title: Populus tremuloides photosynthesis and crown architecture in response to elevated CO2and soil N availability.
abstract: We tested the hypothesis that elevated CO2would stimulate proportionally higher photosynthesis in the lower crown of Populus trees due to less N retranslocation, compared to tree crowns in ambient CO2. 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) CO2and in low and high soil N availability (89 ± 7 and 333 ± 16 ng N g-1day-1net mineralization, respectively) for 95 days using open-top chambers and open-bottom root boxes. Elevated CO2resulted in significantly higher maximum leaf photosynthesis (Amax) at both soil N levels. Amaxwas higher at high N than at low N soil in elevated, but not ambient CO2. Photosynthetic N use efficiency was higher at elevated than ambient CO2in both soil types. Elevated CO2resulted in proportionally higher whole leaf A in the lower three-quarters to one-half of the crown for both soil types. At elevated CO2and high N availability, lower crown leaves had significantly lower ratios of carboxylation capacity to electron transport capacity (Vcmax/Jmax) than at ambient CO2and/or low N availability. From the top to the bottom of the tree crowns, Vcmax/Jmaxincreased in ambient CO2, but it decreased in elevated CO2indicating 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 CO2. Stem biomass segments (consisting of three nodes and internodes) were compared to the total Aleaffor each segment. This analysis indicated that increased Aleafat elevated CO2did 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 CO2.
authors.affiliation: School of Forestry and Wood Products, Michigan Technological University, Houghton, MI 49931, USA, , , , , , US.
authors.forename: M E
authors.initials: ME
authors.lastname: Kubiske
authors.affiliation: School of Forestry and Wood Products, Michigan Technological University, Houghton, MI 49931, USA, , , , , , US.
authors.forename: Kurt S
authors.initials: KS
authors.lastname: Pregitzer
authors.affiliation: School of Natural Resources and Environment, The University of Michigan, Ann Arbor, MI 48109, USA, , , , , , US.
authors.forename: Carl J
authors.initials: CJ
authors.lastname: Mikan
authors.affiliation: School of Natural Resources and Environment, The University of Michigan, Ann Arbor, MI 48109, USA, , , , , , US.
authors.forename: Donald R
authors.initials: DR
authors.lastname: Zak
authors.affiliation: School of Forestry and Wood Products, Michigan Technological University, Houghton, MI 49931, USA, , , , , , US.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Maziasz
authors.affiliation: The University of Michigan Biological Station, The University of Michigan, Ann Arbor, MI 48109, USA, , , , , , US.
authors.forename: A
authors.initials: A
authors.lastname: Teeri
keywords: Key words Carbon allocation ;   Nitrogen ;  Elevated CO2 ;  Photosynthesis ;  Populus tremuloides