Does an Early Spring Indicate an Early Summer? Relationships Between Intraseasonal Growing Degree Day Thresholds

Spring heat accumulation plays a major role in the timing of events such as leaf-out, leaf expansion, flowering, and insect hatch in temperate systems. Accordingly, heat accumulation can serve as a proxy for the timing of plant and insect phenological activity and can be used in a predictive way when the timing of heat accumulation thresholds being reached can be anticipated. This has strong value for a host of planning and management applications. If relationships exist between earlier- and later-season thresholds at a location, then the timing of later-season phenological events that are forced by the accumulation of warmth could be anticipated based on when earlier-season thresholds are met. Using high-resolution daily temperature data, we calculated the coherence in pairs of spring-season heat accumulation (growing degree day) threshold anomalies over 1948–2016. Overall, relationships between thresholds spanning the entire spring season were relatively low, while later season thresholds exhibited much higher correlations. This pattern is generally the result of decreasing variability in heat accumulation with season progression. However, correlation strengths did not follow latitudinal or gradients, revealing that within-season heat accumulation and interannual variability in threshold timing are unique to the specified base temperature and thresholds being compared. We show that the relationships between earlier- and later-season heat accumulation thresholds were sufficient to accurately predict the timing of phenological events in plants in two case examples.