Fire in Upper Midwestern Oak 

Forest Ecosystems: 

an Oak Forest Restoration and 

Management Handbook

Lee E. Frelich, Peter B. Reich, and David W. Peterson 

United States Department of Agriculture

Forest 

Service

Pacific Northwest 

Research Station

General Technical Report

PNW-GTR-914

September 

2015
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Authors

Lee E. Frelich is Director of the Center for Forest Ecology and Peter B. Reich is a 

Regents Professor and the F.B. Hubachek Sr. Chair in Forest Ecology and Tree Physiology at the University of Minnesota, Department of Forest Resources, 115 Green 

Hall, 1530 Cleveland Ave. N, St. Paul, MN 55108. David W. Peterson is a research 

forester, U.S. Department of Agriculture, Forest Service, Pacific Northwest Research 

Station, Forestry Sciences Laboratory, 1133 N Western Ave, Wenatchee, WA 98801.

Cover: Spring prescribed burn in progress on sand savanna with northern pine 

and bur oak, Cedar Creek Ecosystem Science Reserve, Minnesota. Photograph by 

Kalev Jõgiste.
Abstract

Frelich, Lee E.; Reich, Peter B.; Peterson, David W. 2015. Fire in upper 

Midwestern oak forest ecosystems: an oak forest restoration and management 

handbook. Gen. Tech. Rep. PNW-GTR-914. Portland, OR: U.S. Department of 

Agriculture, Forest Service, Pacific Northwest Research Station. 64 p. 

We reviewed the literature to synthesize what is known about the use of fire to 

maintain and restore oak forests, woodlands, and savannas of the upper Midwestern 

United States, with emphasis on Minnesota, Wisconsin, and Michigan. Included 

are (1) known physical and ecological effects of fire on oaks from acorn through 

seedling, established sapling, and mature stages of the life cycle; (2) the use of fire 

to modify competitive interactions between oaks and mesic forest species (e.g., 

maple), between oaks and pines, and between oaks and grasses; (3) interaction of 

fire with other disturbances such as windthrow and harvesting, invasive species, 

and deer browsing; and (4) climate change. Throughout the report, we discuss the 

advantages and limitations of fire use in oak forests. We incorporate lessons learned 

from long-term experiments with fire, from historical evidence of fire over the centuries, and processes in areas where natural disturbances occur. We provide a brief 

summary of the use of fire to restore mixed oak-maple forests, mixed oak forests, 

mixed pine-oak forests, and oak savannas, along with take-home lessons about the 

complex relationships between oaks and fire. 

Keywords: Prescribed fire, oak forest, oak savanna, forest restoration, forest 

management, fire ecology.
Summary

Oak forests and savannas provide valuable ecosystem services, including timber 

production and wildlife habitat, so maintaining or increasing the abundance of 

oaks is a desirable goal for forest management in the Lake States of Minnesota, 

Wisconsin, and Michigan. Fire and climate played important roles in the development, maintenance, and geographical distribution of oak forests before and after 

European settlement. Changes in fire frequency and severity, shifting forest management practices, and invasive species have produced significant changes in forest 

structure and composition over the past century in many formerly oak-dominated 

ecosystems, such that long-term sustainability of oak forests is in doubt. Prescribed 

fire use or restoration of historical fire regimes may help to restore, maintain, or 

increase the dominance of oaks in these temperate deciduous forests, but efforts 

to restore oak forests using fire are constrained by social attitudes toward fire, 

and uncertainties about the frequency and severity of fires needed to reverse past 

changes. Efforts to restore oak ecosystems with fire may be further complicated by 

changes in atmospheric carbon dioxide concentrations, climatic changes, biological 

invasions (e.g., earthworms, buckthorn), and herbivore impacts, which may alter 

plant resource requirements, resource availability, biotic interactions, forest fuel 

dynamics, and potential fire behavior. The purpose of this document is to review 

and summarize scientific knowledge about the role of fire in oak forests, other factors that influence oak-fire relationships, and possible approaches for using fire in 

restoration-based management of oaks in temperate deciduous forests of the Upper 

Midwest and northeastern United States. We first introduce the concept of ecologically significant fire and review the mechanisms by which fire historically regulated 

the balance between oaks and other species at different life-history stages. We then 

review how different stand-development stages, exotic species, plant-animal interactions, and climate change can alter forest structure and composition, forest fuels, 

fire behavior and effects, and the potential effectiveness of fire as a restoration tool. 

We conclude by discussing approaches to restoring fire in a variety of contemporary 

oak forest types, including mixed oak forests, oak-maple forests, oak-pine forests, 

and oak savannas. A suite of oak forest restoration options are available to forest 

managers, including prescribed fire alone, prescribed fire combined with mechanical thinning, planting oak seedlings after removing competing vegetation, and deer 

exclusion. However, the effectiveness of different approaches will differ among 

biophysical and social settings, forest types, and forest management histories.

Historically, different fire regimes in place for centuries created a variety of oak 

forests and savannas. Burning frequently, every one or two decades, or once every 

several decades created savannas, oak forests, and mixed oak-maple forests or oakpine forests, respectively. Many contemporary oak forests and savannas have been 
through at least several decades of fire exclusion, and some invading maples have 

become large enough to resist fires, providing a permanent, and in many instances, 

undesired, seed source. A number of prescribed burns over time will be necessary to gradually push such forests back to a desired condition, especially because 

low-intensity spring burns are commonly done, and in many cases earthworm 

invasions have reduced the forest floor fuel loading, meaning that most fires carried 

out today have a relatively low intensity and small ecological impact. In most oak 

forests, understory shrubs, including invasive nonnatives, and shade-tolerant tree 

saplings create conditions unsuitable for oak establishment. In such cases, several 

burns in consecutive years may be needed to exhaust the sprouting ability of the 

shade-tolerant trees and shrubs, to create the open understory conditions lasting 

for several years that are needed for oak establishment. In oak savannas that have 

grown up into forests, without mechanical removal of some trees, several decades 

of frequent light to moderate burning is necessary to prevent oak reproduction (and 

also reproduction of mesic tree species on some sites) while tree density is gradually reduced by mortality of older trees. 

The ecological impact of a prescribed burn can be considerably altered by 

manipulation of fuels prior to the burn. For example, pulling fuels away from the 

base of large oaks protects them from fire and maintains them as a future seed 

source, while piling fuels at the base of undesired trees like red maple (Acer rubrum) 

can be used to increase their chances of mortality. Other strategies such as timing 

fires to create ideal seed beds just before a bumber crop of acorns, to take advantage 

of a planned thinning, or after a chance wind-disturbance event, along with deer 

management after a fire, can all increase effectiveness of fires in regenerating oak. 

Although oak forest management has been difficult in recent times owing 

to mesophication (the positive feedback cycle in which cool, damp, and shaded 

conditions and less-flammable fuel beds continually favor shade-tolerant species 

and inhibit shade-intolerant, fire-adapted species), increased deer populations, 

and a wetter summer climate, climate change is likely to be a positive factor for 

oak. Warmer summers and increased drought frequency will have a big negative 

impact on mesic species in the Lake States, and smaller negative or even positive impacts on oaks, throwing the competitive balance to oak’s favor. A warmer 

climate may also promote oak invasion into the southern portion of the boreal 

forest. Therefore, fire management for oaks in the region is likely to become more 

feasible and more important. 
Contents

1 Introduction

5 Ecologically Significant Fire in Oak Forests

6 Tree Damage During Fires

12 Fuel Loads for Ecologically Significant Fires

14 Fire as a Regulator of Oak Abundance

14 Historic Fire Regimes and Oak Versus Grass, Maple, and Pine

19 Burning Frequency and Competitiveness of 

Oak Regeneration in Forests

20 Fire Season

21 Multiple Factors Interacting With Fire

22 Plant-Animal Interactions

27 Stand Development

29 Competing Vegetation and Invasive Plants

31 A Case Study of Fire and Red Oak

34 Oak and Climate Change

36 Restoring Fire in Contemporary Oak Forests

36 Mixed Oak Forests

37 Oak-Pine Forests

38 Oak-Maple Forests

39 Oak Savanna

42 The Wildland-Urban Interface (WUI)

42 Conclusions—Some Take-Home Lessons

45 Acknowledgments 

45 Metric Equivalents 

45 Literature Cited

61 Appendix: Common and Scientific Names
1

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

Introduction

Managing forests to maintain or increase the abundance of oaks is a desirable goal 

in many temperate deciduous forests of the upper Midwest and northeastern United 

States. Oak forests and savannas provide a variety of ecosystem services, including 

timber production and wildlife habitat. Oaks produce large quantities of acorns that 

are a major food source for many important wildlife species, including white-tailed 

deer, black bears, eastern gray squirrels and other rodents, and turkeys (Feldhamer 

2002, Smith and Stapanian 2002, Steffen et al. 2002, Vaughan 2002) (common 

and scientific names of all plant and animal species mentioned can be found in the 

appendix). In addition to producing acorns, oaks provide other key wildlife habitat 

features such as cavity trees and dead wood (Healy 2002). Oaks attain large size 

and shed large quantities of leaf litter that heavily influence soil chemistry and 

ecosystem processes. Some oak species are long lived, with individual trees that 

are among the largest and oldest in the region, and many of the remnant old-growth 

forests of the Midwest are oak dominated (Frelich and Reich 2002). Thus, oaks are 

foundational species in ecosystem function across large landscapes. 

In the upper Midwest (defined as Minnesota, Wisconsin, and Michigan for our 

purposes), oak forests currently are most dominant within a band several hundred 

kilometers wide that extends from northwestern Minnesota to southern Wisconsin, 

and southern Michigan (McWilliams et al. 2002). However, as the climate has warmed 

and cooled over the last 11,000 years since glaciers left the region, this band has shifted 

to the south and west during periods of relatively cool, moist climates, and to the north 

and east during times of relatively warm, dry climates (Umbanhowar et al. 2006). 

Fire has been a major contributor to the development and maintenance of oak 

forests for thousands of years (Abrams 1992, 2002). Historically, the region of heavy 

oak dominance was bounded by grasslands with very frequent fires (generally 1- to 

5-year return intervals) to the west, and mesic forests with infrequent fires (generally 

fire-return intervals of 100+ years) in the northeastern portion of the Lake States. 

Within this region however, topographic barriers to the spread of fire created local 

variability in fire frequency that, in turn, produced a mosaic of vegetation types that 

included prairie grasslands, oak savannas and woodlands, mixed-oak forests, and 

mesic deciduous forests (e.g., maple-basswood forests) (Grimm 1984, Williams et 

al. 2004). Fossil evidence of fire (charcoal) is often found together with fossil oak 

pollen in sedimentary sequences from basins in oak-dominated areas. Furthermore, 

it is known that human enhancement of fire frequency, such as burning by Native 

Americans, can convert mesic forest to oak forest or savanna (Clark and Royall 

1995, Dorney and Dorney 1989). Hence, the oak-fire hypothesis—that periodic 

fires played an important role in the historical development and maintenance of oak 
2

GENERAL TECHNICAL REPORT PNW-GTR-914

forests before and after European settlement and that fire regimes with a certain 

range of fire frequencies and intensities are needed to maintain oak—has been a 

mainstay of ecological theory for several decades (e.g. Abrams 1992, Collins and 

Carson 2003, Curtis 1959, Dey and Fan 2009, Johnson et al. 2009). 

Changes in fire frequency and intensity since early settlement of the region have 

produced significant changes in formerly oak-dominated ecosystems (Nowacki 

and Abrams 2008). Harvesting and burning of mesic forests early in the settlement 

period favored oaks over more shade-tolerant but disturbance-sensitive species. 

In other areas, reductions in fire frequency—caused by fewer human ignitions, 

landscape fragmentation, and agricultural land conversion—led to the conversion 

of grasslands and oak savannas to closed-canopy oak forests and allowed shadetolerant mesic species to invade and become dominant in formerly oak-dominated 

forests. As a result, high-quality oak savannas are now rare (Nuzzo 1986), and there 

is concern about increasing losses of oak-dominated forests and their associated 

ecosystem services, resulting from succession under continuing fire suppression 

policies and other factors (Nowacki and Abrams 2008).

Many forests formerly dominated by white oak or red oak have been invaded by 

maple in recent years, a process referred to as mesophication (fig. 1) (McEwan et al. 

2011, Rogers et al. 2008). This mesophication process has been fostered not only by 

Figure 1—A mesophied northern red oak forest in southern Minnesota. Three sugar maple saplings 

stand in front of a large northern red oak. 

Kalev Jõgiste
3

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

fire suppression and exclusion, but by higher deer populations that prefer oaks over 

maples for winter browsing, a wetter summer climate in the last several decades, 

changes in the physical and chemical environment of the forest floor caused by 

maple invasion and, in some locations, invasive plant species such as buckthorn. All 

of these changes favor maple and other mesic-forest species like American basswood, which are more shade-tolerant than oaks, so that they may thrive in the now 

much denser forest understory. In particular, the more shade-tolerant red maple is 

expanding rapidly (Fei and Steiner 2009) and can tolerate many of the same environmental conditions as the oaks. This mesophication has many implications for the 

use of fire to restore oak that are discussed in this report. 

The regeneration window for oaks (Johnson et al. 2009, p. 146), or the range 

of the light gradient over which oaks can successfully compete in the understory, 

is much narrower for the moist site conditions that have expanded to cover a larger 

proportion of the landscape owing to increasing understory density and the wetter 

climate of recent years. Under such conditions, larger gaps with relatively bright 

light are needed for successful oak recruitment. In other words, more forests are 

now in the “recalcitrant” category of oak reproduction accumulation, i.e., stands 

where shade and competition with mesic forest species prevent accumulation of 

large oak advance reproduction over successive acorn crops. And there has been 

a decline in the “ambivalent” or “intrinsic” categories of oak forests, where accumulation of large oak advance reproduction results in modest success on dry-mesic 

sites and high success on xeric sites (Johnson et al. 2009, p. 160). 

