ELEVATED CO2 AND TEMPERATURE ALTER RECRUITMENT AND SIZE HIERARCHIES IN C-3 AND C-4 ANNUALS

In order to understand the implications of changes in global CO2 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 mu L/L) and elevated CO2 concentrations (700 mu L/L) across three temperature regimes; 18 degrees, 28 degrees, and 38 degrees 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 CO2, 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 CO2 differed markedly between the two species. For Abutilon, the C-3 species, serf-thinning and the mean biomass of the survivors increased under elevated CO2. For Amaranthus, survivorship, but not growth, increased under elevated CO2 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 CO2 and temperature suggest that the effects of changing CO2 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 CO2 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 CO2. Because size is often correlated with fecundity, an increase in size hierarchies in response to elevated CO2, 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 hierarchies alone, however, should also consider developmental shifts in response to elevated CO2 that may result in, as in this study, a decrease in the minimum size at the onset of flowering.