Goat prairies at Effigy Mounds: eight plots show trees moving in

Goat prairies are plant communities on bluff edges where the soil is shallow, and at Effigy Mounds National Monument in Iowa they are slowly giving way to trees. The National Park Service's Heartland Inventory and Monitoring Network has tracked eight of them, and what it found points to a need to thin the tree cover.

The soil often becomes thinner closer to the bluff edges where goat prairies are found. NPS

What a goat prairie is

Goat prairies grow on southwest-facing bluff edges where the soil is shallow and limestone or sandstone shows at the surface. They are also called oak openings, juniper glades or hill prairies. Oaks and junipers are typically scattered across them, and plants of prairies, savannas and woodlands grow there. The park has such varied plant communities because of its geology, history and location on the Mississippi River.

Fire used to keep trees out and the prairies open. With little disturbance such as fire in recent times, trees have moved in, and the goat prairies may be turning into deciduous forest.

The monitored sites are a sampling of the park's goat prairies. NPS

How they are monitored

The network began monitoring goat prairies at the park in 1997. Eight permanent plots were set up in 1997 and 2001. Staff measure how closed the tree canopy is, how many plant species are present, and how much ground each species covers. The data from the years when all eight plots were monitored (2001, 2005 and 2017) were compared with weather and fire history.

What the data show

• Fire and weather. The fire record showed very little fire at the eight sites in this period, and the weather was normal to wet except in one year.
• Trees. There were more trees than expected, though each site was unique. Across the sites the monitors found 24 tree species, with oaks dominant. The canopy was 80 percent closed in 2005 and 2017, more than in 2001, which suggests the prairies are becoming forests. Tree basal area, the area the trunks occupy, also rose. Small snag trees, meaning dead trees, held steady, but large snags increased, which the authors read as more trees living to old age.
• Ground flora. Each prairie's low-growing plants were distinct. The number of species at each site fell, but the total across all sites rose, suggesting the prairies are changing in their own ways and growing different from one another. Most species were native, with a few nonnative plants.

Mean percent canopy closure in goat prairie sites. NPS

Keeping them open

The park's goat prairies are resilient but are slowly moving toward deciduous forest. Shallow soils slow tree growth and the bluff-edge position keeps sunlight on prairie and savanna plants, but the prairies would benefit from reduced tree cover. Thinning by cutting, prescribed fire or both would help conserve them, and continued monitoring will show how well tree-reduction work does.

Long-term monitoring will track the status of the prairies. NPS

Sources

• National Park Service, Effigy Mounds National Monument, "Goat Prairie Monitoring at Effigy Mounds National Monument" (https://www.nps.gov/articles/000/efmo_goatprairies.htm), public domain as a work of the U.S. government. The page links a full report, not reproduced here.
• National Park Service; rewritten in hubnx's own words.