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 conce ntrations and temperature for the growth and fitness of individual pla nts, performance must be investigated in relation to the performance o f 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 (7 00 mu L/L) across three temperature regimes; 18 degrees, 28 degrees, a nd 38 degrees C. We created experimental populations of two annual pla nts that differ in their photosynthetic pathway and water use patterns : Abutilon theophrasti (C-3) and Amaranthus retroflexus (C-4). The eff ects of CO2, temperature, and their interactions on population structu re were complex and species dependent. For both species increasing tem perature resulted in higher germination and faster initial growth rate s. These initial temperature responses increased the intensity and rol e of competition in determining stand size and structure. Postemergenc e 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, survivorsh ip, but not growth, increased under elevated CO2 conditions. We attrib ute 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 deployme nt. The patterns of stand development in response to CO2 and temperatu re suggest that the effects of changing CO2 and temperature may be und erstood within mechanistically based models of resource use. Temperatu re regulates the rate of resource use and the onset of interference am ong plants, while CO2 functions both as a resource and a resource regu lator. Although mortality was concentrated later in stand development for Abutilon than Amaranthus, overall patterns of stand size and struc ture were similar for both species; mortality and size inequalities in creased with increasing temperature and CO2. Because size is often cor related with fecundity, an increase in size hierarchies in response to elevated CO2, in conjunction with a decrease in survivorship, may res ult in a smaller effective population size. Our ability to predict cha nges in effective population size due to changing size hierarchies alo ne, however, should also consider developmental shifts in response to elevated CO2 that may result in, as in this study, a decrease in the m inimum size at the onset of flowering.