LATE-HOLOCENE LIGHT-RING CHRONOLOGIES FROM SUBFOSSIL BLACK SPRUCE STEMS IN MIRES OF SUB-ARCTIC QUEBEC

Subfossil black spruce (Picea mariana [Mill.] B.S.P.) stems from perma frost mires in a 250-km(2) area at the arctic tree-line, near the east ern coast of Hudson Ray, were sampled for the construction of a long ' light-ring' chronology spanning the late Holocene. Thirty-three out of the 143 sampled spruce logs were C-14-dated yielding dates between 45 80 and 1540 BP (3550-3010 cal. sc - cal. AD 377-649). Light rings (rin gs with thin-walled latewood cells or showing reduced latewood cell de velopment) were numerous in all the studied stem discs and a set of 47 logs was successfully cross-dated. The cross-dated trees were used to build seven 'floating' light-ring chronologies located between c. 120 0 cal. sc and cal. AD 500 (i.e., c. 1200 to 1000 cal. sc, 900 to 800 c al, sc, 700 to 400 cal. sc, 700 to 500 ed. sc, 100 cal. sc to ed. Ao 3 00, cal. Ao 50 to 450 and cal. AD 400 to 500). The large number of lig ht rings in mire spruces appears to be associated with particular grow th conditions in mires. A comparison of the frequency of light rings i n extant spruce growing in mires and in dry-mesic sites showed that li ght rings were more common in peatland trees during this century. A de layed growing season caused by late peat thaw may possibly explain the difference between the two groups of sampled trees.