The interactive effects of elevated CO2, temperature and initial size on growth and competition between a native C3 and an invasive C3 grass

A controlled environment experiment was conducted to determine the impact of enhanced carbon dioxide and temperature on competition between the C-3 grasses Austrodanthonia eriantha and Vulpia myuros. Plants were grown in mixtures and monocultures to compare the responses both with and without an interspecific competitor. Temperature and CO2 were set at current levels (350 ppm CO2; 20 degrees C day and 10 degrees C night temperature), in factorial combination with enhanced levels (700 ppm CO2; 23 degrees C day and 13 degrees C night temperature). To examine the potential impact of initial seedling size on competition under elevated CO2 and temperature, the two species were combined in mixtures of differing initial sizes. Above-ground growth of all plants was enhanced by increased CO2 and temperature alone, however the combined temperature and CO2 treatment showed a sub-additive effect, where growth was less than expected based on the responses to each factor independently. Austrodanthonia in mixture with Vulpia plants of the same initial size experienced a 27% reduction in growth. Austrodanthonia grown in the presence of an initially larger Vulpia plant experienced a 58% reduction in growth. When the Vulpia plant was initially smaller than Austrodanthonia, growth of the Austrodanthonia was reduced by 16%. The growth of Vulpia appeared to be largely unaffected by the presence of Austrodanthonia. Variation in the CO2 and temperature environment did not affect the pattern of these interspecific interactions, although there was some evidence to suggest that the degree of suppression of Austrodanthonia by Vulpia was less under elevated CO2. These results do not support the initial advantage hypothesis, as Vulpia was always able to suppress Austrodanthonia, regardless of the initial relative sizes of the competitors. Furthermore, the lack of an effect of changing the CO2 or temperature environment on the direction of interspecific competition suggests that the competitiveness of the invasive Vulpia will be minimally affected by changes in atmospheric CO2 concentration or temperature.