CHANGES IN LAND PLANT FUNCTION OVER THE PHANEROZOIC - RECONSTRUCTIONSBASED ON THE FOSSIL RECORD

Major fluctuations in the concentrations of atmospheric CO2 and O-2 ar e predicted by historical long-term carbon and oxygen cycle models of atmospheric evolution and will have impacted directly on past climates , plant function and evolutionary processes. Here, palaeobotanical evi dence is presented from the stomatal density record of fossil leaves s panning the past 400 Myr supporting the predicted changes in atsmopher ic CO2. Evidence from experiments on plants exposed to long-term high CO2 environments and the newly assembled fossil data indicate the pote ntial for genetic modification of stomatal characters. The influence o f tile changes in fossil stomatal characteristics and atmospheric comp osition on the rates of leaf gas exchange over the course of land plan t evolution has been investigated through modelling. Three contrasting cras of plant water economies emerge in the Devonian (high), Carbonif erous (low) and from the Upper Jurassic to the present-day (high but d eclining). These patterns of change result from structural changes of the leaves and the impact of atmospheric CO2 and O-2 concentrations on RuBisCO function and are consistent with the fossil evidence of seque ntial appearances of novel plant anatomical changes. The modelling app roach is tested by comparing predicted leaf stable carbon isotope rati os with those measured on fossil plant and organic material. Viewed in a geological contest, current and future increases in the concentrati on of atmospheric CO2 might be considered as restoring-plant function to that more typically experienced by plants over the majority of thei r evolutionary history. (C) 1997 The Linnean Society of London.