The optimal CO₂ concentrations for the growth of three perennial grass species

Grasslands are one of the most representative vegetation types accounting for about 20% of the global land area and thus the response of grasslands to climate change plays a pivotal role in terrestrial carbon balance. However, many current climate change models, based on earlier results of the doubling-CO₂ experiments, may overestimate the CO₂ fertilization effect, and as a result underestimate the potentially effects of future climate change on global grasslands when the atmospheric CO₂ concentration goes beyond the optimal level. Here, we examined the optimal atmospheric CO₂ concentration effect on CO₂ fertilization and further on the growth of three perennial grasses in growth chambers with the CO₂ concentration at 400, 600, 800, 1000, and 1200 ppm, respectively.

All three perennial grasses featured an apparent optimal CO₂ concentration for growth. Initial increases in atmospheric CO₂ concentration substantially enhanced the plant biomass of the three perennial grasses through the CO₂ fertilization effect, but this CO₂ fertilization effect was dramatically compromised with further rising atmospheric CO₂ concentration beyond the optimum. The optimal CO₂ concentration for the growth of tall fescue was lower than those of perennial ryegrass and Kentucky bluegrass, and thus the CO₂ fertilization effect on tall fescue disappeared earlier than the other two species. By contrast, the weaker CO₂ fertilization effect on the growth of perennial ryegrass and Kentucky bluegrass was sustained for a longer period due to their higher optimal CO₂ concentrations than tall fescue. The limiting effects of excessively high CO₂ concentrations may not only associate with changes in the biochemical and photochemical processes of photosynthesis, but also attribute to the declines in stomatal conductance and nitrogen availability.

In this study, we found apparent differences in the optimal CO₂ concentrations for the growth of three grasses. These results suggest that the growth of different types of grasses may respond differently to future elevated CO₂ concentrations through the CO₂ fertilization effect, and thus potentially alter the community composition and structure of grasslands. Meanwhile, our results may also be helpful for improving current process-based ecological models to more accurately predict the structure and function of grassland ecosystems under future rising atmospheric CO₂ concentration and climate change scenarios.