CO₂ and Plants: Revisited

The decade-long USA research program on the direct effects of CO₂ enrichment on vegetation has achieved important milestones and has produced a number of interesting and exciting findings. Research beginning in 1980 focused on field experiments to determine whether phenomena observed in the laboratory indeed occurred in natural environments. The answer is yes. Data obtained from numerous field studies show mixed response of crop and native species to CO₂ enrichment however. Nearly all experiments demonstrate that plants exhibit positive gain when grown at elevated CO₂; although the magnitude varies greatly. Most crop responses range from 30 to 50% increase in yield. Results from long-term experiments with woody species and ecosystems are even more variable. Huge growth responses (100 to nearly 300% increase relative to controls) are reported from several tree experiments and the salt-marsh ecosystem experiment. Other results from experiments with woody species and the tundra ecosystem suggest little no effect of CO₂ on physiology, growth or productivity. Numerous studies of the physiology of the CO₂ effect are continuing in attempts to understand controlling mechanisms and to explain the variable growth responses. Particular emphasis needs to be given to physiological measures of interactions involving the CO₂ effect and other environmental influences, and to the wide-ranging observations of photosynthesis acclimation to CO₂. Prospects for future research are identified.