J.R. Porter

Climatic variability and the modelling of crop yields

Abstract Crop simulation models are used widely to predict crop growth and development in studies of the impact of climatic change. An important problem is the uncertainty inherent in the construction of the future weather scenarios used as inputs to models. In seeking to couple meteorological information to crop-climate models it must be remembered that many interactions between crops and weather are non-linear. Non-linearity of response means it is necessary to preserve the variability of weather sequences to estimate the effect of climate on agricultural production and to assess agricultural risk. To date, only changes in average weather parameters derived from general circulation models (GCMs) and then applied to historical data have been used to construct climatic change scenarios and in only a few studies were changes in climatic variability incorporated. Accordingly, a computer system, AFRCWHEAT 3S, was designed to couple the simulation crop model for wheat, AFRCWHEAT2, with a stochastic weather generator based on the series approach. AFRCWHEAT 3S provides flexible construction of climatic scenarios and allows changes not only in mean values but also in the variance or type of distribution for a wide variety of weather parameters. Analyses of sensitivity to changes in the variability of temperature and precipitation, as compared with changes in their mean values, were made for locations in the UK and France for winter wheat. Results indicated that changes in climatic variability can have a more profound effect on yield and its associated risk than changes in mean climate.

AFRCWHEAT2: A model of the growth and development of wheat incorporating responses to water and nitrogen

Abstract AFRCWHEAT2, developed from the earlier model ARCWHEATI, is a computer simulation model of the growth and development of wheat for conditions where nitrogen and water may be in sub-optimal supply. The model is described and tested for its ability to simulate crop growth in four field experiments with different sites, growing seasons, sowing dates, amount and timing of nitrogen applications and irrigation. The overall root mean square error for simulated grain yield as a percentage of observed was 13 per cent. A relative error [(observed-simulated)lobserved] of less than 20 per cent occurred in more than 50 per cent of comparisons between observed and simulated values. Errors in simulation of green area index (GAI) and dry matter were small, except for one case. The model simulated adequately crop evapo-transpiration and the ratio between dry matter and amount of water transpired (the transpiration coefficient) but it consistently overestimated the rate of decline in shoot nitrogen concentration. Dry matter accumulation was modelled as being more sensitive to soil nitrogen than to soil water level, in agreement with observations. Generally, model crops continued to absorb nitrogen for a longer time than did observed crops. A future use of AFRCWHEAT2 will be to examine its ability to simulate crops grown under variable conditions of elevated CO 2 and temperature.

Effects of elevated CO2 and drought on wheat: Testing crop simulation models for different experimental and climatic conditions

Effects of increasing carbon dioxide concentration [CO2] on wheat vary depending on water supply and climatic conditions, which are difficult to estimate. Crop simulation models are often used to predict the impact of global atmospheric changes on food production. However, models have rarely been tested for effects on crops of [CO2] and drought for different climatic conditions due to limited data available from field experiments. n nSimulations of the effects of elevated [CO2] and drought on spring wheat (Triticum aestivum L.) from three crop simulation models (LINTULCC2, AFRCWHEAT2, Sirius), which differ in structure and mechanistic detail, were compared with observations. These were from 2 years of free-air carbon dioxide enrichment (FACE) experiments in Maricopa, Arizona and 2 years of standardised (in crop management and soil conditions) open-top chamber (OTC) experiments in Braunschweig and Giessen, Germany. In a simulation exercise, models were used to assess the possible impact of increased [CO2] on wheat yields measured between 1987 and 1999 at one farm site in the drought prone region of Andalucia, south Spain. n nThe models simulated well final biomass (BM), grain yield (GY), cumulative evapotranspiration (ET) and water use efficiency (WUE) of wheat grown in the FACE experiments but simulations were unsatisfactory for OTC experiments. Radiation use efficiency (RUE) and yield responses to [CO2] and drought were on average higher in OTC than in FACE experiments. However, there was large variation among OTC experiments. Plant growth in OTCs was probably modified by several factors related to plot size, the use (or not use) of border plants, airflow pattern, modification of radiation balance and/or restriction of rooting volume that were not included in the models. Variation in farm yields in south Spain was partly explained by the models, but sources of unexplained yield variation could not be identified and were most likely related to effects of pests and diseases that were not included in the models. Simulated GY in south Spain increased in the range between 30 and 65% due to doubling [CO2]. The simulated increase was larger when a [CO2]×drought interaction was assumed (LINTULCC2, AFRCWHEAT2) than when it was not (Sirius). n nIt was concluded that crop simulation models are able to reproduce wheat growth and yield for different [CO2] and drought treatments in a field environment. However, there is still uncertainty about the combined effects of [CO2] and drought including the timing of drought stress and about relationships that determine yield variation at farm and larger scales that require further investigation including model testing.

Modelling CO2 effects on wheat with varying nitrogen supplies.

