Abstract. The North American boreal forest has been developing since the end of the last glaciation approximately 10,000 yr ago. With climate warming and human occupation, it is anticipated that fire danger, ignition, and activity will be increasing, compromising forests' benefits for generations to come. In this study, we show, however, that a century of rapid climate changes and human densification has had the opposite effect in the boreal eastern interior of the North American continent, reducing biomass burning to values below two millennia of historical levels. A multi-millennial fire history was reconstructed for eight forested landscapes from the Lake of the Woods Ecoregion (LWE) located at the boreal-prairie ecotone. Fire history was reconstructed using a combination of archival (period 1920-2010), tree-ring (stand initiations and fire scars: period 1690-2010), and lake sediment charcoal (2500 BP to present) records. The archival record revealed recent large fires (>200 ha) in 1948, 1980, and 1988. An additional 19 fires were identified by the fire-scar record. Fire events in 1805, 1840, 1863, and the 1890s were identified in numerous locations around multiple lakes suggesting that they were of large extents. In accordance with the tree-ring record, the charcoal accumulation rate (CHAR) peak record generally identified the major fires but tended to lag from the tree-ring records by several decades. Within LWE, the long-term charcoal record revealed that CHAR was higher for each lake in the earlier portion of the record including the warm Medieval Climate Anomaly (AD À900 to AD 1000), followed by a progressive decrease toward the cool Little Ice Age period. This decline was abruptly interrupted in the mid-to late 19th century with large synchronized fires, also reported over western and central North America, and resumed approximately four decades later. Fire disturbance level is today below the historical range, despite the accentuated climate warming. Aging of the forest landscape may create biodiversity loss notably in fire-adapted species while at the same time setting the tone for major fires in upcoming decades if no action is taken for managing fuels.

Dendroecological analysis of tree-ring anomalies can facilitate better environmental reconstructions provided that conditions leading to their formation are well understood. White earlywood rings (WER) constitute an anomaly not previously described and, like frost rings (FR), occur often in boreal coniferous species in central Canada. the objectives of this study were i) to examine the distribution of FR and WER in black spruce trees, ii) to measure and compare tracheid dimensions in WER and control rings and iii) to investigate potential causes of WER formation. sampling was conducted in two regions of western Manitoba. At each of four sampling sites, ten black spruce trees were selected for stem analysis. Results indicated that FR were mainly formed in the smaller diameter, thin barked portion of the stem up to a height of 16m, whereas WER were almost uniformly distributed along the stem. tracheids were measured for WER occurring in 1916WER occurring in , 1943WER occurring in and 1970 and WER were found to have earlywood tracheids with significantly thinner cell walls compared to control rings. It is speculated that the formation of WER may be related to reduced hormone and carbohydrate availability in the early growing season. the potential linkage between WER and frost events, or other agents of defoliation, prior to ring formation merits further investigation.

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