Elevated carbon dioxide: favors coexistence for competing species in a trait-based model

Background/Question/Methods

Different plant species have differential response to increases in atmospheric CO₂ concentration (C_a). This differential response could lead to quantitative changes in competition among species and community composition, with flow-on effects for ecosystem function. However, there has been little theoretical analysis of how C_a levels will affect plant competition, and there is currently no consensus regarding how the composition of plant communities might change. Thus, we investigated theoretically how plant competition might change under high C_a.

We applied two theories of competition; resource use theory and resource partitioning theory to a plant carbon and nitrogen cycling model and investigated the outcome of plant competition under low and high C_a.

Results/Conclusions

Both theories predicted that the most important traits for winners in the trait-wise case are high carbon use efficiency, low ratio of root N: C to leaf N: C and high carboxylation rate per unit leaf nitrogen. Resource use theory makes the novel prediction that elevated C_a is unlikely to change species dominance in competition. In contrast, resource partitioning theory predicts that elevated C_a may decrease the dominance ratio of the winning species. Collectively, both theories suggest that elevated C_awill favor coexistence.

We concluded that the logic of plant C: N relations suggest that species diversity should increase with elevated C_a. This novel theory-based hypothesis has potential to help guide design and interpretation of elevated C_aexperiments.