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.