Long-term records of strontium isotopic composition in tree rings suggest changes in forest calcium sources in the early 20th century

• Published in: Global change biology Vol. 11; no. 11; pp. 1926 - 1940
• Main Authors: Drouet, T, Herbauts, J, Demaiffe, D
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01.11.2005; Blackwell Publishing Ltd
• Subjects: acid deposition; air pollution; Atmosphere; atmospheric deposition; Calcium; dendrochemistry; dendrochronology; Fagus sylvatica; forest ecosystem; forest trees; Forests; growth rings; Isotopes; Long term; longitudinal studies; natural Sr isotopes; nutrient availability; nutrient uptake; Quercus robur; strontium; Trees; weathering; Belgium; North America; Europe
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/j.1365-2486.2005.01034.x

Many studies made in Europe and North America have shown an increasing depletion of exchangeable base cations that may cause tree nutritional deficiencies in sensitive soils. We use radial variation of strontium isotope in tree‐rings (87Sr/86Sr ratio) to monitor possible changes in Ca sources for tree nutrition (Sr is used as an analog to Ca). The two main sources of Ca in forest stands are mineral weathering release and atmospheric inputs. Measurements in several forest stands in temperate regions show a steep decrease from pith to outer wood of the Sr isotope ratio from∼1870 to∼1920 except for stands developed on soils with a higher Ca status. This suggests a decrease of the weathering contribution (high 87Sr/86Sr ratio) when cations are displaced from the soil exchange complex by acid deposition at a rate faster than the replenishment of the cation pool by mineral weathering. This displacement enhances the atmospheric contribution, which is characterized by a low 87Sr/86Sr ratio. Tree‐ring chronologies are an exceptional historic‐timing record of chemical changes in the soil environment induced by atmospheric pollution. The reliability of the tree‐ring recorder has been verified with a well‐controlled nutritional perturbation in the context of a limed forest stand (with a known liming Sr isotopic signature). Our data suggest that forest ecosystems were affected by atmospheric inputs of strong acids earlier than previously thought.