Discovering Oak Woodlands: History and Theory.

When volunteer stewards girdled maples in an ancient oak woodland, Forest Preserve staff approved, but some observers expressed strong criticism. Research, study, education and consensus were needed.

We are just beginning to learn the secrets of the tallgrass region's woodlands. We are early, and yet nearly too late. Large parts of remnant oak ecosystem biodiversity are being lost, dying out, year after year, at most sites. So we should continue to study, but we have to act now on what we know.  

Biodiversity conservation began with the prairies. The need was noticed first there. Remnants were small, precious, visibly dying out. We loved those prairies. When George Fell founded the Illinois Nature Preserves System, he recommended prairies as a priority. 

 

Conservation pioneers were inventing something quite new. Much of the science and discipline of ecosystem ecology and conservation had been founded in the tallgrass region. A seminal work was The Vegetation of Wisconsin by John Curtis (1959) – outstandingly good, as science, but for conservation action, embedded within it was a fundamental problem. 

 

Using ecosystem concepts based on that book, Illinois conducted the world’s first intensive Natural Areas Inventory in the 1970s. How much nature was left, it asked for the first time, and where exactly was it? Nearly all the state's prairies had been plowed, its forests cut, its wetlands drained. Even land set aside for conservation could lose its species and quality as shown in the four savanna drawings below:

When the Inventory was completed, it revealed that just 7/100^ths of 1% of the state retained high quality nature – at 610 sites. For many years, this fine Inventory was the basis of Illinois conservation.  Our goal was to protect all the sites, but rarity was a factor in setting priorities. Compare, for example, how much very high quality (Grade A) ecosystem was found on good soil sites that were not too dry and not too wet (“mesic”):

 

Grade A forest:            1,058 acres 

Grade A prairie:           54 acres

Grade A savanna:        0 acres 

 

The Inventory was especially ahead of its time in how it chose to define prairie “nature.” Thanks to the pioneering work of Dr. Robert Betz, Floyd Swink, Ray Schulenberg, and others, the Inventory was able to assess prairie “quality” based in part on plant species conservatism and composition. As with forests or wetlands, evaluators looked for signs of human "disturbance", but they also sought concentrations of species that were rare and normally found only growing with each other in remnants that had all prairie species. These sites were recognized as complete. Using such insights, it turned out to be impossible to find “quality” prairies on good soil that were larger than a few acres.  Thus, very high quality prairies as small as one-quarter acre made it on to the Inventory and were deemed important enough for preservation.  Protecting those precious few 54 prairie acres became an obvious priority.

No parallel approach was used for the forests. Despite the fact that most woodland biodiversity is found among its herbs and associated animals, fungi, etc. – woods were evaluated principally by their trees. You might think - with 1,058 acres of Grade A mesic forest - that 1970s Illinois had roughly twenty times as much Grade A forest as prairie. But it has become increasingly clear that there is very little, if any, very high quality oak forest, our major forest type. 

Prairie destruction had been obvious. Woodland biodiversity was being lost gradually, imperceptibly to most, often from excess shade by invading maples. Many sites accepted by the Inventory, now mostly maple, had ancient oaks in the canopy. Those oaks had typically not reproduced in 100 years and ecologically seemed doomed. Oak woods depend on regular fire. In its absence they had been degrading for more than a century. 

 

Curtis had studied the forests in great detail. We learned much from his excellent, massive, and insightfully mathematically analyzed data. He found sugar maple to be by far the most common mesic forest tree, but his data shows that among them grew oaks: especially white, red, and bur oak. He was looking at a moment in time. But his work was interpreted as defining archetypes and goals for conservation – in perpetuity.

 

It’s not that Curtis couldn’t see what was happening. He described it graphically in the passage below, summarizing what he calls the “normal” replacement of oaks by maples. In describing what he called “an original stand” of white, black, and red oak:

 

“Due to complete fire protection afforded the stand in the last 50 years, the mesic trees began to spread out, basswood going first and farthest, followed by an almost solid wall of young sugar maples. … (T)he shade from the maples brought about the death of the typical oak forest understory. … As the period of exposure to low light lengthened, the oak plants gradually died out altogether, although some persisted for decades in a weak, entirely vegetative condition.” 

 

Of course, as the hundreds of species of oak-associated plants died out, thousands of animal species that depended on them died out too. So what did Curtis mean when he referred to this as “normal”?

