We conducted controlled (chamber) and natural (field) environment experiments on the acclimation of respiration in Quercus alba L. and Quercus rubra L. Three-year-old Louisiana, Indiana and Wisconsin populations of Q. alba were placed in growth chambers and exposed to alternating 5-week periods of cool (20 degrees C mean) and warm (26 degrees C mean) temperatures. We measured respiration rates on fully expanded leaves immediately before and approximately every 2 days after a switch in mean temperature. In a second chamber experiment, 3-year-old potted Q. alba seedlings were exposed to alternating warm (26 degrees C mean) and cool (16 degrees C mean) temperatures at 4-day intervals. Leaf dark respiration rates were measured on days 2, 3 and 4 after each change in temperature. In a third, field-based study, we measured leaf respiration rates in the same three sources of Q. alba and in Arkansas, Indiana and Minnesota sources of Q. rubra before and after a natural 16 degrees C change in mean daily ambient temperature. We observed rapid, significant and similar acclimation of leaf respiration rates in all populations of Q. alba and Q. rubra. Cold-origin populations were no more plastic in their acclimation responses than populations from warmer sites. All geographic sources showed lower respiration rates when measured at 24 degrees C after exposure to higher mean temperatures. Respiration rates decreased 13% with a 6 degrees C increase in mean temperature in the first chamber study, and almost 40% with a 10 degrees C increase in temperature in the second chamber study. Acclimation was rapid in all three studies, occurring after 2 days of exposure to changed temperature regimes. Acclimation was reversible when changes in ambient temperature occurred at 4-day intervals. Respiration response functions, ln(R) = ln(beta0) + beta1T, were statistically different among treatments (cool versus warm, first chamber study) and among sources in a pooled comparison. Pair-wise comparisons indicated statistically significant (P<0.05) differences in cool- versus warm-measured temperature/respiration response functions for Indiana and Wisconsin sources of Q. alba. Log-transformed base respiration rates were significantly lower during periods of higher mean temperatures. Indiana Q. alba showed a significantly higher beta1 when plants were grown at 16 degrees C than when grown at 26 degrees C. Acclimation in Q. alba was unaccompanied by changes in leaf nitrogen concentration, but was associated with a change in leaf total nonstructural carbohydrate concentration. Total nonstructural carbohydrate concentration was slightly, but statistically, lower (13.6 versus 12%, P<0.05) after a 10 degrees C increase in temperature.