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

The strategies of Norway spruce [ (L.) Karst.] to increasing atmospheric CO concentration ( ) are not entirely clear. Here, we reconstructed centennial trajectories of leaf internal CO concentration ( ) and intrinsic water-use efficiency (WUE) from the amount of C in tree-ring cellulose. We collected 57 cores across elevations, soil, and atmospheric conditions in central Europe. Generally, WUE and increased over the last 100 years and the / ratio remained almost constant. However, two groups were distinguished. The first group showed a quasi-linear response to and the sensitivity of to ( = d /d ) 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 increase from -1 to +1.6. Our study shows a marked attenuation of the rise in WUE during the 20 century leading to invariant WUE in recent decades.