Temperature-Independent Diel Variations of Respiration Rates in Quercus alba and Liriodendron tulipifera

Diel cycles of bole and root respiration were observed in white oak and tulip poplar trees with highest rates occurring between 7 P.M. and midnight, and lowest rates between noon and 3 P.M. Calculated Q10 values, using daily mean temperatures and daily mean respiration rates, ranged from 1.9 to 4.8 and averaged 3.2 during several days in April and May. Respiration rates predicted from temperatures, in which Q10 was assumed equal to 2, followed a diel cycle 180 degrees out of phase with measured rates. It is suggested that any use of the classical Q1o relationship to predict respiration rates in a natural forest environment be considered suspect unless other controlling variables are considered. Reducing sugar concentrations in tulip poplar boles followed a similar diel cycle as respiration rates, suggesting a correlation between reducing sugar concentrations and respiration rates. In girdling experiments, respiration rates immediately above the incision of a girdled tulip poplar were up to five times higher than the control, while rates below the girdle dropped to about one-third of the control by the twelfth week after the tree was girdled. Diel respiration cycles were observed both above and below the girdle. We suggest that trees possess a diel scheme of food utilization whereby food catabolism is synchronized with growth processes at night when moisture availability is high.