The fern understory as an ecological filter: Growth and survival of canopy-tree seedlings

We investigated the role of the fern understory of closed-canopy forests as an ecological filter shaping the density, species composition, size struct ure, and spatial distribution of the seedling bank. In New England deciduou s forests we tested the hypothesis that the understory stratum is a selecti ve filter that differentially influences growth and survival of tree-seedli ng species by comparing performance of Acer rubrum, Betula alleghaniensis, and Quercus rubra seedlings in plots where the fern understory was undistur bed, partially removed, or completely removed. We related seedling growth a nd survival to microenvironmental characteristics of experimental plots in order to further explore mechanisms responsible for the filtering capacity of the fern understory. The presence of a fern understory reduced growth and survival of all seedli ng species, but the magnitude of the effect differed among species. Mortali ty resulting from resource limitation in Quercus below the fern understory was balanced by mortality resulting from insect herbivory in fern-free area s. Relative biomass growth rates of all species were negatively influenced by the presence of fern cover, whereas relative height growth rates of Acer and Quercus were uniformly low and insensitive to the presence of fern cov er. Growth and survival rates indicate that only Quercus seedlings can emer ge from the fern stratum in the absence of understory or overstory canopy d isturbance. A trade-off between persistence in low light and maximum growth in understory light levels was observed among species. The relative growth rate of Betula in terms of biomass and height was more responsive to light levels than were relative growth rates of Acer or Quercus, and the growth rate of Betula was higher than that of Acer and Quercus in all light levels . However, survival of Betula below the fern stratum was lower than surviva l of Acer and Quercus. The fern understory has the capacity to selectively filter tree seedlings a s they grow up through it because seedling species respond differentially t o the presence of fern cover. The selective filtering of tree seedlings by the fern understory results in a seedling spatial structure that reflects t he spatial heterogeneity of the fern stratum. The seedling pool below the f ern stratum has a lower seedling density and different species composition and size structure than the seedling pool in fern-free areas.