Response of leaf internal CO<sub>2</sub> concentration and intrinsic water-use efficiency in Norway spruce to century-long gradual CO<sub>2</sub> elevation

The strategies of Norway spruce [<i>Picea abies</i> (L.) Karst.] to increasing atmospheric CO₂ concentration (<i>C</i> _a) are not entirely clear. Here, we reconstructed centennial trajectories of leaf internal CO₂ concentration (<i>C</i> _i) and intrinsic water-use efficiency (WUE_i) from the amount of ¹³C in tree-ring cellulose. We collected 57 cores across elevations, soil, and atmospheric conditions in central Europe. Generally, WUE_i and <i>C</i> _i increased over the last 100 years and the <i>C</i> _i/<i>C</i> _a ratio remained almost constant. However, two groups were distinguished. The first group showed a quasi-linear response to <i>C</i> _a and the sensitivity of <i>C</i> _i to <i>C</i> _a (<i>s</i> = d<i>C</i> _i/d<i>C</i> _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 <i>s</i> 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.