Trees synchronize the cycles of growth and dormancy with the seasonal variations in weather, an essential aspect in ecosystems characterized by wide differences between seasons favorable and unfavorable to the physiological activities. Wood formation, or xylogenesis, is a complex and fascinating example of an intermittent growth process sensitive to temperature that can be studied at several time scales. The period of wood formation is the time window during which the xylem is under differentiation. Environmental factors can then act directly on the cells forming the tree ring and, consequently, on the characteristics of the resulting wood and its properties. The processes of cell production in the cambium and xylem maturation will be described and compared to the limiting factors of cold ecosystems. The results demonstrate the importance of cambial phenology to understand the ecological implications of wood formation for forest environments.