Interspecific competition negates CO₂ benefits for most plant species

Elevated CO₂ (eCO₂) frequently increases plant growth and primary
productivity, and is considered a driver of increased land carbon sinks.
This evidence contributes to a widespread but surprisingly untested
assumption in global change science and policy, and society at large, that
eCO₂ consistently stimulates the growth of plant species within
communities. However, species and community responses depend on species’
growth strategies and abiotic and biotic contexts, and among these, the
role of competition remains underexplored. We therefore ask whether eCO2
enhances growth broadly across species, or primarily in a dominant few? To
address this question of how species interactions are shaped by eCO₂
response and competitive ability, we analysed responses from 97 species in
20 grassland CO₂ enrichment experiments. Herein, we show that
interspecific competition markedly reduced eCO₂ stimulation: while most
species responded positively to eCO₂ in isolation, over half responded
negatively in mixed communities. Despite this, community biomass was often
and on average was increased under eCO₂, driven by dominant species
contributions. eCO₂ did not enhance competition tolerance, and acquisitive
traits—including fast growth and invasion success—conferred a consistent
advantage under both CO₂ enrichment and competitive stress. Our findings
suggest that in species mixtures, which represent the majority of natural
ecosystems, competition causes both positive and negative growth responses
to eCO2 to be ubiquitous and pervasive. We show that eCO₂ does not
consistently reduce competitive vulnerability, but instead, trait-based
filtering in a competitive context determines both which species benefit
from eCO2. These results challenge assumptions built into ecosystem
models; and incorporating these dynamics will improve forecasts of
vegetation response to rising CO₂.