Kinetics of Tracheid Differentiation in Douglas-fir

Seasonal cambial activity and the differentiation of tracheids were studied at five stem levels in three crown classes of Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir). Rates and durations of tracheid expansion and secondary wall deposition were computed. Seasonal changes in radial diameter of cells were attributable both to changes in duration and rate of cell expansion. Seasonal changes in cell wall area were determined by changes in duration of wall deposition. Durations of cell expansion and wall deposition showed a marked trend to increase down the stem, whereas rates of tracheid differentiation showed no clear axial trend. Axial trends in cell dimensions, particularly cell wall area, were mainly determined by durations of differentiation. Rates of cell wall thickening varied among crown classes suggesting that assimilate status is important in determining this process.