The Long‐Term Stability of the Boreal Forest Limit in Subarctic Quebec

Abstract

Using black spruce (Picea mariana) macrofossil remains at the forest—tundra to tundra transition in norther Quebec, Canada, we address the question: did the boreal forest limit shift in unison with climate change during the Holocene? Logs buried in peatlands, charcoal in the organic soil layers on well—drained sites, and dry peat underlying stunted clonal spruce were sampled across a 60—km transect south and north of the modern forest limit. Of 146 spruce logs sampled south of the modern forest limit, 34 were 14C dated between 4580 and 1540 yr BP, and 34 additional logs were cross—dated with 14C—dated spruces (3010—1540 yr BP) using diagnostic tree rings. South of the forest limit, charcoal with abundant spruce fragments was widespread in dry—mesic sites and dated between 1920 and 1140 yr BP. No spruce logs or charcoal were found in all the surveyed sites north of the forest limit. The residence time of humified peat confined to the area occupied by tundra clonal spruces ranged between 3040 and 630 yr BP. The absence of spruce macrofossils in the tundra zone, together with the geographic coincidence of the charcoal limit and forest limit, strongly suggest that the forest limit remained stable during the last 2000—3000 yr BP. Old—aged peat produced by tundra spruces is additional evidence for the stability of the forest limit. Because only spruce is able to accumulate such a thick organic layer, the 14C dates of basal peat indicate that spruce clones developed continuously in the absence of external disturbance at least over the last 3000 yr BP. The stability of the forest limit during war (≈ 2000 yr BP, medieval times, and this century) and cold (≈ 3000 and 1300 yr BP, and the Little Ice Age) periods of the late Holocene demonstrates that mechanisms allowing forest limit advance or retreat are not easily triggered by climatic change.