 

Unburned savannas and woodlands gradually grow dense with buckthorn (above), maple, or other species that fire would have excluded. In the process, most ground level species of plants and animals are lost. In the end, the old oaks die, unable to reproduce in the shade. 

A basic early tenet of ecology as a science was that of succession. It posited that ecological communities, in the absence of what was called "disturbance", would invariably progress toward a “climax community” governed solely by climactic and soil factors. In this model, "undisturbed" nature tended towards a kind of perfection in its climax, while "disturbances" like fire slid communities down from such heights. Damaged nature, for an example a plowed field that was abandoned, would first be colonized by weeds and other hardy plants and gradually over time restore itself to rich nature. 

For Curtis, maple forest represented the principal climax for our region. As succession advanced, an open woods would get shadier, have less wind, and become more moist. Curtis and others referred to “climax” maple forests as “mesic” and to oak forests as “xeric” (the technical term for dry). So, as he put it (with italics added):  

“The xeric forests of southern Wisconsin are seen to be a series of rapidly changing species combinations whose local complexity is the result of progressive and retrogressive processes induced by the biological characteristics of the dominant species and by the repeated interference of outside agents of destruction.” 

 

Curtis deplored the practice of some farmers to burn their oak woods. Nature needed lack of "disturbance". Fire equaled destruction. He loved nature and considered sugar maple forest to be precious, as indeed ancient maple forests are. But he seemed not to worry that oak woodland biota could be lost. He found that savannas had been the most common ecosystem type in the southern half of Wisconsin at the time of Euro-American settlement but were extremely rare in his day. He credited Native Americans with the fires that maintained so much savanna and oak woods, but he didn’t identify fire as a critical element of biodiversity conservation. 

 

With varying amounts of passion and wisdom, early conservationists deplored the impact of “the degrading hand of man.” If fire was a human-caused disturbance and it ceased, our prairies would become woods of some sort, and our oak woods would become maple. Most animal and plant species of this region depend on grassland or open woodland habitats. “Leave nature undisturbed” would not conserve them.  

 

The concepts needed for conservation were gradually being developed. The term “biodiversity” first appeared in publication in 1988. The crucial role of fire was finally codified for scientists in 1985 by Picket and White in The Ecology of Natural Disturbance and Patch Dynamics.

It would necessarily take time to incorporate new understandings into conservation decision-making. 

 

But it was becoming clearer that we had to adopt a longer view. Our planetary heritage of biodiversity was not created in a few thousand years; it took millions. Research indicates that the planet’s grasslands and associated oak ecosystems began to take their current form about five million years ago with the rise of warm-season grasses and fires. Today’s species and genetic richness have been evolving in these communities for those millions of years. Many species are older than that. 

 

Some people point out that the ecosystems on the fertile, new glacial soils of the tallgrass region have only been here for ten millennia, since the retreat of the Wisconsin glacier. But their species, with their intricate interdependencies and relationships, are vastly older. The communities of animals and plants moved back and forth on the continent in response to changing climates. Regardless of where they’ve been over deep time, much of their species and genetic richness survive today in fragmented remnants of the Midwest. 

 

As the Illinois Natural Areas Inventory folks were busy doing the hard work of defining “nature” and identifying the communities that supported it, they needed to do something with the oak woods. Like Curtis, they called them xeric (meaning dry). Thus, the category that likely included some of our best oak woodland was called dry-mesic forest. 

 

Dry-Mesic Grade A forest:      986 acres 

 

But the search for highest quality woodlands did not have the benefit of a Betz-like evaluation of herbaceous plant diversity. Quality was defined more simply as lack of "disturbance" – no past tree cutting or fire. The fact that such areas would lose their original or current species over time seemed not to have been part of the thinking. Perhaps "lack of disturbance" was an opposite of what they should have been looking for (see Endnote 1). In the case of prairies, there was already in the seventies an appreciation of the role of fire. The INAI site reports included recommendations to landowners. In the case of prairies, the need for fire was noted on many sites. In the case of oak forests, it was not. 

 

A concept frequently found in writing on this subject is that of “the mesophytic forest.” This category, as used in the tallgrass region, often seems unhelpful and confusing. “Mesophytic” is from Latin for “medium” (wetness) and “plant.” It is used to indicate maple forest. But the practical difference between oak and maple is fire, not water. Soil types and wetness make important differences, of course. But what the manager or steward controls (and what means life or death to oak woodland biodiversity today) is fire or no fire. Yes, a prairie grove of oaks would get more damp if maples took over – shadier, less wind and evaporation. But the biodiversity of the maple forest would not magically appear. This grove would not increasingly be a natural maple forest; it would be an increasingly degraded oak woodland. 

