Gypsy moth defoliation and N fertilization affect hybrid poplar regeneration following coppicing

The interaction between insect outbreaks and forest health or tree plantation management has been considered rarely from an experimental approach. From 1996 to 1998, we experimentally created a gypsy moth outbreak that defoliated large plots of mature hybrid poplars, Populus xeuramericana, with half of the plots fertilized in a factorial design. The trees were harvested at rotation age in 1999, and we examined the mortality and regeneration of the coppiced (cut at the base) trees in the following growing season. Pre-treatment estimates of tree size strongly predicted the mortality and number of regenerating stems following harvest but not the height of those stems. Defoliation and fertilization each modestly increased tree mortality (10 and 6%, respectively), but the effects were not additive. Only defoliation had strong residual effects on the surviving trees: 25% fewer resprouting stems were produced by previously defoliated trees compared to undefoliated control trees.