New, publicly-available source of phenological data reveals intraspecific variation in the sensitivity of flowering date to climate change: A case study of the California poppy (Eschscholzia californica)

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Katelin D. Pearson, California Phenology Collections Network, California Polytechnic State University, San Luis Obispo, San Luis Obispo, CA, Natalie R. Love, California Polytechnic State University, San Luis Obispo, CA, Tadeo Ramirez Parada, University of California, Santa Barbara, Santa Barbara, CA, Susan J. Mazer, Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, Santa Barbara, CA and Jenn M. Yost, Biology, California Polytechnic State University, San Luis Obispo, CA

Background/Question/Methods
Herbarium specimens are a critical source of phenological data that can be used to identify drivers of variation in flowering within and among species. Specimen-based phenological research has been accelerated by digitization efforts such as the California Phenology Collections Network, which has scored and archived the phenological status of over 1.4 million specimens to date. Using this new, publicly-available data source in the Consortium of California Herbaria’s CCH2 data portal (cch2.org), we obtained data from 993 specimens of the iconic California poppy, Eschscholzia californica Cham., along with climate data from PRISM representing all collection sites, to determine how long-term and interannual climate variation affect flowering dates. We also examined whether phenological sensitivity to climate, and phenological shifts over time, vary across the species’ range.
Results/Conclusions
Using specimen-based phenological data, we discovered that flowering dates in California poppies depended both on long-term and interannual climate variables. Flowering dates were earlier in chronically warm and dry regions and also when conditions were warmer or dryer than average in a given region. However, the magnitude of the effect of interannual variation on flowering date varied across the range of the California poppy, with greater sensitivity to temperature in relatively cooler regions and no discernible sensitivity in relatively warm regions. Consistently, California poppies exhibited significantly earlier flowering dates over the last 120 years, but this advancement was restricted to the cooler portions of its range. Our results provide one of the first accounts of intraspecific variation in both phenological sensitivity to climate and the magnitude of phenological shifts over time. In this study, we demonstrate the utility of aggregated, digitized herbarium specimen data for addressing novel questions about the phenological responses of plant taxa to climate trends. Such data can now be searched and downloaded for California plant taxa in the CCH2 data portal, and we encourage other researchers to leverage these data for a better understanding of plant phenological events and their variation with climate change.