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

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Abstract
Diel cycles of bole and root respiration were observed in white oak and tulip polar trees with highest rates occurring between 1900 h and midnight, and lowest rates between noon and 1500 h. Calculated Q10 [respiration rate increase for each 10.degree. C increase in temperature] values, using daily mean temperatures and daily mean respiration rates, ranged from 1.9-4.8 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 Q10 relationship to predict respiration rates in a natural forest environment be considered suspect unless other controlling variables are considered. Reducing sugar concentrations in tulip polar 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 5 times higher than the control, while rates below the girdle dropped to about 1/3 of the control by the 12th wk after the tree was girdled. Diel respiration cycles were observed both above and below the girdle. Possibly trees possess a diel scheme of food utilization whereby food catabolism is synchronized with growth processes at night when moisture availability is high.