Abstract

Parallel to the increase in atmospheric CO2 from 278 μmol mol−1 in AD 1750 to the current ambient level of 348 μmol mol−1, there have been overall decreases in leaf nitrogen content and stomatal density from 144% and 121%, respectively, in AD 1750 to 100% today of herbarium specimens of 14 trees, shrubs, and herbs collected over the last 240 years in Catalonia, a Mediterranean climate area. These decreases were steeper during the initial slower increases in CO2 atmospheric levels as compared with the relatively faster CO2 increases in recent years. The declines in leaf N content and stomatal density have also been reported in experimental studies on leaves of plants grown under enriched CO2 environments. Meanwhile, the stomatal index and overall carbon and sulphur leaf contents have not changed significantly. Leaf S content was higher in the 1940s samples coinciding with the burning of increased quantities of sulphur-rich coal. Consequently, the epidermal cell density has decreased parallel to the stomatal density and the C/N ratio of leaves has increased, implying possible important consequences on herbivores, decomposers, and ecosystems. An overall decrease in the specific leaf area (SLA) from 184% in the 18th century to 100% today has also been found, as would be expected under CO2 enrichment, but which might also be an artifact of prolonged storage.

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