The influence of elevated CO2 on species phenology, growth and reproduction in a Mediterranean old-field community

We studied the effects on the phenology, growth and reproduction of 19 Mediterranean species, of elevating the atmospheric CO₂ concentration (CO₂]) to twice-ambient. Intact monoliths were taken from an old-field and put, during a six month growing season, into growth chambers in which external climatic conditions were mimicked and CO₂] was regulated. Fruit set time was significantly changed in six species under elevated CO₂] and leaf and branch senescence accelerated in most species. Grasses had fewer leaves and legumes were more branched at peak production under elevated CO₂] than under ambient. Plant seed number was not significantly changed under elevated CO₂], whereas the reproductive effort of grasses was significantly depressed. Reproductive and vegetative characteristics showed related responses to CO₂], as species with enhanced biomass had a hastened fruit set time, a higher number of fruits per plant and a higher reproductive biomass under elevated CO₂] than under ambient conditions, while species with depressed biomass had a delayed fruit set time, a lower number of fruits per plant and a lower reproductive biomass. Our results also show a high interspecific variability in CO₂] response, but some trends emerged at the family level: the production of vegetative and reproductive modules were depressed in grasses and slightly stimulated in legumes.