Diphasiastrum complanatum is a creeping perennial evergreen clubmoss with stalked strobili (cone-like reproductive structures), and flattened, scale-like leaves. Groundcone plants grow in well-drained areas in the lowlands to subalpine zone in Denali, from deciduous and mixed woodlands and shrublands to open dry, grassy sites. The horizontal stems are about 2 mm wide, the upright stems are forked, divergent, flattened and up to 35 cm tall, much branched in the fan-like shape characteristic of the genus. The scale-like leaves are flattened, arranged in four rows, the ventral leaves much smaller than the dorsal leaves. It grows in different habitats than Diphasiastrum alpinum (an alpine plant of moist tundra, meadows and snowbeds) and can be distinguished from alpine clubmoss by its stalked strobili. The other clubmoss in Denali with stalked strobili is Lycopodium clavatum, but that species has tapering linear leaves with hair-like tips.
Diphasiastrum complanatum is an evergreen perennial. Spores are produced in mid-summer.
The visible portion of Diphasiastrum complanatum is a diploid sporophyte. These plants are produced from a haploid lifecycle, which lives underground and is entirely dependent on mycorrhizae for its survival. The haploid gametophyte self-fertilizes and produces an independent, photosynthetic sporophyte (the plant described and pictured). sporophytes produce single-celled spores, which are wind-dispersed and develop into new underground gametophytes.
The larvae of a micro-moth, Catoptria furcatellus, feed on the leaves of species in the family Lycopodiaceae (Emmet 1991 In Biological Records Centre 2008).
The clubmosses (Diphasiastrum and Lycopodium are used by the Dena'ina to make an eye wash (Kari 1995). Other species in these genera which occur primarily in forested areas were used medicinally by indigenous people throughout North America and Europe (Moerman 1998, Schofield 1989).
Groundcedar is a widespread circumpolar boreal-montane species. It occurs across boreal North America from coast to coast, including the northern tier of continental U.S. states. This species occurs in suitable habitat across Alaska from the southern slopes of the Brooks Range southwards (including Denali), although it is absent from the southern Alaska Peninsula and Aleutian chain. It occurs much more frequently on the north side of the Alaska Range in Denali than on the south side.
Throughout its global range, ground-cedar occurs from 0–2000 m elevation. Most occurrences in Denali were below 900 m, although it occurs at 162- 1320 m with an average of 642 m. Its altitudinal distribution is most similar to the clubmosses Lycopodium annotinum and L. clavatum in the park. Slightly over half (59%) of the 97 occurrences in Denali are on slopes with inclines greater than 5 degrees, of these 74% were on south-facing slopes. It occurs most often on mild inclines (5 – 18 degrees) with an average of 9 degrees.