PARENT SESSION
Oral Session #10: Elevated CO₂.
Presiding: D. Hileman
Monday, August 5. 8:00 AM to 11:30 AM. Grand Ballroom East, Radisson.

No evidence for down-regulation of photosynthesis in 11 tree species in a forest understory exposed to Free Air Carbon-dioxide Enrichment (FACE).

Moore, David^*,1,3, Mohan, Jacqueline², DeLucia, Evan¹, ¹ Univeristy of Illinois, Urbana-Champaign, IL³ University of Illinois, Urbana-Champaign, IL² Duke University, Durham, NC

ABSTRACT- The long-term photosynthetic response to elevated [CO₂] exposure is often different from the instantaneous response. In a blocked experimental design consisting of 6 FACE rings, 11 species of deciduous trees in the understory of an unmanaged pine plantation were exposed to ambient and elevated [CO₂] (+200 l l_-1) for approximately 2.5 years. We tested whether down-regulation of photosynthesis occurred in seedlings of a range of species of similar age, and whether species responded differently to elevated [CO₂]. Steady state photosynthesis for seedlings grown and measured at 570 l l_-1 was consistently higher than for those grown and measured at 370 l l_-1 (p<0.0001) with no significant treatment by species interactions (p=0.3603). There was no evidence of photosynthetic down-regulation, and on average photosynthetic rate of seedlings grown under ambient [CO₂] but measured at 570 l l_-1 provided a good prediction of the rate for seedlings grown and measured at 570 l l_-1 (R²= 0.764). The absence of differential species responses to elevated [CO₂] simplifies predictions of future forest photosynthetic responses. However, the percentage stimulation of photosynthesis was greater for species with low photosynthetic rates under ambient conditions than for those with higher rates, suggesting that carbon gain and persistence in the understory may be disproportionately enhanced under future elevated levels of CO₂ for species with intrinsically low rates of photosynthesis.

KEY WORDS: ecophysiology, ecology, photosynthesis, shade