LAUSR

Shifts in flowering phenology is one of the most well‐studied, ecological signs of climate change. However, the extent and magnitude of observed phenological change over time varies significantly among both species and biomes, and general ecological rules predicting long‐term phenological shifts have yet to emerge. Using 2373 herbarium records of 21 common forest understory herbaceous species with diverse bloom times in Pennsylvania, USA, we elucidate which species have adjusted their flowering times the most over the last century. Our results show that spring‐blooming forbs exhibited the strongest phenological shifts over the past century. Long‐term shifts in flowering date were significantly negatively correlated to median bloom times among species. Early spring‐blooming species flowered two days earlier per decade, while flowering time among summer‐blooming remained remarkably consistent. Many studies comparing phenological responses between early‐ and late‐season species have reported similar patterns. However, the global generality and ecological significance of this phenomenon are not yet well understood. Several mechanisms might render spring‐blooming forest understory species more vulnerable to climate change‐driven phenological shifts, including sensitivity to tree canopy leaf‐out and higher phenotypic plasticity. Such hypotheses are not mutually exclusive but require additional research to synthesize patterns and test mechanisms across spatiotemporal scales and biomes. Biogeographic investigations that determine the latitudinal extent to which disproportionately high spring‐blooming sensitivity occurs are needed. The fitness consequences of these contrasting phenological responses also remain largely unexplored. Given the ecological consequences of phenological mismatches within and between trophic layers, a holistic framework explicitly based on phenological niches that predicts phenological sensitivity among species and biomes is needed.