Elevated CO<sub>2</sub>, increased leaf-level productivity and water-use efficiency during the early Miocene

Abstract

Abstract. Rising atmospheric CO2 is expected to increase global temperatures, plant water-use efficiency, and carbon storage in the terrestrial biosphere. A CO2 fertilization effect on terrestrial vegetation is predicted to cause global greening as the potential ecospace for forests expands. However, leaf-level fertilization effects, such as increased productivity and water-use efficiency, have not been documented from fossil leaves in periods of heightened atmospheric CO2. Leaf gas-exchange rates reconstructed from early Miocene fossils which grew at southern temperate and tropical latitudes, when global average temperatures were 5–6 °C higher than today reveal that atmospheric CO2 was ~ 450–550 ppm. Early Miocene CO2 is similar to projected values for 2040AD, and consistent with Earth System Sensitivity of 3–7 °C to a doubling of CO2. While early Miocene leaves had photosynthetic rates similar to modern plants, southern temperate leaves were more productive than modern due to a longer growing season. This higher productivity was likely mirrored at northern temperate latitudes as well, where a greater availability of landmass would have led to increased carbon storage in forest biomass relative to today. Intrinsic water-use efficiency of both temperate and tropical forest trees was high, toward the upper limit of the range for modern trees, which likely expanded the habitable range in regions that could not support forests with high moisture demands under lower atmospheric CO2. Overall, early Miocene elevated atmospheric CO2 sustained globally higher temperatures and our results reveal the first empirical evidence of concomitant enhanced intrinsic water-use efficiency, indicating a forest fertilization effect.

Differences in carbon isotope discrimination between angiosperm and gymnosperm woody plants, and their geological significance

“…Two models of carbon isotope discrimination are currently used to estimate paleo-[CO 2 ] from fossil C 3 plant matter (Schubert and Jahren, 2012;Franks et al, 2014), and both models have been extensively applied to Cenozoic records (Cui and Schubert, 2018;Cui et al, 2020;Reichgelt et al, 2020;Royer et al, 2019). Both formulations are related to the FvCB model (Schubert and Jahren, 2018;Hollis et al, 2019) but di er according to their parameterisation and assumptions.…”

Long-term adjustment of the South America Atlantic Forest to atmospheric carbon dioxide concentration

“…In this regard, paleoclimate data may provide relevant insight for investigating past vegetation changes under naturally varying atmospheric CO 2 concentrations at different time scales. Leaf gas-exchange modeling on temperate and tropical fossil ora from the early Miocene suggests enhanced leaf-level productivity and water-use e ciency under elevated CO 2 (~ 450-550 ppm), which likely contributed to forest survival in climates where currently tropical savannas and grasslands dominate 15 . In the Quaternary, the triple isotopic ratio of atmospheric oxygen recovered from Antarctica ice cores indicates that marine and terrestrial biosphere productivity changes coeval to glacial-interglacial variability, with the latter largely driven by atmospheric CO 2 16 .…”