Overestimated water-use efficiency responses to rising CO2 revealed by tree-ring 13C record

Water-use efficiency (WUE) is a key determinant of carbon and water fluxes at scales ranging from individual plants to continents and thus a key driver of hydro-climatic changes and carbon models. Multiple lines of evidence suggest that WUE of plants increases with atmospheric CO₂, pointing to potential changes in physiological forcing of global carbon and hydrological cycles. Although the increase in forest WUE with atmospheric CO₂ is widespread, declines in tree growth have been observed in different forests. These controversial results highlight the need to re-evaluate the historical trend of forest WUE.

¹³C signatures (i.e., δ¹³C) of plant organic matter is a useful tool for estimating WUE at temporal scales ranging from days to centuries. This estimation relies on photosynthetic ¹³C discrimination models that have different assumptions about the components of isotope discrimination. Mesophyll conductance is a key uncertainty in estimated WUE owing to its influence on diffusion of CO₂ to sites of carboxylation.

We developed a ¹³C-based WUE model that takes into account the effect of mesophyll conductance and tested its performance with experimental data. We applied this model to 464 δ¹³C chronologies in tree-rings of 143 species spanning global biomes. Adjusted for mesophyll conductance, mean sensitivity of WUE to atmospheric CO₂ (0.15 ppm ppm^-1) was considerably smaller than those estimated from conventional modelling (0.23-0.30 ppm ppm^-1). Our results showed a 10-ppm gain in WUE during the 20th century. Ratios of internal-to-atmospheric CO₂ (c_i/c_a) in leaves maintained constant over time but differed among biomes and plant taxa. Our results also suggest that ratios of chloroplastic-to-atmospheric CO₂ (c_c/c_a) are constrained over time, but this result is associated with the sensitivity of g_s /g_m ratio to CO₂ which need further evaluation.

Over the last century, a general increase in forest WUE across the globe has been confirmed. However, our study shows that forest WUE may have not increased as much as previously suggested and that projections of CO₂ fertilization may need to be adjusted accordingly.