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Ice-flood history reconstructed with tree- rings from the southern boreal forest limit, western Québec

Groupe de recherche en écologie forestière and Institut des sciences de l'environnement, Université du Québec à Montréal, C.P. 8888, Succ. Centreville, Montréal (Québec), Canada H3C 3P8

Yves Bergeron

Groupe de recherche en écologie forestière and Institut des sciences de l'environnement, Université du Québec à Montréal, C.P. 8888, Succ. Centreville, Montréal (Québec), Canada H3C 3P8

Historic ice-flood frequency and lake levels were reconstructed for the southern limit of the boreal forest in western Québec by dendrochronological analyses of ice-scarred white cedar (Thuja occidentalis L.) trees growing on the shore of Lake Duparquet. Results reveal a significant increase in major ice floods since the end of the 'Little Ice Age' and, in particular, since the beginning of this century. This rise was also not related to oversampling of younger scarred trees. In addition, time between white-cedar establishment and the first recorded scar has significantly decreased in the last 200 years, and this applies for trees occupying similar topographic conditions. White cedar's increased exposure, in the 150 years, to scarring strongly suggests that Lake Duparquet water levels at breakup are reaching higher elevations. This is supported by an approximate 100-cm rise in maximum ice-scar height since the end of the 'Little Ice Age'. Years leading to abundant ice scars are those with heavy winter and spring precipitation. Cold Novembers and rainy Aprils also favour major ice-flood episodes. Earlier spring melts could also favour pre-mature ice breakup; a situation that may cause higher-amplitude ice floods. Since the 'Little Ice Age', greater penetration of warm and humid air from the south resulting from a northward migration of the Arctic front may be responsible for the increasing major ice floods registered at Québec's southern boreal forest limit.

Key Words: Dendrochronology • ice-scar data • ice floods • proxy climatic indicators • spring breakup • lake levels • 'Little Ice Age' • boreal forest • Québec • Canada.

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