fern understory as an ecological filter: emergence and establishment of canopy-tree seedlings

We investigated the role of the fern understory as an ecological filter that influences the organization of the seedling bank in New England deciduous forests. Microenvironmental variables--including light levels, litter depth, soil exposure, soil moisture, and soil organic matter content--were quantified in experimental plots where the fern understory was undisturbed, partially removed or completely removed and were related to natural recruitment and 1st-yr survival of Acer rubrum, Betula lenta, B. alleghaniensis, Fraxinus americana, Pinus strobus, and Quercus rubra. We conducted a series of three field emergence experiments to test hypotheses regarding mechanisms of fern interference with seedling emergence. The fern understory reduced light levels from 3.4% of full sun to 1.1% of full sun beneath its canopy. Soil exposure was lower and litter depth was greater under fern cover, whereas soil moisture and soil organic-matter content were not affected by fern cover. The understory filter differentially influenced tree-seedling emergence. Fern cover decreased emergence of Betula, Pinus, and Quercus but did not affect the emergence of Acer or Fraxinus. The mechanism of fern interference was species-specific: Betula emergence was reduced primarily by low levels of soil exposure, Pinus emergence appeared to be related to reduced light levels, and Quercus suffered higher levels of seed predation under fern cover. The presence of understory fern cover also differentially influenced 1st-yr survival of natural tree-seedling recruitment. Although seedling survivorship during the first growing season was related to seed size, seedling survivorship below ferns by the end of the 1st yr was independent of seed size. Selectivity of the fern filter is caused by differential response of tree-seedling species to the presence of understory cover and was generally not affected by the species identity of the understory plant. The selectivity of the understory filter can influence the density and species composition of the seedling bank below its canopy and can determine patterns of seedling spatial distribution at the stand level.