Stomatal density and stomatal index as indicators of paleoatmospheric CO(2) concentration.

A growing number of studies use the plant species-specific inverse relationship between atmospheric CO(2) concentration and stomatal density (SD) or stomatal index (SI) as a proxy for paleo-CO(2) levels. A total of 285 previously published SD and 145 SI responses to variable CO(2) concentrations from a pool of 176 C(3) plant species are analyzed here to test the reliability of this method. The percentage of responses inversely responding to CO(2) rises from 40 and 36% (for SD and SI, respectively) in experimental studies to 88 and 94% (for SD and SI, respectively) in fossil studies. The inconsistent experimental responses verify previous concerns involving this method, however the high percentage of fossil responses showing an inverse relationship clearly validates the method when applied over time scales of similar length. Furthermore, for all groups of observations, a positive relationship between CO(2) and SD/SI is found in only </=12% of cases. Thus, CO(2) appears to inversely affect stomatal initiation, although the mechanism may involve genetic adaptation and therefore is often not clearly expressed under short CO(2) exposure times.Experimental responses of SD and SI based on open-top chambers (OTCs) inversely relate to CO(2) less often than greenhouse-based responses (P<0.01 for both SD and SI), and should be avoided when experimental responses are required for CO(2) reconstructions. In the combined data set, hypostomatous species follow the inverse relationship more often than amphistomatous species (56 vs. 44% for SD; 69 vs. 32% for SI; P<0.03 for both comparisons). Both the SD and SI of fossil responses are equally likely to inversely relate to CO(2) when exposed to elevated versus subambient CO(2) concentrations (relative to today). This result casts doubt on previous claims that stomata cannot respond to CO(2) concentrations above present-day levels. Although the proportion of SD and SI responses inversely relating to CO(2) are similar, SD is more strongly affected by various environmental stresses, and thus SI-based CO(2) reconstructions are probably more accurate.