 

Two continua (or “ranges of conditions”) are key to understanding our oak ecosystems. One is the continuum from intense frequent burns to occasional, mild ones. The other is the continuum from dry to wet. Oak woodlands span both continua. The most common tree in wet oak woodlands is the swamp white oak (Quercus bicolor). Wet-mesic oak woods often have mixed swamp white and bur oaks (Quercus macrocarpa). Mesic oak woodlands may be mostly white oak (Quercus alba) – and drier, sandier ones have more black oak (Quercus velutina). Sites with a history of more intense fire (for example those downwind from prairies or savannas) are more likely to have a larger component of bur oak, which is the species with the most resistance to and dependence on fire. Bur oaks have very open (less shady) canopies and, as Curtis pointed out, cannot reproduce in the shade of any other tree species. Red oak (Quercus rubra) seems to be shorter-lived, more vulnerable to fire damage, and able to grow in the shade of other oak species. On many sites, red oak numbers need to be controlled if bur and white oaks are to reproduce. 

 

Oak woodlands are gradually getting a second look and more respect. In 2011, an update of the 1970s Illinois Natural Areas Inventory was adopted by the Illinois Department of Natural Resources (See Endnote 2). That update took a big step toward solving the problem of the missing oak woodlands. The new community classification now includes a distinction between “forests” and “woodlands.” Woodland is now rightly seen as a special type of forest, which depends on fire for survival. As the mesic communities in the new classification are presented:

 

Mesic upland forest: principally sugar maple. Characteristic plants: Blue beech and bladdernut. Characteristic animals: tiger salamander and wood thrush. 

Mesic woodland: principally white and bur oak. Characteristic plants: Viburnums, woodland milkweed, pale vetchling, starry campion. 

 

The conceptual thinking continues to develop. If characteristic animals were to be listed for mesic woodland, species might include red-headed woodpecker, great-crested flycatcher, yellow-throated vireo, and blue-spotted salamander. 

 

The updated Inventory shows Dry-mesic woodland having such characteristic plants as alumroot, New Jersey tea, pale Indian plantain, Wolf’s bluegrass, savanna blazing star, and buffalo clover. Many woodland plant species are on the Illinois Threatened and Endangered lists or are otherwise rare, especially the ecotypes of woodlands. Many have been characterized as “plant refugees.” 

 

Another valuable treatment of tallgrass savannas, woodlands, and forests can be found in Wilhelm and Rericha's Flora of the Chicago Region. About white and bur oak woodlands they write, “There are no intact remnants from which we can piece together their aboriginal character inasmuch as grazing, logging, and fire suppression since settlement have obfuscated their physiognomic structure and ground layer composition.”  

This restored bur oak woodland and savanna now has about 500 plant species, mostly rare today. About half rebounded on their own, and half were restored from seed gathered nearby. Many rare animal species have recovered as well. And yet this restored woodland is still young and evolving rapidly.  

There will be more learning and clarifications. But we can’t wait for them. For those of us who work on biodiversity conservation, we have to act on the basis of what information we have, before it’s too late. With all due respect for the scientific process … and it deserves great respect indeed ... we have to make many decisions on the basis of judgment.  

 

The bottom line is: some of us at some sites need to use fire and other healing restoration to conserve the oak woodlands and savannas, while the biota survives. 

Endnote 1

The following kind-of-fun story illustrates how wrong the anti-disturbance mindset was for both inventory and preserve management. 

As a new Nature Preserve Field Rep, it was part of my job to visit all the Preserves from time to time to "surveil" for any problems. When I visited Reed-Turner Woodland Nature Preserve in Long Grove about 1980, I was greeted by volunteer steward Barbara Turner. She showed me the features of the preserve with passion and affection. One feature I especially wanted to see was cream vetchling (Lathyrus ochroleucus), an Endangered species that once had been common in oak woodlands. Barbara became frustrated as she escorted me to area after area where she said she remembered seeing it - but that day failed to find it. Finally she stopped, looked me in the eye, and said, "Well, Steve, I know that I can show that plant to you, but I'll have to take you to an area I was hoping to avoid. Now, I do know that the Nature Preserves Commission forbids mowing. But we bring many groups of school children here, and for education we really need a place where they can assemble in the woods. So, I do mow, one little area."

