Data from: Overestimated gains in water-use efficiency by global forests

Increases in terrestrial water use efficiency (WUE) have been reported in
many studies, pointing to potential changes in physiological forcing of
global carbon and hydrological cycles. However, gains in WUE are of
uncertain magnitude over longer (i.e. >10 years) periods of time
largely owing to difficulties in accounting for structural and
physiological acclimation. 13C signatures (i.e., δ13C) of plant organic
matter have long been used to estimate WUE at temporal scales ranging from
days to centuries. Mesophyll conductance is a key uncertainty in estimated
WUE owing to its influence on diffusion of CO2 to sites of carboxylation.
Here we apply new knowledge of mesophyll conductance to 464 δ13C
chronologies in tree-rings of 143 species spanning global biomes. Adjusted
for mesophyll conductance, gains in WUE during the 20th century (0.15 ppm
y-1) were considerably smaller than those estimated from conventional
modelling (0.26 ppm y-1). Across the globe, mean sensitivity of WUE to
atmospheric CO2 was 0.15 ppm ppm-1. Ratios of internal-to-atmospheric CO2
(on a mole fraction basis; ci/ca) in leaves were mostly constant over time
but differed among biomes and plant taxa – highlighting the significance
of both plant structure and physiology. Together with synchronized
responses in stomatal and mesophyll conductance, our results suggest that
ratios of chloroplastic-to-atmospheric CO2 (cc/ca) are constrained over
time. We conclude that forest WUE may have not increased as much as
previously suggested and that projections of future climate forcing via
CO2 fertilization may need to be adjusted accordingly.