Early growth decline and vegetation change predict black spruce mortality following spruce budworm outbreaks

Tree mortality is accelerating across boreal forests due to increasing biotic disturbances, but the mechanisms linking defoliation, individual tree stress, and canopy decline remain poorly understood. In eastern Quebec, Canada, a spruce budworm ( Choristoneura fumiferana (Clemens)) outbreak beginning in 2006 has severely defoliated black spruce ( Picea mariana (Mill.) B.S.P.) forests. Yet, many trees were already in growth decline before the outbreak, raising concerns about dieback and long-term resilience under recurring disturbance. We combined tree-ring growth data from 378 black spruce trees (276 living, 102 dead) across 34 study plots with satellite-derived vegetation indices, Normalized Difference Vegetation Index (NDVI) and Normalized Burn Ratio (NBR) (the normalized difference of the Near Infrared and the Short-Wave Infrared bands)—spanning 1984–2022 to assess growth dynamics and canopy stress responses. Nearly all plots underwent severe defoliation; a factor positively associated with a high proportion of dead trees within the plots. Dead trees exhibited long-term, persistent growth decline prior to defoliation onset in 2009. After defoliation began, both live and dead trees showed similar patterns of reduced growth, with early cessation in dead trees. Logistic regression identified an early negative regime shift in tree-ring growth preceding defoliation as a strong predictor of tree mortality. Additionally, satellite data revealed that a 1% decrease in NBR corresponded to a 3% increase in mortality at the stand level. Our findings demonstrate that regime shifts in tree growth and vegetation indices predict mortality, offering a scalable approach for forest monitoring but also providing an early warning of tree mortality. • Divergent pattern of growth between dead and live trees • Early negative shifts in tree ring growth before budworm defoliation signaled high mortality risk • Declines in Normalized Burn Ratio (NBR) tracked tree mortality, supporting its use as a remote sensing tree mortality proxy mortality • Integrating tree-rings and vegetation indices enabled scaling-up detection of growth decline and mortality risk