Predicting fitness in future climates: insights from temporally replicated field experiments in <i>Arabidopsis thaliana</i>

Organisms are already facing climate change. To understand and mitigate the negative effects of climate change on wild and cultivated species, recent research has focused on predicting the fitness of organisms or populations in future climates. The accuracy of these predictions is, however, seldom tested. To test such predictions, we grew a set of 800 genetic families of the annual plant <i>Arabidopsis thaliana</i> in the same field site located in Northern France for 2 consecutive years with contrasted climates. Despite observing, in both years, a clear association between fitness and climatic distance between our field site and the climate of origin of these genetic families, the diverse set of methods we used failed to accurately predict fitness from a year to another. This low accuracy can be explained by the fact that different climatic factors contributed to climate adaptation in different years, which impeded the definition of a meaningful climate descriptor across years. Our results also suggest that populations of <i>A. thaliana</i> from Northern France already suffer from an adaptational lag with respect to climate, and that vegetative growth seems to be a more important trait for climate adaptation than phenology. We discuss the implications of our results for predicting the fitness of wild organisms in future climates and for breeding programs.