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Pseudotsuga menziesii  (Mirb.) Franco (Douglas-fir) seedlings were grown in a 2 × 2 factorial design in enclosed mesocosms at ambient temperature or 3.5 °C above ambient, and at ambient CO 2  concentration ([CO 2 ]) or 179 ppm above ambient. Two additional mesocosms were maintained as open controls. We measured the extent of mycorrhizal infection, foliar nitrogen (N) concentrations on both a weight basis (%N) and area basis (N area ), and foliar δ 15 N signatures ( 15 N/ 14 N ratios) from summer 1993 through summer 1997. Mycorrhizal fungi had colonized nearly all root tips across all treatments by spring 1994. Elevated [CO 2 ] lowered foliar %N but did not affect N area , whereas elevated temperature increased both foliar %N and N area . Foliar δ 15 N was initially –1‰ and dropped by the final harvest to between –4 and –5‰ in the enclosed mesocosms, probably because of transfer of isotopically depleted N from mycorrhizal fungi. Based on the similarity in foliar δ 15 N among treatments, we conclude that mycorrhizal fungi had similar N allocation patterns across CO 2  and temperature treatments. We combined isotopic and Narea data for 1993–94 to calculate fluxes of N for second- and third-year needles. Yearly N influxes were higher in second-year needles than in third-year needles (about 160 and 50% of initial leaf N, respectively), indicating greater sink strength in the younger needles. Influxes of N in second-year needles increased in response to elevated temperature, suggesting increased N supply from soil relative to plant N demands. In the elevated temperature treatments, N effluxes from third-year needles were higher in seedlings in elevated [CO 2 ] than in ambient [CO 2 ], probably because of increased N allocation below ground. We conclude that N allocation patterns shifted in response to the elevated temperature and [CO 2 ] treatments in the seedlings but not in their fungal symbionts.

tree physiology

Foliar nitrogen concentrations and natural abundance of  15 N suggest nitrogen allocation patterns of Douglas-fir and mycorrhizal fungi during development in elevated carbon dioxide concentration and temperature