Diurnal and Seasonal Patterns of Photosynthesis and Respiration by Stems of Populus tremuloides Michx.

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Abstract
The photosynthetic and respiratory rates of 5-7 yr old aspen stems (P. tremuloides Michx.) were monitored in the field for 1 yr to determine the seasonal patterns. The stem was not capable of net photosynthesis, but the respiratory CO2 loss from the stem was reduced by 0-100% depending on the time of year and the level of illumination as a result of bark photosynthesis. The monthly dark respiratory rate ranged from 0.24 mg CO2/dm2.cntdot.h in Jan. to a maximum 7.4 mg CO2/dm2.cntdot.h in June. Individual measurements ranged from 0.02 mg CO2/dm2.cntdot.h in Feb. to 12.3 mg CO2/dm2.cntdot.h in June. Gross photosynthesis followed a pattern similar to the dark respiratory rate. The mean monthly rate was highest in June (1.65 mg CO2/dm2.cntdot.h) and lowest in Dec. (0.02 mg CO2/dm2.cntdot.h). Individual measurements ranged from 0.0 mg CO2/dm2.cntdot.h in winter to 5.5 mg CO2/dm2.cntdot.h in July. Winter studies showed that stem respiration continued down to -11.degree. C, the coldest temperature during this study. Upon warming to -3.degree. C, the dark respiratory rate showed a sudden sharp increase (7- to 12-fold) which required many hours to return to normal levels. No measurable photosynthesis occurred below -3.degree. C. Between -3.degree. and 0.degree. C, the maximal photosynthetic rate was reduced to less than 50% of the respiratory rate, but increased to 89% between 5.degree.-10.degree. C. On a yearly basis, bark photosynthesis in P. tremuloides reduced the stem respiratory CO2 loss by 28.7% on a daytime basis and an estimated 16-18% on a 24 h basis.