Changes in N and S Leaf Content, Stomatal Density and Specific Leaf Area of 14 Plant Species during the Last Three Centuries of CO2Increase

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.