The onset of cambium activity - A matter of agreement?

The diffuse-porous maple and the ring-porous ash showed first cambium cell divisions after bud break contemporaneously in the second half of April. However, in the ash, a few cells scattered throughout the dormant cambium zone had already distinctly enlarged by the end of March. Later on these cells developed further into earlywood vessels prior to the onset of cell divisions. The question about which criteria can be used as an indication of cambium reactivation will be discussed.