In this study, we examined the interactive effects of phosphorus supply and carbon dioxide concentration on the growth of Populus deltoides (cottonwood) seedlings. Cottonwood seedlings were grown for two months at one of six phosphorus supplies (0.5, 0.1, 0.06, 0.02, 0.012, 0.004 mM as KH2PO4), and at subambient (200 ppm), ambient (350 ppm) or elevated (700 ppm) carbon dioxide. There were significant interactions between phosphorus supply and carbon dioxide supply on biomass production, height and total leaf area. Relative to seedlings grown in ambient carbon dioxide, elevated carbon dioxide increased seedling biomass approximately 80% in the two highest phosphorus supplies, but did not significantly affect seedling biomass in the four lowest phosphorus supplies. Elevated carbon dioxide also increased total leaf area and height in the highest phosphorus supply, but reduced leaf area in the four lowest phosphorus supplies. Growth responses in subambient and ambient carbon dioxide were similar across phosphorus supplies. These results suggest that increasing phosphorus limitation reduced the growth response to increasing carbon dioxide. Additionally, increased seedling height and total leaf area in the elevated carbon dioxide, high-phosphorus treatment suggest that increasing carbon dioxide may increase the competitiveness of cottonwood seedlings for light under relatively non-limiting phosphorus supplies. In contrast, reductions in seedling height and total leaf area associated with elevated carbon dioxide in the low phosphorus treatments suggest that elevated carbon dioxide may reduce the competitiveness of cottonwood seedlings for light under more phosphorus-limiting conditions.

Key words: Populus deltoides , elevated carbon dioxide, growth response, phosphorus, subambient carbon dioxide