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).