Abstract
The strategies of Norway spruce [Picea abies (L.) Karst.] to increasing atmospheric CO(2) concentration (C (a)) are not entirely clear. Here, we reconstructed centennial trajectories of leaf internal CO(2) concentration (C (i)) and intrinsic water-use efficiency (WUE(i)) from the amount of (13)C in tree-ring cellulose. We collected 57 cores across elevations, soil, and atmospheric conditions in central Europe. Generally, WUE(i) and C (i) increased over the last 100 years and the C (i)/C (a) ratio remained almost constant. However, two groups were distinguished. The first group showed a quasi-linear response to C (a) and the sensitivity of C (i) to C (a) (s = dC (i)/dC (a)) 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 WUE(i) during the 20(th) century leading to invariant WUE(i) in recent decades.