The fire and oak hypothesis suggests that prescribed fire or restoration of natural fire regimes, particularly low-intensity surface fires, could be used to restore, 

maintain, or increase the dominance of oaks in the Midwest region. However, there 

are various constraints on the use of fire on modern landscapes, including continuing social bias favoring fire suppression, continued expansion of the wildland-urban 

interface, air quality laws, and lack of funding. Where fire can be used, it may not 

be effective for restoring oak dominance within typical management timeframes 

owing to persistence of large trees of mesic species that serve as local seed sources, 

feedbacks of mesic species on understory microclimate and fuels, persistence of 

exotic species associated with closed-canopy forests (e.g., buckthorn, earthworms), 

and resource constraints on understory oak regeneration and persistence (Johnson 

et al. 2009, chap. 3). Therefore, it won’t be possible to restore oak forests to their 

previous status in all, or perhaps even many, cases using fire alone (Abrams 1992).

Efforts to restore oak ecosystems with fire may be further complicated by past 

and future changes in atmospheric carbon dioxide (CO2) concentrations and climate. 

Climatic changes could assist in oak restoration if warmer temperatures produce more 
4

GENERAL TECHNICAL REPORT PNW-GTR-914

favorable conditions for fire, if oaks are better adapted to warmer temperatures than 

their mesic competitor species, or both. Climatic changes are expected to shift the 

biogeographical distribution of biophysical conditions that historically favored oak 

dominance and persistence, which could create mixed responses within the current 

region of oak dominance. Changes in atmospheric CO2 concentrations could also 

create mixed responses by altering the biophysical requirements of oaks and their cooccurring native species (Davis et al. 2007). Finally, climate change could introduce 

new (and changing) suites of co-occurring species as species migrate at different rates. 

To some extent, changes in forest composition and structure proceed along a 

unique trajectory over time, and it may not always be feasible to maintain oak dominance (Frelich 2002). This is the dilemma of managing mid-successional forests. If 

there is no fire, the forest may succeed to species that are less desirable. If there is a 

severe fire, the current crop of mature oak trees may be damaged, although there is 

much flexibility in conditions between these extremes of fire occurrence in which 

oaks can be sustained. Fire also interacts in complex ways with deer, climate, soils, 

invasive species, and native competitors of oak at all stages of the life cycle from 

seedling to mature tree (Frelich and Reich 2002). A whole gradient of forest restoration options are available to oak forest managers, including planting oak seedlings 

after removing competing vegetation and fencing out deer until the saplings are 

beyond their reach; thinning to remove undesirable tree species combined with 

mechanical preparation of the soil surface; thinning combined with fire; prescribed 

fire alone; and no intervention, or “letting nature take its course.” 

The individualistic nature of plant communities over time and space, including 

forests, precludes the possibility of standardized/cookbook silvicultural or fire-use 

prescriptions that will be effective in all stands over an entire region. However, 

much information is available and presented here that can be blended with local 

foresters’ experience to get a picture of which options may have some success and 

how the level of effectiveness of various options is limited by the history of a given 

site, biology of the species present, and site conditions. 

The purpose of this document is to review and summarize scientific knowledge 

about the role of fire in oak forests and other factors that influence oak-fire relationships, and to point out the potential positive effects and limitations of fire use in oak 

forests. Focusing on the Lake States of Minnesota, Wisconsin, and Michigan and 

adjacent areas, this review has a more northern bias than most of the existing body 

of literature on oaks and fire and therefore complements other classic and recently 

published reviews or large-scale studies of oak and fire (Abrams 1992, 2003; 

Arthur et al. 2012; Brose et al. 2013; Buchanan and Hart 2012; Dey and Fan 2009; 

Knoot et al. 2010; McEwan et al. 2011) from centers of excellence in research that 
5

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

exist across the central U.S., including the Ozarks and Cross-Timbers (Oklahoma, 

Missouri, and Arkansas), Ohio River valley (Illinois, Kentucky, Ohio), southern and 

central Appalachians (Pennsylvania, West Virginia, North Carolina), and southern 

New England (Connecticut, Massachusetts). Oak is also important in much of the 

northern tier of states, and with a warming climate, oaks will potentially become 

an even more widespread forest type in the future. Therefore, interactions with the 

boreal forest and northern pine forests are also covered here. 

We start by introducing and discussing the concept of ecologically significant fire 

and its importance in understanding oak-fire relationships and the potential for using 

fire to restore and maintain oak forests. We then review the mechanisms by which 

fire historically regulated the balance between oaks and other species at different 

life-history stages, and assess the continued relevance of those historical relationships 

for modern restoration-based management of oak forests. Next is a section on how fire 

interacts with multiple factors—including stand-development stages, exotic species, 

and plant-animal interactions—as every forest stand has several ecological processes 

happening at once that can work to enhance or diminish the desired effects of fire. 

Presentation of a case study that ties together multiple influences of fire on red oak—

the most important oak species in northern forests—shows that red oak does respond 

to fire with good regeneration today under certain circumstances. We conclude with 

discussions of potential impacts of climate change on oaks and fire, a section on 

restoring fire in a variety of contemporary oak forests (with subsections on mixed oak, 

oak-maple, oak-pine, and oak savanna), and some conclusions or take-home lessons. 

Ecologically Significant Fire in Oak Forests

A fire or series of fires that: (1) removes or reduces abundances of competitors of 

oak or keeps those competitors from entering the stand; (2) damages existing oak 

as little as possible; and (3) creates conditions for future recruitment of oak to the 

canopy, meets the definition of ‘ecologically significant’ for the purpose of restoring and regenerating oak. Ecologically significant fire is largely context-dependent 

because the factors limiting oak dominance and persistence vary among biophysical 

settings, stand-development stages, and forest types. For example, an ecologically 

significant fire for oaks in an oak woodland might have to eliminate duff so that 

germinating oaks can survive, and kill competing red maple saplings or understory 

shrubs. Removing overstory trees of mesic species and reducing total overstory 

canopy cover might also be required to remove competitors, promote persistence 

of overstory oaks, and facilitate seedling establishment in closed-canopy forests. 

Even if some oak seedlings are killed in a prescribed burn, it may still be possible 

to improve conditions for future oak recruitment (Simpson 2010). When restoring 

oak savanna that has grown into woodland, ecologically significant fire may include 
6

GENERAL TECHNICAL REPORT PNW-GTR-914

purposefully damaging some mature oaks and eliminating some (but not all) oak 

reproduction to reduce overstory and midstory cover and promote the development 

of understory grasses and forbs that facilitate frequent, low-severity fires. Therefore, how fires damage trees and change fuel loadings for ecologically significant 

fires are two of the main considerations discussed below.

Tree Damage During Fires

Fires can kill trees by killing the root system, girdling the base of the tree, or completely scorching or burning through the crown. The root system can be damaged 

or completely killed by heat fluxes into the soil or by combustion of organic soils 

(Steward et al. 1990). However, in mineral soils it is uncommon for soil temperatures 

to rise to lethal levels below a depth of about 2 inches (5 cm), unless glowing combustion in microsites such as a pocket of deep duff, pile of branches, or hollow log lasts 

much longer than the flaming front (Dickinson and Johnson 2001). Combustion of 

organic soils is not usually an issue in oak ecosystems, but oaks often occur on rocky 

sites with shallow soils that could expose root systems to mortality during fires if 

fuels are heavy enough to generate long fire residence times and penetration of heat 

pulses deep into soils. A tradeoff exists between rockiness and the ability of the fire 

to spread. For example, extremely rocky sites have non-contiguous fuel beds that 

impede the spread of fire and cause patchiness in fire effects (Signell and Abrams 

2006). Pockets of deep duff between rocks can support glowing combustion of long 

duration that may alternate with pockets that experience few fire effects. Patchiness in 

fire effects allows the coexistence of fire-dependent and non-fire-dependent tree species, such as found by Abrams et al. (1998) in Pennsylvania, where maple, basswood, 

and oak coexist. In contrast, moderately rocky sites may have contiguous duff and dry 

soils, allowing high soil temperatures during fires and more spatially homogeneous 

fire effects, especially across south-facing slopes. These variations in rockiness and 

the effects on fire behavior and tree species coexistence also occur across vast areas 

in Lake States oak forests in the Driftless area in Wisconsin, Minnesota, Illinois, and 

Iowa, and peninsulas and other rocky areas near the Great Lakes. Oaks also occur on 

deep sandy or loamy soils, where root death during fires is minor or spotty. 

Fires girdle trees at the base by either burning through the bark to the cambium or by generating a heat pulse that penetrates through the bark and heats the 

cambium to a lethal temperature. Girdling will often simply top-kill oaks and other 

sprouting-capable hardwoods like basswood and sapling-sized maples, but will 

totally kill species unable to sprout from belowground, such as older sugar maples 

and conifers. Scorching the foliage via rising radiant and convective heat from 

surface fires will kill some trees, but others may persist by producing a new cohort 

of leaves from protected buds, by producing new branches and leaves through 
7

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

epicormic sprouting, or through basal sprouting. Crown fire that directly chars the 

entire tree will generally kill conifers, but may only top-kill hardwoods (Michaletz 

and Johnson 2007). Crown fires and foliage scorch can occur in northern pin oak 

and mixed pine-oak forests on sand plains, leading to top-kill of the oaks, which 

commonly later resprout with multi-trunked trees.

Girdling of trees during low-intensity surface fires was also, under the oak-fire 

hypothesis, an important mechanism by which fire maintained oak dominance. 

Thick bark gives mature oaks more resistance to girdling than many of their 

thin-barked competitors (see box 1). In addition, oak seedlings and saplings often 

have higher rates of resprouting following top-kill than many of their more shadetolerant competitors. Partial girdling during fire typically does not kill trees, but can 

Box 1—Fire and cambial death at the base of trees

To kill cambium beneath the bark of a tree, the fire must either burn through the 

bark, or the radiant heat needs to penetrate the bark and bring the living cambial 

cells underneath to a temperature of 140 °F (60 °C) for at least a minute, or 158 

°F (70 °C) degrees for a shorter time. 

The time required for significant heat applied to the outer bark surface to 

reach and kill the cambium is related to bark thickness (Johnson 1992):

(1) Td = 2.9 BT2

Where Td = time to cambial death (minutes) for radiant and convective heat 

applied to the outside surface of the bark, at standard ambient air and flame 

temperature (68 °F/20°C and 932 °F/500 °C, respectively), and BT = bark thickness in cm.

Using formula (1), we can fill out table 1—note that on warmer days this time 

would be slightly less, on cold days slightly more, and that bark moisture content 

and flame temperature will also vary somewhat from standard conditions under 

which this model was developed, depending on fuel structure, wind, time since 

last precipitation, and other local site factors including slope steepness and aspect. 

Table 1—Time for cambial death (Td, minutes) at standard air and flame 

temperatures. BT = bark thickness

BT (cm) Td (min)

0.5 0.73

1.0 2.9

1.5 6.5

2.0 11.6

2.5 18.1

3.0 26.1
8

GENERAL TECHNICAL REPORT PNW-GTR-914

produce fire scars that provide an entry vector for fungal pathogens that can damage or kill trees and increase susceptibility of trees to mortality in subsequent fires. 

In ecological restoration, fire scar creation may not be of concern, unless long-term 

retention of overstory trees is a concern. However, fire scars are not desired where 

oak is used for lumber. 

Species and age-related differences in bark thickness and heat conductance 

among trees produce differences in tree mortality rates in response to fire. Since 

saplings and shrubs competing with oak—such as buckthorn, red maple, ironwood, 

and hazel—may have paper-thin bark, it is easy to discriminate against them in 

prescribed fires in mature oak forests, leaving the oak seed source and an open 

understory for oak seedling development. Note, however, that oak seedlings and 

saplings may also have thin bark and be killed or top-killed by a fire, and that a 

mature red maple or sugar maple may have bark thick enough so that it may not be 

killed by a fire without raising the intensity and duration of a fire to the point that 

some mature oaks would be killed as well (Hutchinson et al. 2005). Bark thickness differs among oak species as well as between oaks and maples; among oaks, 

bark thickness and tolerance to fire are generally ranked in this order: bur oak 

(most tolerant), black oak, white oak, red oak, and northern pin oak (least tolerant, 

Lorimer 1985). 

Growing to a size that provides bark thick enough to resist surface fires is 

known as reaching a size refuge. This is related to growth rate, with fast-growing 

trees with wide rings having thinner bark. Larger trees usually have narrower ring 

widths and thicker bark and, in addition, the trajectory towards thicker bark with 

increasing size and age of trees varies among species. While all species have thin 

bark in the first few years as a sapling, a transition to thicker bark occurs at younger 

ages for fire-tolerant species such as bur oak, but may take a few decades for red 

oak, and several decades for species such as sugar maple, red maple, basswood, 

and eastern hemlock; and in a few species, the bark may never attain substantial 

thickness (e.g., beech). Large old maples and basswoods can reach a size at which 

bark thickness is equivalent to that of a mature oak, and be able to survive the same 

types of fires as oaks. Large trees of those species can also sustain fire scars and 

survive for decades after being scarred, thus exhibiting more tolerance of fire than 

their classification as ‘fire sensitive’ species implies. Fire-scarred sugar maple, yellow birch, and hemlock trees are common in unlogged forests such as the Porcupine 

Mountains Wilderness State Park, Michigan, where these species are mixed with 

fire-regenerated red oak (Frelich and Lorimer 1991, also see details under “A Case 

Study of Fire and Red Oak” below).
9

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

Some studies have used an alternative measure of fire effects on oak, namely 

the char height, or height of stem blackening, which is generally a surrogate for fire 

intensity and for the total amount of heat the tree experienced. In one study, a blackening height of 5 ft (1.5 m) led to 50 percent mortality of white oak of 5 in (12.7 cm) 

dbh (diameter at breast height), while 10 ft (3 m) of blackening was necessary to 

cause 50 percent mortality of 10-in (25.4-cm) dbh white oaks following dormantseason fires; the same numbers for black oak were about 3 and 6 ft (0.91 and 1.8 m) 

(Loomis 1973). Trees were much more sensitive to fire during the growing season, 

when blackening heights to cause 50 percent mortality were about half that of 

dormant-season fires (Loomis 1973). 

