Oak savanna habitats
provide outstanding conditions for a variety of wildlife. Oak
trees provide nesting sites for birds as well as food for insects.
Acorns provide excellent food for rodents, deer, and wild turkey,
among other species.
Also, wildlife played an important role in presettlement times
in maintaining the integrity of oak savannas. Together with
fire, wildlife aided in cleaning understory brush, thus helping
to create the sparsely wooded character of the savanna.
Restoring oak savanna should be an important wildlife management
objective and has been so recognized by the U.S.D.A.(NRCS) Wildlife
Habitat Incentives Program.
Acorns are the most important wildlife food in Midwest deciduous
forests, “the ecological equivalent of manna
from heaven”. “There are many advantages to oak ecosystems
(timber, recreation, and aesthetics), but we share the belief
of many wildlife biologists that the true value of oak forests
rests in their contribution to wildlife.”.
Of the various species of forest trees, oaks are by far the
most important providers of animal food. The open character
of the savanna and the much greater diversity of forbs and grasses
makes this vegetation type an ideal support system for a large
variety of wildlife.
Trees are used by wildlife in several ways. Den trees
are living trees that contain holes large enough to provide
shelter for wildlife. Mast trees are those
which produce fruit (acorns) usable by wildlife as food. Snags
are dead trees large enough to be used as feeding, perching,
or nesting sites by animals.
Masting in Oaks
The term “mast” refers to the fruit of the oak, beech, and other
forest trees that produce seeds of sufficient bulk and nutritional
value that they can serve as a food for larger animals. Acorn
production varies widely from year to year, and oaks exhibit
years of good acorn production (“mast years”) and years of poor
acorn production (“nonmast years”).
An interesting aspect of masting is that it is a population
phenomenon. In general all the oaks in a stand exhibit high
or low acorn production the same year. Animals that make considerable
use of mast exhibit population fluctuations that track the annual
changes in mast production.
The masting phenomenon is characteristic of trees with large-sized
seeds, and is thought by ecologists to have evolved as a means
of ensuring sufficient seeds for the reproduction of the tree
species. In high mast years, when large numbers of seeds are
produced, there are insufficient animal seed eaters to consume
them all, thus leading to successful establishment of new tree
seedlings in these high-mast years. Because there are more low-mast
than high-mast years, populations of acorn eaters are controlled
by the low mast condition.
In one study of various species of oaks, each species of oak
had its own good mast years, which were different from those
of other species. Over a five year period, there were three
poor mast years, one moderate mast year, and one very good mast
year. The cycle of masting will vary not only with the species,
but with the location and various environmental variables.
High seed production provides an enormous input of food for
seed consumers in forests. High-mast years lead to abundance
of body fat in mammals, thus aiding in their survival during
harsh winter conditions. Because rodents are short-lived creatures,
their populations tend to fluctuate with quantities of mast.
On the other hand, deer, which are longer lived, benefit from
the high mast situation but do not exhibit sharp population
fluctuations that small mammals do.
Despite extensive research on masting in oaks, there is no single
explanation for the annual variability. Weather is an important
factor, but how does it influence masting? Possible environmental
factors include late freezes that might kill oak flowers, variations
in the efficiency of pollination, the initial size of the female
flower crop, and conditions that might affect flower survival.
Larger trees produce more acorns than smaller ones, although
not all large oaks are good acorn producers.
In Minnesota, bur oaks seem to be on a
two-year cycle, whereas white oak and northern red oak are on
a three year cycle. Occasionally, the cycles come together and
all oak species show high masting in the same year, which leads
to an extraordinary acorn production. The year 2000 was such
a year in the Midwest.
Animals whose reproduction cycles depend on mast include squirrels,
bears, and deer. Oak mast is also heavily used by wild turkeys,
ruffed grouse, bobwhite quail, raccoons, and small rodents.
Acorns can also be a major food source for humans. Native Americans
in the foothills of California used acorns as one of their most
important food sources. Acorns are an ideal food source because
they can be stored for long periods of time, and can be readily
processed into a tasty flat bread.
Deer
According to the Wisconsin Department of Natural Resources,
oak savanna was historically the optimum habitat for deer, as
well as for other game species such as bobwhite quail, turkey,
squirrels, and rabbits. Thus, management for oak savanna is
definitely compatible with wildlife management and hunter issues.
Mast is an important factor in deer maintenance, especially
in habitats where deer food is restricted. Mast is most important
during fall but is also heavily used during spring and summer.
