Fire and the Development of Oak Forests
Author(s): Marc D. Abrams
Source: BioScience, Vol. 42, No. 5 (May, 1992), pp. 346-353
Published by: University of California Press on behalf of the American Institute of Biological Sciences
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Fire
the
and
Oak
of
Development
Forests
In easternNorth America,oak distributionreflectsa varietyof
ecologicalpaths and disturbanceconditions
Marc D. Abrams
ak (Quercus)representsone
of the most dominantspecies
groupsin the easterndeciduous forest of North America (Table
1). In certain eastern regions, oak
dominancereflectsthe importanceof
this genus in presettlement forests
(Abrams and Downs 1990, Spurr
1951, Whitneyand Davis 1986). Yet,
in other regions, the currentdistribution of oak greatlyexceedsthat of the
original vegetation (Abrams 1986,
Howell and Kucera1956, Nowacki et
al. 1990). Thus, the developmentof
oak species has occurred through a
variety of ecological pathways and
disturbanceconditions.
The dominanceof oak in presettlement forestsis particularlyinteresting
because most oaks are considered
early to midsuccessionalspecies. Indeed, most recentstudiesindicatethe
potentialfor widespreadoak replacement by more shade-tolerantspecies
in mature forests (Christensen1977,
Lorimer1984, Nowacki et al. 1990).
However,this phenomenonmay vary
with regionaland edaphicfactorsand
be more pronouncedon mesic rather
than on xeric sites (Abrams 1986,
Host et al. 1987, McCune and Cottam 1985, Nowacki and Abrams
1991).
These observations have led researchersto ask the following questions about oak ecology in eastern
Marc D. Abrams is with the School of
ForestResources,PennsylvaniaStateUniversity, University Park, PA 16802.
? 1992 AmericanInstituteof Biological
Sciences.
346
Fire and human
activity have affected
the past and present
ecology of oak forests
factors will likely continue to affect
forestsin the future.
Paleoecology of eastern
oak forests
During the last 18,000 years, dramatic shifts in species assemblages
occurred in eastern North America.
Approximately 18,000 years B.P.,
forests: What factors promoted oak pine (Pinus)dominatedforests in the
dominance in certain presettlement southeast, whereas spruce (Picea)
forests? What factors retarded oak dominated in the northern and cendevelopment in presettlement vegeta- tral regions (Webb 1988). By 10,000
tion types currently dominated by years B.P., oak forests occupied most
oak? What conditions during the ini- of the eastern United States, except
tial postsettlement period perpetuated for the northerntier, which was prioak dominance or facilitated an in- marily pine, spruce, and birch (Betcrease in oak? What factors are cur- ula) forests. Oak forests continuedto
rently causing oak regeneration fail- dominate the eastern United States
ure and promoting the invasion of through 500 years B.P., with pine in
most oak understories by later succes- the southeast and north-centralresional species?
gions.
Oak species not only have a long
Although a suite of factors, such as
climate change, logging, animal and history of dominationin easternforinsect grazing, and disease, have af- ests but also their presence is often
fected the dominance of oak, the an- associatedwith recurringfire. An inswers to these questions may relate vestigationof late Quaternaryvegetadirectly to historical changes in the tion at various sites in the midinfluence of fire in the eastern decidu- Atlantic region revealed long-term
ous biome. Fire, whether it has oc- dominationby pine and oak coupled
curred to a low, moderate, or high with the presenceof charcoal (Watts
degree, seems to be the common de- 1980). On severalof the study sites, a
nominator for the development of oak change from pine to oak domination
forests on upland sites and their past (approximately 9000 years B.P.) coinand present ecological status. This ar- cided with increased charcoal abunticle reviews pre- and postsettlement dance. In a study of Mirror Lake,
forest conditions for the major oak- New Hampshire,peak dominationof
dominated ecosystems throughout white pine and oak occurred 9000eastern North America to discern how 7000 years B.P., when the climate was
the influence of fire and human activ- warmer and drier and charcoal conity have affected the past and present centrationswere most abundant(Daecology of oak forests and how these vis 1985).
