PARENT SESSION
Contributed Oral Session 140: Forest Photosynthesis and Water Relations
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 520 B, Level 5, Palais des congrès de Montréal

Effects of N Addition, Shade, and Elevated CO₂ on Northern Forest Species Differing in Shade Tolerance.

Sefcik, Lesley^*,1, Ellsworth, David¹, ¹ School of Natural Resources and Environ., Ann Arbor, MI, USA

ABSTRACT- Adjustments in photosynthetic capacity have been observed under low N supply in some open-top chamber and Free Air CO₂ Enrichment (FACE) experiments. Leaf net CO₂ assimilation responses of tree seedlings to elevated CO₂ were measured to determine if different levels of understory shade modified these responses. For two growing seasons, we exposed four species of northern hardwood tree seedlings differing in shade tolerance to a factorial combination of atmospheric CO₂ (ambient, and elevated CO₂ at 658 mol CO₂ mol^-1), and shade (2% and 6% full sun) in open-top chambers placed in a forest understory. Long-term enhancement (E) of light saturated CO₂ assimilation (A_sat) due to elevated CO₂ was significant and was 65%. Seedlings also exhibited downregulation of photosynthetic capacity with 11% loss on average. To determine if N addition alleviated the downregulation, we added +30 kg N ha^-1 y^-1 as nitrate to half of the plots. N addition alleviated the photosynthetic downregulation, and seedlings grown in 6% light had greater increases in photosynthetic capacity than those in 2% light. We conclude that in similar N-limited forests, seedlings grown in deep shade may show greater photosynthetic gains from elevated CO₂ exposure than those in less extreme shade due to N limitation as light increases. However, those in higher light may be able to better utilize additional N and this may counteract downregulation caused by long-term CO₂ exposure, which has implications for forest tree species recruitment dynamics.

Key words: Elevated CO₂, Nitrogen, Shade, Northern Forest Species