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.