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

We tested the hypothesis that elevated CO would stimulate proportionally higher photosynthesis in the lower crown of Populus trees due to less N retranslocation, compared to tree crowns in ambient CO. 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 and in low and high soil N availability (89 ± 7 and 333 ± 16 ng N g day net mineralization, respectively) for 95 days using open-top chambers and open-bottom root boxes. Elevated CO resulted in significantly higher maximum leaf photosynthesis (A ) at both soil N levels. A was higher at high N than at low N soil in elevated, but not ambient CO. Photosynthetic N use efficiency was higher at elevated than ambient CO in both soil types. Elevated CO 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 and high N availability, lower crown leaves had significantly lower ratios of carboxylation capacity to electron transport capacity (V /J ) than at ambient CO and/or low N availability. From the top to the bottom of the tree crowns, V /J increased in ambient CO, but it decreased in elevated CO 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. Stem biomass segments (consisting of three nodes and internodes) were compared to the total A for each segment. This analysis indicated that increased A at elevated CO 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.