Leaf respiration at different canopy positions in sweetgum (Liquidambar styraciflua) grown in ambient and elevated concentrations of carbon dioxide in the field

Trees exposed to elevated CO2 partial pressure ([CO2]) generally show increased rates of photosynthesis and growth, but effects on leaf respiration are more variable. The causes of this variable response are unresolved. We grew 12-year-old sweetgum trees (Liquidambar styraciflua L.) in a Free-Air CO2 Enrichment (FACE) facility in ambient [CO2] (37/44 Pa daytime/nighttime) and elevated [CO2] (57/65 Pa daytime/nighttime) in native soil at Oak Ridge National Environmental Research Park. Nighttime respiration (R-N) was measured on leaves in the upper and lower canopy in the second (1999) and third (2000) growing seasons of CO2 fumigation. Leaf respiration in the light (R-L) was estimated by the technique of Brooks and Farquhar (1985) in the upper canopy during the third growing season. There were no significant short-term effects of elevated [CO2] on R-N or long-term effects on R-N or R-L, when expressed on an area, mass or nitrogen (N) basis. Upper-canopy leaves had 54% higher R-N (area basis) than lower-canopy leaves, but this relationship was unaffected by CO2 growth treatment. In August 2000, R-L was about 40% of R-N in the upper canopy. Elevated [CO2] significantly increased the number of leaf mitochondria (62%), leaf mass per unit area (LMA; 9%), and leaf starch (31%) compared with leaves in ambient [CO2]. Upper-canopy leaves had a significantly higher number of mitochondria (73%), N (53%), LMA (38%), sugar (117%) and starch (23%) than lower-canopy leaves. Growth in elevated [CO2] did not affect the relationships (i.e., intercept and slope) between R-N and the measured leaf characteristics. Although no factor explained more than 45% of the variation in R-N, leaf N and LMA were the best predictors for R-N. Therefore, the response of R-N to CO2 treatment and canopy position was largely dependent on the magnitude of the effect of elevated [CO2] or canopy position on these characteristics. Because elevated [CO2] had little or no effect on N or LMA, there was no effect on R-N. Canopy position had large effects on these leaf characteristics, however, such that upper-canopy leaves exhibited higher RN than lower-canopy leaves. We conclude that elevated [CO2] does not directly impact leaf respiration in sweetgum and that barring changes in leaf nitrogen or leaf chemical composition, long-term effects of elevated [CO2] on respiration in this species will be minimal.