Abstract
Rising atmospheric CO2 (ca) has been shown to increase forest carbon uptake. Yet, whether the ca-fertilization effect on forests is modulated by changes in sulphur (Sdep) and nitrogen (Ndep) deposition and how Ndep 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 ca and Sdep. Our data suggest that overall climate had the prevailing effect 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.
Highlights
IntroductionWhether the cafertilization effect on forests is modulated by changes in sulphur (Sdep) and nitrogen (Ndep) deposition and how Ndep affects ecosystem N availability remains unclear
Rising atmospheric CO2 (ca) has been shown to increase forest carbon uptake
In the case of PCA_s, more than 90% of the variance was explained with two principal components (PC_s)
Summary
Whether the cafertilization effect on forests is modulated by changes in sulphur (Sdep) and nitrogen (Ndep) deposition and how Ndep 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. Note that sites were grouped by tree species to better identify the pairing of sites according to similar age and soil type, but the contrasting levels of Ndep (low vs high nitrogen deposition) are shown in Fig. 1 and Supplementary Table S1. Increased Ndep has a fertilization effect on A, thereby contributing to enhancing a c a or climate driven increase in tree iWUE12,13 and forest C-sinks[14,15] in N-limited forests
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