POPULUS-TREMULOIDES PHOTOSYNTHESIS AND CROWN ARCHITECTURE IN RESPONSETO ELEVATED CO2 AND SOIL N AVAILABILITY

We tested the hypothesis that elevated CO2 would stimulate proportiona lly higher photosynthesis in the lower crown of Populus trees due to l ess N retranslocation, compared to tree crowns in ambient CO2. Such a response could increase belowground C allocation, particularly in tree s with an indeterminate growth pattern such as Populus tremuloides. Ro oted cuttings of P. tremuloides were grown in ambient and twice ambien t (elevated) CO2 and in low and high soil N availability (89 +/- 7 and 333 +/- 16 ng N g(-1) day(-1) net mineralization, respectively) for 9 5 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 soi l in elevated, but not ambient CO2. Photosynthetic N use efficiency wa s higher at elevated than ambient CO2 in both soil types. Elevated CO2 resulted in proportionally higher whole leaf A in the lower three-qua rters to one-half of the crown for both soil types. At elevated CO2 an d high N availability, lower crown leaves had significantly lower rati os of carboxylation capacity to electron transport capacity (V-cmax/J( max)) than at ambient CO2 and/or low N availability. From the top to t he bottom of the tree crowns, V-cmax/J(max) increased in ambient CO2, but it decreased in elevated CO2 indicating a greater relative investm ent of N into light harvesting for the lower crown. Only the mid-crown leaves at both N levels exhibited photosynthetic down regulation to e levated CO2. Stem biomass segments (consisting of three nodes and inte rnodes) were compared to the total A(leaf) for each segment. This anal ysis indicated that increased A(leaf) at elevated CO2 did not result i n a proportional increase in local stem segment mass, suggest ing that C allocation to sinks other than the local stem segment increased dis proportionally. Since C allocated to roots in young Populus trees is p rimarily assimilated by leaves in the lower crown, the results of this study suggest a mechanism by which C allocation to roots in young tre es may increase in elevated CO2.