Mountain Forests - Mountains and Plains: The Ecology of Wyoming Landscapes - page 187

osa pine can form a nearly closed canopy with a sparse
understory; on drier sites the forest is more open, or even
savanna-like, and often has a well-developed under-
story of shrubs, grasses, and other herbaceous plants.
open ponderosa pine stands can also be maintained
by frequent surface fires that kill young seedlings and
saplings. the larger, older trees usually are not killed by
surface fires, owing to their thick bark and high crowns,
but they are often scarred by the fire. the scars can be
analyzed using dendrochronology to reconstruct local
fire history over the past several centuries (fig. 11.9).
Such fires burn the tops of understory shrubs and her-
baceous plants, but the roots survive and soon produce
new sprouts, a type of vegetative reproduction that is
lacking in the conifers. Mountain pine beetle outbreaks
also thin ponderosa pine stands, although the result-
ing stand structure is different from that created by fire,
simply because the beetles tend to kill more of the large
trees and fewer of the saplings.
At the scale of an individual ponderosa pine stand,
fires commonly recurred at intervals of a few years to
a few decades prior to disruption of the historical fire
regime in the late 1800s. 55 in some areas these fires were
predominantly low-severity surface burns that helped
maintain an open forest structure. other ponderosa
pine forests were characterized by a variable-severity
fire regime that included a patchy mix of low- and
high-severity burning, both during individual fires and
in individual stands over time. 56 High-severity (stand-
replacing) burning was an important component of the
fire regime primarily on wetter sites—during dry years—
in the ponderosa pine zone, such as at higher elevations
and in mixed-conifer stands where other species, such
as lodgepole pine or Douglas-fir, were co-dominant. in
the colorado Front Range, for example, fires at lower ele-
vations, especially near the prairie-forest ecotone, were
predominantly low-severity surface burns that helped
maintain low-density ponderosa pine forests. At mid-
elevations, ponderosa pine forests burned with variable
severity, so that patches of stand-replacing fire were
interspersed with areas of low-severity surface burn-
ing. At the upper reaches of the ponderosa pine zone,
where ponderosa transitions into lodgepole pine forest,
stand-replacing fires burned large areas of ponderosa
and lodgepole pine forest . 57 Similarly, in the Black Hills,
dense stands and high-severity fires were more common

Fig. 11.9. Ponderosa pine and Douglas-fir with thick bark
often survive surface fires, even though the base of the tree is
scarred. Fire scars develop when the heat scorches a portion
of the cambium, a layer of cells found on the inner side of
the bark that produces new bark and sapwood. Such scars can
be used to determine the years that fires occurred and the
number of years between successive fires. this photo of a pon-
derosa pine was taken in Devils tower national Monument.

historically in the wetter northern part than in the drier
southern part. 58
Severe burns were followed by either gradual or
rapid recovery of forest cover, depending on climatic
conditions and seed availability. in some settings, a
dense stand of ponderosa pine develops after a high-
severity fire; in other settings, such as on dry sites fol-
lowing severe burns, the post-fire stand may be sparse
and slow to re-establish . 59 Ponderosa pine seeds are rel-
atively large and tend to disperse over relatively short
distances; consequently, the interior of a large patch of
stand-replacing fire may become reforested very slowly,
as has been seen after some recent fires in colorado. the