Increasing CO2 and plant-plant interactions: effects on natural vegetation

Plant species and functional groups of species show marked differences in photosynthesis and growth in relation to rising atmospheric CO₂ concentrations through the range of the 30 % increase of the recent past and the 100 % increase since the last glaciation. A large shift was found in the compositional mix of 26 species of C₃'s and 17 species of C₄'s grown from a native soil seed bank in a competitive mode along a CO₂ gradient that approximated the CO₂ increase of the past 150 years and before. The biomass of C₃'s increased from near zero to 50 % of the total while that of the C₄'s was reduced 25 % as CO₂ levels approached current ambient. The proposition that acclimation to rising CO₂ will largely negate the fertilization effect of higher CO₂ levels on C₃'s is not supported. No signs of photosynthetic acclimation were evident forAvena sativa, Prosopis glandulosa, andSchizachyrium scoparium plants grown in subambient CO₂. The effects of changing CO₂ levels on vegetation since the last glaciation are thought to have been at least as great, if not greater, than those which should be expected for a doubling of current CO₂ levels. Atmospheric CO₂ concentrations below 200 ppm are thought to have been instrumental in the rise of the C₄ grasslands of North America and other extensive C₄ grasslands and savannas of the world. Dramatic invasion of these areas by woody C₃ species are accompanying the historical increase in atmospheric CO₂ concentration now in progress.