Wintertime photosynthetic capacity of black spruce (Picea mariana) in boreal forests in interior Alaska

Abstract:

In boreal forests, the growing season is short, and winter temperature is low and fluctuates from considerably below freezing point to intermittent warm spells. Under such conditions, it is important for plants to retain their photosynthetic capacity throughout the winter. To understand the importance of wintertime photosynthetic activity for evergreen boreal coniferous species, the light response curve of black spruce (Picea mariana) was monitored in Fairbanks, interior Alaska (64°86′N, 147°84′W) throughout the winter, and compared with those in the summer. Cuttings of black spruce were collected, and gas exchange of their needles was measured in the incubator set to 0 °C using a gas analyzer (LI-6400, Li-Cor Inc.). A non-rectangular hyperbolic model was fitted to these data, and physiological parameters such as the maximum photosynthesis rate, dark respiration rate and quantum yield of photosynthesis were extracted.

The apparent quantum yield of photosynthesis remained low throughout the winter for black spruce. The maximum photosynthesis rate was downregulated as air temperature fell in early winter, but did not increase in March when air temperature rose. This suggests that photoinhibition may occur more strongly in March than in early winter. The average maximum rates of photosynthesis in winter were almost 10％ of the value measured in summer. On the other hand, the dark respiration rate did not considerably differ between seasons. These results provide new insights into winter photosynthetic activity and its role in boreal forest ecosystems.