Elevated CO2, increased leaf-level productivity, and water-use efficiency during the early Miocene

• Published in: Climate of the past Vol. 16; no. 4; pp. 1509 - 1521
• Main Authors: Reichgelt, Tammo, D'Andrea, William J, Ailín del C Valdivia-McCarthy, Fox, Bethany R S, Bannister, Jennifer M, Conran, John G, Lee, William G, Lee, Daphne E
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 20.08.2020; Copernicus Publications
• Subjects: Analogs; Atmospheric models; Biological fertilization; Biosphere; Carbon capture and storage; Carbon dioxide; Carbon dioxide atmospheric concentrations; Carbon sequestration; Climate; Efficiency; Fertilization; Flora; Forest biomass; Forests; Fossils; Gas exchange; Global temperatures; Greening; Growing season; Leaves; Miocene; Plants; Productivity; Southern Hemisphere; Terrestrial environments; Trees; Tropical climate; Tropical forests; Water use; New Zealand
• ISSN: 1814-9324; 1814-9332
• DOI: 10.5194/cp-16-1509-2020

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. Here, we use leaf gas-exchange modeling on a well-preserved fossil flora from early Miocene New Zealand, as well as two previously published tropical floras from the same time period, to reconstruct atmospheric CO2, leaf-level productivity, and intrinsic water-use efficiency. 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 was similar to projected values for 2040 CE and is consistent with an Earth system sensitivity of 3–7 ∘C to a doubling of CO2. The Southern Hemisphere temperate leaves had higher reconstructed productivity than modern analogs, likely due to a longer growing season. This higher productivity was presumably 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 provide the first empirical evidence of concomitant enhanced intrinsic water-use efficiency, indicating a forest fertilization effect.