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 CO₂ 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 CO₂ (550 µmol mol-1) and warming (+2°C) on the population growth of four plant species important in Australian temperate grasslands is reported. Data collected from the Tasmanian free-air CO₂ enrichment (TasFACE) experiment between 2003 and 2006 were analysed using population matrix models. Population growth of Themeda triandra, a perennial C₄ grass, was largely unaffected by either factor but population growth of Austrodanthonia caespitosa, a perennial C₃ grass, was reduced substantially in elevated CO₂ plots. Warming and elevated CO₂ 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.