Dr. Mark Davis smiled at the dismayed looks on his audience’s faces. He had just announced that the decision to restore any particular ecosystem is not a scientific decision and should not be couched in scientific terms. Hearing him make just this statement, many people might not guess that the Macalester College professor has dedicated a great deal of his research to issues of ecological restoration and is currently involved in restoration projects. However, his position became clear as he continued the talk he was giving to interns at Cedar Creek Natural History Area.

The decision to restore an attribute, or attributes, of an ecosystem is fundamentally a judgment call on our parts, he said, but it is still an important decision to make. We may wish to restore an ecosystem so it once again provides us with a service we value (purifying water or decomposing litter, for example), so it is more diverse, so it remains similar to they way it was during some historical period, so it can remain a habitat for plant and animal species that we care about, or perhaps so it is aesthetically pleasing. All of these possibilities reflect our values.

Where science comes in to play is in guiding the methods for accomplishing the restoration. “The field of ecology becomes important during the implementation stage of a restoration, when ecological knowledge and understanding are necessary to successfully manipulate and manage an ecological system to achieve desired goals,” explained Dr. Davis. It is very unlikely that a restoration effort will succeed if it cannot use scientific findings as instructions.

This means that many experiments must be done to try out and to evaluate different methods for restoration Dr. Mark Ritchie, of Syracuse University, told me. “This whole discussion of restoration, the questions often get very myopic,” he says. “It is treated as an issue of management and general [scientific] principles get forgotten.”

Cedar Creek Natural History Area, a long-term ecological research site in central Minnesota where both Dr. Davis and Dr. Ritchie currently have research projects, turns out to be an excellent place to experiment with restoration techniques, for as much as 90% of the land here could be considered “disturbed.” That is to say, it has been modified by human use, mostly for agricultural purposes like raising crops or grazing cattle. These particular disturbances have now ceased, but their repercussions are still noticeable, and researchers of Cedar Creek are taking advantage of this to perform studies that will hopefully have applications for the field of restoration ecology.

An abandoned agricultural field	In many of the fields at Cedar Creek native plants are declining.

Dr. Jean Knops, of the University of Nebraska, has been working in grasslands at Cedar Creek that were once old agricultural fields. The fields were abandoned at different times and they have different compositions of plants, but Dr. Knops has noticed a common trend amongst them. According to his data, which he presented at the meeting of the Ecological Society of America in July, the composition of the old fields has been changing. All of the fields are showing increasing numbers of exotic invading species while the number of native annual plants they contain is decreasing.

Dr. Knops believes that these changes are due to the combined effects of the homogenization of the soil by plowing and fertilizing for crops, increasing nitrogen in the atmosphere from industry, and also the near absence of fire. Historically both Native Americans and early settlers burned this area very frequently – perhaps even annually – and many native prairie species rely on fire to maintain them. However, fire suppression became the norm in the 1930s because of the dust bowl, and in most places it continues today. With no fires coming through dead plant mass can build up on the ground, which changes the availability of nutrients and facilitates the invasion of exotics, Dr. Knops thinks.

What all this means is that we can’t simply let a field go wild and expect it to return to the state it was in before it was cultivated. “We’ve got to manage nature,” Dr. Knops summarized for me. He recommends more frequent burning and adding the seeds of native plants.

Another native ecosystem that was historically maintained by burning is the oak savanna. Savanna restoration experiments, focusing on burning, have been going on at Cedar Creek since the 1960s. But, this year the research area has received a new grant to expand these projects, so everyone’s excitement about savannas is renewed.

What exactly is an oak savanna? A savanna is like the centaur of the ecology world; instead of being half horse and half human, a savanna is half prairie and half woodland. In fact, it can serve as a transition between the two ecosystems. Savannas are very open with about one large, full oak tree – more specifically bur oak, whose leaves have rounded lobes - every 100 feet. Because of the spacing, the canopy is never closed, and sunlight reaches to the ground allowing prairie plants (and a few select woodland plants) to grow.

