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We examined the effects of elevated carbon dioxide concentration ([CO 2 ]) on the relationship between light-saturated net photosynthesis ( A  sat ) and area-based foliar nitrogen (N) concentration ( N  a ) in the canopy of the Duke Forest FACE experiment. Measurements of  A  sat  and  N  a  were made on two tree species growing in the forest overstory and four tree species growing in the forest understory, in ambient and elevated [CO 2 ] FACE rings, during early and late summer of 1999, 2001 and 2002, corresponding to years three, five and six of CO 2  treatment. When measured at the growth [CO 2 ], net photosynthetic rates of each species examined in the forest overstory and understory were stimulated by elevated [CO 2 ] at each measurement date. We found no effect of elevated [CO 2 ] on  N  a  in any of the species. The slope of the  A  sat —N relationship was 81% greater in elevated [CO 2 ] than in ambient [CO 2 ] when averaged across all sample dates, reflecting a differential CO 2  effect on photosynthesis at the top and bottom of the canopy. We compared  A  sat —N relationships in trees grown in ambient and elevated [CO 2 ] at two common CO 2  concentrations, during late summer 2001 and both early and late 2002, to determine if the stimulatory effect of elevated [CO 2 ] on photosynthesis diminishes over time. At all three sample times, neither the slopes nor the  y -intercepts of the  A  sat —N relationships of trees grown in ambient or elevated [CO 2 ] differed when measured at common CO 2  concentrations, indicating that the responses of photosynthesis to long-term elevated [CO 2 ] did not differ from the responses to a short-term increase in [CO 2 ]. This finding, together with the observation that  N  a  was unaffected by growth in elevated [CO 2 ], indicates that these overstory and understory trees growing at the Duke Forest FACE experiment continue to show a strong stimulation of photosynthesis by elevated [CO 2 ].

tree physiology

Relationships between net photosynthesis and foliar nitrogen concentrations in a loblolly pine forest ecosystem grown in elevated atmospheric carbon dioxide