Possible mechanisms of sugar maple regeneration failure and replacement by beech in the Boisé-des-Muir old-growth forest, Québec

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Abstract
To better understand the causes of sugar maple (Acer saccharum Marsh.) replacement by beech (Fagus grandifolia Ehrh.) in the Boisé-des-Muir old-growth forest (Québec), we studied, i) the effect of beech litter on soil properties, ii) the effect of low light availability on maple survival, and iii) beech regeneration via root sprouts. Soil properties, understory light, seedling emergence and survival, height growth, and density distribution of maple and beech were compared between the old-growth stand and an adjacent managed stand where the abundance of maple regeneration was high. We did not observe any detrimental effect of beech litter on soil properties and maple early establishment. Overall light availability and maple stocking were lower, and maple density declined more markedly with increasing size in the old-growth stand than in the managed stand. The deficient maple regeneration in the old-growth stand might be related to a lack of microsites where light availability is high enough for maple survival, and not to a lack of initial recruitment in this species. In the old-growth stand, beech was located in more shaded microsites than maple but had a higher growth. Size class distributions of maple and beech suggest that beech had a higher survival rate than maple and that beech sprouts had a higher survival rate than beech seedlings. Beech seems to be better adapted than maple to survive in the deep shade of the old-growth stand. However, we expect that maple will regain its dominance in the understory of the Boisé-des-Muir with increasing stand breakup that is likely to occur in the future.