Primary productivity and forage quality of Ginkgo biloba in response to elevated carbon dioxide and oxygen: An experimental approach to mid-Mesozoic paleoecology

Atmospheric composition was unique during the Late Jurassic and Early Cretaceous Periods (∼180-90 mya) due to concurrent elevations of CO2 and O2. Experimental methodology, an under-utilized technique in paleoecology, is used to address the physiological responses of Ginkgo biloba seedlings to these conditions. Experimental results have implications to Mid-Mesozoic paleoecology. Plants were exposed to these atmospheric treatments in hyperbaric chambers: (1) Control: 1 atm pressure, 370 ppm CO2, and 20.9% O2; (2) CO2: 1.25 atm pressure, 2000 ppm CO2, and 20.9% O2; and (3) CO 2&O2: 1.25 atm pressure, 2000 ppm CO2, and 30% O2.;Gas exchange parameters were measured after 24-Hour and 35-Day exposure to evaluate photosynthetic rate and primary productivity. G. biloba photosynthesis was stimulated by CO2, but experienced photosynthetic down-regulation after 35 days. Elevated O2 did not decrease photosynthetic rate.;The concentrations of protein, lignin, sugar, and starch, and the C:N ratio and non-structural to structural carbohydrate ratio of experimental G. biloba leaves were measured to assess foliage quality. Nutritive content was reduced while digestibility was increased in response to elevated CO2 and O2.;Observed changes in G. biloba suggest mid-Mesozoic primary productivity could have been increased 200-300% over control levels; plant growth rate and fecundity may have increased. Changes in foliar quality could have stimulated herbivore and detritivore biomass and affected foraging strategies during the mid-Mesozoic.;Stomatal density and index values of experimental leaves were compared with gas exchange data and stomatal values from mid-Mesozoic Ginkgo fossils to determine if stomatal frequency is a proxy for photosynthesis in fossil and shed leaves. Stomatal frequency correlated with conductance but not photosynthesis. Experimental and fossil values did not compare favorably. Stomatal density and index are inappropriate proxies for photosynthetic rate.;This study used experimental techniques to examine G. biloba physiological responses to atmospheric conditions that existed during the distant past. The use of experimental methods for paleoecological investigation is a powerful technique.