BioScience Vol. 42 No. 5
Table 1. Distributionof some importantQuercusspecieson uplandsites in variousregionsof easternNorth America.
Location
Northeast
GreatLakes
Centralplains
Midwest
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Q. shumardii
+
+
+
Q. stellata
Q. velutina
+
+
+
+
+
+
Species
Q. alba
Q. coccinea
+
+
+
+
+
+
+
+
+
+
+
A warming trend between 65003500 years B.P. in south-centralWisconsin resulted in increasedfire frequency and a change from rich
mesophytic forests to oak savannas
(Winkler et al. 1986). The climate
then became cooler and wetter, resultingin less fireand a conversionof
oak savannas to closed oak forests.
Similarly,a change during the Holocene fromprairieto oak woodlands
and then more recently to mixedmesophyticforests in southern Minnesota was attributedto reducedfire
frequency from increased precipitation and decreased temperatures
(Grimm 1983). However, the global
warmingpredictedfor the next 200500 years may increase oak importance for mixed conifer-hardwood
forests of the northern Great Lakes
and New Englandregions (Overpeck
et al. 1991). Thus, the dominanceof
oak forests, climate, and fire are
seeminglyinterrelatedin the eastern
UnitedStates.
Ecophysiologicalfeatures
of oak
Most upland oak species exhibit
physiologicaladaptationsthat should
facilitatesurvivalon sites exposed to
periodic fire and drought, as well as
on sites that are nutrientpoor. Relative to other hardwoods, fire should
favor oaks because of their thick
bark, sprouting ability, resistanceto
rotting after scarring, and the suitability of fire-created seedbeds for
acorn germination (Lorimer 1985).
Periodicfireshould also checksuccession in oak forests,becausemost later
+
+
+
+
+
Q. virginiana
May 1992
Mid-Atlantic
+
+
+
Q. ellipsoidalis
Q. falcatavar. falcata
Q. imbricaria
Q. laevis
Q. macrocarpa
Q. marilandica
Q. muehlenbergii
Q. prinus
Q. rubra
Southeast
successionalspecies, such as red and
sugar maple (Acer rubrumand Acer
saccharum),exhibit low resistanceto
fire.
Droughtadaptationsin oak include
deep roots, xeromorphicleaves, low
water-potential threshold for stomatal closure, and the ability to adjust osmotically(Abrams1990). Oak
speciescan also maintainhigh ratesof
photosynthesis during drought, and
they may possessthe greaterabilityto
surviveon nutrient-poorsites relative
to other tree species (Reich and
Hinckley 1980). However, one type
of ecologicalstressfor which oaks are
not well adapted is low light. Oak
speciesgenerallyhavelow or intermediate tolerance to shade, and therefore their seedlings do not exhibit
long-term survival or growth in the
conditions of a closed understory
(Burns and Honkala 1990, Crow
1988).
Oak expansion in the tallgrass
prairieregion
Tallgrass prairie dominated by Andropogon, Sorghastrum,and Panicum once occupieda 575,000-squarekilometer, roughly triangular area
bounded by central Texas, eastern
North Dakota, and western Indiana
(Kuchler1964). Isolated pockets of
tallgrassprairiealso existed throughout the northeasterndeciduousforest,
including a more extensive area in
west-centralOhio. Oak savannasor
oak openings were also a common
featureof the tallgrassprairieregion,
and at the time of settlement they
covered11-13 millionha of the mid-
west (Nuzzo 1986). Oak speciesusually found in savannasincludeQuercus macrocarpa(bur oak), Quercus
velutina (black oak), Quercus alba
(white oak), Quercus ellipsoidalis
(northernpin oak), Quercus stellata
(post oak), and Quercusmarilandica
(blackjackoak). Typically, these savannas had a prairielikeunderstory,
and they were thought to have developed from pre-existing forests exposed to frequent burning (Curtis
1959). Presettlementprairie border
forests were also dominated by oak
and were maintainedby recurringfire
(Gleason 1913, Pallardyet al. 1988,
Rodgersand Anderson 1979). However, nearly all of the original tallgrass prairie and savanna have been
convertedto farm land or forests.
