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Phenological scoring of citizen science observations of five native California milkweed species

MacArthur-Waltz, Dylan1; Sommer, Dylan1; Singh, Prabhjot1; Yang, Louie1

Published Oct 09, 2025 on Dryad. https://doi.org/10.5061/dryad.v9s4mw734

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Abstract

Understanding phenological strategies can be complicated by organisms’ use of multiple cues, changing effects of cues over phenophases, and local adaptation. We investigated the importance of spring temperature and rain-year precipitation on phenology in five species of milkweeds (Asclepias spp.) in the western United States, a region with variable temperature and moisture conditions. We categorized participatory science milkweed observations into budding, flowering, and post-flowering phenophases, and matched observations to year- and site-specific climate conditions. We separated climate effects on phenology into variation across sites vs. variation across years to look for evidence of local adaptation in phenology. Warmer mean spring temperatures generally advanced phenology, while wetter rain-years generally delayed phenology. Across all milkweed species × phenophase combinations, spring mean temperature advanced phenology by 2.56 ± 1.71 days spring temp °C -1 (mean ± SD), and precipitation delayed phenology by 0.12 ± 0.079 days cm precip -1. We found evidence for local adaptation in phenology with respect to temperature in three of fifteen species × phenophase combinations, and for precipitation in four of fifteen species × phenophase combinations. All significant local adaptation with respect to temperature and precipitation reflected counter-gradient local adaptation (increasing phenological synchrony across the range). Combining analytical approaches for studying phenology using observation datasets with the wide availability of observational data from participatory science sources offers an exciting possibility to rapidly generate hypotheses about how organisms’ phenology may react to novel cue combinations under global change, and how these reactions may differ based on species traits.