The term "prescribed burn" refers to
a type of burn carried out based on a precisely written burn plan.
A prescribed burn is a controlled burn. Background
on prescribed burns. Pleasant Valley Conservancy conducts
two types of controlled burns, in prairies and in oak savanna
areas. Both are critical management activities, but they have
to be managed in different ways. Most people involved in restoration
ecology are familiar with prairie burns. We concentrate here on
oak savanna burns, which are less well understood.
The most important management tool in oak savanna
restoration is the controlled burn. Savanna burns kill invading
brush, small trees, and brambles, keeping the woods open and encouraging
the growth of grasses and forbs. These are low-intensity burns
that have no effect on the oaks themselves.
History
of controlled burns in wooded areas
Numerous early accounts of visitors and settlers
in our area mention burns in the woods. Farmers knew the value
of burning as a way of keeping the land open for pasture. Two
factors led to elimination of fire in Wisconsin's oak savannas:
1) The Department of Natural Resources and its predecessor the
Wisconsin Conservation Commission discouraged or actively campaigned
against fire in the woods. 2) The contraction of agriculture activity
in southern Wisconsin and increasing urbanization led to the elimination
of farms, and hence, farmers.
The importance of fire in the maintenance of the
oak forest is now well established. Oaks are less easily killed
by fire than most of their associates, and are able to resprout
as small saplings even when they are top-killed. Controlled fire
in an oak savanna is not a high-intensity fire raging through
the tree tops. It is a slowly moving, low-intensity ground fire,
consuming the dry oak leaves as well as savanna grasses and forbs.
(See photo above) It kills invasive shrubs and trees and keeps
the savanna open.
Over thousands of years, Native Americans had
used fire in the woods as an important part of their culture.
Fire was used to encourage berry production, expose acorns for
collection, prepare planting sites, control undesirable pests,
fireproof villages, create and maintain open woodlands and savannas,
concentrate game, and help maintain trails. Fires were ignited
either in spring, before plants had started to grow, or in fall,
after frosts had killed off above ground parts of plants. When
European settlers displaced Native Americans, the use of fire
continued, for many of the same reasons. However, by the late
19th century, widespread logging left vast acreages
with dried slash that ignited easily and burned with unusual power,
creating landscapes prone to high-intensity wildfire. State laws
related to wildfires were enacted to prevent damage to the land
and to humans. In the early 1900s, the newly established U.S.
Forest Service began a major campaign to control forest fires.
"Smoky the Bear" was created as an icon of fire suppression. All
of these activities had unintended consequences, destroying the
values that the oak forest provided, and replacing the oaks with
undesirable trees and shrubs.
Since the early 1970s there has been a gradual
reintroduction of the use of prescribed fire. Even the forest
industry now realizes that in the oak forest fire plays a major
role in removing many weedy trees and shrubs. Prescribed fire
removes the thin-barked shrubs and trees from the midstory and
understory of the forest without harming the dominant oaks. Fire
also consumes the litter, thus promoting the growth of grasses
and forbs, and encouraging the regeneration of oak. Long-term
research studies have shown that annual burns over a number of
years gradually restore the oak savanna to its original state.
Controlled burns
in oak savannas at Pleasant Valley Conservancy
We
have burned the oak savanna in both spring and fall, but the fall
is often the best time. A lot depends upon rainfall and temperature,
and since the leaf litter is the major fuel, on leaf fall. Bur
oaks begin to lose their leaves in mid October and by the end
of the month, or in early November, most of the leaves are on
the ground. If conditions have not been too wet, this is an excellent
time to do a controlled burn on our ridge-top savannas (units
8, 10, 11, and 19).
White oaks hang on to their leaves a lot longer
than bur oaks, and some leaves stay on the tree through the winter.
We have burned our white oak savannas in both spring and fall,
but for the past several years we have had successful fall burns.
Much depends upon the weather.
Savanna burns are much more difficult to do than
prairie burns, because there is a lot more preparation necessary
before a burn can begin, and "mop-up" after the burn takes more
time. Once a prairie is burned, it takes very little time to ensure
that the fire is out and it is safe to leave. A savanna almost
always has standing dead trees which can easily catch on fire.
A standing dead tree can act as a chimney, carrying the fire from
the ground into the upper parts of the tree. Such a "smoker" cannot
be left to burn, even if there is nothing around to catch fire,
because changes in wind may blow sparks into distant parts. The
best way to deal with a standing dead tree is to prevent it from
catching fire in the first place. This means mowing or raking
a wide vegetation- and stick-free zone around the tree. A satisfactory
way is to mow the vegetation around each tree with a brush cutter
and follow this with a leaf blower. It may take a whole day to
"fire-proof" all such trees on a 20-30 acre savanna.
In mop-up, smoldering downed trees can be left
if they are in the middle of a blackened area, but if they are
near the edge they must be put out before the area can be called
secure. If the fire is burning in a standing dead tree, the tree
must be cut down and cut apart so that the fire can be reached
with water. It sometimes takes a long time to secure the area.
Savannas often have a lot of dead branches or
even whole logs on the ground. These logs can block the fire from
moving through the area. If the fire does not carry well through
the unit, because of dead logs or other reasons, it is essential
to walk into the unburned area with a drip torch and restart the
fire. Often one has to "strip" the unit, walking back and forth
with the drip torch, igniting the unit over and over again.
