This study aimed to elucidate the seasonal patterns of cambial activity and the anatomical changes of the secondary phloem of Taxodium mucronatum×Cryptomeria fortunei and to provide anatomical evidence for its cultivation and wood-quality regulation.

Approximately 50-year-old T. mucronatum×C. fortunei trees growing in Nanjing were sampled monthly from April to November at breast height. Stem transverse sections were prepared using the paraffin-embedding method and observed under fluorescence and polarized light microscopy. The number of cambial cell layers, the width of conductive secondary phloem, and the number of callose deposition sites were measured.

The number of cambial cell layers ranged from 3 to 12 during April-November, with 3-4 layers in mid-April, an increase to 8-12 layers during May–June (the annual peak), and a decline to about 3 layers during October-November. This pattern showed a unimodal seasonal fluctuation. The width of conductive secondary phloem increased progressively throughout the growing season and reached its maximum in November (760.40±179.72 μm). Callose deposition between sieve cells increased from 10 sites in April to 22 sites in November, with an average of 19 sites during September–October and a clear rise in fixed callose, indicating that peripheral sieve cells withdrew from the conductive zone toward the end of the growing season.

Under the subtropical monsoon climate of Nanjing, Taxodium mucronatum×Cryptomeria fortunei exhibits a prolonged cambial activity period and maintains a relatively wide belt of conductive secondary phloem during the late growing season. The seasonal progression of callose deposition reflects the gradual transition of sieve-cell function in response to the long growing season. Using representative anatomical indicators and their seasonal dynamics, this study revealed the coordinated annual changes of the cambium and secondary phloem in this species.