Boreal tree growth exhibits decadal-scale ecological memory to drought and insect defoliation, but no negative response to their interaction

1. Interactions between drought and insect defoliation may dramatically alter forest function under novel climate and disturbance regimes, but remain poorly understood. We empirically tested two important hypotheses regarding tree responses to drought and insect defoliation: 1) trees exhibit delayed, persistent, and cumulative growth responses to these stressors; 2) physiological feedbacks in tree responses to these stressors exacerbate their impacts on tree growth. These hypotheses remain largely untested at a landscape scale, yet are critical for predicting forest function under novel future conditions given the connection between tree growth and demographic processes such as mortality and regeneration.
2. We developed a Bayesian hierarchical model to quantify the ecological memory of tree growth to past water deficits and insect defoliation events, derive antecedent variables reflecting the persistent and cumulative effects of these stressors on current growth, and test for their in...