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

Rising atmospheric CO₂ (c_a) has been shown to increase forest carbon uptake. Yet, whether the c_a-fertilization effect on forests is modulated by changes in sulphur (S_dep) and nitrogen (N_dep) deposition and how N_dep affects ecosystem N availability remains unclear. We explored spatial and temporal (over 30-years) changes in tree-ring δ¹³C-derived intrinsic water-use efficiency (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-specific underlying physiological mechanisms reflected the interactions between climate and atmospheric drivers (oak and Scots pine), but also an age effect (Sitka spruce). Most species showed no significant 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 effects), variations in iWUE were significantly associated with changes in c_a and S_dep. Our data suggest that overall climate had the prevailing effect on changes in iWUE across the investigated sites. Whereas, detection of N_dep, S_dep and c_a signals was partially confounded by structural changes during stand development.