Plant community composition and structure monitoring for Fort Laramie National Historic Site: 2011-2015 summary report

Ashton, Isabel; Davis, Christopher

The Northern Great Plains Inventory & Monitoring Program has been monitoring vegetation in Fort Laramie National Historic Site since 2011. This report summarizes those data from 30 locations monitored from 2011-2015. We use these data to explore status and trends in upland and riparian plant communities and the occurrence of rare plants. A summary of the current condition (2011- 2015) and short-term trends in plant communities at Fort Laramie is found in Table E-1. 1. What is the current status of plant community composition and structure of FOLA grasslands (species richness, cover, and diversity) and how has this changed from 2011- 2015? FOLA plays a vital role in protecting and managing some of the last remnants of native prairie in the area. Native grasses, such western wheatgrass and needle and thread, are abundant in some sites, but many areas have a high cover of exotic plants. Native plant diversity is at a low level compared to other grasslands in the region (Table 10), but diversity is spatially variable. We found no significant trends in native diversity or evenness from 1998-2015, but both are threatened by the increasing cover of annual bromes (Table 10). Annual bromes are the most abundant exotic plant species in FOLA and present the largest challenge to FOLA. Continued control efforts will be necessary to maintain native prairie within FOLA. 2. How do trends in grassland condition correlate with climate? Native diversity tended to increase in wet years. The large variability in the climate of FOLA has made it difficult to discern strong patterns linking temperature, precipitation, and plant community structure (e.g. exotic cover, diversity). A longer time series of vegetation data will make it easier to elucidate trends in the future. 3. What, if any, rare plants were found in FOLA long-term monitoring plots? We identified 1 rare plant species in FOLA between 2011 and 2015. This plant is found in such low abundance and in such few plots, it will take many years to determine any trends in rare plant cover. Since to our knowledge, the last rare plant survey was completed in 2004, we recommend targeted surveys of rare plant species be considered when funds are available. 4. What is the composition and structure of riparian forests at FOLA? The riparian forest in FOLA is a fairly diverse assemblage of cottonwood, willow species, honey locust (Gleditsia triacanthos), and green ash. Cottonwoods of all age classes are present (Table 10). Exotic species are common in the understory of the riparian forest, and the current control measures may help reduce their abundance in the future. Young cottonwoods have successful established in 40% of the monitoring plots. This suggests that the successional transition to green ash and box elder dominated forests may be slow.

Plant community composition and structure monitoring for Fort Laramie National Historic Site: 2011-2015 summary report

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