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Karst Geology Supports Rare Plants

Arkansas Natural Heritage Commission - Friday, March 29, 2019

by Theo Witsell, ANHC

There are many different types of native plants found in Arkansas. Some of those species are found only in soils derived from limestone, dolomite, chalk, or other high-pH geology. These soils, called calcareous, are rich in calcium carbonate and the plants that live there are generally called calciphiles, meaning chalk-lovers.

In Arkansas, these plants are found primarily in the Ozark Highlands and in parts of the Gulf Coastal Plain (such as the Blackland Prairie Ecoregion). Many are widespread, though some are rare and restricted to certain uncommon habitats like glades, bluffs, or rich, moist forests. A more exclusive subset of these, however, are found only in association with specific karst landscape (see Underground tales: karst) features like springs, fens, cave mouths, sinkholes, and sinkhole ponds.

Fens are calcareous seepage meadows, fed by mineral-rich groundwater that seeps up through the ground over a broad area. Confined to areas with karst geology rich in springs, seeps, and caves, fens are a rare habitat type in Arkansas and support a number of wetland plants typically found to our north. These include inland sedge (Carex interior), prairie straw sedge (Carex suberecta), rigid sedge (Carex tetanica), willow-herb (Epilobium coloratum), fen rush (Juncus subcaudatus), Allegheny monkeyflower (Mimulus ringens), Virginia mountain-mint (Pycnanthemum virginianum), capillary beak-rush (Rhynchospora capillacea), Riddell’s goldenrod (Solidago riddellii), and shining ladies’-tresses (Spiranthes lucida). Small-fruit primrose-willow (Ludwigia microcarpa), a rare fen species in the Arkansas Ozarks, is typically found in the southeastern Coastal Plain.

Other karst landscape features that support some rare and out-of-range plants are cave mouths and sinkholes. These features often create cool, moist microhabitats that maintain relatively constant conditions, even in summer when the surrounding habitats become hot and dry. This allows them to support species that are typically found in cooler climates to the north and east of Arkansas. Some of our only occurrences of northern ferns like interrupted fern (Osmunda claytoniana), spinulose wood fern (Dryopteris carthusiana), log fern (Dryopteris celsa), and Goldie’s fern (Dryopteris goldiana) are found in these habitats in the eastern parts of the Ozarks.

Sinkhole ponds, formed as sinkholes become plugged and hold water, are one of the rarest karst features in Arkansas. Some sinkhole ponds are ancient features and support a number of species not found elsewhere in the Ozarks. These species may be ancient relicts left over from a time when climatic and ecological conditions were different and these species were more widespread. Example of such out-of-range species include overcup oak (Quercus lyrata), pin oak (Quercus palustris), and cypress knee sedge (Carex decomposita). Sinkhole ponds are our only sites in Arkansas for a few rare plant species including lowland yellow loosestrife (Lysimachia hybrida) and creeping manna grass (Glyceria acutiflora).

Karst ecosystems often support rare, threatened, or endangered plant and animal species. Ecosystems located within karst topography can be disrupted by even the smallest change in water quality, making understanding and protection of these areas a high priority.


Top left -- Allegheny monkeyflower (Mimulus ringens). Photo by John Gwaltney,

Middle left -- A fen in Sharp County, Arkansas. Photo by Eric Hunt.

Bottom left -- A cavern sinkhole at Blanchard Springs Cavern, Stone County, Arkansas. Photo by Eric Hunt.

Above right -- Cypress knee sedge (Carex decomposita). Photo by John Gwaltney,

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