Document: GUN-3-34-79

Longterm effects of N deposition and elevated CO₂ on resource competition of temperate forest seedlings: A community response experiment using open top chambers.

BAUER, G.A.*, G.M.BERNTSON and F.A.BAZZAZ

Harvard University, Cambridge, MA 02138 USA ¹

Abstract:
Temperate forests are predicted to play a key role as important sinks for atmospheric CO₂. This sink could be enhanced by atmospheric nitrogen (N) deposition. The predicted response may vary for deciduous and coniferous trees due to differences in nitrogen use efficiency. We have investigated the effect of elevated CO₂ and N deposition on seedlings of three deciduous (Betula alleghaniensis, Quercus rubra, Acer rubrum) and three coniferous species (Pinus strobus, Picea rubens, Tsuga canadensis). The study comprises a factorial design consisting of two CO₂ treatments (ambient and elevated), two N treatments (ambient N deposition and ambient plus 50kg N ha^-1 y^-1) and three community types (deciduous, coniferous and mixed species communities). We predict that high N deposition will remove or reduce the N-constraint on enhanced productivity in elevated CO₂ by allowing plants to acquire more N. Data on height growth during 1999 indicate however that conifer growth response to elevated CO₂ is greater at low N whereas deciduous seedlings showed a greater response to elevated CO₂ at high N. This growth response was confirmed by data for photosynthetic capacity and N partitioning. At high N availability deciduous trees allocate relatively more N into Rubisco than conifers, thus making it impossible for conifers to use a surplus supply of N at elevated CO₂ for an increase in carbon assimilation. These results suggest that there is a trade-off between the growth of deciduous and coniferous seedlings in response to CO₂ and increased N availability depending on the photosynthetic N use efficiency.

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This abstract is being presented at: 3:30 PM in session:
Oral Session #28: Effects of N Deposition in Ecosystems.