Plant community composition and structure at Mount Rushmore National Memorial: 2011–2017 summary report

Ashton, Isabel; Davis, Christopher; Swanson, Daniel

This report presents the results of vegetation monitoring efforts at Mount Rushmore National Memorial (MORU) by the Northern Great Plains Inventory and Monitoring Network (NGPN) and the Northern Great Plains Fire Ecology Group (NGPFire) from 2010-2017. Field crews collected data relating to species richness, herb-layer height, and abundance of individual native and non-native species, ground cover, seedling and tree densities, and site disturbance at 15 plots and across 57 plot visits. We compared our findings to the range of natural variability seen in the Black Hills and management targets to develop summaries of natural resource condition (Appendix C). We also explored how key metrics have changed over time from 2010 to 2017. In addition to annual monitoring, we also surveyed forest condition in 2010, 2012, and 2016 at 60 randomly located plots. We collected data on tree and seedling density, tree condition, disturbance, and the presence of exotic species of management concern, such as smooth brome and Canada thistle. Monitoring crews identified 232 vascular plant species, most of which are native. Exotic species are present in the park, but remain in low abundance. At most sites, the understory is dominated by bare rock and pine litter with few herbaceous plants in the understory. The average absolute plant cover was 28% and an average of 3.6 native plant species occurred within any given 1 m2 quadrat sampled. Common juniper (Juniperus communis), Richardson’s sedge (Carex richardsonii), Ross’s sedge (C. rossii), and poverty oatgrass (Danthonia spicata) were the most common species observed in monitoring plots. Both plant cover and native species richness in 1 m2 quadrats has increased since 2010. This suggests the thinning project in 2010-2011 was successful in changing forest structure and increasing plant diversity. One risk of the thinning project was increasing the susceptibility of the forest to exotic plant invasions. While exotic species are present in the park, we found them in such low abundance (<1 cover %) that it was difficult to detect an increasing trend. Forests are dominated by ponderosa pine (Pinus ponderosa) trees and the forest density in MORU is now similar to that of historic forest records. This was not the case during the forest survey in 2010, which was completed prior to a large scale thinning project where smaller ponderosa pine trees were removed from much of the park. There was a reduction in basal area following the thinning treatment, but was no change in forest structure between the 2012 and 2016 surveys. Chokecherry (Prunus virginiana), serviceberry (Amelanchier alnifolia), paper birch (Betula papyrifera), and ponderosa pine seedlings are common in MORU. There has been a significant increase in paper birch seedling densities since 2010. Future forest surveys will reveal how many of these seedlings persist and grow to become mature trees. The most frequently observed exotic species of management concern in forest plots was Canada thistle (Cirsium arvense), occurring in 58% of plots in 2016. Woody fuel loads in the park are high, averaging about 27 tons per acre, which exceeds the fire management program’s target range of 2-12 tons per acre. Overall, the forest condition has improved since the 2010 Hazardous Fuel Reduction thinning project. Surface fuel loads remain high, but forest density is now more consistent with historic conditions, the risk of crown fire has been reduced, understory plant cover and native plant diversity has increased, and deciduous species, such as paper birch, are successfully regenerating.

Plant community composition and structure at Mount Rushmore National Memorial: 2011–2017 summary report

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