Complexity of Regeneration Dynamic at the Ecocline between Mixedwood and Coniferous Domains of the Southernmost Boreal Zone in Eastern North America

Abstract

To explain the ecocline between the southern mixedwood and the northern coniferous bioclimatic domains dominated, respectively, by balsam fir (Abies balsamea (L.) Mill.) and black spruce (Picea mariana (Mill.) B.S.P.), 59 field sites and 7010 sample plots (from the Quebec Ministry of Forests, Wildlife, and Parks), with no major disturbances, were selected throughout the two bioclimatic domains. Regeneration (seedlings and saplings), mortality (difference between seedlings and saplings) of balsam fir, and black spruce (saplings) were examined, accounting for parental trees, main soil type (clay and till), summer growing degree-days above 5°C (GDD_5), and total summer precipitation (May–August; PP_MA). Balsam fir regeneration was more depended on parental trees and soil type than black spruce. Balsam fir mortality was related to seedling competition, species composition of the canopy, and the soil type. GDD_5 and marginally PP_MA were beneficial and detrimental for respectively balsam fir and black spruce regeneration. The ecocline mixedwood/coniferous bioclimatic domains was attributed to a northward gradual decrease of balsam fir regeneration and increase of its mortality, due to cooler temperatures, shorter growing seasons, and decrease of the parental trees. However, balsam fir persists above this ecocline, where parental trees populations and good establishment substrates occur.