Direct and indirect effects of a dense understory on tree seedling recruitment in temperate forests: habitat-mediated predation versus competition

Abstract

In forests characterized by a dense woody and herbaceous understory layer, seedling recruitment is often directly suppressed via interspecific competition. Alternatively, these dense layers may indirectly lower tree recruitment by providing a haven for seed and seedling predators that prey on neighboring plant species. To simultaneously test for resource competition and indirect, habitat-mediated effects, we factorially manipulated understory plant cover (removed versus intact) and predation (exclosures versus controls) at three forested sites. We found that vegetation cover created privileged foraging areas that increased seed removal and seedling predation rates. Predator preference was directly related to seed size with larger seeded species including Prunus serotina Ehrh. and Fagus grandifolia Ehrh. removed more readily than smaller seeded species such as Fraxinus americana L. We found strong species-specific evidence for habitat-mediated indirect effects; establishment of P. serotina and Acer saccharum Marsh.was significantly lower under an intact hay-scented fern ( Dennstaedtia punctilobula (Michx.) T. Moore) canopy when small mammals were present. Competition also played a strong role; both P. serotina and Acer rubrum L. survival as well as A. rubrum emergence were reduced under a fern canopy with or without seed predators. The impact of habitat-mediated indirect effects and resource competition appear to vary predictably based upon predator preferences and differences in the timing of woody seed dispersal and germination relative to vegetation cover phenology. Overall, our results suggest that habitat-mediated indirect effects may be common and occur wherever vegetation provides the potential for creating privileged foraging areas.