Abstract: Biodiversity data from herbarium voucher records capture important information on the phenology, distributions, and biotic interactions of species beyond taxonomy and systematics. However, massively accumulated observations—those without link to a tangible material in a herbarium—are fast overwhelming voucher records, and it remains poorly understood how well they represent species’ phenologies and distributions in response to global change. Here, we quantify flowering phenology and geographic distributions of Taraxacum officinale, Achillea millefolium, Encelia farinosa, and Layia carinosa across California documented by voucher and observation records hosted on the Global Biodiversity Information Facility website. We hypothesize that changes in phenologies and geographic distributions by voucher records will be more congruent to expected biodiversity patterns than observation-only data. We record shifts in flowering phenology as the change in mean day-of-year per decade for each species, and distributional shifts as the change in the area and centroid of an alpha hull bounding the geographic coordinates of each species per decade. Herbarium vouchers are more likely to be collected by professional botanists as opposed to observations which tend to be derived from amateur collecting. These differences between the two collected data types may potentially lead to a disagreement between model outputs of species phenological and distributional responses to future climatic changes. Observation-only data collections continue to grow exponentially over time and their use in studies on biodiversity may not be reflective of these plants’ ecologies in response to climatic drivers.
