American beech outgrows sugar maple at the sapling stage regardless of partial harvest intensity in northern hardwood forests

Abstract

Sugar maple (Acer saccharum Marsh) is a key species in the northern hardwood forests of North America and management has historically focused on promoting its regeneration and yielding high-quality timber. A decrease in the abundance of sugar maple to the advantage of American beech (Fagus grandifolia Ehrh.) has been observed over the last decades. Together with the spread of beech bark disease, the understory dominance of American beech has often resulted in depleted, low vigour forest stands (i.e., with a low proportion of healthy trees). To favour regeneration and growth of sugar maple, increasing harvest intensity of partial treatments has been suggested, but the harvest intensity required remains unclear. In this study, we monitored the radial growth of saplings in 12 northern hardwood stands located in the Province of Quebec, Canada. These were commercially harvested with removal intensities ranging from 0% to 80% of the basal area. Using generalized additive mixed models, we investigated the effect of harvest intensity on the growth response of both species. Our results revealed that American beech saplings consistently had a much stronger radial growth response than sugar maple after harvest, regardless of treatment intensity. Consequently, modulating the intensity of partial harvests cannot singlehandedly favour the growth of sugar maple regeneration where American beech saplings dominate the understory. Additional silvicultural interventions are thus needed to control advance understory beech.