United States

Department of

Agriculture

Forest Service

Northeastern

Research Station

General Technical

Report NE-295

Biotic and Abiotic Mechanisms

in the Establishment of Northern

Red Oak Seedlings: a Review

Linda S. Gribko

Thomas M. Schuler

W. Mark Ford
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Abstract

The sustained presence of northern red oak (Quercus rubra) in forests throughout

the eastern and central hardwood regions of the United States often depends on

seed-origin reproduction. However, in some hardwood forests, small northern red

oak seedlings may be sparse for extended periods. This is particularly the case in

the central Appalachians, where seedlings apparently become established in singlecohorts during anomalous years when acorn production is above average. A better

understanding of the mechanisms that drive the establishment of seedlings could

increase the potential for sustaining the species within these regions. This report

summarizes the literature on acorn production, predation by insects and

vertebrates, infection of acorns by microorganisms, and environmental factors that

affect germination. Recommendations for future research are presented.

The Authors

LINDA S. GRIBKO, assistant professor of forest management at West Virginia

University, received a B.S. degree in forestry and wildlife from Virginia Polytechnic

Institute and State University and a Ph.D. degree in forest resources science from

West Virginia University. She is currently engaged in research on the establishment

of Appalachian oak seedlings and interrelationships between biotic and abiotic

components of oak-dominated forests.

THOMAS M. SCHULER, research forester, received a B.S. degree in forest

management, an M.S. degree in silviculture from Colorado State University, and a

Ph.D. degree in forest biology from Purdue University. He is currently engaged in

research on the ecology and silviculture of Appalachian hardwood forests and the

restoration of central Appalachian spruce forests at the Northeastern Research

Station’s Timber and Watershed Laboratory at Parsons, West Virginia.

W. MARK FORD, research wildlife biologist, received a B.S. degree in wildlife and

fisheries science from the University of Tennessee, an M.S. degree in wildlife

ecology from Mississippi State University, and a Ph.D. degree in forest resources

from the University of Georgia. He is currently engaged in research on wildlifehabitat relationships in central and southern Appalachian forests with the

Northeastern Station at Parsons.

Manuscript received for publication 1 February 2002

Cover Photo

Mature red oak acorns in second year of development and current-year acorns

visible in leaf axils of new growth (photo by Paul S. Johnson)
1

Introduction

The sustained presence of northern red oak (Quercus

rubra) in forested ecosystems often depends on seedorigin reproduction (Ward 1966; Arend and Scholz

1969; Sander and others 1976; Watt 1979; Crow 1988),

particularly on highly productive sites. There are

concerns about the ability of oaks and northern red oak

in particular to regenerate from seed (Minckler and

Jensen 1959; Loftis 1988; George and Fischer 1991).

Most research has focused on the inability of small oak

seedlings to grow from the herb to the shrub layer, i.e., to

become well-established advanced regeneration. Forest

managers often ignore the mechanisms involved in the

initial establishment of seedlings because, in many

reported cases, small seedlings (less than 15 cm tall) are

present, but larger ones are not. It is generally believed

that the stock of oak seedlings builds up slowly as small

numbers of acorns escape vertebrate predation and

insect infestation during years of heavy seed production.

When abundant oak seedlings or saplings are present,

the timing of silvicultural treatment is not critical

because there are adequate numbers of seedlings or

saplings for release.

However, in parts of the eastern and central hardwood

regions of the United States — about 235,000 square

miles — small northern red oak seedlings may be sparse

or absent for extended periods. This is particularly

common in the central Appalachians where, rather than

slowly increasing in number, seedlings apparently

become established in single cohorts during anomalous

years of abundant acorn production and favorable

weather (McGee 1967; Gribko 1995a; Schuler and

Fajvan 1999). Silvicultural treatments that favor the

release of oak seedlings must be timed carefully to take

advantage of each regeneration wave. However, there are

few opportunities to release abundant oak seedlings in

most forests. A better understanding of the mechanisms

that drive the establishment of northern red oak

seedlings could increase the potential for sustaining this

species within these regions.

In the eastern and central hardwood regions it is

expected that many forests with a significant northern

red oak component eventually will be dominated by

species such as red maple (Acer rubrum), sugar maple (A.

saccharum), and American beech (Fagus grandifolia). The

specific consequences of such conversions are not

known, though none of these species has the ecological

or economic value of northern red oak. The loss of oak

from forest overstories will lead to reductions in

availability of acorns as a winter food source for

numerous wildlife species, including white-tailed deer

(Odocoileus virginianus), wild turkey (Meleagris gallopavo),

and black bear (Euarctos americanus). Densities of these

species as well as those of many small mammals and

nongame birds are closely tied to oak masting (Bendell

1959; Gilbert and Krebs 1981; Wentworth and others

1992; McShea and Schwede 1993; Wolff 1996;

McCracken and others 1999; McShea 2000). In addition,

red oak lumber and veneer have high monetary value.

The limited availability of oak in these regions would

have a substantial adverse impact on the economic

viability of many value-added forest enterprises,

including the furniture, hardwood flooring, and

cabinetry industries.

Specific factors affecting the initial establishment of

northern red oak seedlings include the lack of: 1) seed

production, 2) viable seed due to insect and vertebrate

predation or infection by microorganisms, and 3) acorn

germination due to environmental factors. Each of these

is discussed with reference to pertinent literature. Much

of the research cited pertains directly to northern red oak

regeneration; examples are drawn from investigations of

other oak and hickory (Carya spp.) species.

Background

Seed Production

There are several methods for assessing seed production

in oaks. The most common entails the use of seed traps

of various size and construction. The simplest trap is a

bushel basket affixed to a stake and placed under a tree

crown (Liscinsky 1984). Downs and McQuilken (1944)

first reported the use of rectangular wood-frame traps

with hardware cloth bottoms and sides to collect acorns.

