Herbarium specimens can reveal impacts of climate change on plant phenology; a review of methods and applications

Studies in plant phenology have provided some of the best evidence for large-scale responses to recent climate change. Over the last decade, more than thirty studies have used herbarium specimens to analyze changes in flowering phenology over time. In thisre view, we summarize the approaches and applications used to date. Reproductive plant phenology has primarily been analyzed using two summary statistics, the mean flowering day of year and first flowering day of year, but mean flowering day has proven to be a more robust statistic. Three types of regression models have been applied to test for changes in phenology; flowering day regressed on year, flowering day regressed on temperature, and temperature regressed on year. Most studies analyzed the effect of temperature by averaging temperatures from three months prior to the date of flowering,but other approaches may be suitable in some cases. On average, published studies have used 55 herbarium specimens per species to characterize changes in phenology over time,but in many cases fewer specimens were used. Geospatial grid data is increasingly being used for determining average temperatures at herbarium specimen collection locations,allowing testing for finer scale correspondence between phenology and climate. Multiple studies have shown that inferences from herbarium specimen data are comparable to findings from systematically collected field observations. Herbarium specimens are expected to become an increasingly important resource for analyzing plant responses to climate change.