stage nematode population during the third simulation year were estimated. The development of resistance varied between locations, time periods and anthelmintic treatment strategies. In general, the simulations indicated a more rapid development of resistance under future climates coinciding with an increase in the numbers of infective larvae on pasture and encysted parasitic stages. This was especially obvious when climate changes resulted in a longer period suitable for development of free-living parasite stages. A longer period suitable for larval development resulted in an increase in the average size of the parasite population with a larger contribution from eggs passed by resistant worms surviving the anthelmintic treatments. It is projected that climate change will decrease the ability to control livestock parasites by means of anthelmintic treatments and non-drug related strategies will become increasingly important for sustainable parasite control.

Seasonal changes in canopy photosynthesis and respiration, and partitioning of photosynthate, in rice (Oryza sativa L.) grown under free-air CO2 enrichment.

An increase in atmospheric CO(2) concentration ( [CO(2)]) is generally expected to enhance photosynthesis and biomass. Rice plants (Oryza sativa L.) were grown in ambient CO(2) (AMB) or free-air CO(2)-enrichment (FACE), in which the target [CO(2)] was 200 micromol mol(-1) above AMB. (13)CO(2) was fed to the plants at different stages so we could examine the partitioning of photosynthates. Furthermore, canopy photosynthesis and respiration were measured at those stages. The ratio of (13)C content in the whole plant to the amount of fixed (13)C under FACE was similar to that under AMB at the vegetative stage. However, the ratio under FACE was greater than the ratio under AMB at the grain-filling stage. At the vegetative stage, plants grown under FACE had a larger biomass than those grown under AMB owing to enhancement of canopy photosynthesis by the increased [CO(2)]. On the other hand, at the grain-filling stage, CO(2) enrichment promoted the partitioning of photosynthate to ears, and plants grown under FACE had a greater weight of ears. However, enhancement of ear weight by CO(2) enrichment was not as great as that of biomass at the vegetative stage. Plants grown under FACE did not necessarily show higher canopy photosynthetic rates at the grain-filling stage. Therefore, we concluded that the ear weight did not increase as much as biomass at the vegetative stage owing to a loss of the advantage in CO(2) gain during the grain-filling period.

Changes in sourcesink relations during development influence photosynthetic acclimation of rice to free air CO2 enrichment (FACE).

Relationships between photosynthetic acclimation and changes in the balance between source-sink supply and demand of carbon (C) and nitrogen (N) were tested using rice (Oryza sativa L. cv. Akitakomachi). Plants were field-grown in northern Japan at ambient CO2 partial pressure [p(CO2)] or free air CO2 enrichment (FACE; p(CO2) ~ 26-32 Pa above ambient) with low, medium or high N supplies. Leaf CO2 assimilation rates (A) and biochemical parameters were measured at 32-36 (eighth leaf) and 76-80 (flag leaf) d after transplanting, representing stages with a contrasting balance between C and N supply and demand in sources and sinks. Acclimation due to FACE was pronounced in flag leaves at each N supply. This was not fully accounted for by reductions in leaf N concentrations, because A/N and Vcmax/N were lower in FACE-grown flag leaves. Acclimation did not occur in the eighth leaf, and A/N and Vcmax/N was not significantly increased in FACE-grown leaves. Soluble protein / sucrose and amino acid / sucrose concentrations decreased under FACE, whereas sucrose phosphate synthase protein levels increased. At flag leaf stage, there was a discrepancy between the demand and supply of N, which was resolved by enhanced leaf N remobilization, associated with the lower Rubisco concentrations under FACE. In contrast to the early growth stage, enhanced growth of rice plants was accompanied by increased plant N uptake in FACE. We conclude that photosynthetic acclimation in flag leaves occurs under FACE because there is a large demand for N for reproductive development, relative to supply of N from root uptake and remobilization from leaves.