Oaks in savannas are great acorn producers, since their branches
are exposed to maximum amounts of light. Also, competition for
nutrients is minimal in savanna oaks. Thus, from the food standpoint,
the savanna habitat is ideal for production of healthy deer.
However, if the deer population is excessive, overbrowsing of
oak grubs and saplings has the potential to cause long-term
reduction in oaks.
Deer compete with a variety of animals for acorns, including
squirrels, chipmunks, woodrats, mice, ducks, woodpeckers, bluejays,
and wild turkeys. Deer show a distinct preference for acorns
from the white oak group (white, bur, swamp white) over the
red oak group (red, black, northern pin).
Wild turkeys
The wild turkey (Meleagris gallopavo), a native species
of the New World, is the largest game bird in North America.
It had almost been eradicated in much of the Midwest but has
made a remarkable recovery in the past several decades. One
important reason for turkey success has been intensive habitat
improvement, supported to a great extent by private landowners.
Although turkeys are adaptable animals, they prefer oak forests,
especially those that are interspersed with openings. Thus,
the oak savanna habitat is ideal. Turkeys are not migratory
birds, but range fairly widely. In southern Michigan, for instance,
the home range of the turkey was estimated to be 1-4 square
miles (around 1000 to 2000 acres).
Turkeys need habitats with good opportunities for roosting and
nesting as well as dependable food sources. The preferred roosting
habitat is a large tree with horizontal limbs, such as an open-grown
(savanna) oak. For nesting, turkeys need moderately dense understory
vegetation that helps conceal the nest but at the same time
allows the hen to survey her surroundings.
Wild turkeys are omnivorous and opportunistic feeders, but acorns
are considered an important food item. In high-mast years acorns
may be the primary food of turkeys during fall, winter, and
spring. During these periods of the year, as much as 30% of
the bird’s diet is acorns. Acorns are eaten by turkeys in preference
to all other foods. Turkeys avoid foraging in habitats with
dense, woody understories; thus the oak savanna habitat is ideal.
Ruffed Grouse
The ruffed grouse (Bonasa umbellus) is another bird
that prefers open woods. According to the Ruffed Grouse Society,
they thrive best in forests that are kept open by frequent clearing
or fire. Grouse and turkey often nest in similar habitats. It
is not unusual to find a ruffed grouse and turkey nests near
each other in the oak savanna.
Cavity-nesting birds oak savannas
A number of desirable bird species nest in tree cavities. Many
species of cavity-nesting birds have declined in the United
States because of habitat reduction.
Cavity-nesting birds may either excavate their own holes, use
cavities resulting from decay, or use holes created by other
species. The trees used by cavity-nesters are dead or deteriorating
specimens, commonly called “snags”. Unfortunately, snags are
often considered undesirable by forest and recreation managers,
and they are often eliminated from the forest. For oak savanna
management snags should be left unless they are in danger of
falling over roads or public trails.
The following is a list of cavity-nesting birds common in the
Midwest.
• Wood duck
• Turkey vulture
• American kestrel
• Barred owl
• Northern flicker
• Pileated woodpecker
• Red-headed woodpecker
• Yellow-bellied sapsucker
• Hairy woodpecker
• Downy woodpecker
• Great-crested flycatcher
• Tree swallow
• Black-capped chickadee
• White-breasted nuthatch
• Tufted titmouse
• Eastern bluebird
• Starling
The majority of cavity-nesting birds eat insects. Because
these birds are a major part of the forest-dwelling bird population,
they play an important role in the control of forest insect
pests.
| This
photo shows an ideal habitat for cavity-nesting birds.. |
|
Birds of Oak Savannas
Sample and Mossman provided a list of birds in selected savanna
habitats in Wisconsin: Birds of Wisconsin Oak Savannas:
Past, Present, and Future, by David W. Sample and Michael
J. Mossman. 1998. U.S. Environmental Protection Agency. http://www.epa.gov/glnpo/oak/Proceedings/Sample.html
Similar connection
to oak savannas probably occurs in Illinois and other Midwestern
states.
