White spruce expansion at the tree line and recent climatic change

Abstract

White spruce (Piceaglauca (Moench) Voss) populations at their northern limit of distribution, the tree line along the east coast of Hudson Bay, expanded significantly during the last 100 years in response to recent climatic warming. Expansion began around 1880 but was more important during the 20th century, especially between 1920 and 1965. Although no major change in the position of the forest limit and the latitudinal tree line occurred, local altitudinal tree lines rose a few tens of meters. The seed regeneration line increased to about 100 meters above the present altitudinal tree line during the same period. Tree density increased below the local tree lines at many sites. There, spruce expansion was stepwise, as indicated by widely distributed clustered cohorts belonging to the 10–29, 50–59, and 70–89 age-class groups. Although the expansion process was stimulated by nearby old seed bearers, long-distance seed dispersal did occur. The development of contagious dispersion during the formation of white spruce groves or forests caused important local ecological changes, particularly in snow patterns and tree growth. Fire selectively influenced tree regeneration during the past centuries. Black spruce krummholz were progressively decimated during the last 400 years before present, although the species was characterized by low postfire regeneration. Vigorous postfire white spruce regeneration was stimulated by favorable climatic conditions 100 years ago in the northernmost forest site. Conclusions from this study support the hypothesis that Holocene tree line displacements were of low magnitude in northern Québec, and that the primary influence of climatic change was on tree density.