Abstract
Background and Aims Natural and anthropogenic disturbances can act as stresses on tree vigour. According to
Manion’s conceptual model of tree disease, the initial vigour of trees decreases as a result of predisposing
factors that render these trees more vulnerable to severe inciting stresses, stresses that can then cause final
vigour decline and subsequent tree death. This tree disease model was tested in sugar maple (Acer saccharum)
by assessing the roles of natural and anthropogenic disturbances in tree decline and death.
Methods Radial growth data from 377 sugar maple trees that had undergone both defoliations by insects and
partial harvest were used to estimate longitudinal survival probabilities as a proxy for tree vigour. Radial growth
rates and survival probabilities were compared among trees subjected to different levels of above- and belowground
disturbances, between periods of defoliation and harvest, and between live and dead trees.
Key Results Manion’s tree disease model correctly accounts for vigour decline and tree death in sugar maple; tree
growth and vigour were negatively affected by a first defoliation, predisposing these trees to death later during the
study period due to a second insect outbreak that initiated a final vigour decline. This decline was accelerated by the
partial harvest disturbance in 1993. Even the most severe anthropogenic disturbances from partial harvest did not
cause, unlike insect defoliation, any growth or vigour declines in live sugar maple.
Conclusions Natural disturbances acted as predisposing and inciting stresses in tree sugar maple decline and death.
Anthropogenic disturbances from a partial harvest at worst accelerated a decline in trees that were already weakened
by predisposing and inciting stresses (i.e. repeated insect defoliations). Favourable climatic conditions just before
and after the partial harvest may have alleviated possible negative effects on growth resulting from harvesting.
