Stable isotope inferred intrinsic water use efficiency and its relation to N sources in temperate tree regeneration with increasing levels of N deposition, precipitation, and temperature

Under ongoing climate change, the mechanisms controlling the vigor and growth performance of tree regeneration are still less understood than for mature trees. Using stable isotope signatures (δ 13 C, δ 15 N), we studied intrinsic water use efficiency (WUE i , with δ 13 C as a proxy) and N relations and their interaction in differently drought-tolerant temperate tree species. We conducted a Germany-wide field study representing independent precipitation, temperature, and N deposition levels, comparing European beech ( Fagus sylvatica ), sessile oak ( Quercus petraea ), silver fir ( Abies alba ), and Douglas fir ( Pseudotsuga menziesii ) in the regeneration stage. At high N deposition WUE i was decreased in all tree species and in beech in particular, as δ 13 C signatures became more negative. This suggests that high N loads give rise to a differentiated discussion of the drought tolerance of tree species depending on the level of N deposition. In the conifers direct uptake of N from atmospheric deposition was important, as indicated by increasing foliar δ 15 N with increasing N concentration. In the broadleaved trees with better decomposable leaf litter, the main effect of N deposition was indicated through low δ 15 N signatures suggesting an intensification of uptake from N mineralization. Foliar δ 15 N signatures, and hence presumed changes in mineralization, were affected by soil chemistry, mean annual precipitation and temperature, but may also be influenced by deposition or other soil properties, which must be acknowledged when considering our results. To complement our results, comparable studies should be conducted for mature forest stands, including ecophysiological measurements of leaf gas exchange or tree water relations. • Intrinsic water use efficiency decreased under high N loads in young temperate trees • In European beech, N deposition increased N uptake from mineralization • In conifers, foliar δ 15 N indicated increased direct N uptake from deposition