The principal
trees of oak savannas are of course the oaks, but shagbark
hickory (Carya ovata), black cherry (Prunus serotina),
and white or jack pine are occasionally present.
In Midwestern
oak savannas the principal oaks are bur oak (Quercus macrocarpa),
white oak (Q. alba), black oak (Q. velutina),
and northern pin oak (Q. ellipsoidalis). Other species
of oaks found in various Midwestern savannas are listed here:
State |
Principal
oaks |
Missouri |
post, bur,
blackjack, black, chinkapin and white |
Minnesota |
bur oak,
Hill's (northern pin) oak |
Illinois |
bur oak,
white oak, northern pin oak, red oak |
Indiana |
black oak |
Ohio |
black oak |
Iowa |
bur oak,
white oak, red oak, swamp-white oak, black oak, chinkapin
oak |
Wisconsin |
bur, white,
black, northern pin, red, chinkapin |
Latin
names: bur oak (Quercus macrocarpa), white (Q.
alba), black (Q. velutina), post (Q. stellata),
blackjack (Q. marilandica), red (Q. rubra),
swamp-white (Q. bicolor), chinkapin (Q. muehlenbergii),
Hill's (northern pin) (Q. ellipsoidalis). See
Silvics of North America for characteristics.
In Wisconsin Curtis
categorized the oaks in terms of the soil characteristics
where they were most common: northern pin (most xeric), black
(better soils and more moisture), red (deep, well-drained
soils; most tolerant of all the oaks), chinkapin (restricted
range in Wisconsin; southernmost; Grant County), bur (2 races,
one without large caps; great range of habitats; acid sands,
deep slit loams, rocky calcareous soils, wide moisture tolerance,
very shade intolerant, very important in southern Wisconsin,
less so in the north, white (slightly less tolerant than bur),
swamp white (wet mesic).
West coast and
southwestern savannas have completely different species of
oaks. California, in particular, has a large number of oak
species, many of which grow only there.
The
two major oak groups
There are two
major oak groups, the red oak and the white oak groups. Oaks
of the red oak group have leaves with pointed lobes whereas
those of the white oak group have leaves with rounded lobes.
Oaks of the red oak group include red, black, and northern
pin. Oaks of the white oak group include white, bur, and swamp
white oak. Hybridization may occur between species of each
of the two oak groups but not across the two groups.
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Red
oak group: black oak leaf and acorn |
White
oak group: white oak leaf and acorn |
White
oak group: bur oak leaf and acorn. The hairy fringe of
the cap is distinctive and makes it easy to recognize
this acorn. The large indentation in the leaf is also
characteristic. |
In the white oak
group, acorns mature at the end of the first season of formation,
whereas those in the red oak group require two seasons to
mature. Another important difference is that the water-conducting
vessels (xylem) of the white oak group are plugged with intrusions
(tyloses), whereas the xylem of the red oak group remain open.
Because of these differences in xylem, wood of the red oak
group fills with water, causing the wood to sink, whereas
wood of the white oak group floats. A consequence is that
wood of the red oak group is more susceptible to decay than
the wood of the white oaks. An additional difference is that
members of the white oak group are resistant to oak wilt where
those of the red oak group are sensitive (see below).
White oak
The white oak
is an outstanding tree and is widespread across eastern United
States. It is of great economic interest in the lumber industry
and is also widely used as staves for the manufacture of barrels.
It is found from southern Maine to northern Florida, and west
to the Iowa/Kansas border. It grows on a wide range of soils
with a wide range of moistures except on the driest sites,
or on poorly drained bottom land. It can be a major or exclusive
component of oak savannas throughout its range.
White
oak savanna in southern Wisconsin. Many of the open-grown
oaks are over 150 years old. |
Bur oak
The bur oak grows
farther to the west than any of the other Midwestern oaks,
but does not grow in the south or along the east coast. Historically,
the bur oak was an important bottom-land species, thriving
where other oak species are not able to grow. Most of these
bottom land oaks have been eliminated, although bur oaks remain
in floodplain forests along the Missouri River in North Dakota
and along watercourses in the Great Plains.
The bur oak remains widespread throughout its range on dry
uplands and sandy plains where it is often associated with
calcareous soils. In the Great Plains it may be a pioneer
tree invading prairie grasslands, and it is frequently planted
there in shelterbelts. It is frequently found as a fringe
between the prairie and upland forest in the prairie/forest
border (eastern Iowa and northern Illinois). In protected
locations along the Missouri River in North Dakota, very old
bur oaks (>400 years) can be found.
Bur
oak savanna in southern Wisconsin. This photo provides
an example of what restoration can do. Ten years before,
this savanna contained a solid stand of buckthorn shrubs
and numerous elm and cherry trees that were crowding the
bur oaks. Selective cutting, extensive use of herbicides
on the buckthorns, and seeding the understory with native
forbs and grasses has brought this savanna to its present
state. |
The bur oak is a slow growing tree and is very intolerant
of shade. However, it is very fire resistant and hence a principal
tree of oak savannas. In Curtis’s detailed
study of tree composition in Wisconsin savannas, the bur
oak was the dominant tree (see table below).
Relative
importance of oaks and other trees in a savanna |
Species |
Importance
value |
Invasiveness |
Fire
sensitivity |
Shade
tolerance |
Bur
oak |
105.1 |
Low |
Very
low |
Intermediate |
Black
oak |
71.5 |
Moderate |
Moderate |
Intermediate |
White
oak |
61.9 |
Low |
Low
to moderate |
Intermediate |
Shagbark
hickory |
19.9 |
Low |
Moderate |
Intermediate |
Northern
pin oak |
9.3 |
Low |
Low |
Low |
Black
cherry |
6.8 |
Low |
Moderate |
Intolerant |
Paper
birch |
6.3 |
Low |
High |
Intolerant |
Quaking
aspen |
4.6 |
High
(clonal) |
High |
Very
intolerant |
Red
oak |
3.9 |
Low |
Moderate |
Intermediate |
Green
ash |
2 |
Low |
High |
Tolerant |
Slippery
elm |
1.7 |
Moderately
high |
High |
Tolerant |
Eastern
red cedar |
1.4 |
Low |
High |
Very
intolerant |
Box
elder |
0.8 |
Moderate |
High |
Tolerant |
Big-tooth
aspen |
0.7 |
High
(clonal) |
High |
Very
intolerant |
Basswood |
0.6 |
Moderate |
High |
Tolerant |
Black
walnut |
0.3 |
High |
High |
Intolerant |
White
ash |
0.2 |
Low |
High |
Intolerant |
American
elm |
0.2 |
Relatively
high |
High |
Intermediate |
The
data on Importance Values are from Curtis, Vegetation
of Wisconsin. The rest of the data are from Silvics
of North American.
Both white
oaks and bur oaks are long-lived trees, and many savanna
remnants in the Midwest have trees over 150 years old,
often even exceeding 200 years. These savannas are found
in areas where logging was not easy to do in the early
years of settlement, and were often on remote parts
of dairy farms. If the savanna was far enough from the
barn, it may not even have been heavily grazed. Although
the understory vegetation may be greatly impoverished,
if the open-grown oaks still remain these
areas are good candidates for restoration.
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The
native range of bur oak (Quercus
macrocarpa) across the United States. In the western
part of its range, it is more commonly found in moister
habitats, such as gullies and ravines. Not shown is its
distribution in southern Manitoba and southern Ontario.
Redrawn from Silvics of North America.
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