Ecophysiological responses of plants to global environmental change since the Last Glacial Maximum

Ecophysiological information on the responses of plants to past global environmental changes may be obtained from Quaternary fossil leaves by measurements of (i) stomatal density, (ii) stomatal dimensions and (iii) ¹³ C discrimination (Δ¹³ C). The stomatal density and stomatal dimensions of leaves can be used to calculate stomatal conductance, while leaf Δ¹³ C values provide independent information on stomatal conductance and plant water use efficiency. In this paper, stomatal conductance is calculated for a sequence of radiocarbon dated fossil leaves of Salix herbacea L. which, together with herbarium and fresh material, represents a time-series spanning from the Last Glacial Maximum (LGM) (16500 yr BP) to the present day. The calculated values were then tested against leaf Δ¹³ C values previously reported for the same material. Our calculations show that stomatal conductance is negatively correlated with increases in atmospheric CO₂ concentration over the last 16500 yr. This represents the first evidence of long-term response of stomatal conductance to increases in atmospheric CO₂ concentration and confirms the response observed in experimental systems exposing plants to lower-than-present CO₂ concentrations in controlled environments. The calculated decrease in conductance was positively correlated with leaf Δ¹³ C values, supporting this interpretation. The mean leaf Δ¹³ C value for the 18th and 19th centuries was significantly (P≥ 0.05) lower than the mean for the interval LGM-Holocene (10000 yr BP) implying an increase in plant water-use-efficiency over this time. These two lines of evidence, together with the stomatal density record from a glacial cycle, and experimental studies growing C₃ plants in glacial-to-present CO₂ concentrations, strongly imply that the water use efficiency of vegetation during the LGM was lower than at present and that it has increased since that time. Further evidence in support of this conclusion comes from the pattern of world vegetation types present during the LGM previously reconstructed using palaeoecological data. This evidence demonstrates that the distribution of vegetation types during the LGM was significantly different from that of the present day and showed a contraction in the area of rain forest and a major expansion of desert areas.