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Diurnal and Seasonal Patterns of Photosynthesis and Respiration by Stems of Populus tremuloides Michx. ¹

^a Department of Botany and Forest Pathology, State University of New York, College of Environmental Science and Forestry, Syracuse, New York 13210

The photosynthetic and respiratory rates of 5- to 7-year-old aspen stems (Populus tremuloides Michx.) were monitored in the field for 1 year 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 to 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· hr in January to a maximum 7.4 mg CO₂/dm²· hr in June. Individual measurements ranged from 0.02 mg CO₂/dm²· hr in February to 12.3 mg CO₂/dm²· hr 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· hr) and lowest in December (0.02 mg CO₂/dm²· hr). Individual measurements ranged from 0.0 mg CO₂/dm²· hr in winter to 5.5 mg CO₂/dm²· hr in July.

Winter studies showed that stem respiration continued down to –11 C, the coldest temperature during this study. Upon warning to –3 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 C. Between –3 and 0 C, the maximal photosynthetic rate was reduced to less than 50% of the respiratory rate, but increased to 89% between 5 to 10 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 to 18% on a 24-hour basis.

³ To whom reprint requests should be addressed.

¹ This research is part of the dissertation submitted by K. C. F. in partial fulfillment of the Ph.D. requirements at this college.

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