Long-Term Trends in Tree-Ring Width and Isotope Signatures (δ 13 C, δ 15 N) of Fagus sylvatica L. on Soils with Contrasting Water Supply

We investigated long-term responses (since 1850) of Fagus sylvatica (Luxembourg; central Europe) to shifts in temperature, precipitation, and nitrogen deposition by analyzing diameter at breast height (DBH) increment, basal area increment (BAI), and tree-ring stable isotopes (δ 13 C, δ 15 N). We compared stands on soils with contrasting water supply (Regosols and Cambisols with an available water capacity of ca. 40 and 170 mm, respectively) and of two different age classes (ca. 60 vs. 200 years). All stands showed a peak in DBH increment in the decade 1978–1987, but a decline in increment growth in subsequent decades. In addition, BAI declined in mature stands in the last two decades. Decreasing increment rates were attributable to an increasing drought limitation of stands, mainly induced by increasing temperatures in the last two decades. Contrary to our expectations, stands on Cambisols showed a similar susceptibility to shifts in temperature and precipitation as stands on Regosols, suggesting a strong adaptation of the respective ecotypes grown at dryer sites. This result was in line with long-term trends for tree-ring δ 13 C signatures, which did not differ significantly between stands on Cambisols and Regosols. Climate impacts on tree-ring δ 15 N signatures were low. High spring precipitation and temperatures caused increasing and decreasing δ 15 N values, respectively, but only in mature stands on Cambisols. Stands on Regosols tended to have lower δ 15 N values than stands on Cambisols. Decreasing δ 15 N values in recent decades suggest an increasing impact of allochthonous N loads with isotopically lighter N.