Article

Plant Community Monitoring at Fort Union Trading Post National Historic Site

closeup of a plant with tiny oval seeds on many delicate stems — Witchgrass (Panicum capillare ) is a native grass documented at Fort Union Trading Post National Historic Site during plant community monitoring.
NPS

Overview

Fort Union Trading Post National Historic Site was established to commemorate the role played by Fort Union as a fur trading post on the Upper Missouri River. The park is small (440 acres) and straddles the North Dakota/Montana border.

During the last century, much of the Northern Great Plains prairie was plowed for cropland, planted with exotics to maximize livestock production, or otherwise developed. This transformation over time from frontier to an agricultural environment at the park unfortunately led to the reduction of native plant species, followed by the introduction or invasion of exotic species.

Today the park is composed of mixed-grass prairie and riparian (riverside) forests along the Missouri River. The Northern Great Plains Inventory and Monitoring Network conducts long-term monitoring of plant communities on the park because it helps us better understand the current health of ecosystems and it can provide an early warning of undesirable changes.

two people crouched in the grass looking at plants in a square thats placed on the ground — Plant community monitoring at Fort Union Trading Post National Historic Site. Staff collect data on plant species present in a 1 x 1-m quadrat at five systematically placed locations along each 50-m transect.
NPS/J Ladd

What We Monitor

We established plant monitoring plots in two areas of the park: the Upland Terrace and the Bodmer Overlook Unit. The Upland Terrace surrounds the fort and has an extensive history of agriculture. The Bodmer Overlook Unit is north of the fort and terrace and is comprised of relatively intact native prairie in rolling hills. A restoration effort began in the 1990s with a goal of bringing back native mixed-grass prairie to the Upland Terrace.

A subset of the plots is monitored each year on a rotating basis. At each plant monitoring plot, we record the native and exotic plant species found and the vegetation cover for each species, a measure of how much ground is covered by the plants. In 2011, we combined our efforts with the NPS Northern Great Plains Fire Ecology Program to establish a single, coordinated set of vegetation monitoring methods.

single grass stem and leaf with horizontal seed head that looks like a hairy eyebrow — Blue grama (Bouteloua gracilis) is an abundant native grass at Fort Union Trading Post National Historic Site. Its seed head looks a little like a hairy eyebrow.
NPS

Plant Communities in the Park

The Fort Union Trading Post National Historic Site plant species list includes 353 species. In 2018, 97 unique plant species were recorded in monitoring plots and at least 190 plant species have been found since plant monitoring began. Forty-four exotic plant species have been identified; most are grasses including Kentucky bluegrass (Poa pratensis), smooth brome (Bromus inermis), and crested wheatgrass (Agropyron cristatum).

Several rare plants have been discovered in monitoring plots, including Alyssum-leaf phlox (Phlox alyssifolia) and white locoweed (Oxytropis sericea), which are both more common in Montana and Wyoming and reach the limit of their eastern distribution in western North Dakota.

The overall condition of the Upland Terrace warrants moderate concern, while the Bodmer Overlook Unit is in good condition with the exception of a higher than desired exotic plant abundance.

More Details

Graminoids (grasses, sedges, and rushes) accounted for most of the plant cover at the park, but forbs (flowering plants that are not woody and not a grass or grass-like), shrubs, and subshrubs (low-growing shrubs) were also present.
The most abundant native plants were western wheatgrass (Pascopyrum smithii) and blue grama (Bouteloua gracilis). Other common native plants were green needlegrass (Nassella viridula) in the Upland Terrace and theadleaf sedge (Carex filifolia) and prairie Junegrass (Koeleria macrantha) in the Bodmer Overlook Unit.
The Bodmer Overlook Unit has a high plant diversity, particularly on the hilltops. Exotic plant cover is higher than desired though; Kentucky bluegrass is particularly abundant.
The park successfully restored the Upland Terrace from an area covered with exotic grass to native prairie. However, exotic grasses are still more abundant than desired and plant diversity is still low, primarily due to a lack of forbs. Native mixed-grass prairie is usually comprised of many more forb species than grass species, though they tend to have much lower cover than grasses do.

two people standing beside a measuring tape in the grass facing away — While several rare plants have been discovered in monitoring plots, our methods are not designed for locating highly dispersed plants.
NPS

Did You Know?

When is a rare plant not a rare plant? Fort Union Trading Post National Historic Site lies across the state boundary of Montana and North Dakota. Large beardtongue (Penstemon grandifloras) and foothill bladderpod (Physaria ludoviciana) were both found on our monitoring plots and they are considered rare in Montana. However, we found these plants just across the state line on the North Dakota side of the park where they are not considered rare!