Unexpected reversal of C ₃ versus C ₄ grass response to elevated CO ₂ during a 20-year field experiment

Abstract

Theory predicts and evidence shows that plant species that use the C photosynthetic pathway (C species) are less responsive to elevated carbon dioxide (CO) than species that use only the C pathway (C species). We document a reversal from this expected C-C contrast. Over the first 12 years of a 20-year free-air CO enrichment experiment with 88 C or C grassland plots, we found that biomass was markedly enhanced at CO relative to ambient CO in C but not C plots, as expected. During the subsequent 8 years, the pattern reversed: Biomass was markedly enhanced at CO relative to ambient CO in C but not C plots. Soil net nitrogen mineralization rates, an index of soil nitrogen supply, exhibited a similar shift: CO first enhanced but later depressed rates in C plots, with the opposite true in C plots, partially explaining the reversal of the CO biomass response. These findings challenge the current C-CCO paradigm and show that even the best-supported short-term drivers of plant response to global change might not predict long-term results.

Strong photosynthetic acclimation and enhanced water‐use efficiency in grassland functional groups persist over 21 years of CO2 enrichment, independent of nitrogen supply

“…Evidence from 13 species during the first 11 years of this experiment (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) pointed toward marked photosynthetic acclimation to eCO 2 , with long-term photosynthetic enhancement sustained at about 10% in eCO 2 -grown plants (Lee, Barrott, & Reich, 2011), in contrast to the 44%-64% instantaneous enhancement observed during the first 2 years of the experiment when ambient CO 2 -grown plants were exposed to an immediate increase in CO 2 concentration ). BioCON and other global change studies have shown that other responses to eCO 2 , such as biomass and soil net N mineralization, may shift through time, with the direction even reversing (Andresen et al, 2016;Reich, Hobbie, Lee, & Pastore, 2018), and photosynthetic responses could undergo similar shifts or help explain these other changing responses over time. A number of questions remain, such as whether responses of FGs have diverged since 2011 (and, if so, whether that divergence matches predictions based on physiology) and whether acclimation has strengthened or diminished depending on potential changes in N availability and sink strength.…”

“…Our study examined leaf gas exchange in plants exposed to eCO 2 over the longest duration in the most species of any study to our knowledge, providing the unique opportunity to examine multidecadal responses in differing FGs within a single experiment. A subset of these data was summarized in Reich et al (2018), which showed annual means through 2017 of the absolute CO 2 effect on photosynthesis in C 3 and C 4 grass monocultures pooled across species and N treatments. A subset of these data was summarized in Reich et al (2018), which showed annual means through 2017 of the absolute CO 2 effect on photosynthesis in C 3 and C 4 grass monocultures pooled across species and N treatments.…”

“…and discussion of photosynthetic responses, as photosynthetic measurements were not the focus of Reich et al (2018). and discussion of photosynthetic responses, as photosynthetic measurements were not the focus of Reich et al (2018).…”

Recent advances in understanding grasslands

“…Also, using a long-term investigation, Reich et al . 21 showed that whilst in the first 12 years of carbon dioxide enrichment C 3 plant biomass increased markedly, C 4 plant biomass did not. This is expected, since C 4 plants are thought to be less limited by carbon dioxide; however, in the latter 8 years of the experiment, the responses switched, with biomass depressed in C 3 plants but not in C4 plants.…”

Comment on “Unexpected reversal of C 3 versus C 4 grass response to elevated CO 2 during a 20-year field experiment”

“…R eich et al (1) explain that positive responses of plant biomass to elevated CO 2 have disappeared in C 3 grasses and appeared in C 4 grasses over the 20 years of the BioCON experiment. They assert, as do the authors of the associated Perspective (2), that these results challenge current expectations of C 3 and C 4 plant responses to elevated CO 2 .…”