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

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 (Asat) 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 Asat. 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 Asat, 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.