White tree rings formed in trembling aspen saplings following experimental defoliation

Abstract

Recent studies of trembling aspen (Populus tremuloides Michx.) in western Canada have shown a correlation between past insect defoliation events and the formation of narrow, abnormally pale-coloured ("white") tree rings. The objectives of this study were to test the hypothesis that defoliation causes the formation of white rings and to examine how defoliation affects ring width and density. We experimentally defoliated 7- to 18-year-old aspen in June, July, or August 1997 and subsequently found that white rings were formed the same year in all aspen that were severely defoliated in early June. These white rings were much narrower than in adjacent trees left as controls, and mean xylem density of the white rings (0.27 g·cm–3) was significantly reduced relative to normal rings (0.35–0.40 g·cm–3). In the year following defoliation, the tree rings remained narrow, but their appearance and density had returned to normal. Aspen defoliated later in the season formed relatively normal rings in 1997, but ring widths were reduced in 1998. The results confirm that white rings in aspen can be a useful retrospective indicator of the severe, early season defoliation that is typical during major outbreaks of forest tent caterpillar (Malacosoma disstria Hbn.) and other insects.