BLACK SPRUCE GROWTH FORMS AS A RECORD OF A CHANGING WINTER ENVIRONMENT AT TREELINE, QUEBEC, CANADA

The environmental conditions prevailing at treeline in subarctic Quebec have been reconstructed over the past 400 yr through a comparative analysis of tree rings and growth forms of black spruce (Picea mariana [Mill.] B.S.P.). Because black spruce growth forms are closely associated with the winter environment, they are a direct response to conditions of low temperature and windblown snow abrasion affecting living tissues at the snow-air interface. Living and dead spruces were sampled along five transects, according to wind exposure (hilltop and protected valley) and aspect (east-, north-, and south-facing slopes). The age structure of supranival shoot populations (stems growing above the snow cover) was closely associated with periods of higher stem survival in winter most likely under snowier and windless conditions. The number of supranival shoots initiated between 1710 and 1810 and between 1880 and the present, respectively, appeared to be related to milder and snowy winters. Spruce growth on slopes and in the valley revealed periods of low tree-ring growth between 1601 and 1663 and between 1700 and 1904, respectively. A long-lasting period of low radial growth between 1697 and 1939 prevailed in the hilltop site. During the 20th century, spruce height increased from 0.8 to 1.6 m on slopes and in the valley, while the basal level of abrasion from windblown snow increased from 0.1 to 0.5 m, suggesting an increasing trend towards warmer and snowier conditions. Abraded spruces growing during the Little Ice Age (1570-1880) were replaced by symmetrical trees during the 20th century. Supranival skirted and whorled spruces which dominated on the hilltop site during the 16th century reverted to infranival cushion and mat growth forms during the Little Ice Age. These stunted spruces were unable to recover during the recent warming because of their inability to catch enough drifting snow to allow vertical growth.