Environmental and Experimental Botany 70 (2011) 151–157 Contents lists available at ScienceDirect Environmental and Experimental Botany journal homepage: www.elsevier.com/locate/envexpbot Does an extreme drought event alter the response of grassland communities to a changing climate? K. Naudts a,∗ , J. Van den Berge a , I.A. Janssens a , I. Nijs a,b , R. Ceulemans a a Research Group of Plant and Vegetation Ecology, Department of Biology, University of Antwerp (Campus Drie Eiken), Universiteitsplein 1, B-2610 Wilrijk, Belgium b King Saud University, Riyadh, Saudi Arabia article info Article history: Received 5 November 2009 Received in revised form 19 August 2010 Accepted 29 August 2010 Keywords: Soil water stress Climate warming Elevated CO2 Net CO2 assimilation rate Productivity abstract Global temperatures and atmospheric CO 2 concentrations are expected to both increase, but their com- bined effect on plant communities has been far less investigated than the single factors of global change. Moreover, drought events are expected to become more frequent and intense in the near future what might alter plant responses to the changing climate. In this study synthesised grassland communities in a current or future climate were subjected to several drought levels (0, 15, 22 and 35 days of drought). The grassland communities were grown in six sunlit, climate-controlled chambers. Three of the chambers were exposed to ambient temperature and CO 2 (current climate), while the other three were continuously warmed 3 ◦ C above ambient temperature at 620 ppm of CO 2 (future climate). The aim of this study was to investigate the effect of drought on the response of grassland communities to a future climate. Therefore, the response to future climate was observed (1) in the absence of drought and (2) in the occurrence of an extreme drought event, both early and late in the growing season. (1) In the absence of drought, plant productivity was positively affected by future climate early in the grow- ing season. Later in the growing season this effect tended to turn negative, resulting in a disappearance of the overall effect of climate at the end of the growing season. (2) During drought there was a stronger decrease in net CO 2 assimilation rate (A sat ) in future than in current climate due to stronger stomatal closure. Consistently, the beneficial biomass response to future climate stagnated during drought. At the end of the season, after a period of recovery, there was no effect of climate on plant productivity. As in the absence of drought, plant productivity was not affected by climate at the end of the growing season. Hence, the occurrence of an extreme drought event during the growing season did not alter the overall response of plant productivity to a future climate. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Most experimental research on responses of plant communi- ties to climate change has focused on single factors, like the effects of increasing CO 2 concentration or elevated temperature. The two fundamental responses of plants to rising atmospheric CO 2 are increased photosynthesis and reduced stomatal conductance (g s ). All other effects of elevated CO 2 on plants and ecosystems are derived from these changes (Long et al., 2004). In the absence of photosynthetic acclimation, elevated temperature increases rates of photosynthesis as long as the plant’s optimal temperature is not ∗ Corresponding author. Tel.: +32 265 22 72; fax: +32 265 22 71. E-mail address: kim.naudts@ua.ac.be (K. Naudts). exceeded (Berry and Björkman, 1980). However, elevated temper- ature may reduce g s , CO 2 uptake and carbon fixation through its effect on soil moisture and leaf water relations (Loik et al., 2000). Single factor studies provide a functional understanding of the impact of various climatic changes, but obtaining a functional understanding of the responses to simultaneously changing fac- tors from these reports alone remains difficult (Beier, 2004). The few examples of studies on the combination of elevated atmo- spheric CO 2 concentration and increased temperature point out that the responses to these simultaneously occurring factors of cli- mate change are not necessarily an addition of the responses to the individual factors (Shaw et al., 2002). The magnitude of plant responses to a changing climate depends on the availability of potentially limiting resources (Campbell et al., 1997). Nitrogen is very often limiting plant productivity and 0098-8472/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2010.08.013