These traps were about 1.0 m2 and covered with 3-cmmesh poultry netting to prevent predation by mammals.

Variations on this trap design were used by Burns and

others (1954), Christisen and Korschgen (1955), and

Goodrum and others (1971). Also used were funnelshaped traps of various size and construction material;

some featured a hardware cloth collection bin at the

apex and a poultry cloth cover (Easley and Chaiken

1951; Gysel 1957; Tryon and Carvell 1958; Klawitter and

Stubbs 1961; Ovington and Murray 1964; Beck and

Olson 1968; Shaw 1968a; Christisen and Kearby 1984).

Marquis and others (1976) designed a variation of the

funnel trap in which a large rectangular sheet of plastic

was affixed to wire supports; one end of the sheet was

elevated, allowing acorns to roll into a collection bin at

the opposite end. Thomson and McGinnes (1963) and

Gysel (1956) compared the efficacy of various

rectangular and funnel traps.

Another common method of assessing acorn production

entails the use of open ground quadrats (Wood 1934;

Kautz and Liming 1939; Allen and McGinley 1947;

Cypert and Webster 1948; Dalke 1953). Open plots can
2

be used with seed traps to assess predation rates (Downs

and McQuilken 1944; Shaw 1968a; Goodrum and others

1971). Other researchers have attempted direct counts of

acorns still on the tree (Verme 1953; Sharik and others

1983). In some cases, individual branches are marked

and inventoried annually (Petrides and others 1953). A

modified ground method in which binoculars are used is

favored by many state wildlife agencies to quickly assess

relative annual production over a wide area (Sharp 1958;

Koenig and others 1994). Indirect observation of crop

size by counts of shed acorn caps was reported by Wood

(1934), Shaw (1968a, 1974), and Auchmoody and

others (1993). Because vertebrate pedators tend to ignore

the caps while foraging, this method can provide a more

accurate estimate of the entire crop size than others cited.

Regardless of the method used, sampling design and

intensity affect how estimates can be used and the

conclusions that can be drawn. Estimates for individual

trees have been made by locating traps and ground plots

under the crowns of mature acorn-producing stems. By

assessing individual-tree production, particularly over a

number of years, one can observe variability within a

single species or among several species (Greenberg

2000). This method also is the basis for studies of the

effects of tree age, size, and other characteristics on acorn

production. Area wide production estimates have been

made by locating ground plots and traps randomly or

systematically through forest stands. Such evaluations are

most useful when wildlife food crops are being studied

(McShea and Schwede 1993), when correlations between

acorn production and silvicultural treatments are of

interest (Beck and Olson 1968), or when estimating the

proportion of acorns in total litterfall (Nielson 1977).

Production usually is reported as number or weight of

acorns per tree or unit ground area. Estimates of zero to

more than 600,000 acorns per hectare have been

reported (Down and McQuilken 1944; Gysel 1956; Gysel

1957; Tryon and Carvell 1962a; Marquis and others

1976; Beck 1977; Christisen and Kearby 1984; Liscinsky

1984; Auchmoody and others 1993). Care must be taken

when comparing study results because sampling

techniques and intensities differ and standard errors

sometimes are not provided. Generally, estimates based

on cap counts are higher than those based on samples

from seed traps, that in turn are higher than estimates

from ground-plot samples. Estimates from samples from

under the crown of individual trees are higher than those

obtained on randomly or systematically located plots.

Placement of traps and plots under more or less

productive portions of the crown also can affect results

(Gysel 1956; Christisen and Kearby 1984).

Relative crop size often is reported using terms such as

bumper (76 to100 percent of the maximum crop), good

(51 to 75 percent), fair (26 to 50 percent), poor (10 to 25

percent), trace (less than 10 percent), and none (Sharp

1958), and there are numerous variations (Snow 1940;

Allen 1952; Uhlig and Wilson 1952; Wright 1953;

Thompson and Lemke 1954; Christisen and Kearby

1984). Wildlife agencies in several southern states use a

scale of 1 to 10 to rate acorn abundance across regularly

surveyed routes within defined areas (Carlock and others

1983). In attempting to standardize the rating sytems,

Auchmoody and others (1993) developed a crop guide

that converts subjective rankings to estimates of acorns

produced per unit ground area and vice versa.

Observers of northern red oak and other acorn crops

have concluded that production variability is

considerable among individual trees and years. Acorn

crops have been characterized as sporadic, infrequent,

and unpredictable (Sharp and Chisman 1961; Watt

1979; McQuilken 1983; Auchmoody and others 1993).

Northern red oak and other oaks of the subgenus

Erythrobalanus generally are more consistent producers

of acorns than are oaks of the subgenus Leucobalanus,

which may fail to produce any acorns for several

consecutive years (Gysel 1956, 1957; Tryon and Carvell

1962a; Christisen and Kearby 1984; Liscinsky 1984). In

most years, northern red oaks produce at least a

moderate acorn crop; Liscinsky (1984) reported

germination failure in northern red oak, black oak (Q.

velutina), and scarlet oak (Q. coccinea) in only 2 years of

an 11-year study.

Some researchers have tried to determine periodicity in

the production of bumper crops of northern red oak

acorns. Christisen and Kearby (1984) reported bumper

crops of northern red oak acorns to be less cyclic than

those of white oak acorns. However, other observers have

noted trends toward heavy crops at 3-, 4-, or 5-year

intervals (Sharp 1958; Tryon and Carvell 1962a; Beck

1977; Sork and others 1993).

