The Lateral Growth of Branches into Small Canopy Gaps: Implications for Competition between Canopy Trees

Abstract

Much research in forest ecology has been devoted to examining the effect of gap formation on regeneration dynamics. However, comparatively little research has examined the process of gap closure, in which larger trees bordering the gap grow laterally to exploit available light. Thus, it remains uncertain whether disturbance disrupts or reinforces the competitive hierarchy established among different species and sizes classes. We quantified the lateral growth of three hardwood tree species with differing autecologies both before and after the formation of small gaps created by single-tree selection. Linear mixed-effect models were employed to link lateral growth to species and stem diameter to examine whether gap formation favors intolerant species and small trees in the canopy. Additional models were also developed to examine the relationship of lateral growth with branch length and tree height. Before gap formation, the mid-tolerant yellow birch grew considerably faster than the tolerant sugar maple and American beech. However, yellow birch was less responsive to gap formation (~16%) than sugar maple or beech, whose lateral growth increased by 42% and 39%, respectively. This suggests that gap formation reinforces the competitive dominance of tolerant species. In contrast, gap formation disrupts the competitive dominance of large trees in the canopy, since the lateral growth of small trees increased five times that of large trees. Thus, small silvicultural gaps bordered by small trees may close too quickly to permit the regeneration of mid-tolerant species. Following the release, small trees also grew faster than their larger counterparts, suggesting that lateral growth declines as the cost of reproduction increases with tree size. However, lateral growth did not vary with tree height or branch length, suggesting that lateral growth does not decline due to increasing support costs or hydraulic limitation.