INTER-GENERIC AND INTRA-GENERIC DIFFERENCES IN GROWTH, REPRODUCTION, AND FITNESS OF 9 HERBACEOUS ANNUAL SPECIES GROWN IN ELEVATED CO2 ENVIRONMENTS

In assessing the capacity of plants to adapt to rapidly changing globa l climate, we must elucidate the impacts of elevated carbon dioxide on reproduction, fitness and evolution. We investigated how elevated CO2 influenced reproduction and growth of plants exhibiting a range of fl oral morphologies, the implications of shifts in allocation for fitnes s in these species, and whether related taxa would show similar patter ns of response. Three herbaceous, annual species each of the genera Po lygonum, Ipomoea, and Cassia were grown under 350 or 700 ppm CO2. Vege tative growth and reproductive output were measured non-destructively throughout the full life span, and vegetative biomass was quantified f or a subsample of plants in a harvest at first flowering. Viability an d germination studies of seed progeny were conducted to characterize f itness precisely. Early vegetative growth was often enhanced in high-C O2 grown plants of Polygonum and Cassia (but not Ipomoea). However, ea rly vegetative growth was not a strong predictor of subsequent reprodu ction. Phenology and production of floral buds, flowers, unripe and ab scised fruits differed between CO2 treatments, and genera differed in their reproductive and fitness responses to elevated CO2. Polygonum an d Cassia species showed accelerated, enhanced reproduction, while Ipom oea species generally declined in reproductive output in elevated CO2. Seed ''quality'' and fitness (in terms of viability and percentage ge rmination) were not always directly correlated with quantity produced, indicating that output alone may not reliably indicate fitness or evo lutionary potential. Species within genera typically responded more co nsistently to CO2 than unrelated species. Cluster analyses were perfor med separately on suites of vegetative and reproductive characters. So me species assorted within genera when these reproductive responses we re considered, but vegetative responses did not reflect taxonomic affi nity in these plants. Congeners may respond similarly in terms of repr oductive output under global change, but fitness and prognoses of popu lation persistence and evolutionary performance can be inferred only r arely from examination of vegetative characters alone.