In seasonal environments, a high responsiveness of development to increasing temperatures in spring can infer benefits in terms of a longer growing season, but also costs in terms of an increased risk of facing unfavourable weather conditions. Still, we know little about how climatic conditions influence the optimal plastic response. Using 22 years of field observations for the perennial forest herb Lathyrus vernus, we assessed phenotypic selection on among-individual variation in reaction norms of flowering time to spring temperature, and examined if among-year variation in selection on plasticity was associated with spring temperature conditions. We found significant among-individual variation in mean flowering time and flowering time plasticity, and that plants that flowered earlier also had a more plastic flowering time. Selection favoured individuals with an earlier mean flowering time and a lower thermal plasticity of flowering time. Less plastic individuals were more strongly favoured in colder springs, indicating that spring temperature influenced optimal flowering time plasticity. Our results show how selection on plasticity can be linked to climatic conditions, and illustrate how we can understand and predict evolutionary responses of organisms to changing environmental conditions.

Data on field observations of flowering time and fitness of the perennial forest herb Lathyrus vernus and on spring temperature from weather station data. The dataset includes 22 years of data (1987–1996 and 2006–2017) from a L. vernus population located in a deciduous forest in Tullgarn, SE Sweden (58.9496 N, 17.6097 E). It includes records from 837 individuals (607 from 1987 to 1996, and 230 from 2006 to 2017), and from 2478 flowering events. The dataset includes the following variables: year: year of the recording id_nr: numeric plant id id: unique plant id (combination of numeric plant id and period) fcode: flowering code (1 if the plant flowered on that year, 0 if not) FFD: First Flowering Date n_fl: number of flowers n_fr: number of fruits totseed: total number of seeds intactseed: total number of intact seeds (not damaged by seed predator beetles) shoot_vol: shoot volume period: old (1987–1996) or new (2006–2017) n_years_fl_fitness: number of years when data on flowering and fitness is available n_years_study: number of years when the plant was included in the study mean_4: Average daily mean temperature of April (calculated from nearby meteorological station data) cmean_4: Mean-centred average daily mean temperature of April NA indicates missing data.