Trends in plant carbon concentration and plant demand for N throughout this century.

Atmospheric CO concentration has increased by 25% over the preindustrial level. A parallel increase in C concentration and decreases in N concentration and δC of plants grown throughout this century have been observed in plant specimens stored in herbaria. We tested our previous results in a study of 12 more species collected in the western Mediterranean throughout this century (1920-1930, 1945-1955, and 1985-1990) and tree rings of Quercus pubescens from the same area. These changes were accompanied by apparent increases in condensed tannin concentration. A decreasing trend in δN both in herbarium material and tree rings was also found, indicating that ecosystems might cope with higher plant N demand by decreasing N losses and increasing N fixation and mineralization. These results may contribute to a better understanding of the effects of global change on carbon and nitrogen cycling.