Climate and atmospheric deposition effects on forest water-use efficiency and nitrogen availability across Britain

Rising atmospheric CO2 (ca) has been shown to increase forest carbon uptake. Yet, whether the ca- fertilization e ect on forests is modulated by changes in sulphur (Sdep) and nitrogen (Ndep) deposition and how ndep a ects ecosystem N availability remains unclear. We explored spatial and temporal (over 30‐years) changes in tree‐ring δ13C‐derived intrinsic water‐use e ciency (iWUE), δ18O and δ15N for four species in twelve forests across climate and atmospheric deposition gradients in Britain. The increase in iWUE was not uniform across sites and species‐speci c underlying physiological mechanisms re ected the interactions between climate and atmospheric drivers (oak and Scots pine), but also an age e ect (Sitka spruce). Most species showed no signi cant trends for tree‐ring δ15N, suggesting no changes in N availability. Increase in iWUE was mostly associated with increase in temperature and decrease in moisture conditions across the South–North gradient and over 30‐years. However, when excluding Sitka spruce (to account for age or stand development e ects), variations in iWUE were signi cantly associated with changes in ca and Sdep. Our data suggest that overall climate had the prevailing e ect on changes in iWUE across the investigated sites. Whereas, detection of Ndep, Sdep and ca signals was partially confounded by structural changes during stand development.