Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis

Abstract

A decrease in the 13 C/ 12 C ratio of atmospheric CO 2 has been documented by direct observations since 1978 and from ice core measurements since the industrial revolution. This decrease, known as the 13 C-Suess effect, is driven primarily by the input of fossil fuel-derived CO 2 but is also sensitive to land and ocean carbon cycling and uptake. Using updated records, we show that no plausible combination of sources and sinks of CO 2 from fossil fuel, land, and oceans can explain the observed 13 C-Suess effect unless an increase has occurred in the 13 C/ 12 C isotopic discrimination of land photosynthesis. A trend toward greater discrimination under higher CO 2 levels is broadly consistent with tree ring studies over the past century, with field and chamber experiments, and with geological records of C 3 plants at times of altered atmospheric CO 2 , but increasing discrimination has not previously been included in studies of long-term atmospheric 13 C/ 12 C measurements. We further show that the inferred discrimination increase of 0.014 ± 0.007‰ ppm −1 is largely explained by photorespiratory and mesophyll effects. This result implies that, at the global scale, land plants have regulated their stomatal conductance so as to allow the CO 2 partial pressure within stomatal cavities and their intrinsic water use efficiency to increase in nearly constant proportion to the rise in atmospheric CO 2 concentration. Significance Climate change and rising CO 2 are altering the behavior of land plants in ways that influence how much biomass they produce relative to how much water they need for growth. This study shows that it is possible to detect changes occurring in plants using long-term measurements of the isotopic composition of atmospheric CO 2 . These measurements imply that plants have globally increased their water use efficiency at the leaf level in proportion to the rise in atmospheric CO 2 over the past few decades. While the full implications remain to be explored, the results help to quantify the extent to which the biosphere has become less constrained by water stress globally. IntroductionThe accumulation of excess CO 2 in the atmosphere over the past century has been associated with a closely related decrease in the

Observed and modelled historical trends in the water‐use efficiency of plants and ecosystems

“…In the example, the photorespiration term contributes to increase Δ 13 C with c a rise by 0.004‰ ppm −1 over the range of 285–400 ppm, in a scenario of constant c i / c a , which is within the range of variability measured in C 3 plants (Schubert & Jahren, ). The trend in iWUE leaf tended to be higher using Equation (11) than (3) for a same Δ 13 C (Figure b), in agreement with Keeling et al (). Note that here we made the assumptions that only c i may change with rising c a , and that T air remains constant over the range of c a .…”

Forest‐Type‐Dependent Water Use Efficiency Trends Across the Northern Hemisphere

“…Increased WUE in forests under eCO 2 has been reported in many previous studies, including those using field experiments (De Kauwe et al, ; Leakey et al, ), tree ring measurements (Dekker et al, ; D. C. Frank et al, ; Saurer et al, ), isotopic compositions of atmospheric CO 2 (Keeling et al, ), eddy covariance (EC) flux tower measurements (Keenan et al, ; Mastrotheodoros et al, ) and model simulations (Cheng et al, ; Huang et al, ; Zhou et al, ). However, these studies have not consistently agreed with the statement that increased WUE under eCO 2 should correspond to increased carbon assimilation and/or decreased water losses in forests.…”

Compiled records of carbon isotopes in atmospheric CO<sub>2</sub> for historical simulations in CMIP6

“…Observations of δ 13 C in ocean dissolved inorganic carbon have been used to investigate anthropogenic CO 2 uptake (Quay et al, 2003) and to evaluate ocean models that include marine ecosystem dynamics (Tagliabue and Bopp, 2008;Schmittner et al, 2013). With terrestrial biosphere models, simulations of the response of plants and photosynthesis to rising atmospheric CO 2 and changing water availability can be evaluated with δ 13 C observations in atmospheric CO 2 or in leaves or tree rings, because a close relationship exists between processes controlling leaf-level isotopic discrimination and water-use efficiency (Randerson et al, 2002;Scholze et al, 2008;Ballantyne et al, 2011;Keller et al, 2017;Keeling et al, 2017). Additionally, observations of 14 C can be used to constrain models of carbon turnover rates in vegetation and soil carbon at plot-level and global scales (Trumbore, 2000;Naegler and Levin, 2009).…”