Supporting dataset for the paper "Climate and atmospheric deposition effects on forest water‑use efficiency and nitrogen availability across Britain" by Guerrieri et al. 2020 Scientific Reports DOI is 10.1038/s41598-020-67562-w (https://www.nature.com/articles/s41598-020-67562-w) The dataset includes carbon, oxygen and nitrogen stable isotope composition measured in tree rings for four species at twelve forests across climate and atmospheric deposition gradients in Britain. Data were used to i) investigate spatial and temporal (over 30‑years) changes in tree intrinsic water use efficiency (iWUE, i.e., the ratio between CO2 assimilation - A, and stomatal conductance- gs); ii) assess the possible species-specific physiological mechanisms (changes in A and/or gs) evaluate whether sites receiving high Ndep experience increase in ecosystem N availability and N saturation (using tree-ring nitrogen isotope composition as a proxy); iv) elucidate drivers of spatial and temporal changes in the isotope-derived physiological and ecological processes (i.e., relative contribution of climate vs. changes in atmospheric CO2 and nitrogen and sulfur deposition). The following files are available: CSV file including: a) Site names; b) Species; c) Latitude; d) Longitude; e) Investigated years (1980-2010); f) Measured isotope-related parameters and tree ring nitrogen %. Isotope-related parameters included in the dataset are: 1) delta13C, 2) carbon isotope discrimination (Delta13C), 3) intercellular CO2 concentration (ci) and the ratio of intercellular to atmospheric CO2 concentrations (ci/ca), delta18O, estimated delta18O in precipitation (delta18OP) according to Barbour et al. 2001 (see Method in Guerrieri et al. 2020 Scientific Reports), oxygen isotope discrimination above the source water (Delta18O), delta15N. PDF file reporting the equations used to calculate the isotope-derived parameters, which can also be found in the paper by Guerrieri et al. 2020 Scientific Reports (accepted)