Flowering time can affect many aspects of a plant’s ecology and fitness (Rathcke & Lacey, 1985; Parra-Tabla & Vargas, 2004; Kover et al., 2009a; Amasino, 2010). Accordingly, natural variation in flowering time has been shown to be under selection (Le Corre et al., 2002; Franks et al., 2007; Korves et al., 2007; Anderson et al., 2011; Munguía-Rosas et al., 2011). Mean global temperatures have risen by around 0.8 °C in the last hundred years and further increases of 2-3 °C are expected by the end of the century (Minorsky, 2002; IPCC, 2007). Climate change is expected to have its strongest and most immediate effects on plant phenology (Forrest &

Miller-Rushing, 2010; Munguía-Rosas et al., 2011), and accelerated phenologies have already been observed in many species (Sparks et al., 2000; Abu-Asab et al., 2001; Menzel et al., 2006; Cleland et al., 2012). There is concern that accelerated phenologies may alter patterns of resource allocation, interactions with pollinators, the size and diversity of the soil seed bank and compromise species persistence (Visser & Holleman, 2001; Minorsky, 2002; Walther et al., 2002; Post & Pedersen, 2008; Hegland et al., 2009). Here, we use a climate manipulation experiment under field conditions, to investigate the consequences of elevated temperature to flowering time and fitness in the plant Arabidopsis thaliana.