Seed germination, seedling emergence, and seed bank ecology of sweet fern (Comptonia peregrina(L.) Coult.)

Abstract

We examined seed-bank seeds of sweet fern (Comptonia peregrina (L.) Coult.), an actinorhizal nitrogen-fixing shrub, to determine their distribution in the soil and to identify the factors that stimulate them to germinate following removal of the vegetation. Seeds were extracted from the soil of adjacent field and forest sites currently lacking sweet fern in Orono, Maine. Both sites contained approximately 2000 seeds·m-2with the greatest concentration at a depth of 60-80 mm. The seeds were 4.0-5.5 mm long, enclosed by a pitted, woody pericarp, and 8% contained embryos. Many seedlings emerged in disturbed plots (vegetation removed and upper soil mixed) in May and June 1997, but none appeared after 24 July. Seeds collected in May and June germinated readily in a growth chamber (30-45% germination) whereas only 2-5% of July- and August-collected seeds germinated indicating induction of secondary dormancy. August-collected seeds showed strong germination after >=15 days of moist chilling at 4°C indicating relief of secondary dormancy by chilling. Temperature fluctuation with an amplitude of 10°C strongly stimulated germination. Presence of annual secondary dormancy cycles and stimulation by strong temperature fluctuation assures that seed-bank seeds germinate under conditions that allow the seedlings to become established.Key words: actinorhizal plants, Comptonia peregrina, germination ecophysiology, secondary dormancy, seed bank, seedling emergence.