Background/Question/Methods Advanced flowering time when temperatures are warmer appears to be a general, but not universal, feature of spring flowering phenology. Most studies, however, are not concerned with the developmental cause of the change in flowering time. The unstated assumption appears to be that earlier flowering is simply due to earlier germination in annuals, earlier emergence in perennial grasses and forbs, or earlier budburst is trees. The extremely high variation in the flowering time of tallgrass species allows an examination of some other developmental causes of variation in reproductive phenology.

Results/Conclusions

In central Oklahoma, the dominant perennial grasses can vary their flowering time by up to two months from year to year. In these species, advanced or delayed spring emergence may or may not correspondingly advance or delay flowering, fruiting or senescence later in the year. In addition to varying the onset of floral development, some species are capable of prolonging their bud phase according to environmental conditions. In forbs and annual grasses, there is a smaller degree of independence between emergence or germination time and flowering time than in perennial grasses. Late-summer-blooming species will flower later with warmer temperatures in some years, possibly depending on the availability of other resources, such as soil moisture. Finally, a few species of forbs are capable of flowering either continuously or 2-3 times in a season if conditions are favorable. This large phenological variability complicates attempts to predict changes in patterns of coincident emergence and flowering in different plant species (which could affect patterns of competition), as well as coincident phenology of plants and their insect pollinators and herbivores.