A new method and insights for estimating phenological events from herbarium specimens

PREMISE OF THE STUDY: A novel method of estimating phenology of herbarium specimens was developed to facilitate more precise determination of plant phenological responses to explanatory variables (e.g., climate). METHODS AND RESULTS: Simulated specimen data sets were used to compare the precision of phenological models using the new method and two common, alternative methods (flower presence/absence and ≥50% flowers present). The new “estimated phenophase” method was more precise and extracted a greater number of significant species‐level relationships; however, this method only slightly outperformed the simple “binary” (e.g., flowers present/absent) method. CONCLUSIONS: The new method enables estimation of phenological trends with greater precision. However, when time and resources are limited, a presence/absence method may offer comparable results at lower cost. Using a more restrictive approach, such as only including specimens in a certain phenophase, is not advised given the detrimental effect of decreased sample size on resulting models.