Moving beyond individual fire effects, management of fire frequency may 

be important in managing oak forest ecosystems. Fire frequency influences the 

number of fires during the lifespan of a plant and the length of the recovery/

growth period between fires. Fire frequency is often related to fire intensity and 

severity because of feedbacks on fuel type and quantity. Finally, fire frequency 

can influence plant nutrient availability through its effects on nutrient pools and 

cycling rates. 

High fire frequencies place a premium on fire resistance or tolerance. High fire 

frequencies typically favor mature trees with thick bark that can protect themselves 

from injury. High fire frequencies can also favor trees and understory species 

with very high resprouting rates. In oak barrens in Minnesota, high-frequency 

prescribed fires (5 to 8 fires per decade) eliminated most non-oak species from both 

the overstory and understory, with understory oaks persisting as “grubs” through 

persistent sprouting after repeated top-kill (Peterson and Reich 2001). A problem 

with very-high-frequency fire regimes is that they prevent oak seedlings and sprouts 

from reaching fire refuge size. Occasional longer fire intervals are needed to allow 

oak reproduction to develop sufficient height and bark thickness to resist being 

top-killed by subsequent fires. Dey and Hartman (2005) found that larger black and 

white oak saplings (>4 in [10 cm] dbh) were very likely (80 to 90 percent) to survive 

(i.e., to resprout even in cases where the tree was top-killed by one or more fires) a 

single burn or 3 to 4 burns, while smaller saplings (1 to 2 in [2.5 to 5 cm] dbh) had 

somewhat lower survival rates (60 to 70 percent) after multiple burns. In Illinois, 

Bowles et al. (2007) found that 17 years of annual dormant-season burning eliminated most understory seedlings and smaller saplings (<2 in [5 cm] dbh), but caused 

loss of only 38 percent of larger saplings (2 to 4 in [5 to 10 cm] dbh), including a 

variety of non-oak tree species like maples and exotic buckthorn, apparently due to 

low fuel loads and low fire intensity. 
10

GENERAL TECHNICAL REPORT PNW-GTR-914

Lower frequency fire regimes allow a wider range of species to persist, but 

may still favor oaks over competitors. Mean fire return intervals of 3 to 10 years 

provide longer periods for vegetation recovery and growth between fires and allow 

many sprouting woody shrubs and trees to persist in the understory. Longer mean 

fire return intervals will allow oaks, and perhaps some other species, to reach fire 

refuge size and eventually replace overstory trees killed by cumulative fire effects 

and other disturbances—details are given in the section “Fire as a Regulator of Oak 

Abundance” below.

Fire may girdle a tree, or scar it by killing the cambium around only a portion 

of the basal circumference. Fire scars occur when larger trees are exposed to fires 

moving at moderate speeds, when vortices form on the lee side of the tree, creating flame duration that can be doubled or more compared to an average duration 

of flame exposure (Gutsell and Johnson 1996). Therefore, in a stand where mature 

trees have bark 1 cm thick, a flame front with general duration of 1.5 minutes might 

not kill the trees, but flame/heat duration could be 3 minutes on the lee side, and 

scar some of the trees. Once a tree is scarred, the callus tissue that rolls in from the 

sides during the tree’s attempt to cover the wound has thinner bark than the rest of 

the base of the tree, and therefore subsequent fires may be able to scar that part of 

the tree over and over. 

Fuels that promote long-duration fires can also lead to scarring or mortality 

from basal scorch, even in very large trees with thick bark (fig. 2). These fuels can 

include down logs (especially if hollow), piles of twigs and branches lying near the 

base of the tree, a broken branch standing vertically against the lower trunk of the 

tree, a pile of duff material caught by the trunk on the upslope side, and accumulations of organic debris beneath the favorite perch of some wildlife species, such as a 

pile of acorn shells and twigs deposited by a squirrel. Branches can be deposited 

near the base of the tree during a storm or harvesting operation. For any of these 

cases, flame duration in a small pocket could easily be 30 minutes to a few hours—

enough to create a fire scar from heat conducted through the bark, to burn through 

the bark, or perhaps even to burn through the base of the tree.

As previously mentioned, there is substantial variability among oak species and 

among individuals within species in their tolerance to fire. White oak and bur oak 

are more resistant to fire than red oak in two ways (Smith and Sutherland 1999). 

First, they are less likely to suffer a fire scar, and second, they are more likely to 

contain the spread of rot around fire scars. Northern pin oak, black oak, and red oak 

are more likely to rot at the base after being scarred and subsequently to blow down 

during a storm one to several decades later. Also, fires may have indirect effects on 

tree mortality. Peterson and Reich (2001) found that bur oaks had lower mortality 
11

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

at higher frequency of fire than northern pin oak on a sand savanna in Minnesota 

(as opposed to the expected higher mortality for higher fire frequencies). The likely 

cause was that pin oaks were getting taller than bur oaks and shading them out at 

low fire frequencies. High fire frequencies, on the order of 6 or 8 fires every decade 

for 4 decades, caused high mortality of the pin oaks, which then opened the canopy 

and lowered the mortality rate for bur oaks. 

Various objectives, such as converting oak woodland to savanna, open grassland to savanna, sugar maple to sugar maple with some oak mixed in, or maintaining oak by getting rid of a maple or ironwood understory, will require different 

considerations when using prescribed fire. The physics of fire and bark thickness 

(described above) for the sizes and species of tree involved will determine whether 

it is possible to save desired trees while simultaneously eliminating undesired trees. 

If there is a large difference in tree size and bark thickness—e.g., 20-in [50-cm] 

dbh oaks with 4-in [10-cm] dbh red maple underneath—then it may be relatively 

easy to prescribe a fire that would at least top-kill the maples without much harm to 

the oaks. If bark thickness is almost the same, then creating differential mortality 

among species will be difficult or impossible.

Figure 2—A freshly formed fire scar at the base of a northern pin oak tree. The large accumulation 

of fuel near the tree was due to windthrow of another pin oak that had been fire-scarred several years 

earlier. Duration of flames from a prescribed fire at tree base was ca 15 minutes but was less than 

1 minute elsewhere on this sand savanna at Cedar Creek Ecosystem Science Preserve, Minnesota 

(Frelich, personal observation).

Kalev Jõgiste
12

GENERAL TECHNICAL REPORT PNW-GTR-914

Fuel Loads for Ecologically Significant Fires

To be ecologically significant, fire must, as described above, kill (or at least top 

kill) shrubs such as hazel and buckthorn, and seedlings/sapling of shade tolerant 

trees such as sugar maple, red maple, and basswood. Generating a flame with 

>900°F (>486 °C) heat that lasts long enough to kill unwanted species requires a 

certain fuel load on the forest floor, and contiguous fuel to carry the fire. Generally, if a fire has a flame length of 2 to 4 ft (0.6 to 1.2 m) and lasts for a minute or 

so as it passes by a given point, it could achieve selective kill of other understory 

species and leave mature oaks undamaged. Oak seedlings and saplings might also 

be top-killed, but if they are established as grubs, with a root at least 0.5 in (1.2 

cm) thick at the root collar, they can resprout and grow rapidly using resources 

stored in the root system, and may perhaps outgrow sprouts of other species 

(Brose 2008). 

If the fuel bed is not contiguous, or tonnage of fuel is too low, the fire may 

merely be a way to remove the duff, which might have some advantage in terms of 

producing seedbed conditions for oak germination, but would not necessarily set 

back shrubs and saplings of competing tree species enough to give oaks a window 

of time to become established. The fuel bed consists of duff, leaf litter (top layer 

of duff), fine twigs (10-hour time-lag fuel), and medium-diameter branches (100-

hour time-lag fuels, 1- to 3-in [2.5- to 7.6-cm] diameter), with abundance of the 

latter especially important for fire intensity. Thousand-hour time-lag fuels (large 

snags and logs) are mostly not consumed in fires, and contribute to fire intensity to 

a moderate degree, although exceptions can occur, such as hollow coarse woody 

debris that has access by oxygen to the interior, creating a large surface area of 

burnable wood. 

A variety of factors affect fuel bed accumulation and contiguity in oak forests. Fuels in eastern deciduous forests have higher decomposition rates than in 

western or boreal conifer forests, and therefore, amount of fuel available does not 

accumulate to huge tonnages over time; furthermore, fuel loads (duff, 1-, 10- and 

100-hour) recover quickly—within a few years—after experimental fire (Graham 

and McCarthy 2006). Annual accumulation of 2 to 3 tons/acre [4.5 to 6.7 tonnes/

ha] of leaf/litter fall is common in oak forests (Kaczmarek et al. 1995), and litter 

accumulation in the forest floor may be two to three times the annual litterfall. 

Lesser accumulations occur on mesic sites with exotic earthworm invasions, where 

decomposition of litter occurs faster than on dry and/or earthworm-free sites—see 

‘Earthworm Invasion’ below. A survey of forest floor mass (Smith and Heath 2002) 

shows a mean of about 6.2 tons/acre (13.9 tonnes/ha; range 2.5 to 31 tons/acre [5.6 

to 69 tonnes/ha]) in mixed hardwood and oak forests in the northern U.S. from 
13

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

North Dakota and Missouri to the east coast. Fuel models for a variety of situations, including savannas with grass and shrub/grass fuels, and hardwood forests 

with forest floor litter fuel beds of various loadings, are summarized by Scott and 

Burgan (2005); discussion of how the models work to predict fire behavior for the 

case of oak forests is given by Waldrop et al. (2006). 

Reports in the literature suggest that fires in oak forests have been ecologically significant when 1 to 6 t/ac (2.2 to 13.4 t/ha) of duff, 1-hour, 10-hour and 

100-hour fuels are present and distributed in a contiguous manner (Chojnacky 

and Schuler 2004, Collins and Carson 2003, Dey and Hartman 2005, Kruger 

and Reich 1997). Even on sites without a contiguous fuel bed (e.g., sites where 

earthworms have eliminated the duff layer), the right amount and contiguity of 

fuel might occur after moderate-intensity windthrow, or after harvesting that 

deposits the needed amount of 1- to 3-in (2.5- to 7.6-cm) diameter (100-hour 

time-lag) fuels to the forest-floor fuel bed, especially if the burn is done in fall 

or early spring when leaf litter is also present. High-intensity fires may occur in 

heavy windfall or harvest slash, which may kill mature oaks and be difficult to 

control. If fireline intensity (I = Kw/m) is predicted to be >5000, larger firebreaks 

and buffer zones with lower fuel loads than are usual for prescribed fires are 

needed. In such cases, water bodies or areas 50 to 100 ft wide [15.2 to 30.5 m] 

with fuel previously removed by a lower-intensity burn or mechanical removal are 

needed to ensure fire will not escape the prescribed zone, or to protect any desired 

residual trees. 

Fires prior to the arrival of early settlers burned in oak savannas and forests 

with a wide variety of fuel loads, probably forming a mosaic of fuel loads across the 

landscape at spatial extents of 1000s of acres. Oak savannas that were burned every 

few years probably had low fuel loads of a few tons per acre composed of grasses 

and small shrubs, with low-intensity and short-duration fires (I in the range of 500 

to 1000 Kw/m, 1-minute duration), leading to high levels of oak seedling death or 

top-killing of well established “grubs,” but little mortality of mature oaks. However, 

elsewhere, high fuel loads were created due to extensive areas of treefall slash 

from windthrow, ice storms, and passenger pigeon roosting areas, deep duff due to 

absence of invasive earthworms, and accumulation of woody growth due to lessfrequent burning than on savannas. Such areas likely had high fuel loads equal to 

that of the tree-tops from seed-tree harvests—many tons per acre, with associated 

high fire intensities and long duration (I in the range of 5000 to 30 000 Kw/m, 5- to 

10-minute duration)—killing mature trees and creating conditions for recruitment 

of even-aged cohorts of oak. 
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GENERAL TECHNICAL REPORT PNW-GTR-914

Fire as a Regulator of Oak Abundance

Historical Fire Regimes and Oak Versus Grass, Maple, and Pine

For centuries, or perhaps millennia, fire has modulated the interaction between oak 

as a dominant species and three main vegetation types: mesic forests of maple, 

hemlock, basswood, and yellow birch; cold-temperate and boreal forests of pine and 

spruce-fir; and grasslands, portrayed conceptually as the ‘Oak Triangle’ in fig. 3. 

Oaks occur within each of these three vegetation types (although only at the southern 

and eastern edges of the boreal and prairie biomes, respectively), with the abundance 

of oaks versus mesic tree species, pines, or grasses regulated by complex interactions 

with fire and soil type (Curtis 1959, Gleason 1913, Grimm 1984, Heinselman 1996). 

There is abundant evidence, based on fire scar analyses, that fire frequency in 

oak forests of central North America has been much lower in the last century than in 

previous centuries, as fire return intervals from 4 to 16 years have increased to several hundred years (Dey and Guyette 2000a, 2000b; Shumway et al. 2001). McEwan 

et al. (2007) showed that the current mixed-oak canopy in Ohio and Kentucky was 

Figure 3—The Oak Triangle, showing the relationships between oak forests, oak savannas, mesic 

forests, and boreal/northern pine forests in the Lake States.
15

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

established during a period lasting several decades with fires approximately every 

6 years (range 1.7 to 11.1 years among stands). Land clearing followed by frequent 

fires led to white and red oak establishment in the Ozark Mountains, followed by 

a period of fire suppression and no oak recruitment (Soucy et al. 2005). Arthur et 

al. (2012) emphasized that several short fire intervals followed by a longer interval 

(i.e., several short intervals of 2 to 5 years followed by a 20- to 30-year interval), 

would go a long way towards pushing back competition in forest understories and 

then allowing oaks to grow large enough to resist damage during the next fire that 

follows the long interval. This observation points out one aspect of the presettlement 

fire regime that variously benefitted oaks, grasses, or mesic tree species depending 

on the situation (see details below): over decades and centuries, fire regimes had 

great variability in intervals between fires. 

