Recently, a novel paleoaltimetry method was presented using leaf stomatal frequency response to the decline in CO2 partial pressure with altitude, and tested on California black oak (Quercus kelloggii) (McElwain 2004). Here, we present new data detailing the influence 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 insufficient to explain the overall increase in stomatal frequency (density and index) with elevation. Furthermore, temperature does not have a significant influence on black oak stomatal frequency in growth chamber experiments. Rather changes in stomatal density and index with altitude appear to reflect an adaptation to counteract the limited photosynthetic potential due to the CO2 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 CO2 concentration can be well-constrained, the stomatal frequency method has the potential for very low error margins.

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