VARIATIONS OF WOOD DELTA-C-13 AND WATER-USE EFFICIENCY OF ABIES-ALBA DURING THE LAST CENTURY

Variations of intrinsic water-use efficiency during the last century w ere investigated based on analysis of delta(13)C in tree rings of Abie s alba from the Jura Mountains (eastern France). To separate the effec ts related to the age of the tree at the time the tree ring was formed from effects due to environmental changes, analyzed wood samples were extracted from a very large sample set including different tree ages and calendar dates of wood formation. For the first 75 yr of the life of Abies alba, delta(13)C of wood holocellulose increases with the age of the tree from -24.4 parts per thousand at age 15 to approximately -22.5 parts per thousand at age 75. Between the ages of 75 and 150 val ues remain constant at -22.5 parts per thousand. Consequently, the eff ect of the tree age on isotopic discrimination has to be taken into ac count in studies on the long-term environmental effects on delta(13)C in tree rings. Divergent trends of delta(13)C during the last century were observed between tree rings formed at age 40 and bulk air data. T he isotopic discrimination Delta varied insignificantly around a mean of 17.3 parts per thousand between the 1860s and the 1930s. It then de creased to 15.8 parts per thousand from the 1930s to the 1980s. Using these results and classical models of carbon discrimination, we calcul ated that the intrinsic water-use efficiency (A/g(w), the ratio of CO2 assimilation rate to stomatal conductance for water vapor), integrate d over the year, has increased by 30% between the 1930s and the 1980s. These results, obtained at the level of mature trees, are consistent with the physiological effects of increasing CO2 concentrations as obs erved in controlled experiments on young seedlings. They are consisten t with the strong increases in radial growth observed for Abies alba i n western Europe over the past decades. However, other long-term envir onmental changes such as increasing nitrogen deposition could cause si milar effects.