Fire intervals in the central United States have been influenced not only by 

climate, but also by anthropogenic factors. Guyette et al. (2002) showed that four 

stages occurred in Missouri oak woodlands/savannas. During the first stage, prior 

to extensive settlement of the area, anthropogenic ignitions were infrequent and 

resulted in low fire frequency. During the second stage when settlement was active, 

human ignitions were very frequent and fuel buildup became limiting. During the 

third stage, fuel fragmentation restricted fire spread, as large parcels of land were 

converted to agricultural land use. Finally, in the modern era, cultural practices 

such as fire suppression and decisions to carry out prescribed fire became the 

dominant factor in the fire regime. These stages also occurred in mixed pine-oak 

and northern hardwood-oak forests further north (Heinselman 1996), but were less 

accentuated due to the overall lower human populations, less conversion of land to 

uses other than forest, and colder climate with a shorter fire season. 

Although aridity alone can preclude trees from grasslands, paleoecological 

studies show that frequent fires can also exclude trees from sites that are mesic 

enough to support tree establishment and persistence (Umbanhowar et al. 2006). 

In the Lake States, the climate can support trees on all but the most severe coarse 

sandy or rocky sites, and recruitment of trees on savannas has occurred even during 

some of the more severe droughts of the last century, as long as a sufficient fire-free 

interval allowed seedlings to grow large enough to resist the next fire (Ziegler et 

al. 2008). Therefore, fire regulates the balance between oak and grass, with highfrequency fire in the range of 1- to 5-year return intervals, when sustained over 

long time periods, leading to lack of oak recruitment and replacement by grasses 

with some shrubs (Hallgren et al. 2012, Peterson and Reich 2001). Frequent burning 

in nutrient-poor sand savannas can lead to lower nitrogen availability, enhancing 

dominance of grasses relative to oaks (Peterson and Reich 2001).
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GENERAL TECHNICAL REPORT PNW-GTR-914

Variability in fire timing and spatial heterogeneity in fire intensity allow some 

oaks to persist in savanna vegetation types (fig. 4). In a savanna in southeastern 

Wisconsin, the mean fire interval over the last 200 years was 6.3 years, but fire 

intervals ranged 2 to 33 years, and the mean interval also changed among presettlement, settlement, postsettlement and modern eras (Wolf 2004). As an example of 

how variability in fire interval may work to allow oaks to persist on a savanna, 

consider the case where the mean fire return interval is 3 years, but it takes 7 years 

for an oak to reach a size such that it can persist through future fires (bur oak is best 

at this, it grows thicker bark early in life, but other oaks can also sometimes persist). 

Variability in fire intervals might lead to annual burns several years in a row, 

followed by 7 or 8 years without a burn, allowing oaks to get established, while 

maintaining a 3-year mean fire interval. Spatial heterogeneity such as that caused 

by gopher mounds that create mineral soil exposures of a few square feet in size, a 

large rock, or small wet area, or differences in slope and aspect might also allow 

some oaks to persist by creating small inclusions where fire intervals are longer 

than that for the area as a whole (Franklin et al. 1997). 

Mean fire intervals in the range of 5 to 15 years usually lead to shrubby 

savanna-woodland vegetation types, often with hickories, elms such as rock elm 

and American elm, walnut, and basswood mixed with oaks on uplands, and cottonwood and red or silver maple mixed with oak in lowland savannas, depending on 

location relative to ranges of tree species (Curtis 1959; Dettman et al. 2009; Frelich, 

personal observation). The common view that savannas consisted of scattered bur 

Figure 4—Relationship of mean fire interval (MFI) and its variability to oak.
17

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

oaks or just a few oak species over grasses is probably overly simplified (Dettman 

et al. 2009), except possibly for extremely nutrient-poor sites with coarse sandy soil 

such as those sites studied by Peterson and Reich (2001). 

Moving to longer mean fire intervals (e.g., 15 to 40 years), variability works 

against oak whereas regular timing works in favor of oak (fig. 4). Here, fire regulates the balance between oaks and maples and other mesic forest species (Henderson and Long 1984, Holzmueller et al. 2009, Hutchinson et al. 2008, Lorimer 1985, 

McCarthy et al. 2001, McClain et al. 2006, Signell et al. 2005). Because it takes 

three to four decades for understory maple saplings to develop bark thick enough 

to resist a fire, a variation in fire interval causing a single 50-year interval, even if 

the mean interval is 20 years, will allow maples to establish and reach a size refuge, 

and shorter fire intervals going on for decades or centuries until the old maples have 

died would then be necessary to get rid of them unless they are mechanically or 

chemically removed. The power of fires every 10 to 20 years going on for centuries 

to favor oaks, as was the case on the landscape we inherited at the time of early 

settlement, should not be underestimated. Also not to be underestimated was the 

mass effect on the landscape of having large areas dominated by oaks, where maple 

seed sources were far away, so that even if some longer fire intervals occurred, 

maples would not necessarily have appeared. This situation is hard to create on the 

modern landscape. 

Fire intervals on the order of a hundred years or more might allow scattered 

groves of oaks to appear within mesic forests, provided that the fires are intense 

enough to wipe out the maple regeneration layer and deer do not browse the oak 

seedlings (fig. 4). Because oaks that do get established can persist for 200+ years, 

scattered groves can easily exist across the landscape even in forests where average return times for fires are 500 to 1,000 years (Frelich 2002). However, note 

that dryness (as regulated by soil type, slope, and aspect), topographic roughness, 

settlement patterns, road networks, and presence of large tracts of public ownership 

can also lead to a mosaic of fire frequencies across the landscape (Yang et al. 2007), 

so that various mixtures of forest types, woodlands, and savannas may be present. 

In Minnesota and Wisconsin, regional water balance determined, at spatial scales 

of >50 mi (>80 km), where the prairie-forest border was located—with grasslands 

in areas where evapotranspiration exceeded precipitation and dense forests in 

areas with an excess of precipitation. At smaller spatial scales of 5 to 10 mi (8 to 

16 km), the location of grasses and forests was fine-tuned by the presence of sandy 

soils, fire breaks (rivers and lakes) and topographic roughness (Danz et al. 2011). 

Grasslands and oak savannas occurred on flat, sandy outwash plains, rolling terrain 

to the southwest of fire breaks, and southwest-facing slopes (alternating with oak 
18

GENERAL TECHNICAL REPORT PNW-GTR-914

woodlands and mesic forest on north and east slopes) in areas with dissected topography (Curtis 1959, Danz et al. 2011, Grimm 1984). Oak woodlands and savannas 

also occupied sites that could have supported forest but that were frequently burned 

by Native Americans, for example sites that are now occupied by the cities of Green 

Bay and Minneapolis (Berland et al. 2011, Dorney and Dorney 1989). Throughout 

the Eastern United States, a large proportion of white and red oak forest and 

savanna grew on sites that are capable of supporting mesic forest, but supported 

oaks owing to regular burning over centuries (Abrams 2003), although clearance 

and pasturing the land by settlers could also create savannas on lands capable of 

supporting forest (McEwan and McCarthy 2008). 

Finally, pine-oak relationships can be somewhat regulated by fire, but climate 

is probably also important. Mature oaks in northern boreal-type climates (e.g., 

northern Minnesota) are short, and they are relegated to understory status in the 

southern margin of the boreal forest, and therefore produce few acorns. In slightly 

warmer climates (northern hardwoods of northern Wisconsin and Michigan), white 

and red pine forests have mixed fire regimes with high-intensity stand-initiation 

fires, moderate-intensity fires, and low-intensity fires every few centuries, every 

several decades, and every one to two decades, respectively (Frelich 2002, Guyette 

and Dey 1995). This mixed fire regime generally allows coexistence of red oak and 

white/red pines, all of which have thick bark when mature, so that they are capable 

of surviving fires even if scarred, and continuing to shed seeds for decades after 

a fire. There is no fire frequency that can eliminate oaks while allowing pines to 

dominate, but the opposite might occur. Oaks can resprout as grubs, and pines 

cannot, so in areas with very high fire frequency, pine reproduction will disappear 

(Blankenship and Arthur 2006, Chiang et al. 2005). Two high-intensity fires within 

an interval too short for pines to bear a seed crop (<10 years) could effectively wipe 

out pines while only top-killing oaks. Some pines could possibly withstand more 

drought than oaks, or more nutrient-poor soils than oaks, or deer might browse 

oaks more than pines, so that a combination of factors could drive forests towards 

dominance by pines in a climate where both oaks and pines can survive. 

Northern pin oak and scarlet oak can be regenerated as even-aged coppice 

stands by crown fire as a result of marscesent leaves (dead leaves that remain 

attached through winter), which also protect the postfire stump sprout’s terminal 

buds from browsing while they are less than 5 ft tall. Coppice stands with multiple trunks are expected and are sometimes mixed with jack pine on sand plains. 

This is one of the few examples of hardwood forest that supports high-intensity 

fire with even-aged stands, in the absence of major blowdown or logging. In contrast, most of the landscape on better soils would tend to support multiaged oak 
19

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

savannas, woodlands, and forests, created by a disturbance regime that kills 10 

to 30 percent of the trees within an acre every one to several decades, continuing 

over centuries. Some even-aged patches (tens to thousands of acres in size) would 

be present resulting from blowdown-fire combinations within these multiaged 

landscapes (Frelich 2002). 

Logging of mixed white pine and oak forests during early settlement tended 

to work against the white pine, which cannot resprout and has no persistent seed 

bank. Red oak and other oaks regenerating after pine removal in a low-deer environment, sometimes with the help of site preparation from slash fires, created red 

oak forests that are common today across the northern tier of states in the Eastern 

United States and adjacent Canada (Leahy and Pregitzer 2003). Subsequent absence 

of logging or fire for several decades combined with increasing deer populations 

allowed establishment of maple and cherry understory seedlings, so that modernday logging is accelerating succession towards mesic forest species on these sites 

(Abrams and Nowacki 1992). Pine can be removed from mixed pine-oak forests by 

two high-intensity fires in a short time or by logging of mature pines followed by a 

surface fire or slash fire that kills pine reproduction. 

Burning Frequency and Competitiveness of 

Oak Regeneration in Forests

Oak sprouting and oak competition with other understory species have complex 

relationships with fire. Single burns may enhance reproductive success of oaks but 

create a short window of opportunity and allow mesic species to return within a 

few years (Brose 2010). Two or three reburns in successive years may be necessary 

to kill resprouting species such as hazel, buckthorn, or red maple, while possibly 

allowing oak grubs to survive and eventually be released (Blake and Schuette 

2000, DeBord et al. 2011). Basically, this process of repeat burning has the objective of exhausting the below-ground storage of energy used to create resprouts—

leading to death of the plants—and doing so for the undesired species but not for 

the advance regeneration of oak. This represents a fine line in the balance between 

oaks and other species (Blankenship and Arthur 2006, Bowles et al. 2007). The 

literature suggests that oaks in general are somewhat better adapted to survive 

repeat burning (Haney et al. 2008); however, repeat fires may also kill some oak 

seedlings and saplings, especially northern red oak, which has thin bark and 

relatively poor sprouting ability compared to other oaks (Collins and Carson 2003, 

Green et al. 2010). Another problem for multiple burns is the presence of sufficient 

fuel for ecologically significant fires during burns that occur within 1 or 2 years 

of the initial burn, especially on mesic sites with earthworms (see “Earthworm 

Invasion” below). 
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GENERAL TECHNICAL REPORT PNW-GTR-914

Even if successful, multiple burning may not create a long window of opportunity 

for regeneration of oak if mature trees of the undesired species are still present, as the 

forest floor seedbank could be readily restocked with maples, hazel, or buckthorn. In 

addition, the increased light levels associated with thinning of the forest midstory by 

repeated fires could also cause an increase in density of the herbaceous layer that can 

compete with oak seedlings at the germination and establishment phases (see “Competing Vegetation and Invasive Plants” below). Long windows of opportunity for 

oak establishment can be created if burns are successful at killing the seed-bearing 

individuals of undesired species, or if those species are removed mechanically prior to 

burning. Eliminating red maple by exhausting the energy of sprouting stems through 

repeated burns without killing regenerating oaks is difficult (Huddle and Pallardy 

1999), and given the variability in the forest, probably not always achievable through 

burning alone, especially on mesic sites. Managers may have to settle for a forest with 

some red maple, ironwood, and other species mixed with oak. 

Fire Season

Fire season can be an important factor influencing oak regeneration through its 

effects on acorn development, deposition, and germination, and subsequent oak 

establishment. Fire season interacts with regeneration phenology to determine 

direct effects of fire on acorns and seedlings. Fire season also alters the importance 

of plant-animal interactions such as the caching of acorns by squirrels. Finally, fire 

season may influence the relative abundance and dominance of oaks and competitor species through previously mentioned differential fire effects on regeneration 

and survival. 

