Abstract: Alnus mandshurica(Call.) Hand.-Mazz. and A. sibirica var. hisruta(Turez.) Koidz. are widely distributed in forest regions of northeastern China as major non-leguminous nodulated tree species with N₂-fixation. This paper summarizes the results of the research on effects of ecological factors on N₂-fixation of alder nodules. The results show that sufficient light condition is essential in raising N₂-fixing capability of the two species. The level of acetylene reduction of nodules partly depends on photosynthesis of the plant. The peak of diurnal course in acetylene reduction appears at 13: 00 hr. after the appearance of higher photosynthetic rate in early morning. The lower capability in N₂-fixation by A. mandshurica growing at high altitude is due to the lower temperature besides genetic factors.