Evolutionary responses of stomatal density to global CO2 change

Stomatal density is known to respond to CO₂ levels during leaf development. Current interest in the increasing concentration of atmospheric CO₂ has stimulated much experimentation on the responses of plants to relatively short-term exposure in artificially high CO₂ levels. Attempts to extrapolate from short-term to long-term responses raise fundamental questions concerning evolutionary change in response to rising global CO₂ levels. We consider the improved water use efficiency observed under elevated CO₂ levels to be the main driving force of natural selection affecting the genotypic component controlling stomatal density. Whether a response is merely phenotypic or becomes incorporated into the genotype depends on two factors: (i) the time scale of exposure and (ii) the generation time of a species. Measurements of stomatal density on fossil leaves of Salix herbacea through a glacial cycle covering the last 140000 years have shown a decrease in stomatal density in response to the rising CO₂ levels of this period. This accords with the shorter-term observations on leaves of trees seen in herbarium specimens where the stomatal density has decreased in response to the rising CO₂ levels of the last 200 years. The results indicate that natural selection over the 140000-year period may have favoured a similar response to that shown by trees phenotypically over the last 200 years. Since there is now some evidence for the genetic control of stomatal density, the role of natural selection affecting it must be considered when translating responses from short-term experiments to predict how stomatal density will be affected by long-term climatic and atmospheric change.