Presettlement tallgrass prairie is
thought to have developedand flourished in an environmentthat included
recurringfireat one- to ten-yearintervals (Axelrod 1985). Frequentfire in
the CentralPlainsregion was a function of lightning strikes as well as
Indian activity, including cooking,
heat, ceramic manufacture,communication,fieldpreparationfor cultivation, combatinginsects, hunting,and
killingwoody vegetation(Day 1953).
Becauseprecipitationin the tallgrass
prairie region is adequate to sustain
forest vegetationon all but the most
xeric sites, it is generallyassumedthat
frequentfires limited forest development (Abrams 1986, Gleason 1913,
Howell and Kucera1956).
After European settlement of the
CentralPlains, fire frequencyand intensity decreased due to road construction, expansion of towns, in347
creased agriculture, intensive cattle
grazing,wildfiresuppression,and recommendationsagainst burningprairie in the mid-1900s (Abrams1986).
Duringthis period,forestsrapidlyexpanded at the expense of prairieveg.'.?*:'..L
fsi?
etation, and the dominant species in
these newly formedforests were gen.
'-4-' Y
*?.---;i
::1.
erallyoak (Figure1, Table 2). Exam":.4i
?*:
.,.a.cui??
of this expansion include the
ples
r
c
formationof blackjackoak and post
oak forests in central Oklahoma;
white oak, shingle oak (Quercusimbricaria),bur oak, and Americanelm
(Ulmus americana)forests in central
Missouri; and bur oak and chinquapin oak (Quercusmuehlenbergii)forests in easternKansas(Abrams1986,
Howell and Kucera 1956, Rice and
Penfound 1959). A reduction in fire
frequency,coupled with drought adaptationsin oak, facilitatedoak inva1.
and
in
forest
a
Figure Quercusmacrocarpa Quercusmuehlenbergii
expanding prairie sion
and survivalafter Europeansetravine in eastern Kansas since European settlement in 1840.
tlement. Birds and small mammals
Table 2. Presettlementvegetationand currentoverstoryand understoryconditionfor variousoak foresttypes in easternNorth America.
Presentcondition
Presettlementvegetation
State
Tallgrassprairie
KS
KS
Overstory
Buroak-hackberry
Chinquapinoak
Hackberry-elm
oak
Elm-redbud-chinquapin
WI
Blackoak-white oak
Blackcherry-boxelder-elm
Savanna
Buroak
Understory
Reference
Abrams1986
Abrams1986
Whiteoak-black oak
Blackjackoak-post oak
Prairieborderforest
Whiteoak
Whiteoak-black oak
White oak-black oak
WI
OK
Whiteoak-shagbarkhickory
Blackjackoak-post oak
Cottam 1949, McCuneand
Cottam 1985
Shagbarkhickory-chokecherry Dorneyand Dorney 1989
Rice and Penfound1959
Blackjackoak-post oak
MO
IL
MI
Whiteoak-sugarmaple
Whiteoak-shagbarkhickory
Whiteoak-black oak-red maple
Sugarmaple
Sugarmaple
Blackcherry-redmaple
Pallardyet al. 1988
Rodgersand Anderson1979
Dodge and Harman1985
Mid-Atlanticregion
Oak-chestnut
Whiteoak
Whiteoak-red oak
Chestnut-redoak
Whiteoak-white pine
Chestnutoak
NJ
PA
VA
VA
PA
PA
Whiteoak-black oak
Beech-redmaple-whiteoak
Whiteoak-red oak
Red oak
Whiteoak-black oak
Chestnutoak-red oak
Red maple-sugarmaple-birch
Red maple-sugarmaple-cherry
Beech-blackgum-redmaple
Sugarmaple-redmaple
Red maple-blackcherry
Maple-chestnutoak-birch
Little 1974, Russell1980
Abramsand Downs 1990
Orwig 1991, Spurr1951
Braun1950, Stephenson1986
Abramsand Nowacki 1992
Nowacki and Abrams1991
Northernforest
Whiteoak-black oak-pine
MA
Blackoak-red oak-white pine
Red maple
WI
WI
WI
Redoak-sugarmaple-redmaple
Red oak-white oak-red maple
Red oak-red maple-paperbirch
Sugarmaple-redmaple
