Changes in graminoid stomatal morphology over the last glacial–interglacial transition: evidence from Mount Kenya, East Africa

Stomatal size and density were measured from graminoid cuticular fragments extracted from dated sediments in two tropical-montane crater lakes on Mount Kenya. The sediments had been dated in other studies and spanned 1500–37 000 calibrated years BP. Changes in the mean size and density of the graminoid stomata were found. Using a coarse signal analysis the two lakes gave fairly similar results, although there was some divergence at the start and end of the time period analyzed. There is some correspondence between the atmospheric CO2 concentration and graminoid stomatal density during the transition from the LGM to the start of the Holocene, where stomatal density decreased while CO2 concentrations increased. All the changes observed may have been plastic responses within existing species at the site or competitive replacements of grass floras. We argue that higher stomatal density may have been a response to falling CO2 levels during the last glaciation, accompanying the replacement of a C3 flora by C4 species. The stomatal size changes exhibited over this time period may have adapted plants to changes in soil water availability. That stomatal morphology changes in a sample flora (not a single taxon) over millennia is a novel finding, and one that may have implications for paleoecological interpretation and the prediction of grass behavior in the future.