No heat-stimulated germination found in herbaceous species from burned subtropical grassland

The inverse relationship between numbers of stomata (stomatal frequency) on tree leaves and ambient CO₂ concentration is increasingly applied for reconstructing past atmospheric CO₂ levels. The abundance of leaf remains of Quercus robur in Holocene peat and lake deposits in Europe makes this species potentially suitable for high-resolution stomatal frequency analysis. In order to quantify the CO₂ responsiveness of the species, the behavior of the stomatal index for Q. robur during the current anthropogenic CO₂ increase is determined on the basis of buried, herbarium and modern leaf material from the Netherlands. The stomatal index (SI), expressing the ratio of the number of stomata in a given area divided by the total number of stomata and other epidermal cells in that same area, is used in order to minimize influences on stomatal frequency of environmental conditions other than CO₂. The sigmoid SI response pattern recorded for Q. robur resembles that of the closely related species Q. petraea, although there is a difference in the timing of the response limitation of the two species to increasing atmospheric CO₂. For calibration purposes only the linear phase of the sigmoidal response curve is taken into consideration in the presented CO₂ response model, which allows confident combination of Q. robur and Q. petraea over the interval from 290 to 325 ppmv CO₂. The model is conservative in reconstructing past CO₂ mixing ratios outside the range of monitored response. As a result of the observed SI response limit, the model predicts CO₂ levels below 325 ppmv with a mean error of 10.2 ppmv, whereas higher CO₂ levels are underestimated.