Abundance of selected savanna bird species in Wisconsin.
| |
|
|
Important
habitat/landscape feature for that bird species |
| Species |
Abund-ance |
Legal |
Open
grassland |
Open
with shrubs |
Scattered
trees |
Semi-closed
canopy |
Shrub
understory |
| |
|
status |
|
|
|
|
|
| American
kestrel |
FC |
|
|
|
*** |
|
|
| Greater
prairie chicken |
CL |
T |
*** |
|
|
|
|
| Sharp-tailed
grouse |
CL |
SC |
|
*** |
|
|
|
| Wild
turkey |
FC |
|
|
|
|
*** |
|
| Red-headed
woodpecker |
FC |
|
|
|
*** |
|
|
| Northern
flicker |
FC |
|
|
|
*** |
|
|
| Eastern
wood-pewee |
C |
|
|
|
|
*** |
|
| Eastern
kingbird |
C |
|
|
|
*** |
|
|
| White-breasted
nuthatch |
C |
|
|
|
|
*** |
|
| Loggerhead
shrike |
R |
E |
|
*** |
|
|
|
| Bells
vireo |
R |
T |
|
*** |
|
|
|
| Chestnut-sided
warbler |
U |
|
|
|
|
|
*** |
| Mourning
warbler |
R |
|
|
|
|
|
*** |
| Indigo
bunting |
C |
|
|
|
|
|
*** |
| Rufus-sided
Towhee |
FC |
|
|
|
|
|
*** |
| Field
sparrow |
FC |
|
|
*** |
|
|
|
| Bobolink |
FC |
|
|
*** |
|
|
|
| Orchard
oriole |
U/R |
|
|
|
*** |
|
|
Abundance given for the southern and western
portions of Wisconsin, where savannas occur.
C = common; FC = fairly common; U = uncommon; R = rare; CL
= casual, local.
Legal status in Wisconsin: T = threatened; E = endangered;
SC = special concern.
Habitat
features
Open grassland = grassland with little or no shrub or tree cover.
Open with shrubs = grassland with scattered shrubs or shrub
clumps (1 dm-tall); less than 30% total shurb cover.
Scattered trees = grassland with scattered open-grown trees,
0-50% tree cover.
Semi-closed canopy = Areas with tree canopy cover greater than
50% but less than 75%. Birds in this group tend to increase
with increasing canopy cover.
Shrub understory = Habitats with a shrub layer beneath a tree
canopy layer.
Red-headed
Woodpecker
One cavity-nester, the red-headed woodpecker, is a species
of special interest because it has declined drastically over
the past decades. Red-headed woodpeckers thrive in areas that
provide open areas for foraging, and well-spaced trees for
nesting. The oak savanna is ideal for this species., and throughout
their range savanna restoration may lead to re-establishment
by these birds.
Breeding bird observations have shown that there has been
a sharp drop in red-headed woodpecker numbers throughout much
of its range. Decline has been linked to habitat loss, changing
land use, changes in vegetation due to succession and fire
suppression, and competition with other cavity-nesting species.
Red-headed woodpeckers nest in trees but throughout the breeding
season obtain much of their food from insects on the fly.
They inhabit oak savannas and open woodlands and flycatch
over open fields and adjacent roads.
In the absence of open oak areas, they forage over roads.
They perch on telephone poles or roadside trees and swoop
down across the open space and catch insects. One reason for
decline is mortality due to impact with motor vehicles, especially
high-sided trucks.
Anecdotal evidence from one Wisconsin oak savanna restoration
indicated that opening up the savanna by removing invasive
trees and shrubs resulted almost immediately in settlement
of the area by red-headed woodpeckers.
| Red-headed
woodpecker (Melanerpes erythrocephalus) in an
open oak savanna area. |
|
Insects
Research on plant/insect specificities has been mostly confined
to prairie plants, but it seems likely that there are many
insects that are specific for savanna plants. However, the
only detailed knowledge available is for the Federally Endangered
Karner Blue Butterfly, which lives specifically on lupine
(Lupinus perennis).
Lupine thrives in partially shaded to open areas with nutrient
poor, generally sandy, soils. Black oak savannas (Quercus
velutina) are often the dominant savanna vegetation in
such sites, and the Karner Blue has a marked specificity for
these savannas.
Restoration of black oak savannas often goes hand in hand
with restoration of the lupine plants that Karner Blues feed
on. The Kitty Todd Preserve, a Nature Conservancy site in
northwestern Ohio, has been the focus of considerable research
on Karner Blues. This site has extensive areas of black oak
savanna which is ideal for lupine and Karner Blues. (See Ohio
Nature Conservancy web site for details)
Another "hot
bed" of Karner Blue restoration is the Central Sands
area of Wisconsin. The Necedah National Wildlife Refuge is
a large, publically accessible area in the Central Sands that
is being managed (among other things) for Karner Blue restoration.
|