Sugarmaple-redmaple
Red maple
Lorimer1984, Whitneyand
Davis 1986
Nowacki et al. 1990
Nowacki et al. 1990
Nowacki et al. 1990
FL
Turkeyoak-scruboak-scrub
hickory
Sweetgum-blackgum-beech
Myers 1985
GA
Longleafpine-slashpine-turkey
oak-scruboak
Beech-oak-magnolia
NC
NC
Whiteoak
Redoak-chestnutoak-whiteoak
Red maple
Red oak-red maple-hickory
Braun1950, Christensen1977
Keever1953
Conifer-northernhardwoods
Whitepine-oak
Whitepine-red pine
Southerncoastalplain
Longleafpine-slashpineturkeyoak
Longleafpine
SouthernPiedmont
Whiteoak-black oak
Oak-chestnut
348
Quartermanand Keever1962
BioScience Vol. 42 No. 5
effect long-distancedispersal of oak
acorns (Crow 1988).
There exist similarities between
tallgrassprairieand oak savannasin
theirtransformationsto closed forests
with decreased fire. For example,
manyof the originalsavannasof central Oklahoma became closed post
oak-blackjack oak forests, whereas
galleryoak savannasin easternKansas became closed oak forests during
the first 50 years after Europeansettlement(Abrams1986, Rice and Penfound 1959). In Wisconsin, bur oak
or white oak-black oak savannas
gave way to black oak, white oak,
and/or hickory (Carya) forests after
settlement (Cottam 1949, Dorney
~ !~
:::~,~
......
and Dorney 1989). Understorygrubs
(multistemmedsprouts) of oak and
hickoryin these savannas,previously
top-killed by recurring fire, rapidly
Figure2. Quercusrubraforestformedafterloggingof Pinusstrobus(notestump)in the
grew after fire exclusion.
fern(Dennstaedtia
late1800sin north-central
punctilobula)
Hay-scented
Pennsylvania.
dominationof the forestflooris attributedto deerbrowsingof competingwoody
Red oak expansion in
plants.
t;...
.t .
_,
:"
'
.
A
^ ....
v
!.; j
.r ?
*
..'.^
.
.j ^
northernhardwood forests
The northernhardwoodforestbiome,
also known as the hemlock-white
pine-northern hardwood region
(Braun1950), extends from northern
Minnesota to New England, including most of New York and northern
Pennsylvania.Presettlementforestsof
the northernhardwoodregion generally containedonly a small oak component. The originalforestsof northern Wisconsin and Michigan were
dominated by various combinations
of hemlock(Tsugacanadensis),sugar
maple, yellow birch (Betula alleghaniensis), beech (Fagus grandifolia),
and mixed pine (Finley 1976, Whitney 1986). An unusuallyhigh amount
of oak (5-75%) in presettlementupland forests, including an oak savanna,in northeasternWisconsinwas
attributedto frequent Indian-caused
firesin the area (Dorey and Dorney
1989). On the Allegheny Plateau,
white oak, black oak, and red oak
represented19% of the presettlement
forestsin westbeech-maple-hemlock
ern New York (Seischab 1990), but
less than 4% in northwesternPennsylvania (Lutz 1930). No oak was
recorded in presettlement sprucebeech-fir forests in northeastern
Maine (Lorimer1977).
There is little doubt that presettlement hemlock-northern hardwood
forests containing almost no oak or
May 1992
pine burned infrequently.The average recurrence interval of fire in
northeasternMaine was 800 years
(Lorimer 1977). Only one fire occurredbetween 1665 and 1967 in a
white pine-hemlock-spruceforest in
New Hampshire (Henry and Swan
1974). In northernlower Michigan,
the average return time for severe
crown fires, estimated from General
Land Office survey records, was approximately 80 years in jack pine,
120-240 years in mixed pine, and
1200 years for hemlock-white pinenorthernhardwoods(Whitney1986).
