The "Enhanced Greenhouse Effect" (EGE) refers to the radiative forcing of the Earth's climate through the anthropogenic loading of the atmosphere with radiatively active gases. Climate modelling studies indicate that an equilibrium doubling of CO2 is likely to cause global mean surface temperatures to increase by 1.4to5.8 °C (Houghton et al., 1990; IPCC 2001). There will be changes in precipitation patterns as well as changes in the frequency and intensity of extreme rainfall events (Houghton et al. 1990). A changing climate will have consequences for agricultural systems, many of which already experience high year-to- year yield fluctuations due to variable weather. Because many weed populations show great genetic variation, it is possible that with a changed climate, weeds will achieve greater competitive fitness against the crop plants with which they compete.
Climate change impacts-assessment studies were carried out on wild oat (Avena fatua L.), a persistent annual weed of the world's temperate cereal growing regions. Growth chambers were used to examine the response of A. fatua to the interactive effects of CO2 fertilisation, soil-moisture deficit and increased temperature. Seed from six Australian near-isogenic lines of A. fatua were germinated and grown in controlled environment growth chambers under either ambient CO2 (357 ppmv) or elevated CO2(480 ppmv) at 20/16 °C or 23/19 °C. Three soil-moisture treatments, - 0.01 MPa (field capacity), - 0.10 MPa, or - 1.00 MPa were imposed. All lines grown under elevated CO2 had higher seed production and greater plant dry-weights though the response of these variates involved a complex of interactions with temperature, soil-moisture and genotype. Plant height varied with line and plants grown at 20/16 °C were taller than those grown at 23/19 °C. At 23/19 °C, time taken to mature was reduced for some A. fatua lines and elevated CO2 reduced the time taken to maturity for some lines at 20/16 °C. There was no significant difference in the level of dormancy developed in freshly-harvested caryopses between the two CO2 treatments but an effect was present in seed that had been after-ripened for 193 days.
Growth analysis studies were conducted to analyse patterns of growth for A. fatua and wheat grown at 357 and 480 ppmvC02 and at two temperatures of propagation (23/19 °C and 26/23 °C). These experiments showed that biomass stimulation under elevated CO2 is present from early stages of growth and is maintained until cessation. Absolute growth rates were higher at 20/16 °C than at 23/19 °C, a finding consistent with earlier growth chamber experiments. Avena fatua grew taller when grown in mixture with wheat but wheat grew taller when grown in monoculture with other wheat plants. Avena fatua also had greater per-plant seed production, greater shoot dry-weight, produced more tillers and had higher Net Assimilation Rates when grown in mixture with wheat than when grown in monoculture with other A. fatua plants. Wheat usually suffered a reduction in these attributes when grown in mixture with A. fatua and had higher grain number, more tillers and greater height when grown in monoculture with other wheat plants.
Addition series experiments were used to examine competitive interactions between A. fatua and wheat during the vegetative (tillering) stages of development at 357 ppmv CO2 and 480 ppmv CO2 and at one increased temperature of propagation and also at one level of soil-moisture deficit. These experiments showed that, for wheat, competition was mostly intiaspecific and A. fatua consistently increased its competitive ability against wheat by a larger factor at 480 ppmv CO2, relative to yield losses at 357 ppmv CO2.
The controlled environment glasshouse facility was used to examine the temperature response of foor A. fatua lines and the Hartog cultivar of wheat. The progeny seed from these experiments were tested to determine if the different temperatures experienced during the period of seed development had any effect on germination behaviour. There were significant declines in plant dry-weights for both wheat and A. fatua at each temperature increment with greater relative losses suffered by wheat. Times to anthesis and maturity were also significantly reduced at each temperature increment. The Toowoomba A. fatua line also experienced reduced seed production at each temperature increment. Germination tests on progeny seed showed that temperature of development influenced dormancy levels with warmer propagation temperatures resulting in seed with lower dormancy levels and faster germination rates.
A simple process-based simulation model (WoatSim) was developed that can simulate changes in A. fatua seed production and timing of development as a function of temperature and rainfall (soil moisture). It also incorporates a CO2 multiplier function to adjust seed production for the year 2070. Simulations for Toowoomba and Rutherglen show that temperature increases will significantly reduce times taken to maturity in the years 2030 and 2070. Without the effects of CO2 fertilisation factored into the models, seed production will be slightly reduced for A. fatua plants at Toowoomba in 2030 and 2070 and slightly increased at Rutherglen. When the effects of CO2 fertilisation are factored into the simulations, seed production increases by 31 to 65% except under the drier extreme in 2070 at Toowoomba where seed production will not change.
These studies have demonstrated that the main climate change variables (CO2 fertilisation, soil-moisture deficit and increased temperature) through the effects on competition, plant growth and seed physiology, will have a significant impact on the population dynamics of A. fatua in Australia.