Comparative cambial dynamics and phenology of Quercus robur L. and Q-pyrenaica Willd. in an Atlantic forest of the northwestern Iberian Peninsula

Wood formation of Quercus robur and Q. pyrenaica is similar along the growing season, mostly differing in the initial earlywood vessels, which are formed later but faster for Q. pyrenaica. We monitored leaf phenology and wood formation during the growing season of 2010 on two oak species (Quercus robur L. and Quercus pyrenaica Willd.) in the northwestern Iberian Peninsula to provide a detailed description of cambial dynamics, with a special focus on earlywood, and to relate phenological phases to simultaneous cambial processes. For this, we used visual survey of buds and leaves, and microscopic observations of microcores taken from stems on the same dates. Sampling intervals were weekly in the early season, and more spaced afterwards. There were no differences in cambial processes among individuals of the same species, whereas some phases of cambial dynamics and phenology were simultaneous for the two species. Cambial reactivation began in middle March, and bud burst was achieved in early May (6 weeks later) for both species, but they differed in the time required for the expansion of vessels in the first row. Vessel enlargement in Q. robur started at the beginning of April, and lasted for 1 month, while it began 2 weeks later in Q. pyrenaica and spanned only 2 weeks. The beginning of secondary wall lignification in the first vessel row was contemporary to bud burst. Earlywood formation lasted until mid-June, when latewood vessels and fibres started to be much more abundant in all individuals. Radial growth ceased in middle September. The total duration of earlywood formation lasted for 3 months in both species, while latewood formation took around two and a half months.