A particularlyinterestingfeatureof
the northern hardwood forests was
the postsettlement expansion of
northernred oak on a varietyof sites
(Figure2, Table 2). Crow (1988) reviewed literature from northern
Michigan indicating that red oak
dominated upland sandy soils that
had been cut over and burned, soils
previously dominated by white pine
and red pine. In Wisconsin, current
red oak stands (with red oak dominance of 37-51%) developedon former hemlock-birch-maple, maplebirch-pine,pine-oak, and mixed-pine
forests that had been clearcut and
burned (Nowacki et al. 1990). Between 1676 and 1750 in Massachusetts, red oak represented7% of the
woodland composition,comparedto
19% in 1981 (Whimey and Davis
1986). In this area, the presettlement
white oak-black oak-pine forests
were cut to clearland for agriculture.
Decades later, abandonedfarm land
was invaded by pitch pine (Pinus
rigida) followed by white pine. Logging of the white pine forests promotedthe dominanceof redoak from
the releaseof understoryregeneration
and widespreaddispersaland establishment of its acorns (Crow 1988,
Whitneyand Davis 1986).
Mixed-oak forests of the
Mid-Atlanticregion
The forests between New Jerseyand
centralPennsylvaniathroughVirginia
were classified by Braun (1950) as
oak-chestnut (Castanea dentata) and
oak-pine, and they includethe Ridge
and Valley and northern Piedmont
provinces. Oak species also dominated presettlementforests of the region (cf. Russell1983). In precolonial
northern New Jersey, white oak,
black oak, chestnut, and hickory
shared dominance in upland forests
(Russell 1980). White oak, red oak,
chestnutoak, hickory,and pine dominatedthe originalforestsin portions
of Virginiaand West Virginia(Orwig
1991, Spurr1951). Presettlementforests in southwestern Pennsylvania
349
Southeasterncoastal plain and
Piedmontforests
The section of the country between
Mississippi and North Carolinawas
classifiedas oak-pinein the Piedmont
and southeasternevergreenforest on
the coastal plain (Braun 1950).
Southeastern evergreen forests include bottomlandhardwoods,beechmagnolia, and evergreenoak types,
but these forests are mostly dominated by longleaf pine (Pinus palustris). Quartermanand Keever(1962)
reviewedevidenceof forestfiresset by
Indians,Europeansettlers,and lightning between 1600 and 1800 on the
coastal plain. Indians in the region
also cleared land for agricultureby
girdling and burning trees. Longleaf
pine is maintainedby periodicfireat a
burnedridge two-to-three-yearinterval,and withforeston a xericandperiodically
Figure3. MatureQuercusmuehlenbergii
out fire this forest type succeeds to
on KonzaPrairiein easternKansasexhibitingslowratesof successional
replacement.
oak-hickory or Southern mixedhardwood forests (Table 2; Garren
contained approximately 63% oak Russell 1983).
1943, Quartermanand Keever1962).
After European settlement, a reandhickory,of which 40% was white
Unique variations of southeastern
oak (Abrams and Downs 1990). gime of recurring logging and fire evergreen forests include sand pine
White oak (39%), white pine (26%), through the 1800s associated with scrub and sandhillvegetation (Myers
hickory (14%), and black oak (11%) charcoal iron production (Pearse 1985). Sandpine scruboccursonly in
also dominated the original valley 1876) and other activities (e.g., land Floridaand is dominatedby a closed
floor forests in central Pennsylvania clearing and producing timbers for overstoryof sand pine (Pinusclausa)
coal mines) perpetuatedor even in- and an understoryof evergreenscrub
(Abramsand Nowacki 1992).
