We compared nitrate (NO₃^--N) assimilation of black spruce (Picea mariana) in summer and winter to test the hypothesis that boreal species have the ability to assimilate NO₃^--N at very low temperature. Nitrate reductase activity (NRA) in needles and fine roots was measured as an indicator of NO₃^--N assimilation. High (30 ˚C) and low (-3 ˚C; surface soil temperature at sampling time) incubation temperatures were applied to winter samples to assess the effect of temperature on the existing enzyme activities in winter. Spruce needles sampled during winter (-20 ˚C air temperature) showed higher NRA than summer needle samples. In addition, winter needles showed the substantial enzyme activity even when assayed at below-freezing temperatures (-3 ˚C). By contrast, fine root NRA showed no difference between summer and winter. Our results indicate that a dominant tree species of boreal forests in interior Alaska, P. mariana, has the physiological capacity to assimilate NO₃^--N in winter, which may extend the active season for N acquisition by this species.