Lo. George et Fa. Bazzaz, The fern understory as an ecological filter: Emergence and establishment of canopy-tree seedlings, ECOLOGY, 80(3), 1999, pp. 833-845
We investigated the role of the fern understory as an ecological filter tha
t 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 quanti
fied in experimental plots where the fern understory was undisturbed, parti
ally removed or completely removed and were related to natural recruitment
and 1st-yr survival of Acer rubrum, Betula lenta, B, alleghaniensis, Fraxin
us americana, Pinus strobus, and Quercus rubra. We conducted a series of th
ree 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 f
ull sun beneath its canopy. Soil exposure was lower and litter depth was gr
eater under fern cover, whereas soil moisture and soil organic-matter conte
nt were not affected by fern cover. The understory filter differentially in
fluenced tree-seedling emergence. Fern cover decreased emergence of Betula,
Finns, and Quercus but did not affect the emergence of Acer or Fraxinus. T
he mechanism of fern interference was species-specific: Betula emergence wa
s reduced primarily by low levels of soil exposure, Pinus emergence appeare
d 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 Ist-yr survival of natural tree-seedling rec
ruitment. Although seedling survivorship during the first growing season wa
s related to seed size, seedling survivorship below ferns by the end of the
Ist yr was independent of seed size. Selectivity of the fern filter is cau
sed by differential response of tree-seedling species to the presence of un
derstory cover and was generally not affected by the species identity of th
e understory plant. The selectivity of the understory filter can influence
the density and species composition of the seedling bank below its canopy a
nd can determine patterns of seedling spatial distribution at the stand lev
el.