A recent meta-analysis of 32 prescribed fire studies conducted across eastern 

North America over the last several decades showed that prescribed fire can be successful as a tool for site preparation for oak seedlings and for release of well-established oaks (Brose et al. 2013). Growing-season fire (between beginning of spring 

leaf expansion and autumn leaf abscission) was found to be more effective at giving 

oak the advantage over other hardwoods, especially if it occurred several years 

after a reduction in overstory density. Apparently, growing-season fire is better at 

setting back mesic hardwood regeneration than dormant-season fire, consistent with 

predictions that follow from an examination of physiological response of oaks and 

other hardwoods to the heat of fire (Abrams 1996, Romero et al. 2009). Analysis of 

lightning-strike activity and precipitation suggests that natural ignitions are most 

likely to occur during the growing season in Ohio (Petersen and Drewa 2006), and 

if this pattern holds for most of the range of oaks, then combined with experimental 

evidence, there is a strong suggestion that growing-season fires should be considered in the mix along with dormant-season fires for oak forest restoration. 
21

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

Damage and mortality to acorns can be high for autumn fires for white oaks 

(which germinate in fall) or during the entire fall-winter-spring dormant period for 

red oaks (which germinate the following spring). Greenberg et al. (2012) found that 

acorns on the surface of the leaf litter suffer high mortality rates during dormantseason prescribed burns, but that those acorns deep in the duff or mineral soil often 

survive owing to insulating properties of the litter/soil. In contrast, growing-season 

fires probably have little impact on developing acorns held high in the canopy, with 

the possible exception of acorns developing on oaks that are intermixed with pines, 

where crown fires may occur. 

In presettlement forests, natural fires in mid summer could have been quite 

intense if they interacted with unsalvaged slash from windfall and passenger pigeon 

damage. White oak acorns falling during the following autumn would have landed 

on a litter layer thinned by fire, making penetration of the germinating radicle into 

mineral soil possible, and would have been covered by the following autumnal leaf 

litter as well, helping the acorns escape detection by deer and keeping the acorns 

moist during germination. Most prescribed burns today have difficulty recreating 

these conditions as a result of the conservative nature of burn prescriptions that 

do not allow fires to take place with very low-humidity and high-wind conditions 

during severe droughts—conditions during which natural fires made their large 

runs with high intensities likely to kill species that compete with oaks. Although 

they would have had immediate negative effects on oaks, oaks were more poised to 

regrow vigorously in such postfire conditions. It may take many low-intensity spring 

prescribed burns to equal the effect of one high-intensity mid-season wildfire, or to 

come close (as their effects will never quite match those of the single hot fire).

Multiple Factors Interacting With Fire

Regenerating oak requires managing the interplay between oaks and competing 

vegetation at the seedling, sapling, and mature stages of the oak life cycle. Success 

of oak regeneration is likely to be context-specific owing to the unique combination 

of variables present on a given site (Brose et al. 2013, Dickie et al. 2007). A thin 

leaf-litter layer and higher canopy openness created by prescribed fires were the 

most important factors predicting success of white oak seedlings in experiments on 

the Cumberland Plateau (Royse et al. 2010) and Piedmont of South Carolina (Wang 

et al. 2005). Increased soil nutrient levels after prescribed burns further enhance 

oak seedling growth in productive forests (Scharenbroch et al. 2012). These conditions last for a short time after a fire and interact with predation on acorns, herbivory, stand development, competing understory vegetation, and invasive species, 

as explained in the following subsections. 
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GENERAL TECHNICAL REPORT PNW-GTR-914

Plant-Animal Interactions

Acorn predation—

Acorn mortality occurs from insects that use acorns as habitat (weevils and 

galls) and from animals that eat acorns on the ground (deer, bears, turkeys) or 

pick acorns from trees and cache them for later use (squirrels and blue jays). 

A combination of acorn weevils (Curculio spp.), pip galls (cynipid wasps), and 

consumption by squirrels, chipmunks, mice, turkey, deer, and bears, can destroy 

most acorns in an oak stand (Bellocq et al. 2005, Marquis et al. 1976). McShea 

and Schwede (1993) found that deer ate 70 percent of marked acorns placed 

in the forest during mast-fall and squirrels and chipmunks also consumed 61 

percent of acorns later in the season, so that predation on acorns significantly 

limited oak reproduction during low mast-producing years. In contrast, a study 

of acorn consumption comparing forested landscapes with woodlots in an agricultural landscape found that deer consumption of acorns was relatively small 

when agricultural foods such as soybeans and corn were available (Nelson et al. 

1988). Brose (2011) reported on the fate of red oak acorns after a bumper crop 

in Pennsylvania. Overwinter survival of unburied acorns was only 20 percent 

owing to desiccation, insect infestation, disease, and consumption by wildlife, 

while 80 percent survival occurred for acorns buried in leaf litter. Thus, burying 

of acorns by squirrels probably leads to survival of those acorns that squirrels 

fail to relocate later.

Despite predation on acorns, germination of oak seedlings is still found in 

many forests. Passenger pigeons no longer consume the huge quantities of acorns 

that they once did (Ellsworth and McComb 2003), so that squirrels, turkeys, deer, 

and bears have probably increased use of acorns. It is likely that the masting pattern 

of oaks evolved to super-satiate acorn predators every few years, so that a number 

of acorns go on to germinate regardless of the species consuming acorns. 

Herbivory—

One of the most significant bottlenecks in the life cycle for most oaks is likely 

that deer eat seedlings a few years after germination, when they are between 6 in 

and 5 ft tall (15 cm to 1.5 m)—susceptibility to browsing usually starts during the 

second or third year after germination. Oaks are favored species by deer, especially red oak (fig. 5). A study of red oaks in Pennsylvania (Brose 2011) found 

that survival for 8 years after acorns were produced was strongly affected by deer 

as well as level of understory density and shade. Seedlings that germinated inside 

a deer fence with partial shade had 56 percent survival after 8 years, while those 

exposed to deer and dense shade had only 2 percent survival. If fencing is used to 

control deer impacts on oak seedling mortality, 2 years of fencing has been found 
23

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

to be ineffective and 6 to 7 years of protection from deer and an understory 

environment without dense shade are needed for oaks to grow beyond the reach 

of deer (Yuska et al. 2008). However, rapidly growing oak seedlings can outgrow 

deer. If an oak seedling/sapling puts on 1 to 3 feet (30 to 91 cm) of new growth 

with a thick leader (at least 1/4 in diameter [0.63 cm]), then deer are likely to take 

only part of the new growth; thus the sapling can make several inches of upward 

growth progress every year, and will eventually reach a height where it is beyond 

the reach of deer. In a dense-shade situation, or in small gaps, the leading twigs 

of oaks are thin and deer can take the entire twig, effectively creating a hedge 

that is chopped off at the same height year after year until the sapling dies. Thus, 

oak saplings with more light are more likely to outgrow deer (Lorimer et al. 

1994). Resistance to browsing will eventually develop through natural selection 

and spread throughout oak populations, as there is variability in palatability 

within natural populations. However, this will take several generations, and in the 

very long meantime, deer can give the advantage to ironwood, red maple, sugar 

maple, pines, and other species that are either not palatable or that have an 

excellent ability to grow rapidly, or resprout repeatedly after burns or browsing 

(Matonis et al. 2011). 

Figure 5—A northern red oak receiving insufficient sunlight to make any progress in height growth 

due to deer browsing. 

Nick Fisichelli
24

GENERAL TECHNICAL REPORT PNW-GTR-914

Earthworm invasion—

There are no native earthworms in most of the northern hardwood region, and 

European earthworms have invaded most of the landscape of Wisconsin, Minnesota, and Michigan. Changes caused by earthworms can influence fire behavior and 

intensity and the forest floor environment experienced by oak seedlings. 

Earthworms are important soil ecosystem engineers and because they can alter 

soil community composition, nutrient availability, and the structure of soil itself, 

these abundant and keystone detritivores may have large invasion impacts. Invasive 

earthworms occur throughout the world (Frelich et al. 2006, Hendrix et al. 2008) 

and are recognized as one of the most important environmental issues of our time 

(Sutherland et al. 2011). Invasive earthworms change soil structure and function by 

consuming the organic horizon (or leaf litter), increasing bulk density and mixing 

of the mineral horizons (Alban and Berry 1994, Hale et al. 2005) and consequently 

affecting nutrient and water cycling in the soil (Bohlen et al. 2004, Costello and 

Lamberti 2009, Hale et al. 2005). These changes to the soil environment have been 

shown to produce many ecological cascades that affect native plants and animals, as 

well as facilitate invasive species that compete with native species (Heimpel et al. 

2010, Nuzzo et al. 2009). Earthworms can affect native plants in many ways, including (1) ingesting seeds; (2) eating germinating seedlings; (3) influencing symbiotic 

fungi that plants need for germination and growth; (4) changing the physical characteristics of the seedbed that affect germination; (5) changing the nutrient status 

of the soil, often decreasing availability of nitrogen (N) and phosphorus (P) to 

plants; (6) changing the water balance of the soil, drying the A horizon; (7) changing the competitive balance among plant species; and (8) interacting with insects 

and mammalian herbivores that affect plant growth (Eisenhauer et al. 2009, 2010; 

Fisichelli et al. 2013; Hale et al. 2005, 2006, 2008). Negative impacts of earthworms 

on rare plants and native plant species richness have been shown (Gundale 2002, 

Holdsworth et al. 2007), as have decreases in tree growth (Larson et al. 2010). 

The two most relevant changes for oak forests caused by earthworms are their 

impacts on the understory environment, including competing vegetation, and on the 

forest-floor fuel bed. Forest-floor conditions created by earthworms favor germination of graminoids in general (grasses and sedges, Eisenhauer et al. 2009), and also 

favor other invasive plant species (buckthorn and garlic mustard, Knight et al. 2007; 

Nuzzo et al. 2009; Van Riper et al. 2010). See details in “Competing Vegetation 

and Invasive Plants,” below. Exotic and native slugs as well as the exotic European 

earthworm, the nightcrawler, are direct predators on germinating seedlings; they 

eat developing leaves (cotyledons) of germinating plants (Eisenhauer et al. 2010). 

However, no studies have been done to quantify these effects on oaks. 
25

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

Invasion of European earthworm species, especially the detritivore species, 

nightcrawlers and leaf worms, is having a large impact on the duff portion of fuel 

loads in deciduous forests (Frelich et al. 2006). This impact varies by stage of 

invasion (box 2), from thick duff at stage 2 to almost no duff at stage 5. Much of the 

Midwestern landscape far from metropolitan areas and waterways used for fishing 

(which are the origin points for earthworm invasion fronts) is in stages 2 through 3 

of invasion, with a thinned but still contiguous leaf-litter (duff) layer, making it 

easier to apply a prescribed fire than at later stages of invasion. However, it seems 

likely that all areas with productive oak forests will eventually progress to stage 5 

of invasion—which can occur on loamy-sand soils with up to 85 percent sand 

(Holdsworth 2007). High stages of earthworm invasion are unlikely in unproductive 

northern pin oak forests and savannas on pure sand. 

Box 2—Stages of earthworm invasion

Earthworm invasion occurs in five stages (Loss et al. 2013; fig. 6): 

Stage 1—earthworm-free, with thick duff and fresh litter, fragmented litter 

and humus layers. 

Stage 2—the duff-dwelling species Small Leaf Worm is present, but duff is 

still thick.

Stage 3—the leaf worm (a strong detritivore capable of consuming the entire 

duff layer) is present, and usually more than one species of angleworm, which 

live in the top 15 in (38 cm) of soil, grazing on organic matter in the A and B soil 

horizons. In stage 3, the litter layer is substantially reduced in mass and thickness but is still contiguous and may still carry a fire, although the amount of fuel 

may not support ecologically significant fire intensities until after leaf fall. 

Stage 4—there are heavy infestations of the leaf worm and angleworm species, and the nightcrawler invasion has begun, with patches of bare mineral soil 

forming by mid-summer. 

Stage 5—the terminal stage of invasion. The nightcrawler, a detritivore that 

eats freshly fallen litter, is the dominant species, and commonly the litter layer 

disappears by mid summer, so there is no permanent duff layer. Instead, the duff 

layer exists temporarily from fall to spring, with much lower mass and potential 

to support an ecologically significant fire than sites at stage 3 or lower of earthworm invasion. At this stage, there may be a large accumulation of twigs (10- 

and 100-hour fuels) that otherwise would have been embedded in the duff where 

they would have rotted away (Frelich, personal observation). However, no studies 

have been published on the characteristics of these fuel beds in earthworminfested areas, and it is not certain if they are contiguous enough to carry fire.
26

GENERAL TECHNICAL REPORT PNW-GTR-914

Figure 6—Forest floors of (A) Stage 1 of earthworm invasion—earthworm-free; note thick and 

contiguous duff; (B) Stage 5 of earthworm invasion at midsummer—note the low fuel loading and 

discontinuity of the duff. 

A

B

Paul Ojanen George Schlaghamersky 
27

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

For the large majority of oak forests now in stage 3 of invasion, the duff is 

likely thin enough for successful oak germination, having a positive effect on oak 

germination in the absence of fire. On the other hand, the earthworms have done 

only half of the work accomplished by fire—fire creates a litter layer of reduced 

thickness that oaks prefer and kills shrubs and competing tree seedlings. Earthworms do this first half, namely creating a thinner organic layer, sometimes eliminating the need for scarification or fire for the purpose of creating an ideal seedbed. 

However, earthworms do not duplicate the second part of fire effects, namely killing 

saplings of trees such as red maple and understory shrubs. 

Stand Development

Stand structure and development have a large impact on success of oak seedlings. 

The intensity of shade and abundance of competing understory species varies as 

stands develop. A large peak in light levels occurs during establishment after standleveling disturbance, dense shade occurs during the even-aged, stem-exclusion 

stage of development, and an increase in light occurs during canopy breakup and 

the transition to the uneven-aged stage, although understory shrubs and shadetolerant tree species can also increase. Modification of stand structure and canopy 

shading are therefore important in restoration-based management of oak forests, 

and fire is an important (but not all-powerful) tool for managing stand structure and 

canopy shading. 

Shade tolerance varies among oak species, but in general, oaks are less 

tolerant of shade than their competitor species. However, not all shade is the same, 

and under certain conditions, oak species can be surprisingly tolerant of shade. 

