ï~~2004
THE MICHIGAN BOTANIST
395
The percentages of non-native species averaged 27% in a study of 22 National
Parks throughout the Midwest (Bennett 1996). A complete plant list for the
county park (the Natural Area and the adjacent 90 acres used for recreation) includes 29 non-native species (Table 7) out of a total of 203 (R.W. Freckmann &
Kirschbaum, unpublished data). Of these 29 species, 8 are considered "ecologically or potentially invasive" (Wetter et al. 1998), but to date have not been
found in the interior of the Natural Area. These data and patterns suggest that the
Powers Bluff Natural Area has shown unusual resistance to invasion by non-native species. Three factors may account for this:
1) High species richness has been suggested since at least the 1950s as an important factor in resistance to invasion (Elton 1958). The mechanism for
such a resistance phenomenon could be the lack of unfilled niches and/or
the extremely efficient use of resources by the current plant occupants. Either could prevent "new" species from becoming established.
2) Lack of large-scale anthropogenic disturbance. Disturbance creates an opportunity for colonization by producing unoccupied space and by freeing
up resources, and thus may be a key to the establishment of exotics
(Hobbs and Hunnecke, 1992). The last major disturbance at Powers Bluff
was 80-100 years ago.
3) Topographic isolation-The Bluff is surrounded by level terrain, the vast
majority of which has been in agriculture since at least the 1930s. Brothers & Spingarn (1992) explain the low levels of alien species in remnants
of old growth forest in Indiana, by suggesting that invasion by non-native
species is limited by dispersal. Crop weeds move only short distances
without human aid, and invasive species from urban or pasture settings are
limited by the long distances between the source site and these forest
patches. Powers Bluff may have avoided non-native invasion because the
source populations are limited.
CONCLUSION
Powers Bluff County Park and Natural Area is a vegetatively and geologically unusual area in Central Wisconsin. Erosion-resistant parent material and
wind-deposited loess merged to provide a diverse topography but relatively
mesic soil conditions. Both the Native Americans and the Europeans have impacted the plant community of the bluff through use of the native plants, logging,
and the introduction of species. The forest at Powers Bluff is an all-aged Southern mesic forest, but contains elements of the Northern mesic forest. We found
that the herbaceous vegetation is exceptionally rich and dominated by species
with a spring/summer phenological pattern. A major component of this late summer flora was wood-nettle, a species which is modal to wet forests. Its local
abundance is due to vigorous clonal spread and competitive abilities under mesic
site conditions.
The plant community of Powers Bluff remains intact, with very few invasive