Oak savanna	Oak woodland

Research has shown that large bur oak trees are somewhat resistant to fire, but the oak seedlings and most other woodland plants aren’t. The frequent, low-intensity burns of the past reduced the number of seedlings that survived to be trees but left the full-grown oaks unharmed, thereby maintaining the spaciousness that characterizes a savanna. The fires also favored the prairie plants in the under story. With the advent of fire suppression many more seedlings survived, and other woody plants could also grow in the fields of the savannas. So, what was once savanna became woodland, thick with growth.

Many scientists have suggested that if an ecosystem was changed because a natural process, in this case fire, was taken away, then simply reintroducing the process should restore the system to its original state. But, “is it as simple as just burning?” asks Dr. Clarence Lehman, director of Cedar Creek’s burn program. Increasing scientific evidence suggests that it isn’t. The scientific term for this is hysteresis; fundamentally it means that if a system changes because its conditions change, changing the conditions back won’t necessarily change the system back as well.

Dr. Lehman has come up with his own hypothesis concerning the process of burning oak woodlands to restore them to savannas. In the open savanna with only prairie plants as fuel most fires won’t get hot enough to damage oak trees, and any dead trees that burn up will be far apart from one another, Dr. Lehman reasoned. Then he imagined a crowded woodland in contrast. Any fallen trees in the woodland are bound to be near other trees, which creates the likelihood that when the dead trees burn, their heat and fire will damage and scar the trees that are near them. These scarred trees will be less healthy, have shortened life spans, fall, burn, and damage other trees in turn. A computer simulation that Dr. Lehman created based on this reasoning showed that by just burning an area that has become thick with trees you could end up killing all of the trees, including the ones you had wished to save.

Fallen branches and dead trees can damage other trees during burns.	Trees damaged by fire

This is still just a hypothesis, but it has many people at Cedar Creek worried. “None of us were thinking about it,” says Dr. Lehman. Now, scientists and students are working out in the field and also with old aerial photos to reconstruct the scarring history of the oaks at Cedar Creek, trying to determine what effect burns have had up to this point. In the future there will be an effort to keep careful records before and after each prescribed burn detailing the conditions of the trees.

Multiple scientific papers have also been published reporting that simply reintroducing fire to degraded oak savannas through prescribed burning was not a sufficient restoration technique. Several of these papers come from researchers at the Necedah National Wildlife Refuge and Quincy Bluff and Wetlands State Natural Area - both in central Wisconsin. Their findings were especially true for land that had gone unburned for substantial amounts of time. A similar pattern is noticeable at Cedar Creek. The burned areas all have more open canopies, but while those that started out by being fairly open are becoming truly savanna-like, others that were very woody or brushy are filling up with tangles of shrubs.

To understand what may be going on it is important to know that plants in fire dependent ecosystems generally have two possible modes of living. Some, like the bur oaks, try to resist the fire above ground. Many, however, live by having a substantial portion of their bulk underground. Most prairie plants use this strategy, but so do some woody shrubs, and so do pin oaks, another type of oak (whose leaves have pointed lobes) that is not traditionally considered a savanna tree.

Before fire is reintroduced to former savanna land most of the prairie plants are pushed out because the tree canopy is too thick and light doesn’t make it to the under story. Shrubs and trees that are better at reaching the light take the prairie plants’ places. When the land is burned many trees are killed and the canopy opens up, allowing light through. The plants that survived underground send up new shoots, which do extremely well because of all the newly available light, and the area becomes packed with brush and saplings. This is why at Cedar Creek many of the burned areas are full of hazel, raspberries, and the invasive buckthorn, and also why in some areas pin oaks are actually taking over from the bur oaks.
Dr. Davis and some of his student have proposed that this oak scrubland could be what is called an alternative ecosystem state. Dr. Katharine Suding and colleagues reviewed the concept of the alternative state this year in the journal Trends in Ecology and Evolution. The review explained that research has shown that some modified ecosystems cannot be changed back to their original states by restoring original conditions for various reasons including: the loss of species, changes in dominance amongst species, invasion by exotic species, changes in the way parts of the landscape are connected, changes in the ways nutrients are used, and others. When this sort of rewiring occurs the ecosystem may become stable in its new form, in which case it may be resistant to being changed back.

