Seed rain, safe sites, competing vegetation, and soil resources spatially structure white pine regeneration and recruitment

We tested the effects of seed rain, safe sites, soil depth, overstory, and shrub layer on the establishment and recruitment of white pine (Pinus strobus L.) in aspen mixedwoods of the western Great Lakes region, U.S.A. Germinant and seedling densities were positively related to seed rain and safe site characteristics that indicate moist conditions: high overstory basal area, decaying wood, and moss cover. Germinant and seedling densities were highest under dense overstory (>16 m₂/ha) and were unrelated to shrub cover. Sapling recruitment was greatest under low overstory density (<16 m²/ha) and low shrub cover (<55%). On shallow soil (∼14 cm), germinants and seedlings commonly occurred on soil depths <5 cm, but large saplings almost always occurred on soil depths >5 cm. On deeper soil (∼21 cm), overstory white pines occupied the shallowest soils (∼18 cm) of all major overstory species, indicating that competition may cause white pine recruitment into the canopy to be lower on deeper soils. White pine populations in the study were initially spatially structured by seed rain and safe sites but sapling growth and recruitment was structured by overstory density, shrub cover, and soil depth.