Herbarium Specimens May Provide Biased Flowering Phenology Estimates for Dioecious Species

Premise of research. Measuring phenology from herbarium specimens has become a widely used, powerful method for investigating the effects of climate change on plants across space, time, and phylogeny. However, it is not known whether and how biases in plant collecting might affect conclusions, particularly for dioecious species.Methodology. We examined the effects of spring temperature and latitude on the flowering phenology of male and female blunt-lobed spicebush (Lindera obtusiloba Blume) plants from across East Asia using 88 herbarium specimens and digitized photographs collected from 1918 to 2018.Pivotal results. Temperature and latitude did not affect the flowering phenology of either sex when analyzed separately. Surprisingly, however, female flowering plants were collected on average 25 d later than male plants. This difference is likely due to male plants being preferentially collected during the flowering season (with showy but more ephemeral flowers) and female plants being collected after the peak flowering season (with nonreceptive flowers persisting later into the season).Conclusions. Researchers investigating phenology using herbarium specimens need to be aware of possible collecting bias in dioecious species and species with persistent flowers. Herbarium collectors and photographers should sample both male and female plants.