Spruce Budworm Defoliation in Young Balsam Fir: The ‘Green’ Tree Phenomenon

During a severe outbreak of spruce budworm, Choristoneura fumiferana (Clem.), some balsam fir trees, Abies balsamea (L.) Mill., remain noticeably greener than others in the same stand. We show that this ‘green’ tree phenomenon is not caused by differential defoliation in which certain trees experience consistently low defoliation pressure throughout an outbreak. Rather, these trees remain ‘green’ because of their prolific, epicormic shoot production which is consistently high during an outbreak, giving them a ‘greener’ appearance than other trees that produce lower numbers of epicormic shoots.