Variable floral phenology: temporal resource heterogeneity and its implication for flower visitors

I investigated within‐ and between‐year patterns of flowering phenology of entomophilous plants in an isolated old field in northern Michigan, USA. The number of open flowers was censused at 5‐d intervals from June‐September of 1984–1986, and the data analyzed by detrended correspondence analysis. DCA axis 1 reflected within‐year phenology, was highly correlated with date of censusing, and implied substantial constancy in flowering sequences. However, DCA axis 2 revealed significant between‐year variation in abundances of flowers; years differed significantly in their scores on axis 2, and these differences were driven by variable plant species abundances during late summer. Within‐year patterns showed that the floral community consisted of three more‐or‐less distinct temporal “guilds” of plants, and that the rate of turnover in community composition throughout the season was not uniform. The appearance of these guilds was not synchronous across years. Insects foraging on these species throughout the floral season can expect constancy in the relative timing of blooming, but are exposed to considerable temporal heterogeneity in flower resource abundance both between‐ and within‐years. Opportunities for coevolutionary specialization are therefore likely to be constrained.