Johnson, S.L. and Lincoln, D.E. 2000. Allocation responses to CO2 enrichment and defoliation by a native annual plant Heterotheca subaxillaris. Global Change Biology 6: 767-778.
What was done
An annual plant (Heterotheca subaxillaris) native to the southeastern USA was gown in controlled environment chambers receiving atmospheric CO2 concentrations of 400 and 700 ppm for up to 27 weeks. The plants were also subjected to one of three defoliation treatments: non-defoliated controls, weekly defoliation of 25% leaf area prior to bolting, or weekly defoliation of 25% leaf area after bolting. Thus, the authors studied the effects of atmospheric CO2 enrichment and defoliation on the growth and reproduction of this native annual species.
What was learned
At both atmospheric CO2 concentrations, defoliation resulted in smaller plants, which compensated for this perturbation by allocating greater biomass to shoots and leaves as opposed to roots. Elevated CO2 enhanced photosynthetic rates in all treatments by approximately 17%; but it only impacted total plant biomass in the non-defoliated controls, where it increased this parameter by 20%. Irrespective of defoliation, elevated CO2 increased reproductive flower biomass and caused flowering to occur much earlier than it did in ambiently-grown plants.
What it means
As the air's CO2 concentration increases, this annual species will likely exhibit increased photosynthetic rates that will enhance reproductive flower development, irrespective of any defoliation that might occur in nature. Moreover, this species will likely begin flowering earlier in the season, thus providing a potential for the production of greater numbers of flowers and enhanced seed production. These consequences, in turn, should strengthen the ability of this species to persist in its current range.
Reviewed 20 February 2002