PARENT SESSION
Oral Session #44: Disturbance ecology of forests: Animals, wind, gaps, edges. Presiding: S. Archer.
Wednesday, August 8, 2001. 8:00 AM to 11:45 AM. Hall of Ideas G.

Comparative productivity and composition of canopy gaps in uneven-aged northern hemlock-hardwood forests .

Webster, Christopher¹, Lorimer, Craig¹, ¹

ABSTRACT- The structural complexity of uneven-aged forests provides important habitat for a number of species, and ecosystem management guidelines often emphasize strategies that mimic natural gap dynamics. However, few studies have addressed the comparative productivity of canopy gaps common in managed uneven-aged forests. In this study, we examined the influence of canopy gap size on gap cohort biomass accumulation in northern hemlock-hardwood forests. Thirty-six canopy gaps ranging in area from 5 - 800 m² and from 3 - 55 years of age were sampled, along with three reference understory plots. Whole tree aboveground biomass per hectare was significantly greater in multiple-tree gaps than in single-tree gaps. However, biomass per unit area approached an asymptote at fairly small gap sizes, and relatively large gaps (600-800 m²) were not more productive than small multiple-tree gaps (80-100 m²). Differences in productivity between single and multiple-tree gaps declined with time since formation. The mean height and individual tree basal area of codominant and dominant gap trees increased asymptotically with increasing gap size, both leveling off at a gap size of approximately 100 m². As gap size increased beyond 225 m², yellow birch (Betula alleghaniensis Britton) replaced eastern hemlock (Tsuga canadensis (L.) Carr.) as the dominant species. Our results suggest that multiple-tree gaps are both more productive and have higher tree species diversity than single-tree gaps, but optimal results probably do not require large openings.

KEY WORDS: gap dynamics, multi-aged forests, Tsuga canadensis, Betula alleghaniensis