NET PHOTOSYNTHESIS AS A FUNCTION OF CARBON-DIOXIDE CONCENTRATION IN PINE TREES GROWN AT AMBIENT AND ELEVATED CO2

Pinus eldarica seedlings were grown in a field of Avondale loam at Pho enix, Arizona within transparent open-top enclosures maintained for 15 months at mean CO2 concentrations of 402 and 788 mu l 1(-1), after wh ich whole-tree net photosynthetic rates were measured at a number of C O2 concentrations ranging from ambient (360 mu l 1(-1)) to 3000 mu l 1 (-1). Rates of the low-CO2-treatment trees saturated at approximately five times their ambient-concentration value; while rates of the high- CO2-treatment trees rose linearly across the entire CO2 range investig ated to more than 10 times their value al 360 mu l 1(-1). These findin gs suggest that long-term exposure to elevated CO2 can increase the ab ility of trees with unrestricted root systems to respond positively to still higher CO2 concentrations.