Hungry and thirsty: Effects of CO₂ and limited water availability on plant performance

Carbon dioxide and water are crucial resources for plant growth. With anthropogenic fossil fuel emissions, CO₂ availability is and has been increasing since the last glacial maximum. Simultaneously water availability is expected to decrease and the frequency and severity of drought episodes to increase in large parts of the world. How plants respond to these two changes will help in understanding plants’ responses to climate of the future. Here we sought to understand how drought affects plant traits responses to CO₂ and whether there are trade-offs in responsiveness to low and elevated CO₂ and drought. We grew seedlings of seven C₃ annuals at past low (160 μl l⁻¹), ambient (450 μl l⁻¹) and elevated (750 μl l⁻¹) CO₂. At each concentration plants were subjected to well-watered conditions (100% soil water availability, SWA), 40% SWA or 20% SWA. We measured biomass allocation, relative growth rate, tissue N concentration, and gas exchange. Compared to well-watered conditions plant size was an important element in the absolute response to SWA decrease, i.e. the smaller, slow growing species were unaffected by drought at low CO₂. Plants allocated less mass to root tissue at low CO₂ contrasting with increased root mass fraction at lower SWA at ambient CO₂. Across all traits measured, we found mostly additive effects of CO₂ and water. As due to climate change regions become more drought prone these results suggest CO₂ fertilization will not counteract the effects of reduced water availability.