Part of a series of articles titled Plant Community Monitoring in Northern Great Plains Network Parks.
Article
Plant Community Monitoring at Fort Laramie National Historic Site
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Overview
Fort Laramie National Historic Site is a small park (833 acres) in southwestern Wyoming near the western edge of the Great Plains and eastern edge of the Rocky Mountains on the boundary of the northern mixed-grass and short-grass prairie region. The park is a mosaic of disturbed old fields, native prairie, and riparian (riverside) forests along the Laramie and North Platte rivers that converge in the park. During the last century, much of the prairie within the Northern Great Plains has been plowed for cropland, planted with exotic species to maximize livestock production, or otherwise developed, making it one of the most threatened ecosystems in the U.S. The park plays an important role in protecting and managing some of the last remnants of native prairie in the area.
Plant diversity and composition in a mixed-grass prairie are determined by a number of factors, including fire and grazing patterns, habitat fragmentation, and weather fluctuations. Fort Laramie National Historic Site also has a mixed history of past land-use practices that have affected the number of native plants growing there. The Northern Great Plains Inventory and Monitoring Network conducts long-term monitoring of plant communities at the park because it helps us better understand the current health of ecosystems and it can provide an early warning of undesirable changes.
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What We Monitor
The Northern Great Plains Network established plant monitoring plots throughout the park in 2011. We monitor plots in grasslands and in riparian forests along both rivers. A subset of these plots is monitored each year on a rotating basis. Additional plots in riparian forests were monitored in 2014 and 2019 to assess the condition of these forests. At each plot we record the native and exotic plant species found, seedling and tree densities, and the vegetation cover for each species, a measure of how much ground is covered by the plants. When larger trees are present, we measure diameter of their trunks, and their status (live or dead) and condition (leaf-discoloration, insect-damaged, etc.).
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Plant Communities in the Park
There are 324 plants on the Fort Laramie National Historic Site plant list. In 2018, 77 unique plant species were found on monitoring plots; one plot in the riparian area could not be monitored because it had eroded into the river and was underwater. More than 200 plant species have been found on plots since plant monitoring first began in 2011. Native plant diversity is variable at the park, but it is low compared to other grasslands in the region. Graminoids (grasses, sedges, and rushes) account for most of the vegetation cover, but forbs (flowering plants that are not woody and not a grass or grass-like), shrubs, and subshrubs (low-growing shrubs) were also present.
Only one rare species, Emory's sedge (Carex emoryi) has been found on our plant monitoring plots since monitoring began. It is considered critically imperiled in Wyoming. Native grasses are abundant in some sites, but many areas have a high cover of exotic species. Fifty exotic plant species have been detected since 2011 and exotic graminoids are particularly abundant. In 2018, exotics were found in every plot monitored and they were abundant in upland plots. Two of the five plots monitored were dominated by exotics. Two species considered noxious in WY were present: Canada thistle (Cirsium arvense) and Scotch thistle (Onopordum acanthium).
The riparian forest at Fort Laramie National Historic Site is small (234 acres) and makes up about 28% of the park. It contains a fairly diverse assemblage of cottonwood (Populus deltoides), willow (Salix sp.), honey locust (Gleditsia triacanthos), and green ash (Fraxinus pennsylvanica). There is a high density and cover of exotic species in the understory and control efforts have been focused in these areas. Young cottonwoods have successfully established in 40% of the monitoring plots suggesting that successional transition to green ash and box elder (Acer negundo) dominated forests may be slow.
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More Details
- The most abundant native grasses at Fort Laramie National Historic Site are western wheatgrass (Pasopyrum smithii), needle and thread (Heterostipa comata), sand dropseed (Sporobulus cryptandrus), and blue grama (Bouteloua gracilis).
- Cheatgrass (Bromus tectorum) and smooth brome (Bromus inermis) were the most abundant exotics. Cheatgrass is a Eurasian annual grass that has been a part of the Northern Great Plains landscape for more than a century, but its invasion in the region has accelerated since 1950. Smooth brome is an exotic perennial grass. Both of these species threaten native plant diversity on the park.
- There are no significant trends (increases or decreases) in the number of native species over time, but plant diversity is threatened by an increasing cover of annual bromes (Bromus sp.).
- Mature cottonwood trees were found in higher densities than other tree species on forest plots. Younger cottonwoods were also fairly common. If the goal is to maintain cottonwood forests and willows along this area of the Laramie and North Platte rivers, management interventions such as watering, bank stabilization, and fencing could ensure that currently present young trees survive to maturity.
- In 2017, we found two species that were not on the park plant list: Carolina draba (Draba reptans) and prairie groundsel (Packera plattensis; also found in 2018).
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Did You Know?
Moderate disturbance in a prairie can contribute to diversity in grasslands, and some species grow better in slightly disturbed areas. However, high levels of disturbance, especially disturbances caused by humans, can allow exotic or weedy species to grow and spread. The most common disturbances at Fort Laramie National Historic Site have been animal-related, including grazing and horse droppings. Disturbances along the rivers are primarily related to flooding. Periodic flooding can actually help cottonwood regeneration in these areas.
For More Information
Visit the Northern Great Plains Network website to read more about plant monitoring at Fort Laramie National Historic Site.
Protocol Contact, Northern Great Plains Inventory and Monitoring Network: Isabel Ashton
Summary by Northern Great Plains Network, updated in 2019
Last updated: November 7, 2019