Variations in the Stomatal Density of Salix herbacea L. under the Changing Atmospheric CO2 Concentrations of Late- and Post-Glacial Time

The rapidly rising CO₂ concentration of the past 200 years has been shown to be accompanied by a fall in stomatal density in the leaves of temperate trees. The present study attempts to investigate the relationship of atmospheric CO₂ change and stomatal density in the arctic-alpine shrub, Salix herbacea, over the longer time span of 11 500 years offered by fossil leaves from post-glacial deposits. Comparisons of fossil material from Scotland and Norway are made with leaves from living populations growing in Austria, Greenland and Scotland. The Austrian material, from an altitudinal gradient between 2000 and 2670 m above sea level, gives added comparison of contemporary differences of CO₂ partial pressure with altitude. The results of our investigation indicate, rather surprisingly, that the rising CO₂ concentration of the past 11 500 years has been accompanied by an increase in the stomatal density of S. herbacea in contrast to the shorter-term observations on the herbarium material of temperate trees. The most likely explanation appears to centre on the temperatures and water availability of the early post-glacial environment overriding the effect of the lower CO₂ regime. However, the scale of the time interval involved may also be significant. Natural selection over the 11 500 year period concerned may have favoured a different response to what is, in effect, an acclimatory response observed in trees within the period of rapid CO₂ rise of the past 200 years.