Abstract
Six decades of temporal changes in the abundance of sugar maple (Acer saccharum Marsh.) were investigated using a network of multi-agency ground plots (MAGPlots) located across Ontario, Québec, and New Brunswick, Canada. Based on a composite dataset of nearly 400 plots mainly composed of sugar maple trees (≥ 50% basal area, m(2) ha(-1)), results showed that the relative abundance (% total sapling basal area) of sugar maple saplings declined significantly over time. On average, the relative abundance of sugar maple saplings decreased significantly between 1970 and 2022. Out of a wide range of potential explanatory variables, including stand conditions, harvest intensity (0%-92% basal area removal), regional ecozones, and climate variables, the relative abundance of American beech (Fagus grandifolia Ehrh.) saplings was the only variable that had a negative effect on the relative abundance of sugar maple saplings. The plot-specific distribution of change between the final and initial measurements over time revealed that many plots showing a decline in relative sugar maple sapling abundance also experienced an increase in relative American beech sapling abundance. The lack of differences between harvested and unharvested plots suggests that beech sapling control in the understory and soil liming treatments may be required to help promote sugar maple regeneration and development.