Needle cell elongation and maturation timing derived from pine needle cellulose δ¹⁸O

Estimates of the timing of Pinus arizonica Engelm. needle development in 1998 and 1999 were derived from the leaf-cellulose δ¹⁸O of weekly growth increments. Significant correlations were noted between time series of local humidity and leaf-cellulose δ¹⁸O for needles growing near Tucson, Arizona. Correlations with temperature were also significant, but much lower, suggesting these variations in cellulose δ¹⁸O were determined mostly by changes in humidity. The timing of all significant correlations lags the timing of the appearance of the new needle growth, and is interpreted as indicating 16-23 d were required for cell enlargement in 1998 and 13-17 d in 1999. Similarly, properties of the environmental time series, when significantly correlated, are interpreted as indicating the duration of cellulose deposition (7-27 d in 1998, 13-21 d in 1999). Variations in stable-isotope back diffusion (the Péclet effect) and the synthesis of cellulose using stored photosynthate are discussed as explanations for departures from a Craig and Gordon-type model of leaf water δ¹⁸O. The Péclet effect, use of stored photosynthate, and variations in the growing-season source-water δ¹⁸O, probably confound the development of a high-resolution paleohumidity proxy from subfossil needle cellulose δ¹⁸O in this region.