Gap expansion in old-growth subarctic forests: the climate-pathogen connection.

We tested the hypothesis considering old-growth subarctic woodlands, free of fire, insect and stand-scale blowdown disturbances, to be at equilibrium with the climate. To do so, we explored the status of Hudsonian woodlands based on the natality/mortality ratio. The gap history of the woodland was reconstructed based on mapping and dating of dead gap-spruces (Picea mariana). Among the 25 gaps studied, 763 dead trees and only 14 saplings were recorded. The center of some gaps remained treeless over the last 1000 yr, and gap area doubled over the last 100 yr. The status of the tree population is in a demographic disequilibrium caused by the small replacement of dead spruces in all of the gaps. Episodes of 'mass' mortality occurred during several decades corresponding to years of favorable tree-ring growth. The natural process of gap-filling appears to be ineffective under current conditions. Good tree-ring growth of dying trees suggests abundant precipitation during the mortality episodes, but precipitation appears to be involved indirectly in the mortality process. The main cause of the widespread tree mortality during the last centuries of gap expansion appears to be biotic in origin. The impact of pathogenic fungal disease linked to late-lying snow cover is proposed for the mortality events.