Seasonal Cambial Activity and Formation of Secondary Phloem and Xylem in White Oaks (Quercus alba L.)

(1) Background: the cambium has seasonal activity, forming earlywood and early phloem with relatively wide conducting cells, which will function during the most favorable season, and latewood and late phloem with narrower conducting cells, which typically function during the less favorable season. However, few studies have focused on when these two contrasting tissue types are formed in relation to climatic conditions. (2) Methods: the senior author of this paper made weekly collections for an entire year of four specimens per collection back in the 1960s, using traditional anatomical methods to study in detail what the cambium was producing progressively. (3) Results: annual growth rings are evident in both secondary xylem and secondary phloem. The cambium resumes activity in early April, with simultaneous formation of wood and secondary phloem. Both latewood and late phloem production are initiated in early June, the peak of the favorable season. The cambium ends its activity in early August. Phloem growth rings are marked by radially narrow sieve elements interspersed among a band of axial parenchyma with dark contents. Most specimens produce only one fiber band per season. This feature may be used as an indirect phloem growth ring marker. Wood growth rings are marked by very wide vessels and thick-walled, radially narrow fibers. (4) Conclusions: growth rings are evident in both secondary xylem and secondary phloem. The trees produce their latewood and late phloem long before the beginning of autumn, indicating that they prepare ahead of the selective regime, a phenomenon most likely dependent on the photoperiod. Living sieve elements are present yearlong.