In the `Des Vieux Arbres' ecological reserve situated within northwestern Québec, 40 band dendrometers were installedon 7 of the major boreal tree species. The late Spring–early Summer daily radial activity registered in 1997 was related todaily weather variables. For each tree species, the daily mean i) cumulative radial increment and ii) radial activity indexedseries obtained by first-difference standardization were analyzed. The results indicate the existence of strong similarities among the 7 species. All showed strong synchronousfluctuations in radius during late winter and early spring. Thisperiod ended with a short but sharp increase in radial increments that marked the passage of water into the stem. Thisinitial swelling, less obvious in Pinus species was followed by a prolonged period of little change in radial activity. Meteorological data indicated that air temperature waspositively related to stem swelling during the late winter–earlyspring period. Both air and soil temperatures became negatively related to radial expansion once the passage of water has occurred in the stem. Starting in early June, all species registered a sustained increase in radial increments possiblyassociated with active cell division. After this, radial expansion was negatively related to air temperature and positively to rainfall.