STOMATAL DENSITY RESPONSES OF TEMPERATE WOODLAND PLANTS OVER THE PAST7 DECADES OF CO2 INCREASE - A COMPARISON OF SALISBURY (1927) WITH CONTEMPORARY DATA

We investigated the possible effect of recent (1927-1995) increases in the concentration of atmospheric CO2 on the stomatal densities of lea ves of a wide range of tree, shrub, and herb species (N = 60) by makin g new measurements for comparison with corresponding data reported by E. J. Salisbury in 1917-a time when ice core studies indicate CO2 conc entrations similar to 55 mu L/L lower than present. A detailed intrasp ecific study of the herb Mercurialis perenius showed plants of M. pere nnis in a Cambridgeshire woodland in 1994 had significantly lower stom atal densities, irrespective of leaf insertion point, compared with th eir 1927 counterparts. Comparisons made across species using evolution ary comparative methods (independent contrasts revealed a significant (P < 0.01) decrease in stomatal density over the past 70 yr. The resul ts of both the inter-and intraspecific comparisons are consistent with the hypothesis that historical CO2 increases have influenced leaf mor phology in a manner consistent with recent experiments and the palaeoe cological record. Further analyses suggested that the strength of the stomatal density response was independent of life form but dependent o n ''exposure'' and the initial leaf stomatal density. Consequently fir mer predictions for future changes in stomatal density across all spec ies, expected as a possible result of authropogenically related CO2 in creases, may now be possible.