ENVIRONMENTAL CONSEQUENCES in rocky mountain coniferous ...

In the spruce-fir stands infested with the eastern spruce budworm in the
Maritime Provinces, fire is also involved in the successional relationships. In
the absence of fire the budworm kill s primarily shade-tolerant trees, and these
species tend to regenerate. If fires occur often within the regeneration period,
the shade-intolerant species regenerate instead, and these species are less preferred
hosts of the eastern spruce budworm (Fl ieger 1970). Essentially, then, in eastern
Canada, in the Lake States, and in other areas where the spruce budworm occurs,
outbreaks may have been less prevalent in primeval times because fire may have
curtailed the expansion of shade-to1 eranl cl imax species.
In the West, the fi re-residue-insect-successional interaction a1 so infl uences
outbreaks of the Douglas-fir tussock moth. Forest managers and investigators have
been concerned that fire exclusion in some coniferous forests may have increased
the susceptibility to the Douglas-fir tussock moth by a1 lowing the number of true
fir understory components to increase (Kickert and others 1976). In studying the
last two tussock moth outbreaks in Oregon and Washington, Williams (1978) found
infestations most common in stands, where fire was excluded. There, fir stands
developed under the pine and eventually dominated the site--a site ordinarily too
dry for either Douglas-fir or true firs. Recently, research has begun to see if
fire could, in fact, be used to reduce Douglas-fir tussock moth damage by preventing
rowth of susceptible Douglas-fir and true firs on sites more suitable to pine
Martin and others 1972).
7
Fire, understory, and residue created by beetle attacks are also involved in
the successional interactions of the western pine beet1 e and ponderosa pine.
Weaver (1943) believed that fires, in combination with pine beetle attacks, frequently
controlled the density, age classes, and composition of ponderosa pine stands. He
also seems to have answered a question asked by land managers: "Would fire exclusion
in ponderosa pine stands yield denser stands and more host material for bark beetles?"
(Kickert and others 1976). Weaver (1951, 1955) indicated that as a result of fire
excl usion, more intolerant white fir, incense cedar, Li bocedrus decurrens Torr. , and
western 1 arch increase and develop into densely reproducing stands that compete
with the overstory pines for moisture, thus predisposing the overstory pines to
bark beetle attacks.
In other situations, wildfires in forest residues created by beetle-killed
pine have killed reproduction, changing the pine habitat to an aspen type, The
pines eventually return, along with fir and spruce; the pines eventually are killed
by the western pine beetle, leaving only the fir and spruce (Craighead 1925).
Another interesting fi re-forest residue-successional interaction involves
insects causing deterioration in fire-killed trees in the Pacific Northwest.
Excessively dry or wet fire-killed trees do not attract insects. In a normally dry
forest, recurring fires keep down regeneration, a1 lowing f i re-kil 1 ed trees to
become too dry. In normally wet coastal forests, fire-kil led trees protected by the
cover of a green forest are too wet for the deterioratian causing insects; however,
if recurring fires destroy the cover, the dead trees will dry out enough to bec~rne
attacked (Kirnney and Furniss 1943).
In a longleaf and slash pine, Pinus caribwa Morelet, forest in the southeqst,
the only known epidemics of engrave-&- spp., occurred in a stand that
had not been burned annually for over 70 years. Transpiration by mixed hardwoods
that in other areas had been removed by prescribed burning a parently weakened the
pine, predisposing them to attacks by the engraver beetles ( I? omarek 1970).