<p>The current study investigated the long-term effects of CO₂ enrichment on cambial activity and wood tracheid traits of <i>Pinus densiflora</i> grown in Open Top Chambers (OTCs). We monitored the cambial activity and analyzed quantitative and qualitative variations in the tracheid at different CO₂ enrichments. The cambial activity in the trees was found to begin in mid-to-late March and end in late October to early November. So, the growing season was a little longer than the monitoring period. The time of maximum growth rate was delayed as CO₂ enrichment increased, thereby suggesting that rising CO₂ concentration may affect intra-annual cambial activity. The quantitative growth (number of cells) increased with higher concentrations of CO₂ in the early years of the CO₂ exposure. However, a decline in growth was observed from the 8th year after exposure. Qualitatively, cell wall thickness was found to be the largest for the highest CO₂ treatment, which was in contrast to previously reported pot experimental results obtained over a couple of growing seasons. The observed quantitative growth increase coupled with the qualitative variations (increased cell wall thickness) may influence the physical properties of wood and carbon storage capacity, and such knowledge is important for forest resource utilization and management planning.</p>