CHANGES IN NET PHOTOSYNTHESIS AND GROWTH OF PINUS-ELDARICA SEEDLINGS IN RESPONSE TO ATMOSPHERIC CO2 ENRICHMENT

Pinus eldarica L. trees, rooted in the natural soil of an agricultural field at Phoenix, Arizona, were grown from the seedling stage in clea r-plastic-wall open-top enclosures maintained at four different atmosp heric CO2 concentrations for 15 months. Light response functions were determined for one tree from each treatment by means of whole-tree net CO2 exchange measurements at the end of this period, after which rate s of carbon assimilation of an ambient-treatment tree were measured ac ross a range of atmospheric COP concentrations. The first of these dat a sets incorporates the consequences of both the CO2-induced enhanceme nt of net photosynthesis per unit needle area and the CO2-induced enha ncement of needle area itself (due primarily to the production of more needles), whereas the second data set reflects only the first of thes e effects. Hence the division of the normalized results of the first d ata set by the normalized results of the second set yields a represent ation of the increase in whole-tree net photosynthesis due to enhanced needle production caused by atmospheric CO2 enrichment. In the solita ry trees we studied, the relative contribution of this effect increase d rapidly with the CO2 concentration of the air to increase whole-tree net photosynthesis by nearly 50% at a CO2 concentration approximately 300 mu mol mol(-1) above ambient.