Historical response of SI for European tree birches (B. pendula and B. pubescens) to global atmospheric CO₂ increase from 287 to 356 ppmv. The training set includes mean SI values for herbarium material (•) collected in The Netherlands and Denmark in the period 1843–1995 and leaf remains from living peat (⋄) in The Netherlands, formed in the period 1952–1995 (17). Historical CO₂ concentrations of 315–356 ppmv are mean spring-season values from Mauna Loa monitoring (http://cdiac.esd.ornl.gov/ndps/ndp001.html), and concentrations of 287–315 ppmv are representative values derived from shallow Antarctic ice cores (http://cdiac.esd.ornl.gov/trends/co2/siple.htm). For analytical method, see text. Regression statistics: n = 63; slope = −0.0846. Goodness-of-fit linear model: R² = 0.78, R = 0.78. Analysis of variance results: F(1–62) = 225.11 (P = 0.000). Statistical analyses were performed with SPSS 8 for Windows (Chicago). B. pendula and B. pubescens display significantly similar SI patterns and can be treated therefore as a single category in stomatal frequency analysis. Species-specific regression statistics: B. pubescens: slope = −0.0871; goodness-of-fit linear model: R² = 0.63, R = 0.60; B. pendula: slope = −0.0724; goodness-of-fit linear model: R² = 0.80, R = 0.79. Trend lines are not given in the figure.

Reconstructed CO₂ concentrations for the time interval between ≈8,700 and ≈6,800 calendar years B.P. based on CO₂ extracted from air in Antarctic ice of Taylor Dome (left curve; ref. 2; raw data available via www.ngdc.noaa.gov/paleo/taylor/taylor.html) and SI data for fossil B. pendula and B. pubescens from Lake Lille Gribsø, Denmark (right curve; see Table 1). The arrows indicate accelerator mass spectrometry ¹⁴C chronologies used for temporal control (Table 1). The shaded time interval corresponds to the 8.2-ka-B.P. cooling event (3–12). Quantification of mean CO₂ concentrations is based on the rate of historical CO₂ responsiveness of the European tree birches (Fig. 1); ±1σ CO₂ estimates are derived from the standard deviation of the SI mean values.