Effects of Armillaria root disease on the growth of Picea mariana trees in the boreal plains of central Canada

Armillaria spp. are a complex of fungal pathogens affecting populations of trees worldwide, including upland black spruce (Picea mariana (Mill.) B.S.P.). In central Canada, upland black spruce stands are severely infected with Armillaria root disease, which can kill trees across wide areas. In 2007–2008, infected dead and asymptomatic living trees in 12 infection centers were sampled in each of two regions for growth and mortality analyzes. In 2009, a subset of 10 infected dead and 10 asymptomatic living trees from two sites per region were selected for stem analysis. Dendroecological techniques were used to examine mortality patterns and growth changes prior to mortality. The onset of mortality in affected stands occurred quasi-synchronously across the sampling regions, though differences existed among individual sites within each region. Mortality in all black spruce stands occurred at an average of 96–99 years. Testing of incremental growth ratios indicated that infected trees experienced a sustained decline in basal area and volume increment 5–15 years prior to death, as compared to asymptomatic trees. This significant decline in growth was expressed in overall tree productivity. Comparing logistic regression curves of cumulative basal area, height, and volume growth revealed significant differences between asymptomatic and infected trees, indicating that the infected trees grew more quickly at a younger age than asymptomatic trees. It is speculated that their increased vigor and larger root systems may have predisposed these trees for infection as more root area was available for fungal contact. In upland black spruce forests, Armillaria root disease is accelerating forest succession by breaking up the even-aged post-fire cohort and contributing to the presence of dead wood on the forest floor.

► Age at mortality and growth loss were quantified in upland black spruce trees infected with Armillaria root disease.
► In all stands, mortality of infected trees occurred at approximately 90 years of age.
► Infected black spruce trees experienced significant basal area and volume growth loss 5–15 years prior to mortality.
► The true date of tree mortality derived at 1.3 m and below was often underestimated due to the presence of missing rings.
► Increased vigor at a younger age may have predisposed trees to infection.