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.

Aims Leaf and root phenology play important roles controlling plant productivity and ecosystem function, yet, few studies link patterns of leaf and root phenology across woody species. Trees with diffuse-porous wood anatomy tend to leaf-out before ring-porous species and we expected that increases in transpiration with spring leaf-out would be coupled with initiation of root production to support uptake of soil resources. Therefore, we hypothesized that the timing of root production would follow patterns of leaf production and wood anatomy.Methods Root production was observed using minirhizotrons and related to leaf phenology across six temperate tree species with different wood anatomy in a common garden. Results As expected, leaves of diffuse-porous species emerged before ring-porous, followed by tracheid species. Root production peaked before bud break in five of the six species and before maximum leaf area index in all species, but did not follow expected patterns with leaf production. Conclusions Our observations did not indicate tight linkages between root and leaf phenology but do highlight the potential for very early season root production and greater variation in the phenology of roots than leaves. Future work should identify the environmental factors and species traits that best explain variation in root phenology.

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Uncertainty about long‐term leaf‐level responses to atmospheric CO2 rise is a major knowledge gap that exists because of limited empirical data. Thus, it remains unclear how responses of leaf gas exchange to elevated CO2 (eCO2) vary among plant species and functional groups, or across different levels of nutrient supply, and whether they persist over time for long‐lived perennials. Here, we report the effects of eCO2 on rates of net photosynthesis and stomatal conductance in 14 perennial grassland species from four functional groups over two decades in a Minnesota Free‐Air CO2 Enrichment experiment, BioCON. Monocultures of species belonging to C3 grasses, C4 grasses, forbs, and legumes were exposed to two levels of CO2 and nitrogen supply in factorial combinations over 21 years. eCO2 increased photosynthesis by 12.9% on average in C3 species, substantially less than model predictions of instantaneous responses based on physiological theory and results of other studies, even those spanning multiple years. Acclimation of photosynthesis to eCO2 was observed beginning in the first year and did not strengthen through time. Yet, contrary to expectations, the response of photosynthesis to eCO2 was not enhanced by increased nitrogen supply. Differences in responses among herbaceous plant functional groups were modest, with legumes responding the most and C4 grasses the least as expected, but did not further diverge over time. Leaf‐level water‐use efficiency increased by 50% under eCO2 primarily because of reduced stomatal conductance. Our results imply that enhanced nitrogen supply will not necessarily diminish photosynthetic acclimation to eCO2 in nitrogen‐limited systems, and that significant and consistent declines in stomatal conductance and increases in water‐use efficiency under eCO2 may allow plants to better withstand drought.