The Origin and Dynamics of Subalpine White Spruce and Balsam Fir Stands in Boreal Eastern North America |
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Authors: | Guillaume de Lafontaine Serge Payette |
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Institution: | 1. NSERC Northern Research Chair, Centre d’études Nordiques, Département de Biologie, Université Laval, 1045 av. de la Médecine, Québec, Quebec, G1V 0A6, Canada
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Abstract: | Associations among the few tree species in the North American boreal landscape are the result of complex interactions between
climate, biota, and historical disturbances during the Holocene. The closed-crown boreal forest of eastern North America is
subdivided into two ecological regions having distinct tree species associations; the balsam fir zone and the black spruce
zone, south and north of 49°N, respectively. Subalpine old-growth stands dominated by trees species typical of the balsam
fir forest flora (either balsam fir or white spruce) are found on high plateaus, some of which are isolated within the black
spruce zone. Here we identified the ecological processes responsible for the distinct forest associations in the subalpine
belt across the eastern boreal landscape. Extensive radiocarbon dating, species composition, and size structure analyses indicated
contrasted origin and dynamics of the subalpine forests between the two ecological regions. In the black spruce zone, the
subalpine belt is a mosaic of post-fire white spruce or balsam fir stands coexisting at similar elevation on the high plateaus.
With increasing time without wildfire, the subalpine forests become structurally similar to the balsam fir forest of the fir
zone. These results concur with the hypothesis that the subalpine forests of this area are protected remnants of an historical
northern expansion of the fir zone. Its replacement by the fire-prone black spruce forest flora was caused by recurrent fires.
In the subalpine belt of the fir zone, no fire was recorded for several millennia. Harsh climate at high altitude is the primary
factor explaining white spruce dominance over balsam fir forming a distinct subalpine white spruce belt above the balsam fir
dominated forest. |
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