PARENT SESSION
Poster Session #11: Elevated CO₂.
Monday, August 6, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

The effects of nearground enriched carbon dioxide on photosynthesis in forests with different land use legacies.

Sipe, Timothy¹, McClaugherty, Charles², Bowden, Richard³, ^{1 2 3}

ABSTRACT- We measured the effects of nearground enriched CO₂ (NEC) on carbon assimilation by three forest herbaceous species (Aralia nudicaulis, Medeola virginiana, Clintonia borealis) across sites with different agricultural land use histories at the Harvard Forest NSF-LTER station in central MA. Average ambient CO₂ concentrations measured at 10 cm above ground in mid-morning hours were significantly higher in permanent woodlots (394 ppm) than in formerly pastured (376 ppm) or plowed (367 ppm) sites. Light response curves showed an overall mean increase of 21% in maximum assimilation from 350 to 450 ppm CO₂. CO₂ response curves measured across the range of ambient CO₂ documented in the field (350-600 ppm) showed a doubling of assimilation at several irradiance levels (50, 200, 800 mol m^-2 s^-1). An increase in CO₂ from 350 to 450 ppm caused mean assimilation to rise by 43% overall (range 20-60%) during simulated sunflecks (500 mol m^-2 s^-1). Species showed some differences in the effects of CO₂ on response curves and sunfleck performance, but the patterns were not strong statistically. Plants consistently showed greater assimilation in formerly plowed or pastured sites than in woodlots. There were no clear site x species interactions. Our results demonstrate that NEC environments have significant implications for photosynthesis in response to both steady-state and fluctuating irradiance, and that land use legacies may influence this relationship for a substantial period of time following disturbance.

KEY WORDS: photosynthesis, enriched CO₂, herbaceous species, temperate forest