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 (Rg) and a 48 % increase in maintenance respiration (Rm). During a 4-d period when CO2 fumi-gation 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 Rg and Rm rate responses may result from changes in growth rates and live to dead tissue ratios rather than from mechanistic alterations of the respiratory processes. Key words: stem respiration, free-air CO2 enrichment (FACE), soluble carbohydrate concentrations, CO2 enrichment, sucrose, Liquidambar styraciflua (sweetgum), growth respiration, maintenance respiration