Montana Field Guide

The sand prairies constitute a very unique system within the western Great Plains. The unifying and controlling feature for this system is that coarse-textured soils predominate and the dominant grasses are well-adapted to this condition. In the northwestern portion of the system’s range, stand size corresponds to the area of exposed caprock sandstone, and small patches predominate, but larger patches are found embedded in the encompassing Great Plains Mixed Grass Prairie, and usually occupy higher positions in local landscapes where former caprock formations have eroded into more subdued and planar topography. In most of eastern Montana, substrates supporting this system have weathered in place from sandstone caprock. Soils can be relatively thin or deep due to varying amounts of downslope movement of weathered sands. Needle and thread (Hesperostipa comata) is the dominant grass species. Other frequent species include little bluestem (Schizachyrium scoparium), often occurring with threadleaf sedge (Carex filifolia) and dominating both sandy sites and actively eroding sites. Prairie sandreed (Calamovilfa longifolia), sand bluestem (Andropogon hallii) and big bluestem (Andropogon gerardii) are sporadically distributed and found generally on the coarsest-textured sands. Other graminoids include bluebunch wheatgrass (Pseudoroegneria spicata), sun sedge (Carex inops ssp. heliophila), and purple threeawn (Aristida purpurea). Characteristic forbs differ by occurrence, but species of scurf pea (Psoralidium species) and Indian breadroot (Pediomelum) species are common. Communities of silver sage (Artemisia cana ssp. cana) or skunkbush sumac (Rhus trilobata) can occur within this system. Wind erosion, fire and grazing constitute the other major dynamic processes that can influence this system.

herbaceous, lowland, sand soil texture, Calamovilfa longifolia, Hesperostipa comata

The distribution, species richness and productivity of plant species within the sand prairie ecological system are controlled primarily by environmental conditions, in particular the temporal and spatial distribution of soil moisture and topography. Soils in the sand prairies can be relatively undeveloped and are highly permeable. Soil texture and drainage, along with a species' rooting morphology, photosynthetic physiology, and mechanisms to avoid transpiration loss, determine the composition and distribution of communities/associations within the sand prairies. Soils are also susceptible to wind erosion. Blowouts and sand draws are some of the unique wind-driven disturbances in the sand prairies, which can profoundly impact vegetation composition and succession within this system. In most of eastern Montana, substrates supporting this system have weathered in place from sandstone caprock; thus the solum is relatively thin, and the wind-sculpted features that are present further east, particularly in Nebraska, do not develop. Graminoid species dominate the sand prairies, although relative dominance can change due to impacts of wind disturbance.

This system is found primarily on sandy and sandy loam soils, generally in areas with a rolling topography, although it can occur on ridges, midslopes and/or lowland areas within a region. In Montana, occurrences are associated with the Great Plains Mixed Grass Prairie, usually occupying higher positions in local landscapes, because sandy members of predominantly marine shale formations constitute the highest and most weathering-resistant points in the landscape.

Needle and thread is the dominant graminoid within this system, regardless of disturbance, and is found on sands and, to a lesser extent. on sandy loams. This species increases in cover with disturbance relative to other graminoids. Prairie sandreed is the only other robust graminoid occurring in abundance and is usually associated with the coarse-textured substrates, becoming nearly a monospecific dominant on sands. Other graminoids such as sun sedge, threadleaf sedge, sand bluestem, little bluestem and big bluestem may be present. Big bluestem and sand bluestem are found only as small patch occurrences in easternmost Montana. Little bluestem occurs preferentially on sites derived from sandstone or porcellanite.

Many of the warm-season graminoids such as western wheatgrass (Pascopyrum smithii), blue grama (Bouteloua gracilis), Sandberg’s bluegrass (Poa secunda), prairie junegrass (Koeleria macrantha) and threadleaf sedge extend to the Rocky Mountain Front occurrences as dominant components on appropriate sites or as a response to disturbance (Kudray and Cooper 2006).

Characteristic forbs differ by region, but species of scurf pea and Indian breadroot are common. Narrowleaf purple coneflower (Echinacea angustifolia) can occur on sandy sites. Very diffuse patches of skunkbush sumac andhorizontal juniper (Juniperus horizontalis) are found on shallow sandy soils, often associated with breaklands. Other shrubs occasionally found within this system include silver sage. Soapweed yucca (Yucca glauca) occurs on sandy upland sites and bottomlands, but usually with less than 15 % cover. In some cases, Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) occurs within or adjacent to this system.