Steven W. Leavitt J.L. Betancourt Peter K. Van De Water 1994 <p><span>Measurements of stomatal density and δ</span>¹³<span>C of limber pine (</span><i>Pinus flexilis</i><span>) 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 CO</span>₂<span>&nbsp;levels. Stomatal density decreased ∼17 percent and δ</span>¹³<span>C decreased ∼1.5 per mil during deglaciation from 15,000 to 12,000 years ago, concomitant with a 30 percent increase in atmospheric CO</span>₂<span>. Water-use efficiency increased ∼15 percent during deglaciation, if temperature and humidity were held constant and the proxy values for CO</span>₂<span>&nbsp;and δ</span>¹³<span>C of past atmospheres are accurate. The δ</span>¹³<span>C variations may help constrain hypotheses about the redistribution of carbon between the atmosphere and biosphere during the last glacial-interglacial cycle.</span></p> application/pdf 10.1126/science.264.5156.239 en American Association for the Advancement of Science Trends in stomatal density and 13C/12C ratios of Pinus flexilis needles during last glacial-interglacial cycle article