Abstract Crop simulation models are an essential tool for testing whether predicted global atmospheric changes are likely to have impact on food production. Any confidence in model predictions must be based on their ability successfully to predict performance in experiments. Accordingly, the predictions of three daily time step wheat simulation models (AFRCWHEAT2, FASSET and Sirius) were tested against data from wheat (Triticum aestivum L.) experiments in AZ in which the amount of applied N and the atmospheric CO2 concentration were both varied. Although there were differences between predicted and observed yields, all the three models predicted yield trends with treatments very similar to those observed. They all predicted, both in absolute terms and in the magnitude of responses, very similar effects of the variations on green area index (GAI), shoot and grain biomass accumulation, and shoot and grain biomass yield to observations and to each other. Comparison of simulated and observed results showed that CO2 effects were expressed through effects on light use efficiency (LUE), whereas N effects were expressed by causing variations in GAI. The exercise showed that the models used have potential for assessing climate change impacts on wheat production.

Climatic change and the growth and development of wheat in the UK and France

Abstract The crop simulation model AFRCWHEAT2 was used to predict the effect of climatic change on the yield and development of winter wheat in the UK and durum winter wheat in France. The model simulates crop responses to climatic variables dnd includes the effect of soil type on water and nitrogen availability to the crop. Crop simulations were made using individual, equilibrium 2 × CO2 general circulation model (GCM) scenarios and composite, time-dependent scenarios of future climatic conditions. AFRCWHEAT2 was most sensitive to changes in CO2 level and temperature, slightly less so to changes in the level of incoming radiation and least to reductions in amount of precipitation. The model calculated higher grain yields for composite time-dependent scenarios for all sites, but yields calculated from different GCM scenarios were more variable. There were no consistent changes in the coefficient of variation (CV) of yield. Management implications are that existing UK winter wheat varieties do not carry sufficient flexibility in their development to allow for adaptation to future climatic change and a more effective option would be to alter sowing date. Yield responses to climatic change cannot be predicted solely on the basis of average changes in climatic conditions but should take account of changes in the variability in the weather parameters to which growth is sensitive. A stochastic weather generator could serve this purpose.

Modelling the effects of climatic change and genetic modification on nitrogen use by wheat

This paper considers how and the extent to which climate change and the balance of nitrogen between crops and soils may interact and how the ability to modify genetically specific crop attributes might affect the overall nitrogen balance of the crop-soil system. The possible effects of each change have been assessed using the AFRCWHEAT2 crop model for wheat. Model output pointed to a decrease in harvest index as a result of coincidental increases in CO2 level and temperature, and the importance of considering not only changes to average but also to the variability of environmental driving variables is illustrated. When mean temperatures were raised the model predicted that more nitrate would be left in the soil at the end of the season but that raising CO2 level could counter this effect. Doubling the variability of temperature had a more complicated effect on the soil N balance with the mean amount of residual soil nitrate predicted to be at about the same level as for the baseline and with only a comparatively small change in its coefficient of variation. These results are interpreted in terms of a bell-shaped response of mineralisation rate to temperature. n nRaising the value of the maximum N concentration (Nmax) in the shoots increased the amount of NO3-N extracted from the soil. However, the overall effects were not simply proportional to the increase in Nmax because the crop could also become limited by the supply of N from the soil.

Leaf demography in willow short-rotation coppice.

Abstract The production, survivorship and death of cohorts of leaves within the canopies of three clones ( Salix burjatica “Korso”, S. viminalis “Mullatin” and S. x dasyclados ) of willow grown as short-rotation coppice were studied in a field experiment. The initial rate of increase in leaf number was fastest for “Mullatin”, which also had the steepest rate of decline in net leaf number. Inter-clonal variability was not reduced by plotting changes in leaf number against thermal time, accumulated above 0°C. Mean number of leaves was highest for “Mullatin” and least for “Korso”. Leaf production occurred in flushes and there were no significant clonal differences in the relative rate of leaf birth: differences in relative rate of leaf death ( r d ) became apparent after day number 230 (1 Jan = 1); thereafter “Korso” had higher values of r d than the other clones. The time course of r d was summarised by a logistic curve and there was no evidence that the initial asymptote nor the maximal rate of change of r d differed between clones. Leaves of “Korso” had minimum and maximum longevities of about 25 days (or about 250°Cd in thermal time) and 65 days (about 1000°Cd), respectively; minimum leaf longevities for the other two clones were similar to that for “Korso” but their leaves lived for up to 100 days (about 1400°Cd). Changes in the age-structure of the leaf populations with time were little influenced by differences in leaf demography between clones. A positive and significant correlation was found between the rate of production of leaves and temperature up to about 14°C. Thereafter, the correlation was either absent or negative. Results are interpreted in terms of the consequences of differences in canopy demography for coppice biomass production.

Ring constrained analogues of the thevinones; Diels-Alder reactions of thebaines with 1-indenone and methylene cycloalkanones

Abstract Thebaine has been shown to react with various in situ generated dienophiles to yield 7,8- and 7,7-ring constrained analogues of thevinones.

Comparison of models to simulate leaf appearance in wheat

Prediction on of leaf appearance in cereals is important for modelling canopy development and timing crop management practices. Four models which aim to predict leaf emergence on the basis of temperature and, in three cases, photoperiod are compared against independent data from a wide range of sites, sowing dates and cultivars of spring and winter wheat. Three of the models behaved bery similarly and had root mean square errors of the order of one leaf in their prediction of leaf appearance. The fourth model performed better in predicting differences in leaf appearance for different sowing dates than it did for latitudes. The prediction of the day of emergence of leaf number seven followed a similar response to that of leaf appearance. These results are discussed in terms of the hypothesised relationships between temperature, photoperiod and ontogeny within the models.