Several studies provide evidence of growth prior to releases from suppression that 

are slow, approximately equal to suppressed sugar maple and hemlock on mesic 

sites—and such periods of suppression prior to release can last 40–90 years—even 

for species such as red oak, black oak, and white oak (Rentch et al. 2003). However, this occurs in forests with oak-dominated canopies and open understories 

(Povak et al. 2008), which have light levels much higher than either mesic maple 

and hemlock forests or oak forests invaded by buckthorn (fig. 7). Unfortunately, 

red maple, ash, and other moderately shade-tolerant tree species can also grow in 

open oak understories. 

Old even-aged stands and stands in transition to uneven-aged canopies, with a 

high density of overstory oak trees and understory shade-tolerant trees such as red 

maple, have little or no successful oak regeneration. Burns in such stands will not 

be successful at allowing oaks to establish unless the understory layer is removed, 

so that light levels increase (Signell et al. 2005). In contrast, the disturbance regime 

of presettlement forests of the central and Eastern United States included patches of 
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GENERAL TECHNICAL REPORT PNW-GTR-914

trees killed by passenger pigeons, blowdown, and fire, leading to multiaged stands 

with more widespread and larger gaps across the landscape than those that form in 

the even-aged stands that are common today (Buchanan and Hart 2012). Although 

some larger gaps are created by natural disturbances in the contemporary forests 

by wind, ice storms, insects, and disease, many stands today have relatively small, 

ephemeral gaps because canopy trees dying from self-thinning in even-aged stands 

have small crown diameters. The resulting gaps can be filled both by lateral expansion of the surrounding trees and rapid upward growth of advance regeneration 

of shade-tolerant species. Thus, smaller gaps tend to favor recruitment of shadetolerant mesic species such as maples over oaks (Cowell et al. 2010, Fei and Steiner 

2009). In a mixed forest in Ohio, large gaps that formed after prescribed burns had 

killed shade-tolerant advanced regeneration were much more effective at recruiting 

white oak saplings than gaps created in unburned forest (Hutchinson et al. 2012). In 

this case, even though shade-tolerant species also germinated after the gaps were 

created, the fact that the larger advanced regeneration of shade-tolerant species 

had been killed by burning gave oaks a chance to compete, probably leading to 

co-dominance by oaks and shade-tolerant species, and at least maintaining the oak 

component of the stand for several more decades. 

Figure 7—Oak regeneration can tolerate open understory conditions (foreground) better than dense 

understories (background). A mixed red and white oak forest in Minnesota is shown with an invading front of understory buckthorn, which has remained green into November.

Alex Roth
29

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

Competing Vegetation and Invasive Plants

A variety of understory vegetation can compete with oak seedlings for light, nutrients and water, from the germination through seedling-establishment stages of the 

oak life cycle (Bowles et al. 2007). Large-seeded species like oaks are more resistant to competition shortly after germination than small-seeded species, because 

the energy stored in the acorn may allow the seedling to grow above herbaceous 

vegetation (Flory and Clay 2010). However, dense understory vegetation in forests 

or grasses in savannas that are taller than oak seedlings can still be significant 

competitors (Davis et al. 1999). Grasses and leaf litter can also form contiguous 

fuels and carry fires that kill small oak seedlings. 

There is a complementary relationship between understory herbaceous and 

woody vegetation. In forests with a dense shrub and sapling layer, the herbaceous 

layer tends to be sparse, whereas without the woody layer, the herb layer can be 

dense and relatively tall. It is difficult to obtain forest floor conditions with sufficient light for oak seedling growth, but without a dense layer of either herbaceous 

or woody vegetation. Deer influence the balance between woody and herbaceous 

understory vegetation; if deer suppress the woody vegetation through winter 

browsing, then the herbaceous layer becomes denser (Abrams and Johnson 2012). 

In general, however, woody understory vegetation has a more negative effect on oak 

regeneration than herbaceous vegetation (Brudvig and Asbjornsen 2008, Brudvig et 

al. 2011), so that fire and other management actions should generally be directed at 

controlling woody understory vegetation. 

There are cases where some species of oak seedlings compete better (or are at 

least associated) with certain types of low shrubs and herbaceous vegetation. For 

example, in Pennsylvania white oak regeneration was most abundant on plots with 

moderate blueberry or huckleberry cover (and it is interesting to note that oaks and 

blueberries are both adapted to fire), whereas red maple was associated with eastern 

hayscented fern (Fei et al. 2004). Such relationships probably also exist in other 

areas, but have not been systematically studied across the region. 

Some herbaceous and woody understory species that compete with oak seedlings have a seed bank that is a historical legacy of past management (Ashton et 

al. 1998). Graminoids (grasses and sedges) and shrubs, such as raspberries and 

blackberries, are especially common in the seedbank of oak forests (Aikens et al. 

2007, Schelling and McCarthy 2007). For example, red raspberry is a buried seed 

species with long-term seedbanks in the soil. Seeds that have been in the soil since 

widespread land clearance during early settlement may still be viable when the 

forest floor is exposed to light. This legacy is continued each time the seeds are 

germinated and allowed to progress to seed-set, by a management action or fire that 
30

GENERAL TECHNICAL REPORT PNW-GTR-914

disturbs the duff and kills woody vegetation, creating a mineral soil seedbed with 

enhanced light at the forest floor (Schelling and McCarthy 2007). Perhaps trying 

to germinate them and then reburn after a short time—prior to seed-set—so that 

the seed bank is exhausted would reduce their abundance, giving the edge to oak 

seedlings. On the other hand, dense raspberry and associated herbs and shrubs that 

are common on many sites after prescribed fires or silvicultural treatments might 

help hide small oak seedlings from deer. 

A number of exotic herbs and shrubs have invaded oak woodlands and 

forests in the Midwest, including reed canary grass, garlic mustard, Tatarian 

honeysuckle, and common buckthorn. Although the invasive shrubs are known 

to have large negative impacts on oak regeneration, there seem to be no systematic effects for exotic herb impacts on oak seedlings (Abrams and Johnson 2012, 

Davis et al. 2012). Reed canary grass on nutrient-rich sites may be an exception, 

as it reaches high densities and is relatively tall for an herbaceous plant (often 3 

to 4 ft [0.91 to 1.2 m]) (Dettman and Mabry 2008). Although fire can reduce reed 

canary grass density, it does not eliminate it, and other control measures may 

be needed (Dettmen and Mabry 2008). This grass is commonly associated with 

lowland sites; however it is also favored on nutrient-rich upland sites with silty 

soils near the prairie-forest border, where it can become abundant after silvicultural treatments and prescribed fires, competing with oak seedlings (Frelich, 

personal observation). 

Invasive earthworms also influence competing vegetation in complex ways. 

They reduce density of understory vegetation in very low-light conditions, but 

promote sedge mats or lawns in forests with moderate light levels, and the density 

of these lawns can be enhanced by deer grazing (Holdsworth et al. 2007, Rooney 

2009). These lawns also compete with oak seedlings, although in this case, the 

short heights of the sedge (less than 1 ft [30 cm]) would be relatively easy for the 

oak seedlings to overcome using energy stored in the acorn. Research needs to 

be done—it is not clear what the impact of sedge lawns on oaks seedlings will be 

(Rooney 2009). If the sedges turn out to be detrimental, they could be treated with 

herbicides for temporary reduction while oak seedlings are established. Sedge 

mats might also be contiguous enough to carry fire when they dry down in the fall. 

If so, then the positive effects may outweigh the negative effects. Also, factors that 

have a negative impact on oaks and other species but more negative against other 

species than against oaks, can have a net positive effect for oak. One possibility 

here is that the sedges might work against maple more than against oak, thereby 

being a negative factor on growth for seedlings overall that creates a net positive 

effect for oak. 
31

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

An adaptive management or even experimental approach may be necessary 

to establish the best system to manage competing vegetation within a multi-factor 

environment in each landscape unit. It may never be possible to provide a prescription that will work everywhere at the regional scale, or even the ecological section/

subsection scale. Competing vegetation seed bank, deer density, deer browsing 

preferences, stand development, and effects of invasive species vary locally. Fire 

will affect and interact with these factors differently, leading to individualized 

effects across the landscape. 

A Case Study of Fire and Red Oak

Among the oak species present in the Midwest, red oak is an especially desired 

species. It grows fast, and over the northern portion of the region it is the only oak 

species that produces high-value timber products. It also has the most ambiguous 

relationship with fire in that although it needs fire to become a dominant tree, fire 

can also, under some circumstances, work against it (Collins and Carson 2003). 

It is the oak species least tolerant of high fire frequency, has the poorest ability to 

resprout after fire, and is perhaps the oak species most likely to persist in the absence 

of fire, although sometimes at low abundance. Red oak does well when a period of 

frequent burning is followed by a few decades without fire (Arthur et al. 2012).

Red oak dominance in many regions today is probably a “one-time deal,” owing 

to the historical dynamics of the passenger pigeon, Native American burning, 

settlement effects, and deer. Frequent burning by Native Americans to purposely 

favor white oak (Abrams 2003), consumption of red oak acorns by the now extinct 

passenger pigeon (Ellsworth and McComb 2003), and frequent prairie fires in 

areas near the prairie-forest border maintained white oak and bur oak as dominant 

species in oak woodlands and savannas prior to early settlement. After settlement, 

Native American burning and passenger pigeon effects disappeared or were greatly 

reduced, and an episode of landscape clearing and burning followed by farm abandonment in an environment with few deer allowed massive recruitment of red oak 

across much of the Midwest and northeastern United States, over a period ranging 

from the late 1800s to the mid-1900s (McEwan et al. 2011). 

In many areas, the environment is no longer suitable for red oak recruitment 

owing to increasing deer browsing and mesophication of the forest, including dense 

understories of shrubs and maple allowed by fire exclusion and a recently wetter 

summer climate in the region (Abrams 1992). A century of fire exclusion and poor 

recruitment has led to an almost complete takeover by mesic forest species on sites 

with good soils (Aldrich et al. 2005). Earthworms, human dispersal, and human 

disturbance to the environment also facilitate the spread of invasive shrubs such 

as common buckthorn and Tatarian honeysuckle (Nuzzo et al. 2009), which form 
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GENERAL TECHNICAL REPORT PNW-GTR-914

very dense understories even in stands without native maple or hazel understories. 

Although red oak seedlings and saplings can tolerate the shade of an open understory for several decades, they cannot tolerate shade in an understory with a shrub 

and sapling layer (Crow 1988).

Prescribed burns do not always allow red oak recruitment to the sapling layer 

or eventually to the canopy (Collins and Carson 2003). For a prescribed burn to 

do that, it must reduce the litter layer and eliminate competing vegetation such 

as shrubs and saplings of other species for a long enough period for the oaks to 

reach the canopy, often in the presence of deer. Although removal of litter and low 

vegetation competition after a prescribed fire often lasts long enough for seedlings 

to become established (Albrecht and McCarthy 2006), it is difficult to achieve an 

increase in light availability for another decade or longer as oak saplings grow 

upwards, unless the prescribed fires are accompanied by formation of some gaps 

large enough so that they do not close in by lateral expansion of mature trees before 

new oak saplings reach the canopy (Alexander et al. 2008). Thus, taking advantage of wind disturbance, mortality owing to old age, or thinning, by combining 

gap formation with prescribed burning is likely to yield better results for canopy 

recruitment (Brose et al. 1999, Rentch et al. 2003). 

Consumption of acorns by deer, bears, squirrels, turkeys and other birds, and 

acorn weevils and galls is important today, but not the limiting factor for red oak. 

Many acorns are buried or cached by squirrels, blue jays, and other animals, and 

not all are retrieved, so that many acorns can survive predation by animals that eat 

them on the surface of the soils (such as deer) and go on to germinate (Garcia et 

al. 2002, Haas and Heske 2005, Johnson and Webb 1989). Adequate germination 

does occur across the range of red oak, but browsing by deer and slow growth rates 

owing to unsatisfactory understory environments are now widely limiting recruitment into the overstory. Mixed forests of red oak with white oak, sugar maple, 

basswood, or white pine (depending on location) are probably more achievable than 

red oak-dominated forests. 

In spite of the foregoing difficulties, red oak can have a positive relationship with 

fire, and still does recruit in areas with suitable conditions. It is instructive to examine those few areas where red oak is still able to regenerate in the last several decades 

and today. One such area is the unlogged forests of the Porcupine Mountains Wilderness State Park, Michigan. This site is in a mesic forest climate, with generally 

high-quality soils that support sugar maple, hemlock, basswood, and yellow birch (a 

few oak stands also occur on rock outcrops, mentioned below). The park lies beyond 

the range of white and black oak, so that red oak is by far the most important oak 

species. Surface fires of natural origin have continued to occur and have burned 
33

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

portions of the park during the last century (Frelich and Lorimer 1991). These surface 

fires have removed the duff layers, killed some overstory sugar maple and hemlock 

trees (or burned through areas with moderate-intensity windfall of the maple and 

hemlock), creating large gaps with light levels equivalent to shelterwood harvests 

with bare soil seedbed. Because the interior of the Porcupine Mountains is in the 

Lake Superior snowbelt, there is little overwinter deer browsing on red oak—deer 

leave the area during the winter and migrate to lower elevation areas near Lake 

Superior where snow is less deep. By the time deer return to the upland hardwood 

forest in the spring, they have switched their diet to grazing on herbaceous plants. In 

addition, because the area was never logged, it is without the legacy effect of the 

settlement-era seed bank of raspberry, reed canary grass, or other shrub and herbaceous competition, and there has been very little intrusion of invasive species. Also, 

the carpet of maple seedlings usually present in such forests is wiped out by fire and 

takes a decade or more to reestablish (an example of this outside the Porcupine Mountains is given by Albrecht and McCarthy 2006), giving oaks plenty of time to recruit 

to the large-sapling size class after fire. The net effect is groves of red oak currently 

ranging in age from 40 to 200 years old, interspersed with sugar maple and hemlock 

in parts of the park prone to these fires—generally upland areas with significant 

lightning-strike frequency, deep duff (which helps carry the fires) owing to lack of 

earthworm invasion, and occasional summer drought, above the Lake Superior 

coastal fog belt (fig. 8; Frelich 2002). There are also occasional red oaks within mesic 

forests that become established in treefall gaps without fire (Collins and Carson 2003, 

Figure 8—Groves of northern red oak embedded within a sugar maple-dominated forest in the 

Porcupine Mountains Wilderness State Park, Michigan. In this May photograph, the dark color of 

oaks without leaves stands out against the light green of sugar maples with expanding leaves. 

