Lenny Kouwenberg

Stomatal frequency adjustment of four conifer species to historical changes in atmospheric CO2

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.

Atmospheric CO2 fluctuations during the last millennium reconstructed by stomatal frequency analysis of Tsuga heterophylla needles

A stomatal frequency record based on buried Tsuga hetero- phylla needles reveals significant centennial-scale atmospheric CO2 fluctuations during the last millennium. The record includes four CO2 minima of 260-275 ppmv (ca. A.D. 860 and A.D. 1150, and less prominently, ca. A.D. 1600 and 1800). Alternating CO2 max- ima of 300-320 ppmv are present at A.D. 1000, A.D. 1300, and ca. A.D. 1700. These CO2 fluctuations parallel global terrestrial air temperature changes, as well as oceanic surface temperature fluc- tuations in the North Atlantic. The results obtained in this study corroborate the notion of a continuous coupling of the preindus- trial atmospheric CO2 regime and climate.