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We tested the hypothesis that elevated CO 2 would stimulate proportionally higher photosynthesis in the lower crown of Populus trees due to less N retranslocation, compared to tree crowns in ambient CO 2 . Such a response could increase belowground C allocation, particularly in trees with an indeterminate growth pattern such as Populus tremuloides. Rooted cuttings of P. tremuloides were grown in ambient and twice ambient (elevated) CO 2 and in low and high soil N availability (89 ± 7 and 333 ± 16 ng N g -1 day -1 net mineralization, respectively) for 95 days using open-top chambers and open-bottom root boxes. Elevated CO 2 resulted in significantly higher maximum leaf photosynthesis (A max ) at both soil N levels. A max was higher at high N than at low N soil in elevated, but not ambient CO 2 . Photosynthetic N use efficiency was higher at elevated than ambient CO 2 in both soil types. Elevated CO 2 resulted in proportionally higher whole leaf A in the lower three-quarters to one-half of the crown for both soil types. At elevated CO 2 and high N availability, lower crown leaves had significantly lower ratios of carboxylation capacity to electron transport capacity (V cmax /J max ) than at ambient CO 2 and/or low N availability. From the top to the bottom of the tree crowns, V cmax /J max increased in ambient CO 2 , but it decreased in elevated CO 2 indicating a greater relative investment of N into light harvesting for the lower crown. Only the mid-crown leaves at both N levels exhibited photosynthetic down regulation to elevated CO 2 . Stem biomass segments (consisting of three nodes and internodes) were compared to the total A leaf for each segment. This analysis indicated that increased A leaf at elevated CO 2 did not result in a proportional increase in local stem segment mass, suggesting that C allocation to sinks other than the local stem segment increased disproportionally. Since C allocated to roots in young Populus trees is primarily assimilated by leaves in the lower crown, the results of this study suggest a mechanism by which C allocation to roots in young trees may increase in elevated CO 2 .

Populus tremuloides photosynthesis and crown architecture in response to elevated CO2 and soil N availability