Pleistocene Relics in the Black Hills

Black Hills of South Dakota and Wyoming, USA.

Tucked away in the Black Hills of western South Dakota and northeast Wyoming are refugia -- safe havens -- for boreal plants that have managed to hang on since the Pleistocene when the boreal zone extended much further south.  Today they inhabit the shadiest coolest spots, often growing on steep north-facing slopes under heavy tree and shrub cover.  Finding them is not easy.

Much of the Black Hills is forested with ponderosa pine and the flora has strong affinities with the Rocky Mountains, but the cool shady gulches of the northern Hills are different.  Here one finds forests of white spruce and paper birch, more like the Great Lakes region, New England and Canada to the north.  There often are many boreal species in these gulches, part of the diverse flora that makes the Black Hills famous as a "botanical crossroads".

Distribution maps for white spruce, paper birch.  Note disjunct occurrences in Black Hills

in western South Dakota, northeast Wyoming.  All maps from Flora of North America.

Ground pine, Lycopodium annotinum.  Courtesy Jerzy Opioła.

Ground cedar, L. complanatum (= Diphasiastrum complanatum).
Courtesy Craig Althen.

Among the boreal plants in the Hills are three species of club moss or Lycopodium:  ground pine, ground cedar and tree club moss.  These are not true mosses but rather “fern allies”.  They are vascular plants, i.e. with a well-developed vascular system which the true mosses lack.  Like ferns they reproduce by spores, rather than by seeds as do “higher” plants.

The “fern allies” are actually a hodgepodge of plant groups, lumped together in the past because relationships among them weren’t clear.  Most current phylogenies split the former allies into two major groups:  the Lycophytes and the Euphyllophytes.  The first group contains the club mosses; the second includes true ferns, horsetails and whisk ferns, as well as seed plants, which split off from the rest more recently.

Phylogeny of vascular plants.  Click to view.  Modified version of this diagram.

The scientific name for the club moss genus, Lycopodium, means wolf’s claw, a very old name referring to the shape of the roots.  Club moss is used sometimes used as a homeopathic remedy in spite of its known irritation to mucous membranes.  The oils of the spores of some species are highly flammable, do much so that these club mosses were used in the past as flash powder for photography and stage lighting! (Reader’s Digest 1997).  Drawing below courtesy U. Minnesota Extension Service.

Note re scale:  the three club mosses in this post are less than 30 cm (12 in) tall.

Tree club moss, L. dendroideum.
Courtesy Kirisame.

Some boreal species, such as white spruce and paper birch, are fairly common in the Black Hills in the right habitat.  In contrast, the three club mosses occur only as widely-scattered small populations, and are managed as species of concern on Black Hills National Forest.

Distribution of Black Hills club mosses.

Patches thinks that if she waits long enough, the little boreal plants will reveal themselves.

Literature Cited

Reader’s Digest.  1997.  Magic and medicine of plants.  Reader’s Digest Association, Inc. [Recommended -- interesting, entertaining, and I especially like the carefully written “statements regarding the scientific evidence about the validity of the uses in folk medicine” for each species covered.]