In one of the few such studies in creased oak dominance in the mid- oaks (Quercus geminata, Quercus
North America,fire historywas eval- Atlantic region (Table 2). In New myrtifolia,and Quercus chapmanii).
uated on a presettlement-originoak Jersey, cutting trees for charcoal fa- This community type burns infrefromMettler'sWoods in centralNew vored oak and birch (Russell 1980). quently, but, when it does burn, the
Jersey(Buellet al. 1954). Six firescars Formerwhite oak-white pine forests high fuel load results in intense,
in the tree were produced between in centralPennsylvaniabecamedom- stand-replacingfires.
In contrast,sandhillvegetationex1641 and 1711, at a mean fire inter- inated almost exclusively by white
val of 14 years. These fires were at- oak and black oak after clearcutting tends from Floridainto North Carotributedto Indianactivityin the area. and burning in the 1800s (Abrams lina; is dominated by longleaf pine,
No fire scars were recorded after and Nowacki 1992). White oak and slash pine (Pinuselliottii), and turkey
1711, representingthe approximate redoak also dominatedpre-and post- oak (Quercuslaevis);and historically
time of European settlement. Other settlementforests near iron furnaces has experiencedfrequent,low-intenaccountsof precolonialfiresin south- in central Virginia (Orwig 1991). sity ground fires (Myers 1985). Until
ern New England and the mid- Shrubforests of pitch pine, bear oak the 1930s, centralFloridahad a long
Atlanticregionhave been reviewedby (Quercus ilicifolia), blackjack oak, history of recurringfire from Indian
Day (1953), Russell (1983), Lorimer and dwarfchestnutoak (Quercuspri- burning for agricultureand hunting
(1985), and Pattersonand Sassaman noides) have been maintained by and from lightning. Fire exclusion
(1988). Although this subjectis con- burningat frequentintervalssincethe fromthe growthof the citrusindustry
troversial, eyewitness accounts and presettlementera in the New Jersey in both sandhilland sand pine scrub
charcoal studies suggest that Indians pine barrens(Little 1974). However, has resultedin little or no overstory
were responsible for increasing fire an increasein oak dominancein the recruitmentof pine species and the
frequency above the low numbers mid-Atlantic region also occurred increaseddominance of turkey oak,
that would have been causedby light- from the loss of chestnut and the scrub hickory (Caryafloridana),and
ning. Precolonialfires were generally exclusionof firefrompine (e.g., Pinus evergreenscrub oaks.
concentratedin, but not restrictedto, strobus,Pinus pungens,Pinus virginOriginal forests of the Piedmont
areas of Indian habitation, and peri- iana,Pinustaeda,and Pinusechinata) Regionwere dominatedby white oak,
odic wildfires, such as reported by and pine-oak forests (Little 1974, black oak, red oak (Quercus rubra
early colonists, would favor oak for- Monette and Ware 1983, Stephenson and Quercusfalcata),scarletoak, post
ests (Pattersonand Sassaman 1988, 1986).
oak, hickory, longleaf pine, and
350
BioScienceVol. 42 No. 5
shortleaf pine (Braun 1950, Nelson
1957). Evidenceof early Indian fires
on the coastalplainand Piedmontwas
presentedby Silver(1990), who stated
that fires were set to enhance soil
nutrientsin fields, stimulate browse
speciesto attractbirdsand deer, clear
land for agriculture,eliminateinsects,
and clearthickundergrowthin forests
to facilitate woodland travel. Large,
accidentalwildfireswere also the occasionalresultof Indianactivity.After
Europeansettlement,virtually all of
the original forests in the Piedmont
regionwere cut, but oak and hickory
still remainthe dominantforest type
(Table2; Braun1950, Keever1953).
Transitionalstatus of
oak forests
In general,oaks are being successionally replacedby other species,despite
observationsthat manyforestscontain
a substantialnumberof oak seedlings
(Table2). In the originaltallgrassprairie region of central Missouri, sugar
maple is replacingwhite oak forests
(Pallardyet al. 1988), whereasin eastern Kansas Celtis occidentalis(hackberry) is replacing former bur oak
forestson mesic sites (Abrams1986).
