MINIMUM OPENING SIZES FOR CANOPY RECRUITMENT OF MIDTOLERANT TREE SPECIES: A RETROSPECTIVE APPROACH

Abstract

Minimum opening sizes and time required for successful canopy recruitment of tree saplings are not well known because the gap capture process is slow and difficult to monitor. In this study, we investigated canopy recruitment in harvest openings created by group selection in hemlock–hardwood forests to determine if yellow birch (Betula alleghaniensis Britt.), a species with intermediate shade tolerance, could successfully reach the canopy via small openings. We measured gap trees in openings ranging from 40 to 1000 m2 and up to 55 years old. Sample trees up to 25 m tall, spanning all stages of the gap capture process, were destructively sampled to determine height–growth rates, total age, and size at time of gap formation. Results indicate that yellow birch comprised >40% of the upper stratum of gap saplings in openings as small as 100–400 m2. The ultimately successful hardwood gap trees, on average, reached crown shoulder height of the mature gap border trees (73% of total canopy height) within 28–37 years after gap formation. Successful trees were mostly advance regeneration at the time of gap formation, but some gap colonists were also successful. Height increments in all hardwood species were strongly size dependent, with saplings reaching a maximum growth rate at a height of 10 m. Extrapolations of current height growth of saplings <3 m tall would substantially overestimate time needed for gap capture, but use of a size‐dependent height increment function provided accurate estimates. Results suggest that inclusion of relatively small openings (200–1000 m2) in uneven‐aged or multicohort harvest operations may be sufficient to perpetuate the diversity of shade‐tolerance classes currently present in these stands.