Single-tree versus group selection in hemlock-hardwood forests: are smaller openings less productive?

Abstract

The influence of opening size on gap cohort biomass was examined in uneven-aged hemlock-hardwood forests in northeastern Wisconsin by sampling 36 canopy gaps ranging from 5 to 800 m2 in area and from 3 to 55 years of age. The standing crop of whole-tree aboveground biomass per hectare was greater in multiple-tree gaps than single-tree gaps of similar age. However, biomass production approached an asymptote at fairly small opening sizes, and 400–800 m2 group selection openings did not have significantly more biomass per unit area than small multiple-tree gaps (80–100 m2). Similarly, mean tree dimensions, recent height growth increments, and basal area increments for most species of gap trees approached an asymptote at an opening size of approximately 100 m2. As opening size increased, yellow birch (Betula alleghaniensis Britton) superseded eastern hemlock (Tsuga canadensis (L.) Carrière) as the species with greatest relative biomass. The selection system appears to be a viable alternative to the even-aged shelterwood system in hemlock forests where browsing pressure is not excessive, but use of group selection may require additional treatments to ensure that some gaps >100 m2 are captured by hemlock.