Natural and anthropogenic disturbances may act as stresses on tree vigour according to Manion’s (1981) conceptual tree disease model: initial vigour of trees is decreased by predisposing factors making these trees more vulnerable to severe inciting stresses which may cause a final vigour decline and tree death. In this study, we tested the validity of this model in sugar maple (Acer saccharum Marsh.) by characterizing the role of natural and anthropogenic disturbances in tree decline and death.
We estimated longitudinal survival probabilities as a proxy of tree vigour from radial growth data from ≈ 300 live and dead sugar maple trees that had undergone both forest tent caterpillar (Malacosoma disstria Hubner) defoliations and harvest disturbances (soil disturbance, sudden increases in light levels). Chronologies of radial growth indices and survival probabilities were compared among trees of different harvest disturbance classes, between periods of defoliation and the post-harvest period as well as between live and dead trees.
Results/Conclusions
Manion’s tree disease model correctly predicts the course of vigour decline and tree death in dead sugar maples. Tree growth and vigour were negatively affected by a first defoliation thereby predisposing the trees to a second outbreak. This second outbreak initiated a final vigour decline that was accelerated during the post-harvest period. However, even the most severe disturbances from partial harvest did not cause, unlike insect defoliation, any growth or vigour declines in live sugar maple but caused instead growth increases.
Hence, disturbances from partial harvest may act at worst as contributing stresses in the decline of already weakened sugar maples but need to be preceded by predisposing and inciting stresses, in our case repeated insect defoliations.