Annual variations in the size of acorn crops are not

completely understood but are thought to be driven

primarily by environmental factors that affect the

survival and pollination of the flower crop (Cecich 1993,

1994, 1997; Cecich and Sullivan 1999). Studies of oak

flowering revealed little yearly variation in initial flower

crop sizes, suggesting that physiology and heredity have

negligible roles in the annual variability acorn crops

(Sharp and Chisman 1961; Sharp and Sprague 1967;

Olson and Boyce 1971; Farmer 1981; Cecich and others

1991; Sork and others 1993). Temperature, wind, late

frost, prolonged rain, and relative humidity can affect

flower development, the opening and closing of anthers,

and pollen dissemination (Sharp and Chisman 1961).

Fertilization rates are higher when pollen shed and the

receptivity of pistillate flowers are closely aligned. Sharp

and Sprague (1967) observed that acorn crops usually

were large when warm weather in late April, which

stimulated early catkin emergence, was followed by cool

temperatures in May. This delayed pollen dispersal until

pistillate flowers matured. Consistently low temperatures

in late April, strong, dry winds in early May, or warm

weather throughout April and May can cause pollen

development and dispersal to be out of synchrony with
3

the receptivity of pistillate flowers (Sharp and Sprague

1967; Cecich 1997). High relative humidity, especially

when associated with prolonged rainy periods, may

delay pollen dispersal (Wolgast and Stout 1977a;

Jovanovich and Tucovic 1975), though no significant

effects have been observed under field conditions (Sharp

and Sprague 1967; Cecich 1997).

Even in years of generally favorable spring weather,

flowers can be damaged or destroyed by a late freeze

(Reid and Goodrum 1957; Beck 1977; Wolgast and Trout

1979; Sork and Bramble 1993), hail (Cecich 1997), or

insects, particularly treehoppers (Membracidae) (Kopp

and Yonke 1973a, b, c, 1974; Cecich and others 1991).

Dormant flower buds also can be damaged or destroyed

when winter temperatures are extremely low (Kramer

and Kozlowski 1979). By early July, virtually all of the

flower crop can be lost due to a combination of

pollination failure and physical damage (Cecich 1993).

The proportion of flowers that actually produce acorns is

reduced further by premature abscission. Although the

abscission of immature fruits is greatest between

pollination and fertilization (Turkel and others 1955),

acorns continue to be aborted throughout summer and

into early fall (Christisen and Kearby 1984). As many as

95 percent of all white oak abscissions were premature in

a 2-year Kentucky study (Williamson 1966). Possible

causes of premature abscission include gypsy moth

(Lymantria dispar) defoliations (Gottschalk 1988),

infestations of seed insects (Galford and others 1988),

and early fall frosts (Kramer and Kozlowski 1979).

Drought was cited as a factor by Christisen and Kearby

(1984) and Sork and others (1993), but not by Sharp

and Sprague (1967) or Goodrum and others (1971).

Drought effects might be confounded by timing, soil

type, and tree species (Kramer and Kozlowski 1979).

Generally, any substantial stress during the 2-year

maturation cycle can reduce crops of northern red oak

acorns. Moreover, it has been hypothesized that different

factors may be responsible for early abscissions from

different branches on the same tree (Stephenson 1981).

Thus, it is not yet possible to predict acorn production

for individual trees more than several weeks in advance

(Gysel 1957; Koenig and others 1994; Cecich 1993).

However, Greenberg and Parrosel (2000) analyzed the

characteristics of southern Appalachian oaks and

developed a statistical method to predict within-year

acorn production for a specified area based on the

proportion of trees with acorns and the basal area of

each species present. Larsen and Cecich (1997)

presented a probalistic model that can be used to explore

the many factors that may cause annual variations in the

production of white oak acorns.

Acorn production by individual trees has been linked

most consistently to inherent genetic capacity (IrgensMoller 1955; Sharp and Sprague 1967; Grisez 1975).

Variation also may be related to tree age (Reid and

Goodrum 1957; Harper and White 1974; Wolgast and

Stout 1977b), size and relative position of the crown

(Downs and McQuilken 1944; Burns and others 1954;

Tryon and Carvell 1962a), size of the bole (Downs 1944;

Goodrum and others 1971; McQuilken and Musbach

1977; Hashizume 1979; Kanazawa 1982; Greenberg

2000), and/or overall tree vigor measured as radial

growth rate (Wood 1938; Burns and others 1954;

Goodrum and others 1971). Investigations of individualtree characteristics and their impact on acorn production

often have produced conflicting results due to

confounding by shading or the inherent production

capacity of the study trees (Goodrum and others 1971).

However, it is generally thought that 40 to 99-year-old

northern red oaks (40 to 50 cm in d.b.h.) with large,

healthy, unshaded crowns and an innate ability to set

flowers produce the largest acorn crops (Toumey and

Korstian 1937; Downs 1944; Verme 1953; Sharp 1958;

Goodrum and others 1971).

The amount of light reaching the crown also is critical as

reduced light affects acorn crops by inhibiting floral

initiation, fruit set, and fruit development (Lassoie and

others 1980). Because both seed number and size can be

reduced by inadequate direct sunlight, oaks in

suppressed and intermediate crown positions usually

produce negligible seed crops (Kittredge and Chittendon

1929; Moody 1953; Gysel 1956). In dominant and

codominant trees, acorn production is greatest in the

upper crown, particularly the south- and west-facing

sides which receive more direct sunlight (Verme 1953;

Sharp and Sprague 1967). The death or loss of large

limbs in the upper crown of large older trees also can

cause acorn production to drop sharply (Sharp 1958;

Goodrum and others 1971).

Site productivity and physical characteristics of growing

site also have been investigated as possible sources of

tree-to-tree or local variation in acorn production (Gysel

1957; Tryon and Carvell 1962a). The results of these

studies have been inconclusive except Gysel’s (1957)

finding that the weight (not number) of acorns

produced by several oak species in the Great Lakes region

was positively correlated with site quality. It generally has

been concluded that most local variation, particularly in

mountainous terrain, is due to frost pockets and other

local weather effects that limit the percentage of pistillate

flowers that are fertilized (Gysel 1956; Farmer 1981).