A burned area that is turning into scrubland	Some woody species take advantage of the increase in light to sprout and grow rapidly.

To evaluate this possibility further and to look for better methods of restoring a degraded savanna Dr. Davis’ students are using a multi-pronged approach. In addition to burning they are experimenting with removing all the brush and also seeding areas with savanna species. Though the experiment is still small, they have set it up so that each of the strategies is tried by itself as well as in combination with the others. The students hope that in time they will be able to increase the amount of area they are testing.

All the difficulty in manipulating the landscape raises a very particular and nagging question: was all this land actually oak savanna in the past? It is not really possible to say for sure – some old remaining trees definitely look like they were once savanna trees, but there are other areas at Cedar Creek that look like they may never have been burned at all.

There are several different types of oak growth possible, and probably a bit of each of them historically existed at Cedar Creek. This includes the classic oak savanna and the true oak woodland, as well as oak scrubland (as Dr. Lehman pointed out to me, “people don’t really like scrubland, but maybe birds do,” and indeed Dr. Davis has found that the highest density of bird species at Cedar Creek is found in the scrubland). This could also include ‘pin oak parkland,’ which looks like a savanna in its openness but is not technically a savanna because the oaks are pins not burs. This sort of system is maintained by the grazing of herbivores.

A restored bur oak savanna	Bur oak leaves (left) and pin oak leaves (right)

Testing the role of herbivores in restoration is yet another direction in which Cedar Creek research is heading. Herbivores could play an important part in renewing savannas, and they may be equally important in the grasslands. In most areas of the upper Midwest, an abandoned, unburned field will fill with tree seedlings and become forest fairly quickly. However, at Cedar Creek many of the unburned fields have managed to resist being invaded by trees for an impressively long time. Part of this may be due to the intense competition for resources as the soil at Cedar Creek is rather sandy and bereft of nutrients. But, Dr. Ritchie suggests another possibility: the deer. Cedar Creek has a very high deer population, and deer eat small woody plants, so they may be chomping down almost every tree seedling that crops up in the fields. Dr. Ritchie is testing this idea by fencing off areas in the fields, in the forests, and in the savannas with fences that will keep deer out.

Dr. Ritchie is also in charge of a sheep experiment at Cedar Creek. That’s right; sheep are the animals of choice for Cedar Creek’s first foray into researching how grazing changes the composition of plant communities. This experiment is new and has no published results yet, but several people who sorted the flora clipped from the grazed plots told me that the plants they found were so different from what they had intuitively expected, they had worried briefly that they were sorting plants from the wrong field.

A fence to keep out deer	Another deer exclosure

Historically, it is believed that this area was grazed by bison and by elk rather than by sheep, and ideally someday Cedar Creek will have experimental bison roaming in the fields and under the oaks. However, “this will involve a lot of boring logistics,” admits Dr. Ritchie, including extra funding, veterinarians, and fences. Dr. Lehman maintains that bison require much less care than cows (bison can stay outside in the winter, for example). But, he cautions, he has heard stories that a bison can jump over a 6-foot fence from standing!

It is possible to see right away how some manipulations change a landscape, but it doesn’t do anyone much good to create a system that looks nice if it doesn’t function reliably. This is why there is also a great deal of research at Cedar Creek evaluating how the cycle of nutrient acquisition and loss changes in restoration attempts as well as how below ground communities like symbiotic fungi or important soil bacteria respond.

Ecosystems are full of complexity, and it appears that ecosystem restoration is too. The findings of all these experiments at Cedar Creek, as well as others that involve techniques such as fertilization, herbicide, and species diversity must be synthesized to make them truly useful as restoration guidelines. Then, throw in global aspects like climate change and greenhouse gases, and things become even more complicated. “We’re figuring out what the questions are and setting up experiments,” says Dr. Ritchie. “It’s a new direction.”

As long as we continue to value certain ecosystems and wish to maintain them, the path in this new direction could lead a long way into the future, and at least in some experiments at Cedar Creek that future will be burning bright.