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

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.