The effects of elevated CO and light environment on growth and reproductive performance of four annual species.

To predict changes in the range and density of plant species as a consequence of elevated atmospheric CO , it is essential to characterize the effect of elevated CO on key components of plant life history stages such as rate of establishment and maturation of individuals, reproductive output and offspring fitness. We measured vegetative response, phenology, reproductive output and seed quality of four annual C plant species, grown from seed to senescence under ambient and elevated (ambient+200 ppm) CO and ambient and reduced (33% shade) light. Few previous studies have included all stages of a plant's life as well as characteristics of the next generation. Elevated CO had no effect on the vegetative attributes of Cardamine hirsuta, Spergula arvensis or Poa annua whereas Senecio vulgaris produced longer leaves and greater biomass. Both Senecio and Poa had faster maturation times. The vegetative response of Senecio was not translated into increased seed output, although seed mass and carbon∶nitrogen ratios were significantly increased. By contrast, Poa showed no vegetative response to elevated CO , but had significantly increased seed production. Thus we found no evidence for a simple translation from vegetative response to elevated CO into reproductive response. There was also no consistent light-mediated response to elevated CO among the four species. However, the effect of reduced light (33% shade) on vegetative and reproductive output was consistent across the four species and significantly stronger than the effect of elevated CO . On the basis of this glasshouse study, we predict that Poa would be most likely of the four species to show significant increases in population size and migration potential, as a result of increased reproductive output, under elevated atmospheric CO . However, this response may be relatively small compared with variation in growth and reproduction as a result of environmental heterogeneity in resources such as light.