Chinquapinoak still dominatesxeric
sites in easternKansas,but it has exhibitedlow recruitmentinto the treesize class relative to that of redbud
(Cercis canadensis) and elm. The
slowerreplacementof chinquapinoak
(Figure3) is probably related to the
greater influence of fire as well as
droughton those sites (Abrams1985).
In the northernhardwoodsregion,
most northern red oak and mixedoak forests currentlyexhibit poor recruitmentof new oak individualsinto
the canopy due to understorydomination by maple and cherry (Crow
1988, Host et al. 1987). Along
edaphic gradients in northern Wisconsin and lower Michigan, sugar
maple and red maple dominatedoak
understorieson mesic and dry-mesic
sites, respectively(Host et al. 1987,
Nowacki et al. 1990). Red maplealso
dominatedthe understoryof red oak
and chestnut oak forests in Massachusetts and New York, and it was
expectedto exhibit increasingcanopy
importancein the absence of disturbance (Lorimer1984).
However, it has been argued that
low numbersof northernred oak reMay 1992
remnant,mesic Quercusalba forestin
Figure4. SophiaWoods,a presettlement
in a latestageof oak replacement
southwestern
by beech(foreground),
Pennsylvania,
and
maple.
tulippoplar,
generationdo not necessarilytranslate
to a lack of futurered oak overstory
dominance. After clearcutting of
mixed standsin centralNew England,
redoak mayexhibitlow seedlingnumbersand slow earlygrowthrelativeto
red maple and black birch (Oliver
1978). However, high mortalityand
deceleratedheightgrowthovertimeof
mapleand birchmay allow red oak to
maintaindominancein the matureforest. It is importantto note that there
are many examples where red oak
seedlingsare not expected to recruit
into largersize classesin closedforests
or to outgrowthe competingtree speciesafterclearcutting(seediscussionin
Nowacki et al. 1990).
The regenerationlayerof manyoak
forests in the mid-Atlanticregion is
also dominatedby later successional
species (Table 2). In northern New
Jersey, sugar maple, red maple, and
sweet birch (Betula lenta) exhibited
increasing importance on unbured
oak sites (Little 1974). Blackgum
(Nyssa sylvatica),sassafras(Sassafras
albidum,andredmapledominatedthe
understoryon mesic oak sites in Virginia (Ross et al. 1982). A presettlement-originwhite oak forestin southwestern Pennsylvaniawas in a late
stage of oak replacementby beech (F.
grandifolia),redmaple,and tulippoplar (Liriodendrontulipifera),presumably facilitatedby a long-termreduc-
tion in fire frequency (Figure 4;
Abrams and Downs 1990). Selective
loggingduringthe 1930s and 1940s in
that stand acceleratedthe speed of
obtainingcanopydominancefor these
non-oak species.Similarly,red maple
and black cherry (Prunus serotina)
dominatedthe understoryand subcanopy in mature white oak and black
oak forests in central Pennsylvania,
and they fully occupied the canopy
afterloggingof the oak overstory(Figure 5; Abramsand Nowacki 1992).
Relative to other areas of eastern
North America, studies are scarce
concerningsuccessionaltendenciesin
oak forests of the southernPiedmont
region. In one such study of a white
oak-hickory forest in North Carolina, red maple increased in nearly
every tree size class over a 22-year
period, and those trees are expected
to form a dominantpart of the future
overstory (Christensen1977). Other
forests in North Carolina,comprised
of red oak, chestnutoak, white oak,
and hickory (which dominated former oak-chestnutstands), also had a
substantialnumberof red mapleseedlings and saplings(Keever1953).
Oak as a late successional
foresttype
Althoughmost recentstudiesindicate
the transitionalnature of oak, a few
351
later successionalspecies and a lack
of adequateoak regeneration,leaving
the ecological status of these stands
unresolved (Orwig 1991). Oakdominatedforestson dryridgesites in
Kansasand Missourihave comprised
substantially lower importance of
later successional species compared
with mesicoak forests,as do Appalachianoak forestsin southwesternVirginia (Abrams 1986, Pallardyet al.
