The timing of spring rehydration and its relation with the onset of wood formation in black spruce

Stem and root radius variations in black spruce were monitored between August 2004 and October 2005 with automatic point dendrometers to determine if the timing of growth initiation corresponded to the rehydration of tissues in spring. Phases of contraction, expansion and increment were defined in a time series following the stem cycle approach. The cycles were classified into different types according to their characteristics and the temperature recorded during each phase. These classifications enabled a division of the year into three periods: winter shrinkage (1), spring rehydration (2) and summer transpiration (3). Rehydration lasted about one month (32–47 days), from mid-March to mid-April and trees were fully rehydrated 39 days before radial growth occurred in terms of cell enlargement, which started on May 27 in the stem and May 30 in the root. Growth initiation was not limited by the spring rehydration. The importance of temperature as a limiting factor to growth initiation was confirmed by meteorological data and the classification of cycles.