Fuel Concepts - Wildland Fuel Fundamentals and Applications - page 175

burned, exhibited fire behavior below some threshold parameter. If fuel loadings are
used as measures of longevity, then the length of time the treated stand stays below
a threshold fuel loading might be used as a measure of longevity. Alternatively, if
fire behavior attributes are used, then the measured fuel conditions can be input to
fire behavior models, such as BEHAVE or NEXUS, to compute fire behavior met-
rics, such as rate of spread, fire intensity, and torching index, that are then used to
estimate longevity by comparing against a corresponding threshold value.
The problem is that countless fire behavior and fuel characteristics can be used
to evaluate longevity, and among them they have different (1) rates of change over
time, (2) threshold and importance values to management, (3) degrees of correlation
with each other, and (4) spatial distributions. Commonly used canopy fuel treat-
ments involve removing trees (reducing CBD and increasing CBH), but this may
open the stand to increases in incipient herbaceous vegetation resulting in a flashy
surface fuel layer that has a higher rate of spread than the previous fuelbed (Kane
et al. 2010 ). An example of this dilemma concerns the treatment of ponderosa pine
ecosystems that historically experienced frequent fires, but as a result of fire exclu-
sion, these forests are now densely populated with trees (high CBD, low CBH) that
heavily shade the forest floor making it mostly devoid of shrub and herbaceous fu-
els with thick duff and litter layers and high accumulations of woody fuels. If spread
rate were used as a measure, then the spread rates before the treatment might actual-
ly be lower than the spread rates in the herbaceous-dominated posttreatment stand.
In other treatments, tree removal may result in slash that increases surface woody
fuel loadings creating conditions where decreases in crown fire potential from CBH
reductions may be offset by the increases in surface fire intensities and concomitant
deeper soil heating (Pollet and Omi 2002 ). Moreover, the use of multiple fire behav-
ior and fuels measures to evaluate longevity might produce contradictory results.
Values of one fire or fuel metric might always exceed threshold values even though
the values of other fire and fuel metrics remain under their threshold values. Some
surface fuel threshold values might be so high, for example, that a target low torch-
ing index might be impossible.
A more holistic approach is needed to replace the notion of treatment longevity
with a broader view of fuel in a landscape and successional ecology framework.
First, longevity, by definition, demands a consideration of scale and context. Most
managers and scientists tend to evaluate longevity at the stand scale because this
is the scale of treatment. However, to be fully effective, fuel treatments need to be
implemented at landscape scales to take advantage of adjacency, contagion, and
connectivity in fuel patch heterogeneity. It makes little sense to only treat a 10-ha
ponderosa stand, for example, when it is surrounded by a 10,000-ha dense forest
with heavy surface fuels. The treated ponderosa pine stand might be effective lon-
ger if it is surrounded by other treated stands. And since fuel deposition rates are
dictated by vegetation development, a consideration of the vegetation species and
structure before and after the treatment is critical to ensure that treatments are ap-
propriate for the biophysical conditions so that they are effective longer.