Growth and Canopy Architecture of Shade‐Tolerant Trees: Response to Canopy Gaps

Patterns of aboveground growth, branching, and leaf display were examined in saplings of Acer saccharum Marsh. (sugar maple) and Fagus grandifola Ehrh. (beech) to determine (1) the responses of aboveground growth rates to variation in forest light regimes and (2) the importance of branching and leaf display patterns to the ability of saplings of these two species to respond to changes in forest light regimes produced by canopy gaps. For both species, rate of height growth, lateral growth, and the production of new shoots in even the low gap light levels created by small canopy gaps (15—75 m 2 ) were as much as an order of magnitude greater than growth rates of saplings beneath closed canopies. However, saplings of both species showed little response to further increases in gap light levels. The strong response of maple saplings to low gap light levels was correlated with an increased in sapling leaf area index and the efficiency of leaf display (measured as leaf area per unit length or surface area of branches). In beech, the more modest response to low light levels in small gaps and the higher growth rates than maple beneath a closed canopy were paralleled by a lack of significant increase in beech leaf area indices in small gaps and a higher efficiency of leaf display beneath a closed canopy than in small gaps. Thus, the magnitude of the response of these two species to small canopy gaps is correlated with their degree of plasticity in patterns of branching and leaf display. Both species can be considered small—gap specialists in the sense that their combinations of shade tolerance, growth responses, and canopy architecture make them particularly successful at exploiting small canopy gaps. However, the two species differ in their placement on a gradient in the degree to which woody plants respond to canopy disturbances.