Biomass allocation in old-field annual species grown in elevated CO2 environments: no evidence for optimal partitioning

Increased atmospheric carbon dioxide supply is predicted to alter plant gro wth and biomass allocation patterns. It is not clear whether changes in bio mass allocation reflect optimal partitioning or whether they are a direct e ffect of increased growth rates. Plasticity in growth and biomass allocatio n patterns was investigated at two concentrations of CO2 ([CO2]) and at lim iting and nonlimiting nutrient levels for four fast- growing old-field annu al species. Abutilon theophrasti, Amaranthus retroflexus, Chenopodium album , and Polygonum pensylvanicum were grown from seed in controlled growth cha mber conditions at current (350 mu mol mol(-1), ambient) and future- predic ted (700 mu mol mol(-1), elevated) CO2 levels. Frequent harvests were used to determine growth and biomass allocation responses of these plants throug hout vegetative development. Under nonlimiting nutrient conditions, whole p lant growth was increased greatly under elevated [CO2] for three C3 species and moderately increased for a C4 species (Amaranthus). No significant inc reases in whole plant growth were observed under limiting nutrient conditio ns. Plants grown in elevated [CO2] had lower or unchanged root:shoot ratios , contrary to what would be expected by optimal partitioning theory. These differences disappeared when allometric plots of the same data were analyse d, indicating that CO2-induced differences in root:shoot allocation were a consequence of accelerated growth and development rates. Allocation to leaf area was unaffected by atmospheric [CO2] for these species. The general la ck of biomass allocation responses to [CO2] availability is in stark contra st with known responses of these species to light and nutrient gradients. W e conclude that biomass allocation responses to elevated atmospheric [CO2] are not consistent with optimal partitioning predictions.