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

The article investigates canopy recruitment in harvest openings created by group selection in hemlock-hardwood forests. The researchers measured gap trees in openings ranging from 40 to 1000 square meters and up to 55 years old. Sample trees up to 25 meters 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 greater than 40% of the upper stratum of gap saplings in openings as small as 100-400 square meters. The ultimately successful hardwood gap trees, on average, reached crown shoulder height of the mature gap border trees within 28-37 years after gap formation. Successful trees were mostly advanced 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 meters. Extrapolation of current height growth of saplings less than 3 meters tall would substantially overestimate time needed for gap capture, but use of a size-dependent height increment function provided accurate estimates.