TRENDS IN STOMATAL DENSITY AND C-13 C-12 RATIOS OF PINUS-FLEXILIS NEEDLES DURING LAST GLACIAL-INTERGLACIAL CYCLE/

Measurements of stomatal density and deltaC-13 of limber pine (Pinus f lexilis) needles (leaves) preserved in pack rat middens from the Great Basin reveal shifts in plant physiology and leaf morphology during th e last 30,000 years. Sites were selected so as to offset glacial to Ho locene climatic differences and thus to isolate the effects of changin g atmospheric CO2 levels. Stomatal density decreased approximately 17 percent and deltaC-13 decreased approximately 1.5 per mil during degla ciation from 15,000 to 12,000 years ago, concomitant with a 30 percent increase in atmospheric CO2. Water-use efficiency increased approxima tely 15 percent during deglaciation, if temperature and humidity were held constant and the proxy values for CO2 and deltaC-13 of past atmos pheres are accurate. The deltaC-13 Variations may help constrain hypot heses about the redistribution of carbon between the atmosphere and bi osphere during the last glacial-interglacial cycle.