Trends in stomatal density and ¹³C/¹²C ratios of Pinus flexilis needles during last glacial-interglacial cycle

Measurements of stomatal density and δ¹³C of limber pine (Pinus flexilis) needles (leaves) preserved in pack rat middens from the Great Basin reveal shifts in plant physiology and leaf morphology during the last 30,000 years. Sites were selected so as to offset glacial to Holocene climatic differences and thus to isolate the effects of changing atmospheric CO₂ levels. Stomatal density decreased ∼17 percent and δ¹³C decreased ∼1.5 per mil during deglaciation from 15,000 to 12,000 years ago, concomitant with a 30 percent increase in atmospheric CO₂. Water-use efficiency increased ∼15 percent during deglaciation, if temperature and humidity were held constant and the proxy values for CO₂ and δ¹³C of past atmospheres are accurate. The δ¹³C variations may help constrain hypotheses about the redistribution of carbon between the atmosphere and biosphere during the last glacial-interglacial cycle.