Leaf trait plasticity alters competitive ability and functioning of simulated tropical trees in response to elevated carbon dioxide

The response of tropical ecosystems to elevated carbon dioxide (CO₂ ) remains a critical uncertainty in projections of future climate. Here we investigate how leaf trait plasticity in response to elevated CO₂ alters projections of tropical forest competitive dynamics and functioning. We use demographic vegetation model simulations to quantify how plasticity in leaf mass per area and leaf carbon to nitrogen ratio alter the responses of carbon uptake, evapotranspiration, and competitive ability to a doubling of CO₂ in a tropical forest. Observationally constrained leaf trait plasticity in response to CO₂ fertilization reduces the degree to which tropical tree carbon uptake is affected by a doubling of CO ₂ (up to -14.7% as compared to a case with no plasticity; 95% confidence interval [CI _95% ] -14.4 to -15.0). It further diminishes evapotranspiration (up to -7.0%, CI _95% -6.4 to -7.7), and lowers competitive ability in our simulations. Consideration of leaf trait plasticity to elevated CO₂ lowers tropical ecosystem carbon uptake and evapotranspirative cooling in the absence of changes in plant type abundance. However, `plastic' responses to high CO₂ which maintain higher levels of plant productivity are potentially more competitively advantageous, thus, including changes in plant type abundance may mitigate these decreases in ecosystem functioning. Models that explicitly represent competition between plants with alternative leaf trait plasticity in response to elevated CO ₂ are needed to capture these influences on tropical forest functioning and large-scale climate.