spruce budworm (choristoneura fumiferana clem.) late-instar dispersal and impact on host-species natural regeneration in a boreal mixedwood forest of ontario
abstract
understory feeding by the spruce budworm ( choristoneura fumiferana
clem.) has potential to alter the species composition of established regeneration
and therefore the value of the future forest. this study sets out to test
the hypothesis that the spruce budworm kills more balsam fir (abies balsamea
(l.) mill.) than spruce (picea spp.) seedlings, thereby increasing the spruce
component of the future forest. four 1 0-ha study plots were established in a
boreal mixedwood forest near black sturgeon lake ontario in 1993. a spruce
budworm outbreak had been underway in this area for more than 10 years and
was expected to collapse at any time. branch samples were collected from
mature balsam fir, white spruce (picea glauca [moench] voss), and black
spruce (picea mariana [mill.] b.s.p.) trees to compare foliage defoliation and
budworm densities. water traps were set beneath host trees to capture late-instar
larvae as they dispersed to the understory. stand characteristics were
documented through stem tallies of trees and seedlings. all host seedlings
(diameter < 3 em) were assessed for budworm feeding damage.
stem tallies indicated that the spruce budworm had increased the
relative importance of spruce by killing more balsam fir in the overstory. this
influence became progressively more pronounced among smaller diameter
classes. however, mortality ceased abruptly at the seedling level. despite
fairly high dispersal numbers under host trees (40-160 larvae per m2), budworm
damage to regeneration was minimal and evidence of a selective influence on
species composition could not be shown. the reason for a lack of damage to
regeneration remains unknown, but a number of possible reasons are provided.
over 90% of all seedlings in the understory were less than 30 em tall. almost
97% of all regeneration was balsam fir. possible reasons for the lack of spruce
regeneration are provided and possible changes to the species composition of
the forest in the future are considered. it is concluded that a harvesting system
other than clearcutting may be necessary to insure adequate spruce regeneration
in the boreal mixedwood forest.
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- retrospective theses [1604]