Variations of wood δ¹³C and water-use efficiency of Abies alba during the last century

Variations of intrinsic water-use efficiency during the last century were investigated based on analysis of δ¹³C in tree rings of Abies alba from the Jura Mountains (eastern France). To separate the effects 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, δ¹³C of wood holocellulose increases with the age of the tree from -24.4‰ at age 15 to approximately -22.5‰ at age 75. Between the ages of 75 and 150 values remain constant at -22.5‰. Consequently, the effect of the tree age on isotopic discrimination has to be taken into account in studies on the long-term environmental effects on δ¹³C in tree rings. Divergent trends of δ¹³C during the last century were observed between tree rings formed at age 40 and bulk air data. The isotopic discrimination A varied insignificantly around a mean of 17.3‰ between the 1860s and the 1930s. It then decreased to 15.8‰ from the 1930s to the 1980s. Using these results and classical models of carbon discrimination, we calculated that the intrinsic water-use efficiency (A/g(w), the ratio of CO₂ assimilation rate to stomatal conductance for water vapor), integrated 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 observed in controlled experiments on young seedlings. They are consistent with the strong increases in radial growth observed for Abies alba in western Europe over the past decades. However, other long-term environmental changes such as increasing nitrogen deposition could cause similar effects.