The lack of acorn production has not been reported as

directly responsible for sparse seedling establishment

(Walters 1990). In fact, Tryon and Carvell (1962b) found

that density of oak seedlings was independent of acorn

production on a site in the central Appalachians.

However, selective harvest of large dominant and

codominant northern red oaks and increasing mortality

of mature oaks due to drought and attacks by

opportunistic organisms will significantly impede future

seed production (Healy and others 1997).
4

Insects, Vertebrates, and Microorganisms

Insects

Three important insect pests have been identified in

autumn collections of northern red oak acorns: Curculio

weevils (Coleoptera: Curculionidae), larvae of an acorn

moth, Melissopus latiferreanus (Lepidoptera:

Oleuthreutidae), and gall-forming wasps of the genus

Callirhytis (Hymenoptera: Cynipidae) (Gibson 1969,

1982; Oak 1992). These insects can breed in otherwise

sound acorns that are developing on the tree (Gibson

1982). Although never implicated in complete

germination failures, high Curculio infestation rates were

reported by Brezner (1960), Marquis and others (1976),

Gibson (1982), Galford and others (1991a), Gribko and

Jones (1995), and Steiner (1995). A much lower

incidence of Melissopus latiferreanus (Brezner 1960;

Dorsey and others 1962; Galford and others 1991a;

Gribko and Jones 1995) and wasp-forming galls,

especially Callirhytis fructosa, (Dorsey and others 1962;

Gibson 1982; Gribko and Jones 1995) was reported,

though populations of these species may be locally high

in some years (Myers 1978; Gibson 1972, 1982).

It is generally assumed that most “weeviled acorns” do

not produce viable seedlings. Insecticidal and

mechanical treatments have been suggested to control

infestations (Dorsey 1967; Stockton and Morgan 1979;

Crocker and Morgan 1983; Crocker and others 1987,

1988; Galford and others 1988). However, several

authors found small numbers of Curculio-damaged

acorns that produced seedlings. These usually were

acorns with an undamaged embryo and most of the

endosperm intact (Downs and McQuilken 1944; Oliver

and Chapin 1984; Weckerly and others 1989b). It is

thought that infested acorns that remain viable are

vulnerable to infestation by secondary insects, fungi,

bacteria, and other microorganisms, though data from

the Allegheny Plateau in West Virginia suggest that large

numbers of weevil-infested northern red oak acorns can

produce viable, healthy seedlings (Gribko 1995a).

Other insects present in northern red oak acorns often

are ignored due to their low numbers or to the erroneous

perception that they act primarily as scavengers of the

frass and detritus left in acorns vacated by Curculio weevil

and acorn moth larvae. These insects cannot breach the

acorn shell and must use damaged, previously infested,

or germinating acorns for oviposition sites. The most

important of these secondary insects are weevils of the

genus Conotrachelus (Coleoptera: Curculionidae), an

acorn moth, Valentinia glandulella (Lepidoptera:

Blastobasidae), and a nitidulid sap beetle, Stelidota

octomaculata (Coleoptera: Nitidulidae). Each has been

observed in small numbers of damaged northern red oak

acorns in autumn and in greater numbers of germinating

acorns in spring (Brezner 1960; Dorsey and others 1962;

Gibson 1964; Gibson 1982; Galford 1986a; Galford and

others 1988, 1991b; Gribko 1995b).

Widespread infestations of germinating northern red oak

acorns by these insects can result in germination failure

(Galford and others 1991a), severely reduced

germination rates (Krajicek 1960; Galford 1986a;

Galford and others 1988; Auchmoody and others 1994),

or reduced vigor of seedlings (Gribko 1995b). Valentinia

glandulella was observed feeding primarily on the surface

of the cotyledons; its most serious impact on

germinating acorns may be the attraction of more

destructive Conotrachelus weevils and nitidulid sap beetles

(Galford 1986a). Cool wet springs and late freezes may

limit damage from these insects (Gribko 1995b).

Other secondary insect pests of northern red oak acorns

are dipterous larvae of the families Drosophilidae

(pomace flies), Cecidomyiidae (gall gnats),

Mycetophilidae (fungus gnats), Psychodidae (moth

flies), and Anthomyiidae (anthomyiid flies) (Winston

1956; Dorsey and others 1962). All are found in acorns

that have been vacated by weevil or lepidopterous larvae

or in the cotyledons of badly diseased or decomposed

acorns. These insects act primarily as scavengers or

fungus feeders (Winston 1956; Dorsey and others 1962;

Borror and White 1970).

Although they do not infest the cotyledons of acorns,

two nonnative weevils, Cyrtepistomus castaneus (Asiatic

oak weevil) and Barypeithes pellucidus, cause extensive

damage to germinating northern red oak acorns and

seedlings by feeding on emerging radicles, roots, and

shoots (Galford 1986b; Ferguson and others 1992). A

millipede, Ptyoiulus impressus, was observed feeding

heavily on emerging radicles (Galford and others 1992).

Vertebrates

Acorns are low in crude protein, fiber, calcium, and

phosphorus, but high in digestible cell contents and

nitrogen-free extract (Waino and Forbes 1941; Reid and

Goodrum 1957; Short 1976; Lassoie and others 1980).

