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

• Main Authors: T. Reichgelt, W. J. D'Andrea, A. C. Valdivia-McCarthy, B. R. S. Fox, J. M. Bannister, J. G. Conran, W. G. Lee, D. E. Lee
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-08-01
• Series: Climate of the Past
• Online Access: https://cp.copernicus.org/articles/16/1509/2020/cp-16-1509-2020.pdf

<p>Rising atmospheric <span class="inline-formula">CO₂</span> is expected to increase global temperatures, plant water-use efficiency, and carbon storage in the terrestrial biosphere. A <span class="inline-formula">CO₂</span> 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 <span class="inline-formula">CO₂</span>. 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 <span class="inline-formula">CO₂</span>, 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&thinsp;<span class="inline-formula">^∘</span>C higher than today, reveal that atmospheric <span class="inline-formula">CO₂</span> was <span class="inline-formula">∼450</span>–550&thinsp;ppm. Early Miocene <span class="inline-formula">CO₂</span> was similar to projected values for 2040&thinsp;CE and is consistent with an Earth system sensitivity of 3–7&thinsp;<span class="inline-formula">^∘</span>C to a doubling of <span class="inline-formula">CO₂</span>. 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 <span class="inline-formula">CO₂</span>. Overall, early Miocene elevated atmospheric <span class="inline-formula">CO₂</span> sustained globally higher temperatures, and our results provide the first empirical evidence of concomitant enhanced intrinsic water-use efficiency, indicating a forest fertilization effect.</p>