Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 4 1 2007 10.5194/bg-4-27-2007 http://www.biogeosciences.net/4/27/2007/ http://www.biogeosciences.net/4/27/2007/bg-4-27-2007.html http://www.biogeosciences.net/4/27/2007/bg-4-27-2007.pdf 27 36 2007-01-12 Effects of climate warming and declining species richness in grassland model ecosystems: acclimation of CO₂ fluxes S. Vicca sara.vicca@ua.ac.be P. Serrano-Ortiz H. J. De Boeck C. M. H. M. Lemmens I. Nijs R. Ceulemans A. S. Kowalski I. A. Janssens Research Group of Plant and Vegetation Ecology, Department of Biology, University of Antwerp (Campus Drie Eiken), Universiteitsplein 1, 2610 Wilrijk, Belgium Department of Applied Physics, University of Granada, C/ Fuentenueva S/N, 18071 Granada, Spain To study the effects of warming and declining species richness on the carbon balance of grassland communities, model ecosystems containing one, three or nine species were exposed to ambient and elevated (ambient +3°C) air temperature. In this paper, we analyze measured ecosystem CO₂ fluxes to test whether ecosystem photosynthesis and respiration had acclimated to warming after 28 months of continuous heating, and whether the degree of acclimation depended on species richness. In order to test whether acclimation occurred, short term temperature response curves were established for all communities in both treatments. At similar temperatures, lower flux rates in the heated communities as compared to the unheated communities would indicate thermal acclimation. Because plant cover was significantly higher in the heated treatment, we normalized the data for plant cover. Subsequently, down-regulation of both photosynthesis and respiration was observed. Although CO₂ fluxes were larger in communities with higher species richness, species richness did not affect the degree of acclimation to warming. 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