Greater deviation in daily temperature from historic normals translates to shorter durations of reproductive spring phenophases for understory plants

As plants continue to respond to global warming with phenological shifts, our understanding of the importance of extreme, but short-lived, heat events has lagged relative to our understanding of plant responses to broad shifts in mean climate conditions. Here, we explore the importance of extreme daily temperatures in driving phenology across fourteen species of spring-flowering woodland herbs spanning a broad geographic range. We uniquely harnessed the combined power of community science and public gardens, engaging more than 30 volunteers to monitor 198 individual plants biweekly across five botanic gardens in the midwestern and southeastern U.S. We tested two hypotheses. First, that the duration of individual phenophases would be shortened by high daily temperatures relative to the 30-year historical means (1991-2020). Second, these durations would vary among species. Our findings support both hypotheses. We observed significant inverse relationships between higher positive deviations of daily temperature from historic conditions, and the duration of three reproductive phenophases: ‘First Bud,’ ‘First Ripe Fruit,’ and ‘Early Fruiting’. Similar, though non-significant, trends were noted for several other early season phenophases. Additionally, significant differences in mean phenophase durations were detected among the different species, though these differences were inconsistent across plant parts (vegetative, flowering, fruiting). 

Synthesis. Results underscore the sensitivity of reproductive plant phenophases to short-term extreme daily temperatures relative to historic means early in the growing season. This suggests that climate change, characterized by increasing frequency and intensity of temperature extremes, could induce rapid and potentially disruptive shifts in the phenology of spring wildflowers. The inclusion of multiple species and geographically distinct locations permits broad generalization of the results across North American spring-flowering understory herbs in various environments. The methodology (fine-scale temporal monitoring of phenology by community scientists) is widely applicable and could be adopted by public gardens globally to enhance our understanding of climate change impacts while simultaneously engaging with the public.

Methods
This dataset was collected by trained public volunteers at 5 botanical gardens/arboreta (Chicago Botanic Garden, Glencoe, IL; Holden Arboretum, Kirtland, OH; Missouri Botanical Garden, St. Louis, MO; Dawes Arboretum, Newark, OH; Huntsville Botanical Garden, Huntsville, AL) from March - October 2023. The data are observations of phenology (i.e., stage of phenology or 'phenophase') recorded by volunteers on a twice-weekly basis using the Chicago Botanic Garden smartphone app 'Budburst.'