Stem respiration increases in CO₂-enriched sweetgum trees

Stem respiration in trees results in substantial CO₂ release to the atmosphere. Stem respiration is an important component of the carbon budget of forest stands that could be perturbed by CO₂ enrichment of the atmosphere. We measured stem respiration in a free-air CO₂ enrichment (FACE) experiment in a 15-yr-old sweetgum (Liquidambar styraciflua) plantation. The trees were exposed to ambient or 1.4× ambient [CO₂]. Stem respiration of 12 trees in ambient [CO₂] and 12 trees in CO₂-enriched plots was continuously monitored over a period of 1 year. CO₂ enrichment caused a 23% increase in annual growth respiration (R_g) and a 48% increase in maintenance respiration (R_m). During a 4-d period when CO₂ fumigation was turned off, both respiration and stem sucrose concentrations declined in the CO₂-enriched trees. Concentrations of other soluble sugars did not change. We conclude that elevated [CO₂] caused stem respiration to increase and that the increase was driven by increased substrate supply from leaves. We suggest that both R_g and R_m rate responses may result from changes in growth rates and live to dead tissue ratios rather than from mechanistic alterations of the respiratory processes.