Digestible cell contents include lipids (or fats), starches,

soluble protein, nonprotein nitrogenous compounds,

sugars, and pectins (Short 1976). Nitrogen-free extract,

the substance that remains after crude fat, protein, ash,

and crude fiber are subtracted from the dry matter,

contains soluble carbohydrates and vitamins B and C

(Reid and Goodrum 1957; Short 1976). Acorns are a

high-energy, easily digested food for squirrel (Sciurus

spp. and Glaucomys spp.), white-tailed deer, wild turkey,

black bear, deer mice (Peromyscus maniculatus), whitefooted mice (Peromyscus leucopus), blue jay (Cyanocitta

cristata), and many other species of wildlife (Van Dersal

1940; Hamilton 1941; Mosby and Handley 1943; Cypert

and Webster 1948; Martin and others 1951; Reid and

Goodrum 1957; Duvendeck 1962; Korschgen 1962;

Nixon and others 1968; Beeman and Pelton 1980;

Lassoie and others 1980; Pekins and Mautz 1987). It has

been demonstrated that the density of seed predators

may be set by the size, frequency, and quality of mast

crops. The reproductive success and condition of white-
5

tailed deer (Wentworth and others 1992), gray and fox

squirrel (Allen 1943; Christisen and Korschgen 1955;

Uhlig 1955), small mammals (Jameson 1955; Gore

1988), and black bear (Eiler 1981) can be linked to the

previous year’s mast crop.

Mammalian and avian acorn predators generally can be

divided into species that consume acorns before they

drop from the tree (predispersal) and those that eat them

when they reach the ground (postdispersal). Squirrel,

chipmunk, mice, several species of birds, and even whitetailed deer have been observed eating acorns as they

ripen on the tree. The predispersal loss of acorns to

vertebrate predation can have a significant impact on

germination (Sharp 1958; Goodrum and others 1971).

Removal/consumption rates have ranged from 18 to 27

percent (Cypert and Webster 1948; Tryon and Carvell

1962a). Damage rates of approximately 25 percent were

reported by Downs (1944, 1949) and Cypert and

Webster (1948).

Open plots or a combination of open and screened plots

have been used to assess the impact of vertebrates once

acorns reach the forest floor. Most investigators reported

high levels of acorn predation for a variety of oak species

(Table 1). Small mammals and squirrel were assumed

responsible for the depredations or were observed

removing acorns from the study plots. Screening devices

and barriers have been used to segregate the effects of

different wildlife species (Table 2). The results of these

studies confirmed that small mammals such as mice and

chipmunks remove a large proportion of acorns from the

forest floor. In Pennsylvania, white-tailed deer also have

a substantial impact on acorn removal (Marquis and

others 1976).

Vertebrates that feed on seeds such as acorns may act as

seed predators or seed dispersers or both. White-tailed

deer, turkey, and bobwhite quail (Colinus virginianus) are

acorn predators because they destroy the seeds that they

consume. Dispersal agents are vertebrates that enhance

plant reproduction and distribution by transporting

seeds from the parent plant (Janzen 1971; Sork 1983).

Dispersal occurs when a proportion of the acorn crop is

cached by small mammals and birds. Species that

scatterhoard acorns at shallow depths in the forest floor,

e.g., squirrel, blue jay, and mice, usually are given credit

for some dispersal even though they eat large numbers of

acorns and other nuts (Darley-Hill and Johnson 1981;

Sork 1983, 1984; Miyaki and Kikuzawa 1988; Thorn and

Tzilkowski 1991). Species such as chipmunks that larderor nest-hoard are not considered effective as dispersal

agents (Smith and Reichman 1984).

Despite their propensity to scatterhoard, the role of

squirrel and mice in the dispersal of significant numbers

of acorns is questionable. Squirrel may successfully

recover virtually every buried nut by spring (Cahalane

1942) and their habit of biting off the tips of acorns

(notching), destroys the viability of large numbers of

cached nuts (Wood 1938; Barnett 1977). White-footed

and other species of mice usually eat acorns where they

are found rather than storing them for later use (Smith

and Reichman 1984; Sork 1984), and nearly every acorn

scatterhoarded by wood mice (Apodemus speciosa and A.

argenteus) (Miyaki and Kikuzawa 1988) and Peromyscus

spp. (Sork 1984) is later recovered and consumed or

placed in nest burrows. Often, acorns that are not

retrieved are damaged severely by mice and thus

rendered inviable (Smith 1962). Miyaki and Kikuzawa

(1988) observed a maximum dispersal distance of 27 to

37 m for Apodemus, and Sork (1984) estimated that 36 to

58 percent of northern red oak acorns were cached or

consumed by small rodents within approximately 18 m

of the parent tree.

Microorganisms

Fungi, bacteria, and other microorganisms associated

with northern red oak acorns have received little

attention. Winston (1956) completed the only published

account of microbiotics in northern red oak acorns and

described several genera of organisms in limited detail.

Researchers have identified organisms in other species of

oak. For example, drippy nut disease (Erwinia quercina)

has been isolated from California live oak (Quercus

agrifolia) and interior live oak (Q. wislizenii) acorns

(Hildebrand and Schroth 1967). The fungus that causes

black rot (Ciboria batschiana) has been identified in

English oak (Q. robur) and sessile oak (Q. petraea) acorns

(Delatour and Morelet 1979), and Vozzo (1984) isolated

Epicoccum purpurascens and Fusarium solani fungi from

water oak (Q. nigra) acorns.

Acorn germination

Like all acorns in the Erythrobalanus subgenus, northern

red oak acorns are characterized by a period of

dormancy before germination (Bonner and Vozzo

1987). In laboratory trials, it was necessary to stratify

fully hydrated northern red oak acorns for 4 to 6 weeks

in moist storage at 2° to 5°C to break dormancy. Sound,

undamaged northern red oak acorns have a moisture

content of 50 to 60 percent and germinative capacity of

75 to 90 percent (Korstian 1927), germination of

Erythrobalanus acorns is best when the moisture content

falls to 40 percent and concentrations of crude fat and

total carbohydrates peak (Bonner and Vozzo 1987).

