EFFECTS OF ATMOSPHERIC CO2 ON LONGLEAF PINE - PRODUCTIVITY AND ALLOCATION AS INFLUENCED BY NITROGEN AND WATER

Longleaf pine (Pinus palustris Mill.) seedlings were exposed to two co ncentrations of atmospheric CO2 (365 or 720 mu mol mol(-1)) in combina tion with two N treatments (40 or 400 kg N ha(-1) year(-1)) and two ir rigation treatments (target values of -0.5 or -1.5 MPa xylem pressure potential) in open-top 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 r oot system than for the shoot system. In water-stressed trees, elevate d CO2 increased root biomass only at the final harvest. Root:shoot rat ios were usually increased by both the elevated CO2 and low-N treatmen ts. In the elevated CO2 treatment, water-stressed trees had a higher r oot:shoot ratio than well-watered trees as a result of a drought-induc ed 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 con trolling resource for the growth response to elevated CO2 in this stud y. Although some growth enhancement was observed in water-stressed tre es in the elevated CO2 treatment, this response was contingent on avai lable soil N.