Lee Frelich
34

GENERAL TECHNICAL REPORT PNW-GTR-914

Frelich 2002). In addition, red oak in the Porcupine Mountains can coexist with mesic 

forest species in areas of rocky terrain where there are microsites not quite suitable 

for dominance by the mesic species but that allow persistence of oaks, which then 

also serve as seed sources for gaps and nearby burns, as described by Frelich (2002) 

and Abrams et al. (1998).

Thus, we know that red oak today is capable of reproduction given the right 

conditions, but that these conditions are not as common across the landscape as they 

were during the early 1900s (Crow 1988). This case illustrates the usefulness of 

natural areas, where we can learn from intact natural processes that lead to regeneration of desired tree species. Conditions that now occur in the Porcupine Mountains were unwittingly created on a massive scale by the settlement of eastern North 

America, so that red oak forests were created over vast acreages during the late 

1800s and early 1900s. We have taken that situation with abundant and now aging 

red oak stands as normal, but in fact, as explained by McEwan et al. (2011), this may 

be a unique situation in the last several centuries, and it may not be possible to for 

red oak dominance to continue without alteration of landscape conditions. 

Oak and Climate Change

An increase in mean annual temperature of about 5 °F (2.8 °C), along with a slight 

increase in precipitation, is expected in the western Great Lakes Region by 2069 

(Galatowitsch et al. 2009). The increase in precipitation is unlikely to balance 

higher evaporation, so the net impact will likely be towards a drier climate. The net 

effect is equivalent to a latitudinal shift such that a given location will experience 

climates currently prevailing 250 to 300 mi [400 to 482 km] to the south-southwest 

(Galatowitsch et al. 2009). 

In the northernmost region of occurrence for the oak species group, including 

northern Wisconsin, upper Michigan, northern lower Michigan, and northeastern 

Minnesota, oaks could benefit substantially from a warmer climate. Here oaks compete with boreal conifers, red, white, and jack pine on sandy or rocky sites, as well 

as mesic northern hardwoods on sites with deep, loamy soils. Oak in these locations—as demonstrated by a modest increase in importance value (the average of 

relative density and relative volume in a stand)—has already begun a slow expansion during the last two decades (Fei and Yang 2011). Limits to oak growth imposed 

by a short growing season and extreme winter minimum temperatures will lessen 

over time in these areas as the climate warms, allowing oaks to better compete with 

boreal species and northern pines. Northern hardwoods including oaks can outgrow 

boreal species as understory saplings, when mean summer temperatures (June, July, 

August) exceed ca 64 °F (18 °C), although deer browsing can raise this threshold to 
35

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

68 °F (20 °C) (Fisichelli et al. 2012), owing to deer preference for hardwoods over 

white spruce and balsam fir as winter browse. Thus, increasing deer populations in 

the far north may retard the expansion of oak into the boreal forest. 

As the climate warms, much of the Midwest could enter a postmesophication 

era, where maples may no longer have the advantage as a result of longer summer 

dry periods and warm early springs (Belden and Pallardy 2009, Fei et al. 2003). 

Longer summers and more frequent droughts in this northern region will narrow 

the niche of sugar maple and associated species along a soil-moisture gradient, confining them to progressively more silty/clayey soils, while expanding the niche of 

oak onto sandy loams now occupied by mesic forests (Bapikee 2013). The effectiveness of fire in favoring oaks will also increase as the warmer climate begins to work 

in concert with fire to favor oaks over mesic tree species. A warmer climate is also 

likely to favor increasing proportions of oaks in oak-pine mixtures on sandy soils. 

We can deduce this from the existing gradient of decreasing pine abundance from 

north to south across the Midwest, as shown by Curtis (1959). 

At the same time oak has been expanding in the northernmost region of occurrence, it has been declining along the prairie-forest border region that cuts northwest 

to southeast across Minnesota and Wisconsin into northern Illinois, Indiana, and 

southern Michigan (Fei and Yang 2011). A century of fire suppression, fragmentation of the landscape, wetter summers for the last few decades, dense understories, 

and high deer populations have allowed mesophication of oak forests, typified by 

invasion of sugar maple, basswood, and other mesic forest species (McEwan et al. 

2011). This trend is likely to reverse with a warmer climate, although the magnitude 

of reversal will depend on the extent to which the other factors mentioned above 

continue to work against oak. Deer, in particular, are likely to be a major force 

opposing the expansion of oaks, although their impacts will vary with deer density 

across the landscape and will eventually be overridden at some level of climate 

warming that disfavors mesic tree species. Thus, the postmesophication era will 

play out differently across the landscape, creating a mosaic of differential oak success potential, because factors that oppose the general trends toward oaks caused by 

a warmer climate vary at the spatial scale of stands to ecological sections. 

It is not certain at this point whether a warmer climate will cause the region 

dominated by oak to simply shift north, losing out to grasses along the southwest 

and replacing mesic and boreal forests in the north, as opposed to causing wider 

ecotones within which oaks will be mixed with grasses and mesic/boreal forest 

trees. Iverson et al. (2008b) suggested that the oak-hickory forest type will expand 

into the current northern hardwoods and boreal forests without retraction along the 

southwest part of the range, while Frelich and Reich (2010) point out that a number 
36

GENERAL TECHNICAL REPORT PNW-GTR-914

of factors (drought, fire, windstorms, insect pests, diseases, invasive species, and 

deer) other than climate change may work against trees, magnifying the impacts 

of climate change. Matthews et al. (2011) have made an early attempt to model 

how multiple factors may affect tree species abundance in the region as the climate 

warms. The latest available maps of change in abundance across the Eastern United 

States for tree species of interest are available in Prasad et al. (2007 to ongoing). 

Managers can use these maps to see how dominant tree species may potentially 

change by the end of the century for low and high climate change scenarios. 

Most analyses of future climate point to a longer fire season. However, it is 

unknown how this will affect the number of days that meet conditions for prescribed fire. A larger number of days on which fire could occur throughout the year 

could be counterbalanced by an increase in number of days where fire weather 

conditions are too severe for prescribed fire. This could also lead to more wildfires 

that may reduce the need for prescribed fires. We don’t have the ability to model 

future fire behavior in the oak region as can be done for the boreal forest, where 

much more research has been done on how climate will affect future fire frequency 

and intensity in a changing climate. 

Restoring Fire in Contemporary Oak Forests

Forest managers are basically trying to push succession ahead, keep it the same, or 

push it backward. Maintaining existing oak ecosystems generally requires maintaining the current successional status (i.e., stopping forward movement), and fire is a tool 

that can do that alone or in combination with harvesting. A thinning-fire combination 

can prevent succession, possibly keeping the forest at an early- to mid-successional 

state, which is generally the goal of oak management. Such combinations of disturbance may even set succession back—generally the goal for mixed oak and maple 

stands. Although it is not possible to create perfect conditions for oaks everywhere 

within one site, or on all types of sites across the landscape (Iverson et al. 2008a), there 

are a number of factors that affect oak regeneration at various stages from germination, establishment, and recruitment. The list of factors varies from region to region 

and stand to stand. It is up to the manager to create the relevant list for a given stand 

and push as many factors as possible in favor of oak. Fire will also affect these factors 

and have direct effects on oak, as summarized nicely by Johnson et al. (2009, p. 157). 

Mixed Oak Forests

In parts of the upper Midwest, it is possible to maintain a mixture of oak species. 

Mixtures of northern red, white, and black oak are possible in southern Wisconsin, 

Minnesota, and Michigan. Mixtures of red, bur, and northern pin oak are possible 

in the southern boreal region of northern Minnesota. The advantages of mixed oak 
37

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

forests are masting in different years, so that acorns as a food source for wildlife 

will be more even over time, and supporting a greater diversity of ecosystem 

processes and habitat structures. To enhance all oaks, fire can play a role in limiting 

understory density, especially density of mesic species like maple and shrubs. 

Several factors have moderate to pronounced differential effects among 

oak species:

• Among oak species, deer will work against red, black, and northern pin 

oaks relative to white and bur oaks. 

• South aspects will generally support more oak species (white oak, northern 

red oak, black, and bur oak) than north aspects (generally only red oak). 

• Varied gap sizes will take advantage of differential shade tolerance among 

oaks. Northern pin oak and black oak require more light than northern red 

oak, with other oak species intermediate in shade tolerance.

• Higher fire frequencies favor white oak and bur oak, while lower frequencies favor the red oak group. 

• The continued spread of oak wilt would favor the white oaks over the red oaks.

Oak-Pine Forests

There are two types of oak-pine covered here: (1) jack pine–northern pin oak on sandy 

soils, which has some features analogous to sand savannas of pin oak, bur oak, or 

black oak; and (2) red and white pine mixed with northern red oak on dry-mesic sites. 

The main complicating factor in managing fire in oak-pine mixtures is that the 

fuel structure will support high-intensity fires. Particularly in northern pin oak–jack 

pine on sand, it is hard to control the fire frequency, as uncontrollable wildfires 

are common, and prescribed fires are risky, owing to the fuel loading on the forest 

floor, low heights to canopy base, and low foliar moisture content (including marcesant leaves of pin oak during the dormant season), all increasing the chance of a 

surface fire becoming a crown fire. These forest types require high-intensity disturbance to regenerate, like boreal forest. This is a problem, as most of the areas with 

oak-pine forest in the Midwest are also in a wildland-urban (WUI) interface (see 

“Wildland-Urban Interface” section below), with small towns and cabins embedded 

in the forest. High-intensity fires are also possible in white/red pine and northern 

red oak forests, owing to dense brush and sometimes balsam fir in the understory. 

For a single high-intensity fire, oaks will resprout and initially overtop pines 

after fire, but that situation will reverse a decade later when pines put on more height 

growth. If a prescribed burn is done within a few years after a high-intensity wildfire, 

when all seeds of pine have either germinated or died, but there is no seed crop on the 

young pines, the young pines may be killed, virtually eliminating pines from a stand. 
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GENERAL TECHNICAL REPORT PNW-GTR-914

Oaks farther from pines in high-intensity fires are less likely to be killed during 

fires (Johnson et al. 2009). Therefore, if coexistence of pines and oaks is desired, 

especially in the jack pine–northern pin oak type, a mosaic of forest types may work 

better than having pines and oaks mixed together at the scale of individual trees.

For other situations where the structure allows prescribed fire due to higher 

canopy base heights and relatively low canopy bulk density, including many red 

oak mixtures with white and red pine, it is relatively easy to manage the balance 

between pine and oak. Following in concept the example of land use by early settlers that created now-mature northern red oak forests (see “Historic Fire Regimes 

and Oak Versus Grass, Maple, and Pine” above), cutting pine followed by burning 

to remove duff and brush will likely allow establishment of a new cohort of oaks. 

Light conditions are generally conducive to oaks on the jack pine-pin oak sand 

sites. However, on account of fire suppression, red pine, white pine, and red oak 

forests today often have dense understories of hazel, red maple, and balsam fir. This 

understory interferes with oak and pine reproduction and, like red maple removal in 

mesic forests, burns during 2 or 3 consecutive years are required to eliminate it.

Oak-Maple Forests

In managing for mixed oak-maple composition, the goals are usually to stop any further advance in, or reduce the dominance of, maples and other mesic species in the 

forest. The oak component will mostly be composed of northern red oak across the 

northern part of the region, but could include white oak and black oak in the southern part of the Lake States or northern pin oak to the west. On north- and east-facing 

slopes, maintaining an oak component in mixture with maple is probably all that can 

be accomplished—if one or two cohorts of oak are recruited into the canopy each 

century, coexistence can be maintained. The manager should decide under what 

conditions it is practical to fight maple, versus when it is futile, or perhaps even work 

with maple. It may be prudent to wait and see if climate change begins to help push 

the successional trajectory of the forest away from maple. On south- and west-facing 

slopes, it is much easier to create or maintain oak dominance by using fire.

Low-intensity fire can make positive contributions toward these goals by: 

• Removing a carpet of sugar maple seedlings that often exists. 

Reestablishment of a dense carpet may take a decade or more. 

• Killing maple saplings with thin bark up to 3 to 4 in dbh (7.5- to 10-cm). 

Fires in 2 or 3 consecutive years may be necessary to kill resprouts of red 

maple, which can outgrow red oak in some cases.

• Removing the duff layer, providing a 1- to 2-year window of opportunity 

for oak seedlings to germinate, especially if the fire is timed to match a 

mast year for acorn production. 
39

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

Killing large red and sugar maples with fire is difficult, as they can develop 

thick bark. Therefore, rapid restoration to oak could be accomplished by a combination of thinning to remove maple or discrimination against maple during a burn by 

piling fuel against the base of maple trees to kill them and reduce future maple seed 

input. A slower restoration could be achieved by prescribed burns every several 

years to prevent a maple seedling layer from becoming established, while waiting 

for fire-scarred large maples to rot at the base and fall over. 

Complicating factors can include earthworms that remove the duff layer, making it difficult to run a fire through a stand, even though this situation may also help 

acorn germination. Winter deer browsing also works in favor of maple, as oaks are 

generally more preferred by deer than sugar maple. Larger gaps (500 to 2000 ft2

 [46 

to 186 m2]) created by windthrow, selection cutting, or thinning, followed by fire, 

will allow oaks to grow faster than maples and even to outgrow deer in some cases. 