Around the edges of that area, sure enough, grew the cream vetchling side by side with fire pink (Silene virginica) and other rare plants that Barbara had wanted to show me. I also noticed a few small stumps, suggesting that saplings growing out into the assembly area had been trimmed back as well. By Nature Preserve standards of the time, this area had been illegally disturbed and damaged.  

I told Barbara that conservationists were beginning to explore the possibility that we had lost our savannas through lack of fire. Might that be true of oak woodlands as well? Barbara, a thinker, remembered the woods during her childhood being much sunnier and more filled with beautiful rare flowers. In time, Reed-Turner Woodland would become the first oak woods Nature Preserve to experiment with and benefit from controlled burns.  

Endnote 2

In 2011, the Illinois Department of Natural Resources published an update of The Standards and Guidelines for The Illinois Natural Areas Inventory. Under the leadership of John Taft, it defined the newly recognized communities of dry-mesic and mesic woodland, below:

Dry-mesic woodland -- Soil conditions intermediate between dry and mesic; not as limiting for tree growth compared to dry woodland. Periodic fire is necessary for maintenance of composition and open structure. Subcanopy structure depends on recent fire history. Intergrades with dry woodland and dry-mesic upland forest. Formerly classified, in part, as dry-mesic upland forest (White and Madany 1978). Distinguished from dry-mesic upland forest by spreading growth forms of the dominant overstory trees and predominance of characteristic understory woodland species. 

Distribution: Statewide on ridges, level and rolling uplands, and south and west-facing upper slopes. 

Dominant plants: 

Canopy - Quercus alba, Quercus velutina, Quercus stellata, Quercus macrocarpa (bur oak), Carya spp. (C. glabra [pignut hickory], C. ovata [shagbark hickory], C. ovalis [shellbark], C. tomentosa [mockernut hickory]); 

Ground Layer - Bromus pubescens (woodland brome), Muhlenbergia sobolifera (rock satin grass), Solidago ulmifolia (elm-leaved goldenrod). 

Characteristic plants: 

Understory - Amelanchier arborea (Juneberry), Rosa carolina (pasture rose), Rhus aromatica(aromatic sumac), Ceanothus americanus (New Jersey tea), Cornus florida (flowering dogwood), Corylus americana (hazel), Ostrya virginiana (hop hornbeam), Viburnum prunifolium (black haw). 

Ground Layer – Amphicarpa bracteata (hog peanut), Anemone virginiana (tall anemone), Apocynum androsaemifolium (spreading dogbane), Arnoglossum atriplicifolium (pale Indian plantain), Asclepias purpurescens (purple milkweed), Asclepias quadrifolia (whorled milkweed), Aster spp. (e.g., A. drummondii, A. sagittifolius), Bromus pubescens (woodland brome), Dasistoma macrophylla (mullein foxglove), Desmodium nudiflorum (bare-stemmed tick trefoil), Dichanthelium latifolium (broad-leaved panic grass), Elymus hystrix (bottlebrush rye), Frasera caroliniensis (American columbo), Helianthus strumosus (pale-leaved sunflower), Heuchera americana (tall alumroot), Krigia biflora (false dandelion), Carex spp. (e.g., C. albicans, C. cephalophora, C. hirsutella, C. pensylvanica, C. retroflexa), Liatris aspera (rough blazing star), Liatris scariosa var. nieuwlandii (savanna blazing star), Monarda bradburiana, Poa wolfii (Wolf’s bluegrass [State Threatened]), Porteranthus stipulaceus (Indian physic), Pycnanthemum pilosum (hairy mountain mint), Silene stellata (starry campion), Solidago nemoralis (old field goldenrod), Solidago speciosa (showy goldenrod), Taenidia integerrima (yellow pimpernel), Tradescantia virginiana (Virginia spiderwort), Trifolium reflexum (buffalo clover [State Threatened]), Triosteum perfoliatum (late horse gentian), and Veronicastrum virginicum (Culver’s root). 

Characteristic animals: Eumeces laticeps (broad-headed skink) 

Mesic woodland -- Soils deep and rich; periodic fire required for maintenance of woodland structure. Examples include mesic savannas converted to woodland with > 50% canopy cover as a result of fire absence or low frequency. Formerly classified, in part, as mesic savanna and mesic upland forest (White and Madany 1978). Intergrades with mesic savanna; following extended fire absence, converts to mesic upland forest. 

