In the past dozen or so years ecologists have begun to
realize that the future of a prairie remnant is strongly influenced by its past
history, and these “legacy effects” are important in restoration work.
I stumbled on these concepts of historical ecology while working
on the restoration of Black Earth Rettenmund Prairie, a State Natural Area. Before
it became a Prairie Enthusiasts’ site, this outstanding natural area was owned
by the Nature Conservancy (TNC), and TNC had extensive files on its history.
Also, there is an extensive sequence of air photos to follow the changes through
In 1937, when the first air photo was taken by the U.S. Soil
Conservation Service, the prairie was almost completely devoid of woody
vegetation. During the four decades between the early period and 1980 there was
a gradual increase in wooded area, and the rate accelerated between 1980 and
1987 (see graph below). The early shrub/tree development took place in the south part of the
prairie, which is adjacent to a fairly extensive undisturbed woodlot. By 1949,
the woody patches had increased, and some of the trees in the South unit were
now quite large. (The stumps from some of these trees still remain today.)
Although only approximate, the rate of increase in
woody vegetation has some resemblance to a growth curve. There is an extended
lag phase in the early years, but beginning in the mid 1970s there seems to be
a “tipping point” after which the site seems to “explode” with woody
vegetation. (The terms “tipping point” and “explode” were first suggested to me
by Randy Hoffman. According to him, left unchecked other prairie remnants in Wisconsin show
this same phenomenon.
The graph shows that at the peak, almost half of the site was wooded.
Upon acquisition in 1986, this preserve became a major
project of the DNR and TNC. Tree and shrub removal and frequent prescribed
burns by the DNR and TNC were used
in attempts to restore the prairie.
Although these efforts led to substantial success, the
legacy of the woody vegetation remains, considerably complicating restoration
efforts. New shoots still appeared in former aspen areas. Sumac and gray
dogwood were particularly troublesome, but brambles and grape were also a
problem. One former wooded area had been invaded by willow and hazel, which
have proved difficult to eradicate.
I gave a presentation on my work at the North American Prairie
Conference in Winnipeg, Manitoba, in 2012. My paper has now been published and can be downloaded as a PDF using this URL: This Blog Post is a brief summary.
|A view looking SW from Fesenfeld Road, showing
the extensive clone of aspen on the top and sides of the north unit. Note also
the large clone of sumac on the lower slope (above the cropped field). 1986 photo
from TNC archives taken by a real estate assessor.|
A view of the “Saddle” area of Black Earth Rettenmund
Prairie, probably taken in the late 1970s. In addition to the substantial
brush, the edge of a large aspen clone can be seen in the lower right corner.
Photo kindly provided by Cliff Germain, retired head of the Bureau of
Endangered Resources, Wisconsin Department of Natural Resources.
|Woody vegetation for different years as measured on air photos by GIS. (a) Thumbnails for 1937, 1987, and 2010. (b) Change in woody areas with time over the 73-year period.|
The present work suggests some principles of
importance for restoration ecology.
let things get out of hand. During the early stages, shrub establishment
may seem benign and not worth worrying about. This is obviously wrong. At first,
shrubs and trees are easy to remove. However, once the explosive phase of
growth has set in, not only is the biomass needed to be removed much larger,
but the whole population is growing at a vastly increased (quasi exponential)
importance of frequent fires. Fire is now a widely accepted component of
prairie and savanna management systems. However, there is a tendency to assume
that fire on a three- to four-year cycle may be sufficient. This is a mistake. Beginning
at the time of TNC acquisition, burns have been conducted on the various units
on the average every second year. Despite this frequency, shrub growth remains
a problem. Sumac, prairie willow, brambles (Rubus
spp.), and gray dogwood in particular continued to thrive. Aspen shoots
continue to appear in former aspen areas. In those areas where the legacy of shrub
growth remains, annual fire should be considered. The historical GIS data will help locate the areas where annual
fires should be carried out.
burns alone are not enough. At Black Earth Rettenmund Prairie, with its
biennial burn cycle, sumac, gray dogwood, prairie willow, etc. have remained or
have developed into problems. There are strong reasons to believe that all
clonal shrubs, even if they are native, should be eradicated by a multi-year
program of herbicide use. If the prairie is burned in alternate years in the
spring, then the shrubs should be treated with herbicide in the late fall of
the second year after the burn (that is, before the upcoming spring burn).
of clonal shrubs is essential, even if native, because annual stewardship
cannot be guaranteed to continue into the indefinite future.
effects must be taken into consideration when planning management
strategies. Areas that were woody in 1986, when restoration began, continue to
inhibit the established of true prairie species, especially graminoids. These
areas not only need tree and brush removal, but planting with seed collected
elsewhere on the site.
of herbicide use. The prejudices against use of herbicides remain. The
first seven years of restoration work at Black Earth Rettenmund Prairie were
wasted by trying to control shrubs without the use of herbicides. (The State Natural Area Board had refused permission to use herbicides.) Although trees can often be controlled without herbicide (by girdling), shrubs, with
their paramount ability to resprout, cannot be controlled without herbicide.
collecting and overseeding are essential. A site that has been wooded for
an extended period of time almost certainly has lost most or all of its warm
season grasses, as well as some of the more light-demanding forbs. The seed
bank for such a site is uncertain, and probably highly variable. Without
overseeding, restoration of such a site may take years. Even if there is a good
seed source near by, spontaneous overseeding will be random and, depending on
the quality of the seed, the success will be highly variable. Even 25 years
after the initial restoration work, formerly wooded areas at Black Earth
Rettenmund Prairie still remained impoverished, even though they have been reseeded with native grasses and forbs. Although each restoration
project must be analyzed individually, if the site was wooded it can safely be
assumed that overseeding is advisable. In fact, why not plant? Unless this is a
research site, it seems better to plant than to wait and see what happens.
Overseeding is easy and relatively inexpensive, and the sooner it is begun the
better. The only reason not to overseed may be if a local seed source is not
For some reason, Black Earth Rettenmund Prairie did
not become infested with red cedar, as have all the other hillsides in the
area. This was fortunate, because the shade imparted by red cedar is much worse
than that of aspen or shrubs. Cedar-infested sites quickly lose all of their
species diversity. In one study at Konza Prairie, herbaceous cover decreased
over 99%. (See Briggs,
John M., Knapp, Alan K. and Brock, Brent L. 2002. Expansion of woody plants in
tallgrass prairie: a fifteen-year study of fire and fire-grazing interactions. American
Midland Naturalist 147: 287-294.)
(I am grateful
to Wayne and Sharon Gaskill, stewards for TNC, for sharing with me their detailed stewardship