Stomatal density responds to the glacial cycle of environmental change

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Abstract
Examining the response of stomatal density to past changes in atmospheric CO$_{2}$ concentration helps us to understand how plants adapted to palaeoenvironmental change, and so helps in predicting their response to future global environmental change. Stomatal density is an important physiological parameter that underpins the productivity of terrestrial vegetation by affecting both rate of carbon uptake and water use efficiency. Previous work on temperate tree species showed a decrease in stomatal density in response to a 60 p.p.m.v. increase in atmospheric CO$_{2}$ concentration over the past 200 years. However, such a short timespan largely excludes the genetic component of plant adaptation to change in CO$_{2}$ concentration. We present the first record of stomatal density from fossil leaves extending over 140 ka. Our record for the arctic-alpine dwarf shrub Salix herbacea L. extends back to the penultimate glacial stage, spanning two intervals when atmospheric CO$_{2}$ concentration was considerably lower (by ca. 170 p.p.m.v.) than at present. Our results demonstrate a decrease in stomatal density in response to long-term increases in atmospheric CO$_{2}$ concentration, and are in accordance with previous short-term observations and experiments. This implies that relaxation of the stress imposed by low atmospheric CO$_{2}$ concentration has enabled terrestrial plants to exhibit an adaptive response to the other limiting factor of water availability by reducing stomatal density and hence improving water use efficiency.