The role of resource interactions and seedling regeneration in maintaining a positive feedback in hemlock stands

1 When a species influences conditions at a site to promote recruitment of conspecifics, the community may remain dominated by that species until the next major disturbance. We examined the occurrence of possible mechanisms for such a positive feedback in one community where it may be important: forests dominated by eastern hemlock (Tsuga canadensis). 2 Intact soil divots were collected from a hemlock stand in central Massachusetts, USA, and placed in shade-houses at either 1.5% or 60% full sun. Soil was amended with nitrogen (5 g m(-2) year(-1)), lime (1 kg m(-2) year(-1); to increase soil pH) or left untreated. Emergence, survival and growth of seedlings arising from the soil seed bank were subsequently monitored. 3 Light availability had a very large influence on seedling regeneration dynamics. Birch (Betula spp.) seedlings were most sensitive of all species to light availability, with low light reducing emergence by 43% and substantially decreasing seedling growth (by 99%) and survival (by 94%). In contrast, light treatments had smaller effects on performance of other species [hemlock and white pine (Pinus strobus)]. Birch survival was also increased by nitrogen addition, but only in low light. Liming had only marginal effects on seedling performance. 4 Differential seedling responses to resource availability led to dominance of final seedling communities by hemlock in low light, unamended soils (i.e., under conditions naturally found in hemlock stands), but by birch in high light, as well as in low light, nitrogen-amended soils. 5 Positive feedbacks in hemlock stands are mediated both by availability of light and nitrogen, and by species' life-history strategies.