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.
Evidence of changing intrinsic water-use efficiency under rising atmospheric CO2 concentrations in Boreal Fennoscandia from subfossil leaves and tree ring δ13C ratios
