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. 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.

Plant Proxy Evidence for High Rainfall and Productivity in the Eocene of Australia

“…Increased availability of CO 2 effectively reduces the drought threshold for plants, as plant reduce their stomatal conductance and thus transpire less water in response to increased CO 2 (e.g., Cernusak, 2020;Cernusak et al, 2019). Such an increase in water-use efficiency under higher than modern CO 2 has also been observed in organically preserved early Miocene leaf fossils (Reichgelt et al, 2020) and would result in biosphere expansion into areas that would otherwise be too dry (Herold et al, 2011;Zhou et al, 2017). Finally, higher than modern NPP may be attributable to a direct carbon fertilization effect, which is higher carbon assimilation due to increased photosynthesis as a result of the higher availability of atmospheric CO 2 (e.g., Z.…”

Puzzle cell shape emerges from the interaction of growth with mechanical constraints

“…(2018), and data from 45 leaves of 13 early Miocene species ( Suppl. Table 1 ) (Reichgelt et al ., 2020). The plotted relationship between min axis and lobeyness showed a moderate or higher positive correlation (≥ 0.3) in the majority of species (235 out of 327, 72%) (Figure 2G).…”

“…The Miocene plant fossils analyzed in this study were obtained from datasets published by Reichgelt et al (2020). These fossils were preserved in turbidite deposits within the Foulden Maar diatomite core.…”

The evolution of extant South American tropical biomes

“…early Eocene, middle Miocene) have always been associated with increases in CO 2 without a decrease in precipitation or increase in the length of the dry season (Ufnar, 2002; Ufnar et al ., 2004, 2008; Wing et al ., 2009; Jaramillo et al ., 2010b; Royer, 2010). Physiological and paleontological studies indicate that plants at higher temperatures (up to 10°C) both increase WUE and are more efficient at photosynthesis provided that levels of both CO 2 and soil moisture are high (Berry & Björkman, 1980; Aber et al ., 2001; Lloyd & Farquhar, 2008; Niu et al ., 2008; Cernusak et al ., 2011; Reichgelt et al ., 2020). Genes regulating photosynthesis have deep evolutionary roots, suggesting that physiological function is likely to be similar in Eocene and modern plants.…”