Recently, a novel paleoaltimetry method was presented using leaf stomatal frequency response to the decline in CO 2 partial pressure with altitude, and tested on California black oak (Quercus kelloggii) (McElwain 2004). Here, we present new data detailing the infl uence of other climatic variables on leaf stomatal frequency change with altitude in the context of more fully characterizing how stomatal frequencies can be used to infer paleoelevations. A clear increase in stomatal density and stomatal index is observed with increasing elevation for Q. kelloggii (black oak) leaves, and Nothofagus solandri var. cliffortioides (mountain beech) growing over an altitudinal transect on the slope of Mt. Ruapehu (New Zealand). Modern leaves growing in full direct sunlight versus shaded diffuse light for both species show substantial differences in stomatal density and index, however, growth chamber experiments that vary light intensity have revealed that the magnitude of natural increase in radiation with altitude is likely insuffi cient to explain the overall increase in stomatal frequency (density and index) with elevation. Furthermore, temperature does not have a signifi cant infl uence on black oak stomatal frequency in growth chamber experiments. Rather changes in stomatal density and index with altitude appear to refl ect an adaptation to counteract the limited photosynthetic potential due to the CO 2 partial pressure decrease, further limited by shorter growing seasons and/or increased UV radiation. Our review of the uncertainties associated with the stomatal frequency paleoaltimeter from the literature, together with results from the new plant growth experiments indicate that if sea-level paleoatmospheric CO 2 concentration can be well-constrained, the stomatal frequency method has the potential for very low error margins.

The species-specific inverse relation between atmospheric CO(2) concentration and stomatal frequency for many woody angiosperm species is being used increasingly with fossil leaves to reconstruct past atmospheric CO(2) levels. To extend our limited knowledge of the responsiveness of conifer needles to CO(2) fluctuations, the stomatal frequency response of four native North American conifer species (Tsuga heterophylla, Picea glauca, Picea mariana, and Larix laricina) to a range of historical CO(2) mixing ratios (290 to 370 ppmV) was analyzed. Because of the specific mode of leaf development and the subsequent stomatal patterning in conifer needles, the stomatal index of these species was not affected by CO(2). In contrast, a new measure of stomatal frequency, based on the number of stomata per millimeter of needle length, decreased significantly with increasing CO(2). For Tsuga heterophylla, the stomatal frequency response to CO(2) changes in the last century is validated through assessment of the influence of other biological and environmental variables. Because of their sensitive response to CO(2), combined with a high preservation capacity, fossil needles of Tsuga heterophylla, Picea glauca, P. mariana, and Larix laricina have great potential for detecting and quantifying past atmospheric CO(2) fluctuations.

The majority of the stomatal frequency-based estimates of CO 2 for the Holocene do not support the widely accepted concept of comparably stable CO 2 concentrations throughout the past 11,500 years. To address the critique that these stomatal frequency variations result from local environmental change or methodological insufficiencies, multiple stomatal frequency records were compared for three climatic key periods during the Holocene, namely the Preboreal oscillation, the 8.2 kyr cooling event and the Little Ice Age. The highly comparable fluctuations in the palaeo-atmospheric CO 2 records, which were obtained from different continents and plant species (deciduous angiosperms as well as conifers) using varying calibration approaches, provide strong evidence for the integrity of leaf-based CO 2 quantification.

A stomatal frequency record based on buried Tsuga heterophylla needles reveals significant centennial-scale atmospheric CO 2 fluctuations during the last millennium. The record includes four CO 2 minima of 260-275 ppmv (ca. A.D. 860 and A.D. 1150, and less prominently, ca. A.D. 1600 and 1800). Alternating CO 2 maxima of 300-320 ppmv are present at A.D. 1000, A.D. 1300, and ca. A.D. 1700. These CO 2 fluctuations parallel global terrestrial air temperature changes, as well as oceanic surface temperature fluctuations in the North Atlantic. The results obtained in this study corroborate the notion of a continuous coupling of the preindustrial atmospheric CO 2 regime and climate.

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