Synchronicity in the timing of spring phenology within and across species in the same taxon: Evidence from the first flowering date data of 67 crabapple cultivars

Phenological events in early spring within and across plant species of the same taxon often exhibit synchronicity. However, the mechanisms underlying this phenology remain poorly understood. Here, we hypothesized that the temperature-dependent developmental rates of related species are linear and have a common intersection point (the common-intersection hypothesis, CIH). This hypothesis predicts a linear relationship between the lower developmental threshold (also called the base temperature in phenology) and the accumulated degree days required for the day of first flowering (FFD). We tested the CIH using a nine-year data set to estimate the base temperature and the accumulated degree days for each of 67 crabapple cultivars. After excluding four cultivars with abnormal estimates, a significant linear relationship of the two thermal parameters was validated for the remaining 63 cultivars. All developmental rate lines shared a common intersection point at 10.1°C with a corresponding developmental duration of 46.4 days. The validation of the CIH provides clear evidence that the observed synchronicity in flowering is fundamentally governed by a shared temperature-dependent developmental response among related cultivars.