Defining Burn Units
The basic spatial entity in this analysis is
the management unit, or for fire planning purposes, the burn
unit. A unit is defined as a single parcel of land which
receives management treatments according to a particular regime.
To give a simple example, a preserve may be divided into two
parts, each receiving a fall burn every three to five years, but
in no instance are both parts burned in the same year. Each of
the two parts in this example is a separate burn unit.
A single unit may be divided into sub-units
if the sub-units are contiguous and treated in close sequence.
Another term for divisions within a contiguous fire unit is
compartments. In the example just given, suppose each of the two
units is larger than could be burned safely at one time. For
logistical purposes, each unit could be broken into sub-units to
be treated sequentially within a short period of time, where the
order of treatment is irrelevant to the desired treatment
effects. In most cases, however, all of a unit is exposed to a
management treatment in one time frame.
A general guideline is to delineate two or
more management units which encompass each priority species and
community to be treated with a distinct treatment and schedule.
The delineation would be done for each site, group of sites, or
the portion of one site included in a Fire Regime Proposal. In
this way, no one entire habitat or population would be burned in
a given treatment.
If, for instance, fall and spring burns are
two treatments to be applied in a site with only one community
element, the guideline would call for dividing the site into at
least four units, two for each treatment, with each unit
including representation of the community element. If two
different treatments are to be used in a site with two community
elements, four units would be the minimum but more might be
necessary depending upon how the occurrences of these
communities are spatially distributed. This approach provides
some protection against temporary negative fire effects, since
unburned areas may function as refugia. It also supplies
comparison areas to help managers and others discern fire
effects (keeping in mind that management fire plans seldom have
the design needed to ascribe causation with a known degree of
confidence).
Apart from the habitat needs of high priority
species, the scale and configuration of units has a large
influence over management efficiency. In general, efficiency is
increased if units are as large as possible without exceeding
containment capabilities. This is because the cost of fire
management tends to be proportional to the length of unit
perimeters while benefits are proportional to unit areas. Units
with approximately square or rectangular shapes are easier and
more efficient to burn than irregularly shaped units. Straight
line unit boundaries are easier to work than curving boundary
lines. However, highly regular boundaries may be aesthetically
offensive or ecologically inappropriate.
Ideal unit boundaries do not traverse slopes, boggy areas, or
topographic depressions. Keep in mind what might happen if the
fire should cross a unit boundary and escape. Would the fire
make a run upslope? Would wet areas impede control efforts?
Although it is tempting to employ stream channels as firebreaks,
if the stream is at the bottom of a canyon or gully, fire can
race upslope should it jump the unit boundary.
Often a good approach is to use watershed boundaries to
delineate units, with firebreaks placed on the high ground
dividing watersheds. Be aware, however, that significant species
and communities may be narrowly restricted to ridge tops and
hence be severely impacted by firebreak preparation. Other
consequences may include the threat of erosion. A firebreak may
be placed downslope from the ridge top, just outside of the
watershed containing the burn unit. Potential problems with this
are the difficulty of working on slopes and the potential for
burning material to roll downslope, spreading the fire outside
of the unit. There are no perfect solutions to these problems,
but if the important factors are understood, an acceptable
alternative can usually be found.
There are different views about whether to have permanently
located fire unit boundaries or to relocate them for each burn.
Permanent locations concentrate impacts from firebreak
preparation. This may be better or worse than distributing and
diluting impacts over more area. Much depends on the roads,
trails, natural firebreaks which already exist, and the type of
vegetation you are dealing with.
Natural or pre-existing fire barriers such as rock slides,
roads, trails, ponds, fuel type changes, etc. obviate certain
cost, management and impact problems. In placing unit boundaries
along pre-existing fire barriers, and in certain relationships
to topography and barriers to access, always consider the
contingency of fire escaping. Natural barriers may present
greater problems from escapes than manager-produced firebreaks.
Formulating
a Burn Schedule
Similar types of considerations exist in
formulating treatment schedules. A very regular schedule, such
as, "burn in April once every five years," facilitates planning
and makes it easier to interpret fire effects, but the lack of
variability may be ecologically inappropriate. Temporal and
seasonal variability may be essential to maintain community and
landscape diversity. Regular schedules also place burdens on the
manager if conditions do not permit effective burning.
Flexibility in schedules makes it more likely that treatments
will be accomplished when conditions are suitable. There are
several approaches to developing a burn schedule. You will have
to decide which is best for your situation.
One approach to defining a burn schedule is to state the
quantitative parameters that will be used to trigger the need
for a subsequent burn. This allows the manager latitude in
selecting the time for the next burn based on previous burn
results and the environmental or fuel conditions that produced
those results. For example, suppose a previous burn did not kill
encroaching hardwoods to the degree expected. This might warrant
a re-burn at a relatively short interval, or under drier or
windier conditions. On the other hand, a better than expected
kill may allow for a longer interval before the next burn. This
trigger approach is best applied when a site goal involves
maintaining the habitat of a single species or when the site is
in a restoration phase.
Another approach is to focus on known fire regime attributes
(i.e. burn frequency, severity, seasonality, scale) that will
maintain the desired community or landscape mosaic at the site.
A range of fire regime variables might be derived from available
knowledge; e.g. a consensus of opinion supports a fire regime
for a particular savanna type that includes a return interval
averaging 4 years, within a range of 1 to 8 years. Exceed this
range or repeatedly deviate too far from the average and the
community will likely change and species might be lost. The
manager can then track time since last burn for the community
types within each burn unit. In planning for a burn season,
treatment priority may be directed toward those units where the
fire regime will soon deviate beyond its normal range and mean.
Variation in frequency or seasonality can be incorporated by
developing a weighted or constrained random numbers generator or
table. Prescription parameters should be broad enough to allow
variation in fire intensity and burn severity. Keep in mind that
the variation in fire regime components may be more important
ecologically than average properties.
What about the situation where there are several different
communities within a burn unit, each maintained by a different
fire regime? For example, one community is characterized by
frequent surface fires occurring throughout the year, the other
by infrequent fires that normally occur only under seasonally
limited severe burning conditions. In this situation, burns must
be designed so that the community requiring infrequent fire does
not burn too frequently, at the same time allowing for frequent
burning of the other community such that a dynamic ecotone is
maintained between the two. Flexible burn units, or burning
subunits within fixed boundaries, may facilitate this need.
Developing a burn schedule for a mosaic of communities usually
requires mapping burned and unburned areas each time fire has
been applied, so that fire histories can be analyzed for
community occurrences across the landscape. GIS-based decision
making tools are becoming available for this analysis which will
use manager-defined fire regime parameters to flag areas needing
fire or areas that are in danger of burning too frequently.