In recent decades, the final frost dates of winter have advanced throughout North

America, and many angiosperm taxa have simultaneously advanced their flowering

times as the climate has warmed. Phenological advancement may reduce plant fitness,

as flowering prior to the final frost date of the winter/spring transition may

damage flower buds or open flowers, limiting fruit and seed production. The risk of

floral exposure to frost in the recent past and in the future, however, also depends

on whether the last day of winter frost is advancing more rapidly, or less rapidly, than

the date of onset of flowering in response to climate warming. This study presents

the first continental-scale assessment of recent changes in frost risk to floral tissues,

using digital records of 475,694 herbarium specimens representing 1,653 angiosperm

species collected across North America from 1920 to 2015. For most species, among

sites from which they have been collected, dates of last frost have advanced much

more rapidly than flowering dates. As a result, frost risk has declined in 66% of sampled

species. Moreover, exotic species consistently exhibit lower frost risk than native

species, primarily because the former occupy warmer habitats where the annual

frost-free period begins earlier. While reducing the probability of exposure to frost

has clear benefits for the survival of flower buds and flowers, such phenological advancement

may disrupt other ecological processes across