1988, Ross et al. 1982). Thesestudies
suggestthat the speed of oak replacement is much slower on xeric versus
mesic sites and that late successional
oak forests are probably limited to
certain areas at the western extreme
of the easterndeciduousbiome or on
very xeric or nutrient-poorsites further east.
Conclusions
Figure5. Acer rubraand Prunusserotina
forest formedafter the 1936 logging of a
Quercusalba-Quercus velutinaforest in
centralPennsylvania.
studies suggest some stability in this
forest type (Table2). Blackjackoakpost oak forests derivedfrom savannas in Oklahomaand blackjackoakblack oak forests on extremelyxeric,
uplandsites in Illinoisdid not exhibit
signs of being replacedby late successional species (Adams and Anderson
1980, Dooley and Collins 1984).
Mixed oak forestsof blackoak, white
oak, chestnut oak, and scarlet oak
may representa climaxassociationon
upland sites in the New Jersey pine
barrens (Little 1974). Xeric ridge
communitiesdominated by chestnut
oak in southeasternPennsylvaniaalso
exhibited no strong successionaltendencies (Keever 1973), although
chestnutoak stands on similar ridge
sites in central Pennsylvaniahad a
sapling class dominated by striped
maple (Acer pensylvanica),red maple, and sweet birch, with little oak
(Nowacki and Abrams1991).
Despite a regenerationlayer dominated by black cherry, box elder
(Acer negundo), and elm, none of
these speciesseemedcapableof being
a self-replacing climax in a xeric
white oak-black oak woods in Wisconsin (McCune and Cottam 1985).
Similarly, second-growth white oak
forests in Virginia had a scarcity of
352
Paleoecologicalstudies indicate that
oak dominationof easternforestsoccurred during warmer and drier climatic periods at the beginningof the
Holoceneepoch and that these conditions are thought to have increased
the incidenceof fire. Indian burning
practices and other disturbancefactors may have elevated oak dominancein certainpresettlementforests.
Furtherincreasesin oak occurredafter Europeansettlement,whose activities included fire exclusion in tallgrass prairie and southeasternpine
forests;loggingand burningof northern pine-hemlock forests; and the
charcoaliron industry,land clearing,
and chestnut blight in the midAtlantic region. Thus, the postsettlement distributionof oak greatly exceededthat of the presettlementera in
various regions of eastern North
America.However,the evidenceindicates that oak is not a typical dominant in late successionalforests, and
its stabilityis probablylimitedto sites
of extremeedaphicor climaticconditions or areas that are periodically
burned.
Many recentlyformed oak forests
contain a substantialnumberof oak
seedlings that do not recruit into
larger size classes, suggesting these
forestsmay be a one-generationphenomenon. Major oak-replacement
speciesthroughoutthe easternUnited
States include sugar maple, red maple, and black cherry. Although today's oak regenerationproblemscan
be attributedto a suite of factors, a
primary factor is the physiological
limitationimposedon oak in a closed
understorydominatedby latersuccessional species.
Regions of easternNorth America
that lacked oak domination before
Europeansettlementprobablyburned
too frequently (tallgrass prairie) or
too infrequently (hemlock-northern
hardwoods)for oak to prosper.Presettlement oak forests of southern
New England, the mid-Atlantic region, the midwest, and the southern
Piedmontmust have burnedat some
intermediatefrequency(e.g., 50-100
year intervals) that promoted the
dominance and stability of oak, includingthose forestsat some distance
from Indiansettlements.
Loss of oak dominancein numerous forests in which fire has been
excluded during the twentieth century, and the lack of such patternsin
forestsperiodicallyburned,shouldbe
considered important indirect evidence that fire played a vital role in
maintaining oak dominance before
Europeansettlement.If in the current
oak forests factors antagonistic to
oak regeneration,such as a lack of
fire, persist into the twenty-firstcentury, a retrenchmentof oak dominance would seem inevitable.
Acknowledgments
I thank Craig Lorimerand KurtPregitzer for stimulatingdiscussionsand
criticalreviews of an earlierdraft of
the manuscript.
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