Threshold responses in Yellowstone grasses across a broad CO2 gradient

Background/Question/Methods

Atmospheric CO2 concentration has doubled since the last glacial maximum 20,000 years ago and is predicted to more than double again by the end of this century. Characterizing plant responses to past CO2 conditions is critical for predicting the capacity of plants to continue responding to rising CO2 in the future. Invasive plant species, which often possess traits associated with rapid growth, are hypothesized to respond more strongly to rising CO2; however, few studies have compared the effects of both low and high CO2 on native and invasive plants. Fast-growing invasive species may eventually become limited by other factors, such as physiological tradeoffs or nutrient availability at high CO2. In this study, we tested plant physiological and growth responses to 200 (historic), 400 (modern), and 800 (future) ppm CO2 in two native and two invasive grass species. We also compared plant growth under high and low nutrient conditions within each CO2 treatment to account for potential nutrient constraints on plant-CO2 responses.

Results/Conclusions

As expected, low CO2 constrained plant growth for all species. Surprisingly, there was no consistent difference in growth or CO2 effects between native and invasive species. Instead, the two fastest growing species, Bromus inermis (invasive) and Bromus carinatus (native), were most responsive to historical changes in CO2. However, plant growth increased only marginally for these species at elevated CO2, supporting the hypothesis that plants may eventually reach a threshold in their response to rising CO2. Nutrient availability constrained plant responses to CO2 for all species in the study, but especially for the faster growing Bromus species. Differences in physiological traits among plant species or tradeoffs that shift under low and elevated CO2 may help explain these results and provide a framework for predicting CO2 responses in the future.