CARBON-ISOTOPE FRACTIONATION IN TREE-RING EARLY AND LATE WOOD IN RELATION TO INTRA-GROWING SEASON WATER-BALANCE

We determined the stable carbon isotope composition (delta(13)C) of ce llulose extracted from early and late wood in Douglas fir [Pseudotsuga menziesii (Mirb.) France] tree rings, Data were obtained for the peri od 1962 to 1981, at the start of which the trees were 20 years old, A water balance model was used to calculate daily stand transpiration an d water deficit, The model incorporates site factors (soil water avail ability, slope and aspect) and environmental variables (solar radiatio n, air temperature and rainfall), There was far greater variability in late wood than in early wood delta(13)C, In wet years, late wood delt a(13)C was significantly lighter (by as much as 2 parts per thousand) than early wood delta(13)C but in dry years this difference was revers ed, Differences between spring and summer cumulative transpiration acc ounted for almost 60% of the variability in differences between early and late wood delta(13)C. We found excellent correspondence between su mmer cumulative transpiration and late wood delta(13)C, with estimates of transpiration accounting for up to 93% of the variability in delta (13)C, Correlations between early wood delta(13)C and spring transpira tion were generally poor (r(2) < 0 . 4), but we were able to identify those exceptional years in which there had been a very dry spring, Our results indicate that, while tree ring delta(13)C correlates reasonab ly well with basal area increment, it is a far better indicator of int er- and intra-annual variability in water availability than radial gro wth.