Phosphorus supply drives nonlinear responses of cottonwood (Populus deltoides) to increases in CO2 concentration from glacial to future concentrations

P>Despite the importance of nutrient availability in determining plant responses to climate change, few studies have addressed the interactive effects of phosphorus (P) supply and rising atmospheric CO2 concentration ([CO2]) from glacial to modern and future concentrations on tree seedling growth. The objective of our study was to examine interactive effects across a range of P supply (six concentrations from 0.004 to 0.5 mM) and [CO2] (200 (glacial), 350 (modern) and 700 (future) ppm) on growth, dry mass allocation, and light-saturated photosynthesis (A(sat)) in Populus deltoides (cottonwood) seedlings grown in well-watered conditions. Increasing [CO2] from glacial to modern concentrations increased growth by 25% across P treatments, reflecting reduced [CO2] limitations to photosynthesis and increased A(sat). Conversely, the growth response to future [CO2] was very sensitive to P supply. Future [CO2] increased growth by 80% in the highest P supply but only by 7% in the lowest P supply, reflecting P limitations to A(sat), leaf area and leaf area ratio (LAR), compared with modern [CO2]. Our results suggest that future [CO2] will minimally increase cottonwood growth in low-P soils, but in high-P soils may stimulate production to a greater extent than predicted based on responses to past increases in [CO2]. Our results indicate that the capacity for [CO2] stimulation of cottonwood growth does not decline as [CO2] rises from glacial to future concentrations.