Defining cambial activity: the limitations of indirect indicators and the need for direct cellular markers

The vascular cambium is a key lateral meristem responsible for secondary growth in woody plants, producing secondary xylem and phloem. Understanding its activity is crucial for studies on plant phenology, carbon sequestration, and environmental responses. However, defining the precise period of cambial activity remains challenging due to reliance on indirect indicators, such as cambial zone width or the presence of undifferentiated cells adjacent to the cambium. These parameters often misrepresent the true timing of cambial cell division, conflating it with subsequent differentiation processes. This study critically examines the limitations of indirect indicators and advocates for a more precise definition of cambial activity strictly as the process of cellular division. Direct cellular markers, such as mitotic figures, phragmoplasts, and newly formed tangential walls, provide more accurate assessment of cambial activity. By distinguishing cell division from differentiation, we can refine growth periodicity analyses and improve our understanding of environmental influences on cambial function. We review the structure and activity of the vascular cambium, demonstrating how indirect indicators can misrepresent cambial activity dynamics and lead to errors in determining its onset, duration, and cessation. By integrating direct cellular markers, we propose a more accurate methodology for assessing cambial activity, improving phenological studies and providing a clearer framework for evaluating plant responses to climatic variability.