Relationships Between First Flowering Date and Temperature in the Flora of a Locality in Central England

1. A data set of 36 years (1954-1989) of observations on first flowering dates (FFD) of 243 species of angiosperms and gymnosperms in one locality in southern central England is presented and analysed. 2. Individual FFDs ranged from 1 January to 17 August, and species varied considerably in the standard deviation of their FFD. The most variable species were mainly annuals and there was a negative relationship between mean FFD and variability, early-flowering species being the most variable. 3. For 219 of the 243 species, it was possible to fit regression equations for FFD to some set of monthly mean temperatures of the preceding months. These fits were generally best for woody plants and geophytes. February temperature was overall the most important determinant of flowering time. Sixty per cent of species flowering between January and April were affected by temperature 1-2 months before flowering; for summer (May onwards) flowering species, temperatures up to 4 months previously were important. 4. High spring temperatures advanced flowering by a mean of 4 days per degree. In contrast, both spring- and summer-flowering species were retarded in flowering by high temperatures in the previous autumn. 5. These relationships were used to simulate the effects of climatic warming: an overall increase of 1°C in each month would advance flowering in some species and retard others, by as much as 6 weeks. Retarded species were early-flowering, advanced species late-flowering. These results suggest a high degree of dependence of flowering time on temperature, and the variation between species implies that responses to climatic warming may be difficult to predict