Short-term effect of defoliation on terpene content in Thuja plicata

Abstract

Insect herbivory or mechanical wounding in conifers can induce monoterpene biosynthesis. Low risk of herbivory, coupled with low availability of resources, is hypothesized to favour induced responses and to decrease constitutive defences. We studied the response to defoliation in western redcedars (Thuja plicata) from two regions: the Haida Gwaii archipelago, where mammalian herbivores were lacking until black-tailed deer were introduced at the end of the 19th century and previous work indicated that trees were less well defended, and the north coast mainland (British Columbia, Canada). We predicted that higher induced defences in the island population would compensate for reduced constitutive defences. We used one- and two-year-old nursery-grown seedlings to test i) whether defoliation would cause a short-term chemical response in island western redcedar and ii) whether mainland western redcedars that have always been exposed to large mammalian herbivores respond differently. The concentration in monoterpene and diterpenes did not vary significantly in response to defoliation over the 5-day period analyzed regardless of the defoliation intensity or the plant’s origin.