The seasonal dynamics of cambial activity, wood formation, occurrence of false rings (FR), and tangential bands of resin canals (RC) were investigated in Pinus halepensis from three Mediterranean dry and semi-arid ecosystems in Spain. We collected intact tissues of phloem, cambium, and outer xylem at monthly intervals throughout 2004 from each time six trees at the three sites.Cell divisions in the cambium in all trees started before our first sampling in mid-March and ceased between November and December. Cambial activity was characterized by two maxima; one in spring and another in autumn.Trees still grew in summer but at a very low rate. The first solitary RC were formed in May or June and tangential bands of RC in June or July. In general, tangential bands of RC were observed in wider growth rings. The formation of tangential bands of RC seems to be induced by drought in the second half of the growing season. FR were formed as a result of cambial reactivation in autumn and were observed in growth rings that contained more than 50 cells in a radial row. It appears that summer drought and early autumn precipitation play an important role in false-ring formation.