Distribution: Statewide on high terraces, at the base of moraines, and other locations that are fire prone; scarce due to fire absence and conversion to forest structure. 

Dominant plants: 

Canopy - Quercus alba, Quercus macrocarpa, Q. rubra. 

Characteristic plants: 

Understory - Prunus virginiana, Viburnum spp. (e.g., V. lentago, V. prunifolium, V. rafinesquianum). Ground Layer - Asclepias exaltata, A. purpurascens, Bromus pubescens (woodland brome), Camassia scilloides (wild hyacinth), Carex spp. (e.g., C. jamesii, C. hirtifolia, C. radiata), Dichanthelium spp. (e.g,. D. clandestinum, D. latifolium), Echinacea purpurea (purple coneflower), Eupatoriadelphus purpureus (purple joe-pye weed), Festuca subverticillata (nodding fescue), Heliopsis helianthoides (false sunflower), Diarrhena americana (beak grass), Lathyrus ochroleucus (pale milkvetch [State Threatened]), Podophyllum peltatum (May apple), Silene stellata. 

Congratulations to the Illinois DNR folks who assembled this good contribution to our increasing recognition of woodland communities. Two details in the above deserve comment.  It could be misleading to claim that “mesic savannas convert ... to woodland ... as a result of fire absence or low frequency” and that mesic woodland without fire “converts to mesic upland forest.” It would be more accurate to say that in most cases a rare global conservation priority (oak woodland) would be replaced by a poor quality common maple forest. 

If high-quality examples of both communities were present side-by-side, evolution towards a quality maple forest does seem possible. But more likely in most cases, lack of fire would result instead in degraded savanna or woodland. It may be accurate to say that they develop the structure of the new community. But if the species of that new community are not present nearby, the rare conservative biodiversity that's lost in the absence of fire would not likely be replaced by anything comparable. Generally speaking, the goal of conservation should be to maintain high-quality communities with their existing biodiversity intact. This is especially true for a global conservation priority, like oak woodland. 

Both Illinois and Wisconsin have contributed a lot of leadership to be proud of and continue to improve the conceptual frameworks and priorities that guide conservation. Wisconsin DNR defines the ecology and needs of the fire-dependent “Oak Openings” well here. Indeed, Wisconsin ranks savanna (which they refer to as “Oak Openings” - following Curtis) as that state's only Globally Endangered (“G1”) community. G1 means “critically imperiled” globally and “at very high risk of extinction or elimination.” WI DNR lists oak woodland as “GNR S1?” - meaning that its global priority is not well understood (NR - not rated) and it is likely a top priority in that state - but with a question mark. Oak woodlands have been getting additional attention in Wisconsin recently as a result of the Army Lake Woodland experience. Many of us hope that these rare communities will increasingly get the priority and stewardship that they need and deserve. 

 

References

Abrams, M.D. 1992. Fire and the development of oak forests. Bioscience. 42: 346-353. https://www.personal.psu.edu/faculty/a/g/agl/1992%20Bioscience%2042%20346%20353.pdf

Oak Ecosystems Recovery Plan (Chicago Wilderness) https://www2.illinois.gov/dnr/conservation/IWAP/Documents/Chicago%20Wilderness%20Oak%20Ecosystem%20Recovery%20Plan.pdf

Curtis, John T. The Vegetation of Wisconsin: as Ordination of Plant Communities. 1959

Andrew Hipp, Paul S. Manos, and Jeannine Cavender-Bares: https://www.scientificamerican.com/article/how-oak-trees-evolved-to-rule-the-forests-of-the-northern-hemisphere/

John White, Conceptual Foundation and Philosophical Framework for the Illinois Natural Areas Inventory, Fourth Edition, 2008

Wilhelm, Gerould and Laura Rericha, Flora of the Chicago Region, 2017. See especially the analysis of sugar maple issues in their treatment of Acer saccharum. 

Wilhelm, Gerould, What is a Savanna? Savanna/Woodland Symposium. Kansas City, MO. 2002

Acknowledgments 

The 1800 to 2010 savanna drawings are by Paul Nelson and appeared in The Tallgrass Restoration Handbook by Packard and Mutel (Island Press). 

For editing and proofing, thanks to Christos Economou and Eriko Kojima.