Competitive advantage of black spruce over balsam fir in coniferous boreal forests of eastern North America revealed by site index

Abstract

The boreal zone of northeastern North America is characterized by mixedwood forests dominated by balsam fir (Abies balsamea (L.) Mill.) in the south and by coniferous forests dominated by black spruce (Picea mariana (Mill) B.S.P.) in the north. Site index values of balsam fir and black spruce were compared in 364 sites spread across the boreal zone of northwestern Quebec to determine if the northward dominance shift from balsam fir to black spruce could be explained by a difference in height growth. Site index of both species decreased along a south-north gradient, although the trend was only significant for balsam fir on clay deposits. Site index values shifted from being significantly higher for balsam fir in the boreal mixedwood forest, to being slightly (but not significantly) higher for black spruce in the coniferous forest. Mean annual temperature had a significant positive effect on site index for both species, and precipitation of the growing season had a significant negative effect only for balsam fir. The competitive advantage of black spruce over balsam fir in coniferous forests is due to a greater tolerance to cooler temperatures and water-logged soils. © 2014 Society of American Foresters.