Germinative capacity is reduced substantially by further

desiccation (Korstian 1927; Hodges and Gardiner 1993),

and viability may be lost when the moisture content falls

to 15 to 25 percent and remains there (Korstian 1927;

Bonner and Vozzo 1987). Because acorns must imbibe

water after dormancy for the pericarp (seedcoat) to

rupture (Peterson 1983), dry spring weather can be

particularly damaging. Rehydration after a period of

desiccation can restore viability (Bonner and Vozzo

1987), suggesting that wet spring weather might reverse

the effects of dry winter conditions.
6

Table 1.—Summary of studies of removal of acorn crops from the ground by vertebratesReference Oak species Technique N Period Percent Vertebrate responsible

observed removed

Wood (1938) Quercus prinus Acorn cap counts Varieda Acorn drop to 87.0 Pitymus pinetorum,

November Sciurus carolinensis

Cypert and Webster Q. nigra Marked acorns on open- Varieda 24 hours 5.6 Peromyscus spp.,(1948) Q. phellos ground plots Glaucomys volans,

Quiscalus quiscula,Colaptes auratus

Christisen and Q. stellata Acorn traps and open- Varieda Acorn drop to 21.5 Not specifiedKorschgen (1955) Q. alba ground plots compared January 37.2

Q. velutina 4.0

Q. marilandica 18.9

Krajicek (1955) Q. rubra Unprotected on top of 640 Late fall to 100.0 Unspecified rodents

litter or on soil surface mid-May

Gysel (1957) Good sites: Q. rubra, Acorn traps and open- Varieda Acorn drop to 8.0 Sciurus niger (based on

Q. alba ground plots compared December 17.0 photographic devicesMed. Site: Q. velutina, 13.0 and direct observation)

Q. alba 18.0Poor sites: Q. velutina 47.0

Krajicek (1960) Q. velutina Litter kept intact 320 per Late fall to 99.8 Unspecified rodents

Q. macrocarpa Litter removed species mid-May 100.0Q. albaQ. rubra

Tryon and Carvell Q. alba Acorn traps and open- Varieda Acorn drop to 58.6 Not specified

(1962a) Q. rubra ground plots compared mid-November 50.2Tryon and Carvell Q. alba Acorn traps and open- Varieda Acorn drop to 50.0 Not specified(1962b) Q. rubra ground plots compared mid-NovemberShaw (1968a) Q. petraea Acorn traps and cap Varieda Acorn drop to 63.0 Not specified

counts compared mid-November

aReported as proportion of total acorn crop consumed.
7

Table 2.—Summary of research using screens, barriers and other devices to exclude vertebrate acorn predators based on body sizeReference Oak species Technique RemarksKorstian (1927) Q. rubra (other Acorns planted under Unspecified, small rodents removed 100 percent of planted acorns.

species also used) screens that failed to No difference in survival on plots screened from larger vertebrates

exclude small rodents and unprotected plots.

Shaw (1968b) Q. petraea Buried and unburied Exclosures accessible only to small mammals suffered same rate of

exclosures with and loss as open-control plots. Wood mice (Apodemus slyvaticus) and

without top screening bank voles (Clethrionomys glareolus) were responsible.

Marquis and others Q. rubra Exclosures used to 53 percent germination of acorns fully protected from deer and(1976) exclude vertebrates rodents; 9 percent germination if protected from deer only; 2 percent

based on body size germination if not protected. Unspecified small mammals and

white-tailed deer (Odocoileus virginianus) were responsible.

Kikuzawa (1988) Q. mongolica Frames with different Exclosures accessible only to small mammals suffered same rate of loss

sizes of mesh used to as open-control plots. Clethrionomys rufocanus, Apodemus argentius,

protect acorns and Apodemus speciosus were responsible.

Borchert and others Q. douglasii Exclosures used to Exclosures accessible to smallest vertebrates suffered the highest(1989) exclude vertebrates predation rates. Botta pocket gophers (Thomomys bottae), deer mice

based on body size (Peromyscus maniculatus), and other unspecified species of mice were

responsible.

Gribko (1995a) Q. rubra Exclosures used to Exclosures accessible only to small mammals suffered same rate of loss

exclude vertebrates as open-control plots. White-footed mice (Peromyscus leucopus), deer

based on body size mice, eastern chipmunks (Tamias striatus) and southern flying

squirrels (Glaucomys volans) were responsible.
8

Temperature also is critical during dormancy and

germination; no germination occurs until temperatures

are consistently above 4.5°C (Korstian 1927).

Germination is best when nighttime temperatures

average 10°C and daytime temperatures average 18°C.

Both excessive heat and excessive cold can damage

acorns (Korstian 1927; Hodges and Gardiner 1993).

Temperatures of 26.5°C at night and 35°C during the

day are too high for optimal germination (Korstian

1927). Periods of fluctuating temperatures, especially in

the spring, can be particularly detrimental (Gribko

1995b). Early thaws often stimulate premature radicle

emergence; subsequent subfreezing temperatures kill

back the tender radicles. Affected acorns may send out a

second radicle though, in the absence of cool, wet

weather in spring, the early splitting of the seed coat

exposes the cotyledons to desiccation reducing the

germination rate.

A layer of leaf litter or soil greatly enhances acorn

germination by providing a cool, moist, stable

environment (Korstian 1927; Shaw 1968b; Sander and

others 1983; Crow 1988). This protective layer reduces

desiccation and encourages the growth of

microorganisms that may aid germination by softening

the pericarp (Bonner and Vozzo 1987). Because acorns

from oaks of the subgenus Leucobalanus germinate in

autumn when litter depths peak, they may derive the

maximum benefit of litter accumulations. In studies of

chestnut oak (Quercus prinus), litter was essential for

germination even under dense overhead shade (Barrett

1931). Seedling establishment was best when the depth

of loose unincorporated leaf litter was 2 to 4 cm. Olson

and Boyce (1971) suggested that deeper, looser litter at

the time of germination may explain why Carvell and

Tryon (1961) found that white oak regeneration was five

times greater than northern red oak regeneration given

equal seed crops.

