Response of leaf internal CO2 concentration and intrinsic water-use efficiency in Norway spruce to century-long gradual CO2 elevation

The strategies of Norway spruce [Picea abies (L.) Karst.] to increasing atmospheric CO2 concentration (Ca) are not entirely clear. Here, we reconstructed centennial trajectories of leaf internal CO2 concentration (Ci) and intrinsic water-use efficiency (WUEi) from the amount of 13C in tree-ring cellulose. We collected 57 cores across elevations, soil, and atmospheric conditions in central Europe. Generally, WUEi and Ci increased over the last 100 years and the Ci/Ca ratio remained almost constant. However, two groups were distinguished. The first group showed a quasi-linear response to Ca and the sensitivity of Ci to Ca (s = dCi/dCa) ranged from 0 to 1. Trees in the second group showed nonmonotonic responses with extremes during the peak of industrial air pollution in the 1980s and s increase from -1 to +1.6. Our study shows a marked attenuation of the rise in WUEi during the 20th century leading to invariant WUEi in recent decades.