Winter photosynthesis of red spruce from three Vermont seed sources.

We evaluated winter (January through March) carbon assimilation of red spruce (Picea rubens Sarg.) from three Vermont seed sources grown in a common garden in northwestern Vermont. Although CO(2) exchange rates were generally low, net photosynthetic rates increased during two prolonged thaws. Significant correlations between CO(2) exchange rates and multiday air temperature means supported our observations of enhanced gas exchange during extended periods of elevated temperature. Increases in photosynthesis during thaws occurred before observed increases in leaf conductance, indicating that initial changes in photosynthesis were probably not associated with changes in stomatal aperture. Results of correlations between photosynthetic rates and PAR suggested that solar irradiance did not have a strong effect on winter carbon capture. Rates of net photosynthesis differed among seed sources. Trees from the Mt. Mansfield source had the highest average rates of photosynthesis and, at times, rates for individual trees from this source approximated those occurring during the growing season. Because seed sources differed in photosynthetic rates but not in leaf conductance, we conclude that differences in winter photosynthesis among seed sources were primarily attributable to factors other than changes in stomatal aperture.