The influence of recent historical changes in atmospheric CO 2 have been investigated by two methods: 1, the responses of leaf development and physiology as indicated by leaves stored in herbaria and 2, by investigating the differential growth responses of populations originating from naturally different CO2 concentrations. Herbarium leaves indicate that stomatal density and leaf nitrogen have decreased over the last 150 to 200 years, while water use efficiency, estimated from leaf (~13C and historical measurements of climate, has increased.
Natural populations of Boehmeria cylindrica were found growing at sites, in Florida, with CO2 mole fractions varying naturally from 350 #mol mol -1 to 505 #mol mol -1. Plants were grown in the controlled environment, using seeds originating from populations occurring in the different CO2 mole fractions. Plants from the different ambient CO2 mole fractions showed different rates of growth and different non-linear responses of the shoot to root ratio in response to changes in the CO2 mole fraction from 350 to 675 #mol mol-1
The proposal that plants originating from high altitude will show greater stimulations of growth with an increase in CO2, than plants from low altitude, was rejected in experiments which simulated the atmospheric pressure at altitudes of 0 and 2000m, at CO2 mole fractions of 350 and 700/~mol mol-1 and on populations of Plantago major originating from altitudes of 0 and 3335 m.