Western oakfern is a small, broad fern, found growing in the shade of thickets, in moist forests, rock outcrops and meadow areas in Denali (most common south of the Alaska Range crest). Leaves grow up to 30 cm tall from a rhizome. The plant is delicate looking, light green in color, and the stems have few scales. The pinnae are broadly triangular, the lowest pair typically much larger than the others. All leaves are fertile. On the back of the leaves, the brown sporangia are not covered with an indusium (unlike similar looking Cystopteris montana). These round sori are produced close to the margins of the leaves.
Gymnocarpium dryopteris is a perennial species, with deciduous leaves. Spores are produced in mid-summer.
Western oakfern, like all ferns, is spore-producing. Spores germinate into tiny haploid gametophytes, which can be fertilized to become new, diploid ferns. This is a tetraploid, fertile species, originally a hybrid from two different species. One of the parent species, G. disjunctum, is almost exclusively out-crossing, but the breeding system of G. dryopteris has not been studied (Kirkpatrick et al. 1990).
Gymnocarpium dryopteris is a widespread circumpolar species. In North America, its range is boreal from Alaska to Labrador and south into New England, with scattered localities in the western United States. The species is a tetraploid hybrid between the diploid species G. appalachianum and G. disjunctum, but G. dryopteris is now more widespread than either of its parent species (Pryer and Haufler 1993). Gymnocarpium dryopteris is abundant on the south side of the Alaska Range in Denali, in valleys and into the mountains, and also occurs (albeit less frequently) on the north side in the Park, particularly in and near the Kantishna Hills.
Occurring on a wide range of slopes, this species has been most often found on moderate inclines (but is disproportionately likely to be found on extremely steep slopes). Western oakfern has a preference for southern aspects. Specimens on south-facing slopes also occur at slightly higher elevations, on average. Plants in Denali were found at low to moderate elevations, with an average altitude of 563 meters.