Increasing water-use efficiency mediates effects of atmospheric carbon, sulfur, and nitrogen on growth variability of central European conifers

Climate controls forest biomass production through direct effects on cambial activity and indirectly through interactions with CO₂, air pollution, and nutrients availability. Atmospheric concentration of CO₂ and sulfur or nitrogen deposition could exert a robust indirect control on wood formation, since they influence the stomatal regulation of transpiration and carbon uptake, that is, intrinsic water use efficiency (iWUE). Here we provide 120-year long time series of iWUE, tree growth, climatic and sulfur and nitrogen(SN) deposition trends for two widespread tree species, Pinus sylvestris (PISY) and Picea abies (PCAB), at their lower and upper distribution margins in Central Europe. The main goals were to explain iWUE trends using theoretical scenarios and climatic and SN deposition data and to assess the contribution of climate and iWUE to the observed growth trends. Our results show that after a notable increase in iWUE between the 1950s and 1980s, the positive trend slowed down. Substantial rise of iWUE since the 1950s resulted from a combination of an accelerated increase in atmospheric CO₂ (C_a) and a stable level of leaf CO₂ (C_i). The offset of observed iWUE values from the trajectory of iWUE growth proportional to increase in C_a (constant C_i/C_a scenario) was explained by trends in SN deposition (all sites) together with the variation of drought conditions (low-elevation sites only). Increasing iWUE over the 20th and 21st century improved tree growth at low-elevation drought-limited sites. In contrast to low-elevation sites, recent warming was the main reason for the growth increase at high-elevation PCAB. We propose that SN pollution should be considered to explain the steep increases in iWUE of conifers in the 20^th century in a broader area of Central Europe and in other regions with a significant SN deposition history.