Plant-specific response to herbivory: Simulated browsing of suppressed Balsam fir on Isle Royale

Document Type

Article

Publication Date

1-1-1996

Abstract

Balsam fir (Abies balsamea) is a shade-tolerant conifer that experiences striking growth suppression from browsing of the understory of mixed, boreal-type forests of Isle Royale, Michigan, where it forms an important component of moose (Alces alces) diet in winter. Suppression increases with increasing canopy cover and forest age. However, response to an individual, severe browsing episode cannot be generalized in terms of suppression. Such shorter-term responses are best investigated by experimentation. Artificial removal of foliage from understory balsam fir in two sites on Isle Royale and one mainland site showed that damage such as reduced terminal growth and complete mortality was actually highest among least suppressed trees. Canopy cover produced a secondary effect, in which trees experiencing > 60% cover were unable to compensate for removal by clipping. Among less suppressed trees, competition for limited light produced an apically oriented morphology that countered the laterally oriented morphology associated with suppression. Higher vulnerability to the artificial browsing occurred because trees were no longer protected by snow cover. By the second growing season following clipping, surviving clipped trees even in the most damaged sites resumed the same growth as controls. Over the long term, suppression may be a means by which fir remains alive when subjected to continuous browsing. Over the shorter term, delayed growth recovery following severe browsing in an unsuppressed understory may also contribute to sustainability in Isle Royale's plant-herbivore system.

Publication Title

Ecology

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