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Citation. Lee, T. D. 2001. Physiological and growth responses of grassland species to elevated carbon invasibility and increased N supply with emphasis on symbiotic N fixers vs. non-fixers. Ph.D. Thesis, University of Minnesota.

Abstract. Although rising atmospheric concentrations of carbon dioxide (CO 2 ) and increasing nitrogen (N) deposition are well-documented global changes, their interactive effects on vegetation are not well understood. This thesis characterizes the physiological and growth responses of perennial grassland species to combinations of atmospheric CO 2 and N treatments. Mechanisms operating at leaf, wholeplant, and community levels were studied using species adapted to N-limited habitats with emphasis on species capable of accessing atmospheric N through symbiotic N 2 fixation.

The response of leaf-level gas exchange was measured for 13 species in field plots exposed to ambient (368 μmol mol -1 ) and elevated (560 μmol mol -1 ) CO 2 concentrations combined with unamended and enriched (+4 gN m -2 yr -1 ) N treatments. All species showed pronounced photosynthetic acclimation resulting in minimal stimulation (7%) of photosynthesis with CO 2 enrichment. Elevated CO 2 decreased stomatal conductance (24%), leading to increases in intrinsic water-use efficiency. Increased N supply did not affect leaf-level responses to elevated CO 2 . The substantial acclimation of photosynthesis was associated with decreases in stomatal conductance and leaf N in response to CO 2 enrichment.

To further investigate the effect of N availability on the CO 2 response, growth and physiological responses to elevated CO 2 were compared between an N 2 -fixer and a non-N 2 -fixer across a range of N additions in a growth chamber study. The N 2 -fixer derived 32% more N from symbiotic N 2 fixation and accumulated 80% more biomass, regardless of N addition, under elevated compared to ambient CO 2 . In contrast, the growth response to CO 2enrichment of the non-N 2 -fixer was limited at low N.

The hypothesis that N 2 -fixers will alleviate N-limitations on the CO 2responses of plants and communities was evaluated in multi-species field assemblages. Photosynthesis and plant and soil N status were enhanced by the presence of an N 2 -fixer, however this did not facilitate greater responses of non-N 2 -fixers to elevated compared to ambient CO 2 .

Interspecific variation in acclimation of photosynthesis to CO 2 enrichment and N availability, and the contrasting growth responses of N 2 -fixers and non-N 2 -fixers, will be important determinants of the response of vegetation to future environments. Furthermore, species interactions may critically modify plant and community responses to these global changes.

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