While not always affording protection from vertebrate

predators, a thin layer of soil enhances germination of

northern red oak acorns on exposed slopes in dry years

(Gribko 1995a). Acorns of several species buried in a

California foothill woodland germinated regardless of

aspect, shade, or early spring weather; those sown on the

surface were more susceptible to overheating and

desiccation (Griffin 1971). A planting depth of 3 cm in

mineral soil, approximately the depth acorns are cached

by gray squirrel (Sciurus carolinensis) (Thorn and

Tzilkowski 1991), was suggested by Sander and others

(1983). It is important that site conditions be considered

because acorn burial in consistently wet soils can reduce

the germination rate (Shaw 1968b).

Acorn germination usually is good in uncompacted,

moist, well-drained mineral soil that allows penetration

by the developing radicle (Griffin 1971; Matsuda and

McBride 1986; Walters 1990). Because radicles are

unable to penetrate soil that has been compacted at the

surface (Korstian 1927), soil disturbance can enhance

the establishment of oak regeneration (Carvell and

Tryon 1961; Arend and Schloz 1969) and soil

scarification has been suggested to improve initial and

possibly long-term survival of seedlings (Minckler and

Jensen 1959; Scholz 1959). In the absense of

compaction, the humus layer generally should be left

intact because it keeps the soil surface porous and retains

moisture (Carvell and Tryon 1961).

Thick, dense, unicorporated litter or grass that suspends

the acorn above the soil surface prevents the radicle from

reaching mineral soil (Arend and Scholz 1969; Griffin

1971; Borchert and others 1989). Burning has been

suggested to remove excessive litter (Johnson 1974; Brose

and others 1999) and create a more favorable site for

acorn burial by vertebrates (Galford and others 1988).

However, substantial numbers of acorns were destroyed

in fall burns (Korstian 1927; Auchmoody and Smith

1993).

Initial survival and early growth of seedlings depend

primarily on stored food reserves in the cotyledons

(Grime and Jeffrey 1965; Musselman and Gatherum

1969; Kormanik and others 1998). Accordingly, many

observers have reported strong correlations between

acorn weight and the size and vigor of first-year seedlings

(Korstian 1927; McComb 1934; Shaw 1968a; Farmer

1980). Soil moisture also is critical in the first year (Core

1971), particularly before the development of taproots

and in the presence of competing herbaceous vegetation

(Carvell and Tryon 1961; Kolb and others 1990).

Initially, shading has little negative impact on oak

seedlings and may even enhance establishment in dry

environments (Barrett 1931; Borchert and others 1989).

However, once cotyledon reserves are depleted and

seedlings depend on photosynthate, light often becomes

limiting (Crow 1988). The mechanisms involved in the

long-term survival and development of oak seedlings are

beyond the scope of this report, but it is worth noting

that absent management intervention, northern red oak

seedlings persist in the understory only for several years

(McGee 1967; Olson and Boyce 1971; Loftis 1988;

Hodges and Gardiner 1993).

Interactions of Factors Affecting Seedling

Establishment

The interaction of factors that affect oak seedling

establishment has been the focus of much speculation

but little study. For example, it has been suggested that

populations of Curculio and Conotrachelus weevils may be

regulated by predation by small mammals on larvae,

larvae-infested acorns, and pupae. Weevil larvae of both

genera exit acorns after feeding and burrow into the soil

to pupate. Depending on the species, they may

overwinter as pupae or as adults. In laboratory trials,

Peromyscus and northern short-tailed shrew (Blarina

brevicauda) readily burrowed through soil to a depth of

15 cm to prey on pupae (Semel and Andersen 1988).

Mice consumed about 5.4 larvae per hour versus 6.5 for

shrew. On the basis of these results, it was suggested that
9

predation on larvae during and after periods of low

acorn production might retard the growth of the weevil

population, though there have been no controlled field

investigations. In addition, gray squirrel (Weckerly and

others 1989a) and Peromyscus (Semel and Andersen

1988) ate significant numbers of both weevil-infested

and noninfested acorns during laboratory trials.

Unfortunately, the ecological implications of these

observations have received little attention.

It also has been suggested that the impact of acorn

insects may depend at least partially on the size of the

acorn crop (Downs and McQuilken 1944; Christisen

1955; Beck and Olson 1968; Schopmeyer 1974;

Christisen and Kearby 1984). Research to date indicates

that light acorn crops are more heavily infested by

Curculio and Conotrachelus weevils than heavy crops

(Gibson 1972; Marquis and others 1976; Galford and

others 1988). These observations may indicate that acorn

weevils do not adapt their breeding behavior to take full

advantage of bumper crops; as a result, populations may

be regulated primarily by the size of the smallest acorn

crops. The dynamics of acorn weevil populations have

not been studied in detail so few conclusions should be

drawn.

The effect of crop size on vertebrate acorn predation also

is of interest. The synchronous production of large seed

crops followed by one or more years of small crops may

allow oaks to take advantage of mammalian seed

dispersal (Downs and McQuilken 1944). The irregular

production of large crops is assumed to satiate

populations of acorn predators (Janzen 1971). In years

of low or average production, many vertebrates act as

predators, whereas in years of high production, they

function as dispersal agents when hoarding and then

failing to use excess acorns. Sork (1983) listed two

criteria that must be met if satiation is functioning in a

seed predator-prey system: seed disappearance must be a

function of seed density, and seed predation should

eventually decline at high seed density. In studies

involving pignut hickory (Carya glabra) (Sork 1983) and

northern red oak (Sork 1984; Gribko 1995a), there was

no evidence of predator satiation. High seed production

slowed but did not stop total predation of nuts and

nearly total predation was observed even in areas where

acorn production was very high.

