Effects of sample size on model accuracy
Although sample size exhibited significant effects on model accuracy, the magnitude of the changes in MAE was < 2days , and remained consistent across predictions of flowering onset, median, and termination DOYs (Fig. 3). In all cases, MAE declined with larger sample sizes, but MAE exhibited < 2 days improvements as sample size increased from 100 to 1000 specimens in all cases (Fig. 3, Table S2). Increases in model performance as sample size increased above 300 were minimal, and never exceeded a one-day reduction in MAE (Table S2).
Increased magnitudes of unexplained (i.e., stochastic) variation in phenological timing among populations within a species also were associated with increased MAE as expected (Fig. 3), but exhibited similar relationships to sample size as noiseless models. This implies that unexplained phenological variation inherently degrades the accuracy of phenoclimate models, but the effects of unexplained variation in phenological timing cannot be remedied by greater quantities of sample data.