SPECIES-DEPENDENT EFFECTS OF SEED PREDATION AND GROUND COVER ON SEEDLING EMERGENCE OF OLD-FIELD FORBS

Seed predation may limit seedling emergence by reducing seed supply but in old fields suppression of seedling emergence by dense ground cover could override any potential effect of seed predation. Experiments were conductcd with sown seeds of three old-field forbs (Medicago lupulina L., Centaurea nigra L., Taraxacum officinale Weber) to test whether seed predation affected seedling emergence in densely vegetated hollows and on less densely vegetated ridges in an abandoned pasture. Some seeds were protected from potential seed predators (i.e., ants, rodents and birds) while other seeds were not protected. Unprotected seeds disappeared faster in hollows than on ridges. Excluding ants or ants plus rodents and birds reduced the rate that seeds disappeared in hollows more than on ridges. Excluding only rodents and birds had little effect on the rate that seeds disappeared. Fewer seedlings emerged in hollows than on ridges when seeds were not protected from predators. Excluding ants plus rodents and birds increased seedling emergence for Centaurea nigra and Taraxacum officinale, especially in hollows. In contrast, seedling emergence of Medicago lupulina did not increase in hollows when seed predators were excluded. Dense ground cover in hollows likely suppressed seedling emergence of Medicago lupulina. Our results indicate that seed predation reduces seedling emergence for some but not all old-field forbs. It is inappropriate to assume that seed predation always reduces seedling emergence.