Obstacles to herb-layer recovery in an old-growth forest fragment: Deer, invasive earthworms, or propagule supply?

Loss of herb layer species in forest fragments has been reported in many studies.  Cited causes of declines include deer browsing, earthworm invasion, and limited supply of propagules to replace lost species or augment sparse populations.  However, few studies have simultaneously tested the degree to which these factors are obstacles to recovery of herb layer diversity.  We tested the effect of deer exclusion, earthworm reduction, and propagule augmentation on herb layer cover, diversity, and establishment in an old-growth oak-maple-basswood fragment of the “Big Woods” in Minnesota (U.S.A.).  After 8 years of deer exclusion, there was no recovery of the herb layer as measured in eighty 1.8 m² plots.  However, censuses of large plots (2000 m²) revealed an increasing number of species growing inside as compared to outside deer exclosures, suggesting that deer exclusion can facilitate slow recovery of some species.  Electroshocking of plots over two years resulted in a significant (p<0.001) 46-70% reduction in total earthworm mass as compared to unshocked plots.  However, this did not contribute to herb layer recovery.  Nor did propagule augmentation lead to successful establishment of most species.  Planted seedlings of the tree, A. saccharum, and six of eight forb species had <10% survivorship after 2 years and survivorship was not affected by earthworm reduction either year.   Herbivory by the invasive slug, Arion cf. fasciatus, was a major source of mortality.  This is an overlooked factor that may be a significant obstacle to seedling establishment at this and other fragments of the Big Woods.