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

Leaf gas exchange responses of 13 grassland species to elevated CO₂ and increased N supply.

Lee, Tali¹, Tjoelker, Mark², Ellsworth, David³, Reich, Peter¹, ^{1 2 3}

ABSTRACT- Understanding how elevated CO₂ concentration and increased N deposition interact in affecting physiological processes of different plant species and functional groupings is important to predict community response to global change. We address this issue within a long-term prairie grassland field study (BioCON) that controls CO₂ concentrations using free-air CO₂ enrichment (FACE) in combination with soil N treatments. Thirteen perennial species, representing four functional groups: C₃ grasses, C₄ grasses, legumes, and non-leguminous forbs, were grown in monoculture under current ambient and elevated (560 mol mol^-1) CO₂ concentrations and low (unamended field soil) and high (addition of 4 gN m^-2 yr^-1) N treatments. Across three growing seasons (98-00), all species showed pronounced photosynthetic acclimation to elevated CO₂, resulting in minimal stimulation of photosynthesis (A) averaging 18% in C₃ grasses, 11% in forbs, 7% in legumes and 5% in C₄ grasses grown under elevated compared to ambient CO₂. Elevated CO₂ significantly decreased stomatal conductance (g_s) leading to an average 46% increase in water-use efficiency (A/g_s). The effects of CO₂ and soil N supply did not interact significantly and the effects of soil N were inconsistent across years. Differential responses between functional groups were few and due exclusively to the legumes or the C₄ grasses. For these species with varying ecophysiology, the substantial acclimation of photosynthesis was greater in magnitude than in most field studies, and was associated with the combined effects of decreased g_s and decreased leaf N concentrations in response to growth under elevated CO₂.

KEY WORDS: elevated CO2, photosynthesis, acclimation, grassland