Changes in radial increment in stems and roots of balsam fir [Abies balsamea (L) Mill] after defoliation by spruce budworm

Radial increment along the stems and the root systems of 20 balsam fir (Abies balsamea Mill.) was analysed as an indicator of resource allocation after a spruce budworm outbreak. The trees were located in a natural forest, 130 km north of Lake Saint Jean (Quebec) in the boreal forest zone. The dynamics of the forest were determined by fire and insect outbreaks. The 20 analysed firs showed clear signs of the last two spruce budworm outbreaks. Tree-ring width was reduced one year earlier in the crown region than at the stem base, and was more variable in the root system. Many tree-rings were missing at the stem base section and especially in the root system (2099 missing rings). Recovery started with a time delay of one year from the upper part of the crown and continued downwards in the stem. The roots and the crown recovered within a longer period.