The intensity of vertebrate acorn predation also may be

related to specific site factors. Acorns of several species

tend to be removed more quickly from forests with intact

canopies (Kikuzawa 1988; DeLong and Yahner 1991).

Site productivity also may play a role in vertebrate

predation rates; Gysel (1956) observed higher rates of

vertebrate crop damage on less productive sites. However,

an intact canopy on a high-quality site does not always

lead to low rates of predation and high seedling

recruitment. In the Western United States, seedling

recruitment of blue oak (Quercus douglasii) was positively

associated with increased canopy cover and more mesic

microclimate at low elevations but negatively associated

with these factors on a higher elevation site with more

overstory oaks (Borchert and others 1989). As acorns

were more available, vertebrate predation apparently was

more pronounced and the advantages of improved

microsite were eliminated. Likewise, in the Appalachians,

a high proportion of large northern red oaks in the

overstory and the relative availability of acorns of any

species resulted in higher rates of predation of northern

red oak acorns (Gribko 1995a).

Acorn predation by vertebrates also may be affected by

weather conditions. During two winters of light snow in

West Virginia, the entire northern red oak acorn crop was

removed by small mammals (Gribko 1995a). By

contrast, much of the crop remained in place on the soil

surface during a winter of deep consistent snowpack,

resulting in a wave of seedling establishment.

Summary

Despite the volume of information available, a clear

understanding of the mechanisms involved in the

successful seedling reproduction of northern red oak and

other oak species is needed. The knowledge to date and

gaps in this knowledge can be summarized as follows:

• Northern red oak acorn production is highly variable

and the mechanisms involved are only partially

understood. Late spring freezes and insects that

damage flowers reduce crop size. Methods for

predicting the size of acorn crops are only beginning

to be developed and are limited to empirical withinyear estimates.

• Site conditions and microsite variability are assumed

to have minimal influence on seedling establishment,

though factors such as soil moisture have not been

studied rigorously.

• In some years, especially years of low production, a

large proportion of the acorn crop is infested with

Curculio weevils and other insects. The potential for

healthy seedling production from infested acorns has

not been investigated thoroughly. Moreover, the

population dynamics of these insects, including the

effects of weather, small mammals, and site

conditions, are poorly understood.

• Conotrachelus weevils and nitidulid sap beetles infest

large numbers of northern red oak acorns as they

germinate in the spring. However, there are few data

on the population dynamics of these insects and the

degree of regulation afforded by small mammals and

unfavorable spring weather.

• Little is known about the role of microorganisms and

secondary insects that potentially affect acorn viability.
10

• Large numbers of acorns are removed from the ground

and directly from the tree by vertebrate predators.

However, little is known about the effect of acorn

availability on vertebrate foraging behavior or whether

small mammals are effective as dispersal agents. There

is some evidence that site productivity and canopy

cover affect vertebrate predation rates, but the

mechanisms driving these effects are not known. The

impact of winter weather patterns, particularly

constancy of snow cover, on the behavior of vertebrate

acorn predators may have important consequences yet

remains largely unexplored.

To sustain oak forests throughout the eastern and central

hardwood regions, long-term research is needed on the

biotic and abiotic factors that affect establishment of

northern red oak seedlings. Objectives should include

the intensity of predation on acorns by vertebrates; the

impact of autumn and spring infestations of insects on

acorn crops (with an emphasis on weevils of the genus

Curculio and Conotrachelus); the role of pathogens that

infect northern red oak acorns and their impact on acorn

crops; the effects of soil moisture, soil temperature, and

weather patterns (including depth and duration of

snowpack) on seed germination and seedling

establishment; the relationship between crop size and

seedling establishment; and how these factors interact.

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Wood, O. M. 1938. Seedling reproduction of oak in

southern New Jersey. Ecology. 19: 276-293.

Wright, J. W. 1953. Notes on flowering and fruiting of

northeastern trees. Stn. Pap. No. 60. Upper Darby,

PA: U.S. Department of Agriculture, Forest Service,

Northeastern Forest Experiment Station. 38 p.
Printed on Recycled Paper

Gribko, Linda S.; Schuler, Thomas M.; Ford, W. Mark. 2002. Biotic and abiotic

mechanisms in the establishment of northern red oak seedlings: a review.

Gen. Tech. Rep. NE-295. Newtown Square, PA: U.S. Department of Agriculture,

Forest Service, Northeastern Research Station. 18 p.

This review of literature pertaining to the biotic and abiotic mechanisms in the

establishment of northern red oak seedlings is designed to aid forest managers and

researchers interested in sustaining the oak component of the eastern and central

hardwood forest regions. Recommendations for future research are presented.

Keywords: northern red oak, acorn production, acorn dispersers, acorn

germination.
Headquarters of the Northeastern Research Station is in Newtown Square,

Pennsylvania. Field laboratories are maintained at:

Amherst, Massachusetts, in cooperation with the University of Massachusetts

Burlington, Vermont, in cooperation with the University of Vermont

Delaware, Ohio

Durham, New Hampshire, in cooperation with the University of New Hampshire

Hamden, Connecticut, in cooperation with Yale University

Morgantown, West Virginia, in cooperation with West Virginia University

Parsons, West Virginia

Princeton, West Virginia

Syracuse, New York, in cooperation with the State University of New York,

College of Environmental Sciences and Forestry at Syracuse University

Warren, Pennsylvania

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