ENVIRONMENTAL CONSEQUENCES in rocky mountain coniferous ...

I
not seem to be directly caused by heat of fire (Ahlgren 1974). The direct effect of
fire on forest floor macrofauna is greater in the forest environment than in grass-
lands, not only because of the more abundant fuel in forested areas (Ahlgren 19-74],
but because "grassland" species probably have evolved with more frequent fires.
Perhaps one of the more significant implications of harvesting and residue
management practices on forest floor macrofauna is the indirect effect of these
forest practices on the interrelationships of several groups of forest floor arthro-
pods with the western spruce budworm, Choristoneura occidentalis Freeman, the most
widespread and destructive forest defoi iator in the northern Rocky Mountains. There
are perhaps two implications: (1) many species of forest floor arthropods are
predators of the spruce budworm, particularly in the egg, larval, and pupal stages;
(2) some species of budworm parasites may be "forest floor" inhabitants for only a
portion of their 1 ife cycle, perhaps using understory broadleaf vegetation or forest
residues as an alternate host or substrate.
The role of predators and parasites as full-time or part-time inhabitants of
the forest floor is not one of "control 1 ing" outbreaks of the western spruce budw~rm
(or any of the other coniferophagous Choristoneura species) but perhaps in regulating
low-level populations. Although dozens of native insects and spider species eat
spruce budworm (mostly their eggs and small larvae), none is recognized as a prospec-
tive agent for biological control (Miller and Varty 1975), and "there is little hope
of using parasites and predators in a biological control context against the budworm
species" (McKnight 1976). In full -blown epidemics parasites and predators do not
seem to influence defoliation (Varty 1976) and have little influence in moderatin
explosive populations (Renaul t and Miller 1972; A1 len 1968; Mlller and Varty 1975 3 .
This is because predators and parasites (1) are unable to respond to changes in pest
abundance, and (2) have critical habitat requirements and quick1 y reach population
levels beyond which they cannot rise, no matter how easy it is to find budworm prey
(Mi 1 ler and Varty 1975).
Predators and parasites may help to regulate budworm numbers when there are
only a few thousand larvae per acre--perhaps 5,000-50,000 larvae--or when budworm
populations are just above the norms of epidemic status of about 100,000 larvae per
acre (Varty 1976; Miller and Varty IgJ'5). In non-epidemic years, the eastern spruce
budworm is usually scarce, inconspicuous and totally harmless; evidently natural
control mechanisms can be effective for decades (Miller and Varty 1975),
Of the forest floor arthropods influenced by harvesting and residue management
that could be involved in natural ly regulating spruce budworm populations, several
species of ants, spiders, and ground beetles probably are the most significant. In
the Lake States where the ant species Formica exsectoides Is extremely abundant, it
undoubtedly helps maintain low populations of the jackpine budworm (A1 len 1968).
This Allegheny mound ant is associated with the jackpine budworm only where soils
are suitable (sandy and well-drained) for nest building and where stands are open
enough to permit sunshine to reach the forest floor (Allen and others 1970). Another
species, Camponotus noveboracensis, is a frequent predator of the jackpine budworm
on the boles and branches of jackpine in stands where excessive ground vegetation
and lack of insulation on the forest floor do not prohibit nest building (Allen and
others 1970).
In a Montana study of ants and the western spruce budworm, Bain (1974) found
nests of Formica criniventris located on the edges of clearings or other open spaces
in ~tands-~~l~unli~ht, while nests of another species, F. obscuri es
associated with more shady conditions. In the present study, t +' e significant were resurgence
of ants following residue removal by prescribed burning could have some residue
management impacts. Ants are apparently highly adapted to hot, xeric conditions of
early postfire topsoil and their cryptic habits enable them to survive fire below