1 The very cold, wind-exposed coastal environment along the eastern Hudson Bay coast (northern Quebec, Canada) has resulted in a treeline which runs parallel to the coast (i.e. north-south). We have measured changes in the longitudinal position of the black spruce (Picea mariana (Mill.) B.S.P.) treeline during the last centuries, using tree ring analysis of spruce stem development. 2 Treeline trees (greater than or equal to 2.5 m high) were sampled along transects in valley sites of four different rivers draining towards Hudson Bay, from the present treeline to the forest limit. The year of initiation of supranival stem growth (i.e. when stems first developed above the snowpack) was determined for trees, whorled growth forms and dead shrubs. Supranival tree stems developed synchronously in the valleys, from the late 1800s to the mid 1900s, in the area presently occupied by treeline trees, indicating that only krummholz (shrubby spruces) were present at these sites during the Little Ice Age. 3 A corresponding displacement of the treeline of about 12 km towards Hudson Bay has occurred in the area since the late 1800s, most likely as a result of recent warming. The recent treeline shift also resulted from the development of vertical tree stems from pre-established krummholz, suggesting that stem growth was a direct response to favourable summer growth conditions and reduced wind-driven snow abrasion and supercold wind-chill. The absence of treeline trees originating from seeds indicates that the recent warming has not been of sufficient magnitude or duration to promote sexual reproduction.
