Reconstruction of Millennial Forest Dynamics from Tree Remains in a Subarctic Tree Line Peatland

Abstract

Tree ring and growth form sequences of 319 black spruce (Picea mariana [Mill.] BSP.) stems buried in a treeless peatland at the arctic tree line of northern Québec were used to reconstruct the development of a woodland in response to climate change and fires during the last 2500 years. A high frequency of diagnostic tree rings (light rings and narrow rings) allowed the cross-dating of 142 individuals and the construction of a master chronology spanning AD 690–1591. Three floating chronologies covering 964 (≈178 BC–AD 785), 349 (≈587–239 BC) and 210 (≈1274–1065 BC) years were also developed. The buried stems were classified as arborescent (75%), small fragments of unknown origin (13%), stumps (11%), and portions of stunted stems (1%). The high frequency of arborescent individuals indicates that the buried spruces were the remains of a former forest. The forest bordered the peatland between 600 BC and AD 1568, and successfully regenerated after two fires around 350 BC and 10 BC. Winter-damaged trees dominated during periods of suppressed growth at AD 760–860 and AD 1025–1400, whereas undamaged trees were more frequent during periods of rapid growth around AD 700–750, 860–1000, 1400–1450, and 1500–1570. Fast growth between AD 860–1000, along with the concurrent establishment of symmetrical trees, suggests well-defined boundaries for the Medieval Warm Period in northeastern Canada. The forest abruptly shifted to an open krummholz (stunted spruce) after the last fire in AD 1568, indicating that a climatic threshold inhibiting postfire regeneration was crossed between the second and the last fires. The maintenance of this old growth forest over 1500 yr, in the absence of external disturbances except climate change, was probably due to the buffering effect of aggregated trees on wind-drifted conditions at the snowpack line. With the exclusion of the forest influence on microclimate and local growth conditions, the AD 1568 fire caused the forest vegetation to shift to krummholz. The amplitude of these ecosystem changes at tree line does not mirror that of climate change. Hence, it is concluded that climate and vegetation reconstructions from proxy indicators cannot portray full ecological impact, because vegetation change at tree line is nonlinear relative to climate change.