Land management: using historical data to detect problem areas

Black Earth Rettenmund Prairie is one of the best prairie remnants in southern Wisconsin yet it still has lots of areas with troublesome shrubs. Sumac, aspen seedlings, gray dogwood, grapevine, etc. occur in certain sites across the prairie, although most areas are high-quality prairie and remain virtually shrub-free. This is so even though this preserve has been burned at least biannually since the mid 1980s (about 30 years).

Fortunately, the Nature Conservancy (TNC) left a historical legacy for this preserve that helps greatly in explaining this situation. At the time that TNC acquired the preserve in 1987 a map was prepared showing the shrub and tree distribution across the site. Also, a number of photopoints were taken, such as the one above of the North unit. Note the large stand of trees.

All of the trees and large shrubs on the North unit were removed by TNC volunteers in the late 1980s and early 1990s. Also, all the large trees along the fence rows and property boundaries were removed at the same time.

When Kathie and I started managing the prairie, we worked with volunteers to remove remaining shrubs from the Saddle, Narrows, and South unit. At present there are no large trees or substantial shrubs anywhere on the preserve. However, there are still many small shrubs, including quite a few sumac clones and a lot of prairie willow. There are also areas where tiny aspen shoots still come up.

As Kathie and I have worked on shrub control with volunteers over the past ten years I have had the TNC historical documents in mind. Once I felt competent in using ArcGIS, I realized that this program would be a perfect tool for connecting present-day shrub distribution with the historical data.

As described in an earlier post, I recently did a GPS survey of sumac on the whole prairie. I then brought these data into GIS as a layer. I then georeferenced air photos from the 1980s, and the TNC map from 1987, and brought them in as additional GIS layers.

Everything lined up beautifully! The points for the present-day sumac stands fell right on top of the historical shrub and tree data from the 1980s.

I should emphasize again that today there are no trees or large shrubs present anywhere at Black Earth Rettenmund Prairie. What the current shrub data show us is where the trees and large shrubs "used" to be.

These data show that the land retains "memory" of what had once been.

These data have important management applications. Historical data, especially old air photos (available for almost any site in Wisconsin) can help one anticipate where problems might arise, and remind one to pay special attention to these areas.