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Stomatal frequency adjustment of four conifer species to historical changes in atmospheric CO₂¹

²Laboratory of Palaeobotany and Palynology, Utrecht University, 3584 CD Utrecht, Netherlands; ³Department of Geology, Field Museum of Natural History, 1400 S. Lake Shore Drive, Chicago, Illinois 60605 USA; ⁴Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN UK; ⁵Department of Geography, University of Leicester, Leicester LE1 7RH UK

The species-specific inverse relation between atmospheric CO₂ concentration and stomatal frequency for many woody angiosperm species is being used increasingly with fossil leaves to reconstruct past atmospheric CO₂ levels. To extend our limited knowledge of the responsiveness of conifer needles to CO₂ 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 CO₂ 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 CO₂. In contrast, a new measure of stomatal frequency, based on the number of stomata per millimeter of needle length, decreased significantly with increasing CO₂. For Tsuga heterophylla, the stomatal frequency response to CO₂ changes in the last century is validated through assessment of the influence of other biological and environmental variables. Because of their sensitive response to CO₂, 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 CO₂ fluctuations.

Key Words: CO₂ • conifers • leaf morphology • North America • quantification methods • stomata • stomatal density

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