Effects of atmospheric CO2 on longleaf pine: productivity and

were exposed to two concentrations of atmospheric CO2 (365 or 720 μmol mol) in combination with two N treatments (40 or 400 kg N ha year) and two irrigation treatments (target values of −0.5 or −1.5 MPa xylem pressure potential) in opentop chambers from March 1993 through November 1994. Irrigation treatments were imposed after seedling establishment (i.e., 19 weeks after planting). Seedlings were harvested at 4, 8, 12, and 20 months. Elevated CO2 increased biomass production only in the high-N treatment, and the relative growth enhancement was greater for the root system than for the shoot system. In water-stressed trees, elevated CO2 increased root biomass only at the final harvest. Root:shoot ratios were usually increased by both the elevated CO2 and low-N treatments. In the elevated CO2 treatment, waterstressed trees had a higher root:shoot ratio than well-watered trees as a result of a drought-induced increase in the proportion of plant biomass in roots. Well-watered seedlings consistently grew larger than water-stressed seedlings only in the high-N treatment. We conclude that available soil N was the controlling resource for the growth response to elevated CO2 in this study. Although some growth enhancement was observed in waterstressed trees in the elevated CO2 treatment, this response was contingent on available soil N.