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 for Avena sativa, Prosopis glandulosa, and Schizachyrium 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.