Reference dataset: the creation of sample locations
representing known population-level phenological distributions and
individual phenological parameters
We simulated phenological data for 1200 hypothetical “species” in the
continental USA that varied in the attributes of their individual- and
population-level flowering phenology. For each of these simulated
species, we selected 1000 locations within the continental United
States, each representing a local population observed during a single
year from which a simulated specimen was later obtained. The coordinates
for each location, year, and associated mean annual temperature in the
year of collection were randomly selected without replacement from
4-km2 PRISM pixels (PRISM Climate Group 2011) between
the years 1901 to 2020, and were restricted to locations with 1991–2020
temperature normals of 1–20 °C and mean annual precipitation normals
for the same period of 60–3800 mm.
Each species generated this way was assigned a series of attributes
defining its individual- and population-level flowering phenology. The
peak flowering date of an individual was assumed to coincide with its
mean flowering date. We then defined a linear equation describing the
relationship between the mean date of peak flowering among individuals
within a population and local temperature conditions. Each species was
assigned a median population flowering DOY of 50 at 0˚C (i.e., the
intercept) as well as a phenological responsiveness (i.e., slope) of
median flowering DOY to mean annual temperature: advancing by 1, 4, or 8
days per increase in °C. Next, each species was assigned a short,
moderate, or long duration of the flowering period by each individual
within each population (15, 30, or 60 days, representing the duration of
time each individual plant was in flower. Then, we assigned each species
a low or high magnitude of intrapopulation variation in phenological
timing (i.e., in peak flowering DOYs) among individuals (based on normal
distributions with standard deviations (σ) of either 10 or 30 days),
representing the magnitude of variation in the flowering times of early-
to late-flowering individuals within each local population. Fifty
species were simulated for each of these 18 combinations of phenological
responsiveness, flowering duration, and intrapopulation variation in
phenological timing (Table S1).
To accommodate the possibility that the magnitude of variation in
phenological timing within a population could depend on local climate
conditions, we also simulated 50 species with temperature-sensitive
intrapopulation phenological variation (σ) ranging from 10 to 30 days.
For these species, σ of the DOY among individuals in a given population
increased by 1 day for every 1 °C increase in the mean annual
temperature of its location (this class of σ is labelled as “variable”
in Table S1 and in the figures cited below). For these simulated
species, individual flowering duration was fixed at 30 days.
Additionally, to accommodate the possibility that individual flowering
durations could exhibit linear relationships with local climate
conditions, we also simulated 50 species that exhibited individual-level
variation in flowering duration resulting from changes in temperature
(increasing by 1 day per °C increase in mean annual temperature, and
ranging from 10 days to 30 days). For these species, the degree of
intrapopulation variation in peak flowering dates was held constant at σ
= 30 days (i.e., high intrapopulation variation).