The Arboretum’s Greene Prairie is a 47-acre restoration that lies along the southern boundary of the Grady Tract, south of the Beltline. Renowned botanist Dr. Henry Greene single-handedly planted more than 130 species from 1942 through the early 1960s. He took great care to match specific plant traits to the appropriate soil and moisture type; conditions grading from wet in the southwest to dry in the northeast. The result is an exquisite restoration—one of the finest shortgrass prairie examples in southern Wisconsin.
However, the prairie is not without its management challenges. Over the past several decades stormwater has coursed through approximately 10 acres of the southwest corner, displacing native plant species with a nearly monotypic invasion of reed canary grass (Phalaris arundinacea). In the drier northeast corner, a patch of invasive clonal shrub species such as quaking aspen (Populus tremuloides), smooth sumac (Rhus glabra), gray dogwood (Cornus racemosa), and black oak (Quercus velutina) grubs had grown to approximately 10 acres and were continuing to spread across the prairie, displacing native prairie plants. Most of the woody species at Greene Prairie are native, including all of the ones discussed here. However, due to their clonal growth they are considered invasive and are well adapted at filling in areas devoid of canopy cover, such as prairies.
Although the invasion by reed canary grass and woody species was apparent, the prairie had not been systematically sampled since 1992, so we did not know the diversity or abundance of native plants or if there were other invasive species that we should keep an eye on. With these issues in mind, in 2012 we set out to determine the quality of the plant community at Greene Prairie. I enlisted a graduate student, two undergraduates, and a volunteer to help sample. We had three goals:
- Systematically sample the entire prairie, recording all plant species found in more than 300, 1×1 m2 plots;
- Conduct an intensive sample of the plant community in the largest contiguous area that was invaded by native but invasive clonal shrubs and trees before removing them;
- After shrub and tree removal, repeat the intensive sample of the plant community in the same area as #2 to detect the effects of removal.
In the spring and summer of 2012, we sampled 142 1×1 m2 plots in the shrub-invaded area. We recorded 173 total species, of which 153 were native. The mean species richness (number of plant species) per plot was 12, and mean native species richness was 10. The plants encountered most frequently were species of non-native bluegrass (Poa pratensis and P. compressa). Other common species included native species: Alleghany blackberry (Rubus alleghaniensis), quaking aspen, rattlesnake-master (Eryngium yuccifolium), big bluestem (Andropogon gerardii), and mountain mint (Pycnanthemum virginianum).
In addition to richness, we also measured species abundance at each plot. Abundance was measured using the point-cover method. At the center of each plot, a rod was placed vertically and all the plant species it touched were recorded as “cover.” Plant species that touch the rod are more likely to have canopies (i.e. leaves and branches) that create cover over other areas of the plot. Bluegrass species, quaking aspen, black oak, big bluestem, sedge species (Carex), and smooth sumac were the most abundant.
In the winter of 2012–13, Quercus Land Sterwardship, Inc., was hired to remove the shrubs from the northeast corner of the prairie by using a forestry mower (a skid-steer with a large rotating blade on the front) as part of a Dane County Partners in Recreation and Conservation grant. Resprouts were also treated with herbicide the following year. We monitored the cut stumps and found little to no above-ground survival after herbicide application.
In the spring and summer of 2013, we surveyed 181 1×1 m2 plots throughout the rest of the prairie, including the area invaded by reed canary grass, in order to assess the status of native plant diversity. However, since there were very few native species found in the reed canary grass invasion area, it will be left out for the purposes of this discussion. The high-quality prairie held 201 species, 182 of them native. Mean species richness per plot in the high-quality prairie was 19 and mean native species richness was 18. Similar to the shrub-invaded portion of the prairie, non-native bluegrass was the most commonly encountered species. Native species such as rattlesnake-master, blue-joint grass (Calamagrostis canadensis), mountain mint, spiderwort (Tradescantia ohiensis), and saw-tooth sunflower (Helianthus grosseserratus) were also common.
Finally, in the spring and summer of 2016, we re-surveyed the area that had been invaded by shrubs to see if the plant community had changed over the last four years. Overall, thirteen more species were recorded after shrub removal (for a total of 186), all of them native. Mean species richness per plot had increased to 15 and native richness increased to 13. Some of the most common plants found in 2012 were still common: bluegrass, blackberry, big bluestem, and mountain mint. However, other species, such as spiderwort, Canada goldenrod (Solidago canadensis), and bee balm (Monarda fistulosa), had become more common. This was likely because more light reached the ground after shrub removal.
With the exception of blackberry, woody species decreased in occurrence from 2012 to 2016. The occurrence of quaking aspen decreased from 46 to 31 percent—the most significant of the woody species. Smooth sumac decreased from 30 to 20 percent and black oak grubs decreased from 16 to 4 percent. Gray dogwood stayed steady at 32 percent occurrence. Blackberry stayed surprisingly constant at 54 percent occurrence.
However, the number of plots where woody species occurred is just part of the story. Possibly more important is their abundance over the entire invasion area. The overall abundance of quaking aspen decreased from 32 percent to 13 percent of the sampled area. Shrubby oaks decreased from 22 to 6 percent, gray dogwood decreased from 15 to 9 percent, and smooth sumac decreased from 15 to 1 percent. In most cases, although shrubs and trees may have been found in more plots than we would have liked, they were only represented by a few stems rather than a high density of established plants.
Overall, the prairie is in great shape . . . notwithstanding the area invaded by reed canary grass. Although non-native species of bluegrass dominate the prairie, they do not seem to be diminishing native species richness or abundance. Removing shrubs from 10 acres of the prairie increased the number of native prairie plants. In fact, almost all of the new shrub growth came from underground rhizomes (horizontal underground stems) which clearly were not affected by herbicide. Although the woody species have begun to re-invade, they are generally less common than they were four years ago. Plans are already in place to remove them again this winter and use prescribed fire to help keep them at bay.
The reed canary grass monotype is a problem without an easy solution. Unfortunately, stormwater runoff is going to be part of the prairie for the foreseeable future. Our goal is to prevent the reed canary grass from spreading, and so far our monitoring efforts indicate that we are achieving that goal.
Management of Greene Prairie remains a top priority at the Arboretum. We reduce shrubs, judiciously treat with herbicide, use prescribed fire, and add native plant seeds, among other tools, to help keep the prairie one of the most exceptional restorations in southern Wisconsin now and into the future.
—Brad Herrick, Arboretum ecologist