Developing millennial tree ring chronologies in the fire-prone North American boreal forest

A denser global network of millennial tree ring chronologies is needed to verify whether the warming of the last century is anomalously rapid or of large amplitude in the context of the last millennium. The North American boreal forest in particular has been poorly represented in long climatic reconstructions based on tree rings, because of short tree longevity, as well as frequent and severe wildfires. In this study, we exhaustively sampled tree remains in the littoral zone of two lakes and partially sampled seven additional lakes in the northern boreal forest of eastern Canada to verify if highly replicated millennial tree ring chronologies could be developed. In total, 989 black spruce (Picea mariana (Mill.) BSP.) tree remains were sampled, of which 529 could be crossdated, providing a master chronology spanning the AD 676-1991 time period. Radiocarbon dating of a floating chronology indicates that maximum residence time of tree remains outside of sediments is at least 1500 years. Past fires halted or reduced wood inputs at least once at all but one site and thus influenced the replication of local chronologies. However, fire effects can be minimized using appropriate site selection criteria (old-growth forest on the leeward shore, sharp deepening of the littoral zone, burial of tree remains by fine sediments), such that it would be possible to develop a network of several highly and evenly replicated multi-century local chronologies. These tree ring chronologies would have desirable properties for extracting low-frequency tree growth trends and for reconstructing forest disturbance histories. Copyright # 2013 John Wiley & Sons, Ltd.