Changes in N and S Leaf Content, Stomatal Density and Specific Leaf Area of 14 Plant Species during the Last Three Centuries of CO2 Increase

Parallel to the increase in atmospheric CO² from 278 µmolmol^–1 in AD 1750 to the current ambient level of 348 µmolmol^–1, there have been overall decreases in leaf nitrogencontent 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 duringthe initial slower increases in CO₂ atmospheric levels as comparedwith the relatively faster CO₂ increases in recent years. Thedeclines in leaf N content and stomatal density have also beenreported in experimental studies on leaves of plants grown underenriched CO₂ environments. Meanwhile, the stomatal index andoverall carbon and sulphur leaf contents have not changed significantly.Leaf S content was higher in the 1940s samples coinciding withthe burning of increased quantities of sulphur-rich coal. Consequently,the epidermal cell density has decreased parallel to the stomataldensity and the C/N ratio of leaves has increased, implyingpossible 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 beenfound, as would be expected under CO₂ enrichment, but whichmight also be an artifact of prolonged storage. Key words: Carbon dioxide increase, leaf nitrogen content, leaf sulphur content, stomatal density, last centuries