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.