Fuels from thinning or windthrow may allow a much higher-intensity fire than 

could ordinarily occur in this forest type, which would have a much higher efficacy 

in favoring oaks over maples, but have the downside of more restrictive prescription 

conditions and fire/smoke management. 

Oak Savanna

Three different goals occur: (1) restore savannas that have “grown up” into oak 

forests back to savannas; (2) introduce scattered oak trees into open grasslands to 

create savannas (this latter goal often occurs on recently abandoned farmland); and 

(3) maintain the rare existing savannas, preventing invasion by shrubs and trees. 

Most oak-dominated landscapes originally had a mosaic of stands with various 

degrees of canopy closure, and given the mosaic of environmental variables that 

influence favorability to trees versus grass, such as soil type, slope, and aspect, a 

mosaic of woodland, shrubby savanna, and grass with scattered trees may be more 

realistic than all savanna. 

Regular fires every 2 years are generally required to maintain grasses (Considine 

et al. 2013), which in turn provide the necessary continuity of fuel for fires to spread 

at an intensity that will top-kill oak seedlings (Johnson et al. 2009). Spatial and 

temporal variability of fire allow occasional recruitment of oaks (black oak, northern 

pin oak, bur oak, and white oak) to a size where they can survive the next fire. Only 

a handful of oaks need to be established and recruited to the canopy each decade, 

even on a 10-ac (4-ha) tract of savanna, to maintain a savanna level of tree cover. 

Restoring “grown up” savannas requires one of two strategies. The first is to 

maintain a high fire frequency for several decades, during which older trees die 

without successfully reproducing, leading to reduced tree density over time. In the 

second, thinning is followed by a relatively high-intensity fire to create a large, 
40

GENERAL TECHNICAL REPORT PNW-GTR-914

quick push towards savanna conditions, followed by frequent low-intensity fires for 

maintenance. Recent case studies support these strategies. Haney et al. (2008) found 

that a single high-intensity fire had a larger impact on restoring savanna conditions 

on a black oak sand savanna than several lower intensity burns over 20 years, 

whereas Peterson and Reich (2001) showed that 40 years of almost-annual burning 

accomplished the same goal on a sand savanna with northern pin oak and bur oak 

(fig. 9). Brudvig et al. (2011) found that mechanical removal of woody vegetation 

followed by maintenance of an encroachment-free state by frequent prescribed fires 

and promotion of diverse understory plants native to oak savanna constituted an 

effective strategy for restoring the desired savanna vegetation. 

Note that older trees in the red oak group (northern pin and black oaks) will 

rot around a fire scar and eventually fall down. Therefore, continuation of highfrequency fire for several decades will lead to more dominance by bur and white 

oaks, as was the case for areas frequently burned by Native Americans. Pushing 

fuel that will burn for a long time (several 1- to 3-in diameter [2.5- to 7.6-cm] pieces 

of wood and duff) up against the bases of trees you want to remove, or pulling such 

fuels away from the trunks of trees you want to save, as a prefire treatment could be 

very effective. This could be done in combination with a harvest that would supply 

movable 1- to 3-in (2.5- to 7.6-cm) diameter fuel. Or, if timing of a prescribed burn 

can be flexible, a prescribed fire within a year or two after a windstorm or ice storm 

that provided copious fuels on the forest floor that could be manipulated may lead 

to maximum fire effect. In any case, a goal of getting fire to last for 12 minutes or 

more at the base of larger trees that are marked to be killed would be effective for 

a bark thickness of 0.75 in (2 cm). Good models do not yet exist for predicting fire 

duration, so local experience needs to be developed. 

For restoration of trees in grassland, there are two factors to take into account. 

First, competition with grasses for natural regeneration or planted oak seedlings 

will be severe (Davis et al. 1999); removal of the sod or use of mulch or mats to 

keep grasses at bay around seedlings may be needed. Second, lack of mycorrhizae 

could be an important limiting factor for tree establishment (Dickie et al. 2007). 

Thus, supplements of forest soil to reintroduce mycorrhizae and protection from 

deer and competion with grasses and forbs may be needed for oak establishment. 

Establishment in this context means that oaks are tall enough to be above heights 

that deer can reach, above the heights of grasses and forbs, and large enough in 

diameter and bark thickness to withstand any future fires. Once this size is reached, 

the tree and grass mixture can be maintained by frequent fire, and given the long 

lifespan of many oak species, only a few new trees need be recruited per acre per 

decade to maintain the tree component. 
41

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

Figure 9—Prescribed fire in a pin oak–bur oak sand savanna, Cedar Creek Ecosystem Science 

Reserve, University of Minnesota. A. Spring burn in progress. B. Four decades of almost-annual 

burning has allowed substantial progress toward restoration of oak woodland to savanna, with death 

of pin oaks due to rotting basal fire scars (note fallen pin oak on left) and release of old bur oaks. 

Photos: Kalev Jõgiste.

A

B

Kalev Jõgiste Kalev Jõgiste
42

GENERAL TECHNICAL REPORT PNW-GTR-914

The Wildland-Urban Interface (WUI)

The region with highest oak abundance also corresponds to areas with high density of human settlement. Suburban and second-home development has spread 

into many oak forests and savannas throughout the Midwest. Therefore, where 

prescribed fires occur and where the smoke goes have become issues in the use of 

fire for oak management. For a review of these issues, and models available to help 

manage smoke distribution during fires, see Achtemeier (2009). High-intensity fire 

with a strong updraft may take smoke higher into the atmosphere so that level of 

exposure to smoke for local residents may be minimal. Similarly, fires conducted 

during windy days have a higher dilution factor as wind spreads the smoke. 

Intensity and windspeed, however, are two factors that must have strict limits for 

any prescribed fire. The types of fires commonly prescribed in oak forests generally have smoke close to the ground with limited dilution by wind. Furthermore, 

being in a Wildland-Urban Interface also restricts the range of fire prescriptions 

that are acceptable, because higher intensity fires that may be desired to kill latesuccessional and invasive species, require wider firebreaks, hold over longer, and 

generate more smoke than low-intensity grass fires. Consequences of an escape are 

much larger than for fires not occurring in a Wildland-Urban Interface. Therefore, 

wind direction and speed and fuel conditions for fire prescriptions may be quite 

restricted. This may necessitate greater reliance on mechanical treatment than fire, 

as the prescriptions necessary for ecologically significant fire may not be possible. 

Conclusions—Some Take-Home Lessons

1. Do not underestimate the legacy effects of long-term fire regimes that were 

in place prior to or during early settlement, where decades to centuries were 

available to create oak-dominated stands (McEwan et al. 2007). It probably 

will take a long time to reverse the effects of several decades of fire exclusion that have led to today’s conditions in many oak forests. 

2. The impact of several burns in a short time is generally underestimated, and 

potential for a single burn overestimated, especially in the context of a forested site that has experienced fire exclusion for several decades to a century. 

Also, the impact of variation in return intervals between fires in moderating 

the competitive balance among oaks and other tree species has been underestimated. The concept of cumulative disturbance severity tells us that either 

several burns or one burn with very high intensity would be necessary to 

push a forest (where fire has been excluded) back to a condition where future 

low-intensity fires would be effective in maintaining desired conditions. 
43

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

3. The needed cumulative disturbance severity to push successional status 

of oak-maple forests back to oak may also have to be achieved by some 

combination of mechanical treatments and fire. If maples and other mesic 

species have reached a size refuge (thick bark resistant to fire effects), only 

continued high-frequency fire that prevents regeneration until the old trees 

die will remove those species from the forest. The old mesic trees could die 

from natural causes such as rotting from fire scars leading to blowdown, 

although a silvicultural treatment designed to favor oaks could also be helpful for both forests (Brose et al. 2008) and savanna restorations (Brudvig 

and Asbjornsen 2008). 

4. Learn to predict when mast years will occur (a function of size of oak 

flower crop, weather, and time since last mast year; Sork and Bramble 

1993, Sork et al. 1993) and time fires so that they create optimal conditions 

for a large crop of oak seedlings, and cause minimal damage to the acorn 

crop itself. 

5. Take advantage of windthrow and ice storms. Use crowns on the ground as 

buffers against deer if practical. Windthrow and fire interaction could also 

be used to carry out a prescribed fire with higher than usual intensity if 

adequate fire breaks and other aspects of the local situation allow. 

6. The balance between relative sprouting ability of smaller trees of species you want to disfavor (red maple, sugar maple, and others) and oaks is 

an important consideration, especially as it relates to the number of burns 

needed to kill nondesired species while allowing oaks to survive. Northern 

pin, bur, white, and black oaks will fare better in this balance than red oak. 

It may not be a bad thing to have some basswood, red maple, ironwood, or 

other species mixed in with oaks. It is probably almost impossible to adjust 

the sequence of events (in terms of silvicultural method followed by burning) that will create perfect conditions for oaks only. 

7. Oak seedlings that are well established, to the point of a grub with significant underground stored energy, will more likely survive top-killing by 

fire, and resprouts will be poised to grow rapidly. Therefore, an effective 

strategy is to use prescribed fire several years after a canopy disturbance 

(whether a silvicultural treatment such as thinning or windthrow), so that 

postfire oak seedlings will resprout vigorously. Oaks can take advantage of 

single-tree sized gaps, but only if advance regeneration of mesic tree species like maple is not present. Otherwise, larger gaps are needed. 
44

GENERAL TECHNICAL REPORT PNW-GTR-914

8. Altering the fuel structure may help tremendously in increasing the effectiveness of a prescribed burn, possibly mimicking the effects of a more 

severe fire than was actually carried out. This objective of increasing the 

effect of a prescribed fire may be achieved in several ways; for example, by 

altering fuel structure to kill oaks (to restore forest to savanna) or to save 

oaks and kill other tree species (to restore a maple-oak forest to higher percentage of oak). 

9. Learn about germination ecology of competing shrubs, trees, and herbaceous species; there is always the possibility of inventing unique ways to 

discriminate against them, and in favor of oak. Try to exhaust the seed 

bank of undesired species. Competition varies tremendously across the 

range of oak in the Midwest, and studies have not been done that show how 

to vary management activities to regenerate oak under all conditions across 

the landscape. 

10. Mesophication, also called mapleization of oak forests in some areas, has 

occurred owing to the wetter summers of recent years, reduced fire frequency and intensity, and faster litter decay (Nowacki and Abrams 2008). 

This leads to less-intense fires, possibly too low in intensity to have a significant ecological effect. However, this situation may start to reverse with 

a warmer climate and higher drought frequency, which would start to favor 

oak over maple. 

11. A number of changes have occurred in the environment in the last several 

decades, including earthworm invasion, sedge mats, invasive plant species 

that change understory density, deer grazing, and climate change. These 

multiple factors often have both positive and negative impacts on oak, and 

numerous interactions with fire. Also, their occurrence varies locally and 

regionally throughout the oak-forest region. 
45

Fire in Upper Midwestern Oak Forest Ecosystems: an Oak Forest Restoration and Management Handbook

Acknowledgments 

This synthesis was funded by The Joint Fire Science Program (grant number PNW10-JV-11261927-049). We thank Eli Sagor, Alan Haney, and Daniel Dey for reviewing and greatly improving the manuscript. 

Metric Equivalents 

When you know: Multiply by: To find:

Inches (in) 2.54 Centimeters (cm)

Feet (ft) 0.305 Meters (m)

Miles (mi) 1.609 Kilometers (km)

Acres (ac) 0.405 Hectares (ha)

Square feet (ft2) 0.0929 Square meters (m2)

Tons (ton) 0.907 Tonnes or megagrams

Tons per acre 2.24 Tonnes or megagrams per hectare

Degrees Fahrenheit 0.56(°F – 32) Degrees Celsius

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Appendix: Common and Scientific Names

Common name Scientific name

Vertebrate animals:

Bear (American black bear) Ursus americanus 

Blue jay Cyanocitta cristata

Deer (white-tailed deer) Odocoileus virginianus

Squirrel (eastern gray squirrel) Sciurus carolinensis

Passenger pigeon Ectopistes migratorius

Turkey, wild Meleagris gallopavo

Invertebrate animals:

Nightcrawler Lumbricus terrestris

Leaf worm Lumbricus rubellus

Small leaf worm Dendrobaena octeadra

Angleworms Aporrectodea and Octolasion spp.

Herbaceous plants:

Garlic mustard Alliaria petiolata

Hayscented fern, eastern Dennstaedtia punctilobula

Sedge Carex spp.

Reed canary grass Phalaris arundinacea

Shrubs:

Blueberry, lowbush and sour-top Vaccinium angustifolium and A. myrtilloides

Buckthorn, common Rhamnus cathartica
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Common name Scientific name

Hazel Corylus cornuta and C. americana

Honeysuckle, Tatarian Lonicera tatarica

Huckleberry Gaylussacia baccata

Raspberry and blackberry Rubus spp.

Red raspberry Rubus ideaus

Trees:

Ash Fraxinus spp.

American basswood Tilia americana

Birch, yellow Betula alleghaniensis

Cherry Prunus serotina 

Cottonwood Populus deltoides

Elm Ulmus spp.

Elm, American Ulmus americana

Elm, rock Ulmus thomasii

Fir, balsam Abies balsamea

Hemlock, eastern Tsuga canadensis

Ironwood Ostrya virginiana

Hickory Carya cordiformis and C. ovata

Maple Acer spp.

Maple, red Acer rubrum

Maple, silver Acer saccharinum

Maple, sugar Acer saccharum

Oak Quercus spp.

Oak, black Quercus velutina

Oak, bur Quercus macrocarpa

Oak, northern pin Quercus ellipsoidalis

Oak, red Quercus rubra

Oak, scarlet Quercus coccinea

Oak, white Quercus alba

Pine Pinus spp.

Pine, jack Pinus banksiana

Pine, red Pinus resinosa

Pine, white Pinus strobus

Spruce, white Picea glauca

Walnut, black Juglans nigra


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