Cedar invasion on hill prairies in the Midwest

A comparative survey of 1949 and present-day air photos will likely show that in our area the south- or south-west facing slopes that were once hill prairies are now red cedar (Juniperus virginiana) forests.

John Curtis (Vegetation of Wisconsin), who did not have the benefit of the last 50-60 fire-free years, called these "cedar glades". Given the lack of fire since Curtis day, it is likely that all his cedar glades no longer exist and are now close-canopy cedar forests.

To see an example of this sort of closed canopy cedar landscape, take a drive west on US 14 from Cross Plains to Black Earth. Every hill on the north side of the road (except Festge Park, which has been restored) has a dense cover of cedars in the region of the hill that was a prairie in Curtis' time.

Study these cedar-covered hills and try to imagine them burning almost every year, as they almost certainly did in 1949 (or earlier). Somebody, probably the landowner, simply lit a match.

It appears that cedar encroachment eventually reaches a tipping point, after which the whole hill prairie disappears almost in a flash. If cleared and burned quickly enough, the prairie might still be saved, but a dozen years of closed-canopy cedar forest and all prairie species are likely gone. (The cedar growth rate can be quantified by GIS from a series of air photos, using a technique that I described in detail in this paper.)

However, it is interesting that today not all south- and south-west slopes have closed canopies. Some are open enough (despite lack of fire) that Curtis would still call them "glades." Initial cedar establishment is probably a random event, depending primarily on transport of seeds by birds. And some sites may be more favorable to cedar growth than others.

Cedar hillside that has not yet closed in. 20 years ago this hill was completely open.
Rocky Dell Road near US 14, east of Cross Plains, WI

In fact, it seems that not all south- or south-facing slopes even get invaded by cedar. In some cases, other woody species such as buckthorn, honeysuckle, walnut, and or elm take over. Perhaps the soil, steepness of the slope, or the aspect differs in those hills. But most south slopes in our area are cedar covered.

At Pleasant Valley Conservancy, when we started clearing the south slope in 1997, there was a scattering of cedar, and a scattering of red pine (planted by a former landowner), but large buckthorn, honeysuckle, and slippery elm were more common. (We removed 100-year old elms and lots of large buckthorns.) But there were still open areas, including two prairie remnants that were completely free of woody vegetation. It's not clear why these areas had survived, and in fact they were in such good shaped that it only took a single burn to bring them back to life. (See photos)

It is uncertain if a closed-canopy cedar hillside can be restored to prairie. Since the original prairies on these hillsides had probably been there since pre-settlement times, it is likely that there is still a seed bank. However, the initial flush of prairie growth after clearing would be from the rootstock, primarily of grasses. According to John Weaver's work in Nebraska, the rootstock of some species of grasses do not survive very long although others may survive for some years, gradually dying out. Since many of the former prairie remnants in our area have been under closed canopy and essentially in the dark for at least 50 years, it seems likely that there is little of the original prairie left. To recover prairie now will require extensive seeding.

It takes a lot of effort to restore hills with closed canopy. The best example is the Nature Conservancy's Spring Green Preserve, which has been undergoing restoration since 1971 (more than 40 years!).  Kathie and I participated in large work parties at Spring Green back in the late 1990s and early 2000s. Some of these involved up to 100 people, with often 7 or 8 chain saws running simultaneously. The hills are now clear and are a great example of what can be done. And the prairies on these hills have recovered surprisingly well.

South slope prairie remnant. Bird's foot violet, lead plant, prairie dropseed,
and other conservative species had survived here without help,
and flourished after the area had been burned.

Another prairie remnant on the south slope. The woody vegetation surrounding this prairie is evident. (All later removed)
This photo was taken in the fall after the first burn the previous spring.
One burn is all it took to stimulate the growth of this great stand of Indian grass.

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