Elevated CO2 and temperature alter recruitment and size hierarchies in C3 and C4 annuals

In order to understand the implications of changes in global CO"2 concentrations and temperature for the growth and fitness of individual plants, performance must be investigated in relation to the performance of other plants within a population. In this study we examined patterns of recruitment, mortality, and size structure of monospecific stands in response to ambient (400 @mL/L) and elevated CO"2 concentrations (700 @mL/L) across three temperature regimes; 18@?, 28@?, and 38@?C. We created experimental populations of two annual plants that differ in their photosynthetic pathway and water use patterns: Abutilon theophrasti (C"3) and Amaranthus retroflexus (C"4). The effects of CO"2, temperature, and their interactions on population structure were complex and species dependent. For both species increasing temperature resulted in higher germination and faster initial growth rates. These initial temperature responses increased the intensity and role of competition in determining stand size and structure. Postemergence responses to elevated CO"2 differed markedly between the two species. For Abutilon, the C"3 species, self-thinning and the mean biomass of the survivors increased under elevated CO"2. For Amaranthus, survivorship, but not growth, increased under elevated CO"2 conditions. We attribute differences in response between species not only to photosynthetic pathway, but also to differences in the onset of competition mediated through differences in plant form and in resource uptake and deployment. The patterns of stand development in response to CO"2 and temperature suggest that the effects of changing CO"2 and temperature may be understood within mechanistically based models of resource use. Temperature regulates the rate of resource use and the onset of interference among plants, while CO"2 functions both as a resource and a resource regulator. Although mortality was concentrated later in stand development for Abutilon than Amaranthus, overall patterns of stand size and structure were similar for both species; mortality and size inequalities increased with increasing temperature and CO"2. Because size is often correlated with fecundity, and increase in size hierarchies in response to elevated CO"2, in conjunction with a decrease in survivorship, may result in a smaller effective population size. Our ability to predict changes in effective population size due to changing size due to changing size hierarchies alone, however, should also consider deve