Stomatal responses in deciduous oaks from southern Europe to the anthropogenic atmospheric CO2 increase; refining the stomatal-based CO2 proxy

Herbarium specimens of Quercus robur L. from 25 localities on the Iberian Peninsula covering the time period from 1901 to 2003 were studied to develop a suitable stomatal index (SI)-atmospheric CO2 inference model that met the specific demands of the abundant fossil material from this geographical region. Significant SI changes were observed under the atmospheric CO2 increase since the onset of industrialization, resembling a sigmoidal response. The highly comparable SI response rates of Iberian and Northwest European data sets confirmed that Q. robur has reached its response limit at 330 ppmv, and adaptation of Quercus to changing CO2 concentration is in principle the same. A 3.5% SI difference between both data sets is likely to be a result of the differences in irradiation level at the high and low latitudes. An adjusted SI-CO2 calibration data set for the Northern Iberian Peninsula is provided for the CO2 range from 295 ppmv to 330 ppmv, where the best statistical fit for the CO2 inference model is achieved when data are restricted to an altitudinal range of 0-1000 m a.s.l. Geographical extension of the model is attained by the incorporation of the month irradiance average (W/m2) as an independent factor. Additional pore length increase was observed over the same CO2 range. Usage of the Stomata-Pore Coefficient (proportion of pore length over the Stomatal length) was introduced in order to give a better description of the response of stomatal dimensions to the rising atmospheric CO2 concentrations.