Local Seed Source Availability Limits Young Seedling Populations for Some Species More Than Other Factors in Northern Hardwood Forests

Abstract

Light is generally regarded as the most important factor affecting tree regeneration in selection managed forests. For decades this paradigm guided forestry in the Great Lakes region, where small harvest gaps were used to promote shade-tolerant species. However, recent attempts to regenerate a more diverse forest community, by varying harvest gap size, have been less successful, bringing the predominance of light into question. Here, we investigate the influence of light availability along with local seed production potential [Σ(diameter2/distance2)] (SPP), substrate availability, site quality, and nontree vegetation (NTV) on 8 tree species' initial density and survival in 19 stands located throughout the Northern Lower and Eastern Upper Peninsula of Michigan. Larger-seeded, shade-tolerant to mid-tolerant sugar maple, red maple, and white ash seedlings were at least twice as abundant as those of any other species. Light availability was not a strong factor predicting seedling density, except for sugar maple, which showed a negative response to increasing light. In contrast, SPP had a strong positive influence on yellow birch, eastern hemlock, black cherry, and white ash seedling density, with evidence of local and regional seed source limitations. Substrate availability was an important factor influencing white pine and paper birch seedling density, as each species was significantly more abundant in plots dominated by bare mineral soil than in plots dominated by leaf litter. Collectively, these results suggest that from seed through two growing seasons, SPP and substrate availability can exert a stronger influence on seedling layer diversity than light availability created by harvest gaps.