In 31 German spring phases, temperature sensitivity did not decrease from 1952–1966 to 2012–2025, and the 1980–1994 to 1999–2013 decrease agrees with a forcing-only model
Question: did spring phenology in Germany become less sensitive to temperature (days per °C) with warming, and is a decrease larger than a forcing-only thermal-time model predicts? Data: Deutscher Wetterdienst annual-reporter wild-plant network, 31 spring phases of 27 taxa, 3.9 million station-years at 6,581 stations, 1952–2025, with HYRAS-DE 1 km daily temperature. Method: we fitted a forcing-only model for each phase on 1952–1985 data only and calculated within-station slopes of event day on fixed-window preseason temperature for observed and model dates. Intervals come from a year bootstrap shared by all phases. Results: mean observed sensitivity was 4.6, 4.6, 5.2, 4.3 and 4.4 days/°C in five 15-year windows. The 2012–2025 / 1952–1966 ratio was 0.95 (95% CI 0.78–1.15), and its CI was below 1 for no phase. Between 1980–1994 and 1999–2013, sensitivity decreased (0.81, 0.67–0.96). The model predicted 0.93 (0.80–1.05), and the difference was not significant (observed/model 0.87, 0.73–1.05). The observed change was smaller than the model change in most phases, but the across-phase difference was within the CI in 11 of 12 contrasts (six variants: base temperature 0 or 5 °C, chilling-forcing model, common stations, re-anchored preseason). A chilling term did not improve the fit. After 1985, observed dates were 3.1 days (2.5–3.6) earlier than the forcing forecast (4.2 days in 2012–2025). This agrees with sci:sha256:5e63bb36c8ace247e7020a6acd9cf5d6e4f40feaffa50a9c8082d6ca2dff5bf3. The size depends on the model (0.9–3.4 days). It is similar in former East and West Germany, and winter warming does not explain it. Limitations: grid temperature error can attenuate early observed slopes more than later ones; the windows are short; the models are simple; observer and programme effects cannot be separated from biological change.
phenology · temperature sensitivity · thermal time · growing degree days · chilling · leaf unfolding · flowering · Germany · Deutscher Wetterdienst · HYRAS · climate change · out-of-sample forecast
References
6- supportsTemperature models fitted to 1952–1990 German leaf-out data predict 1991–2025 leaf-out 1.7 to 5.1 days too late for seven tree species sci:sha256:5e63bb36c8ace247e7020a6acd9cf5d6e4f40feaffa50a9c8082d6ca2dff5bf3
Independent replication with the same DWD and HYRAS sources but different code, 31 phases and a 1985 training cutoff: observed dates after the training years are earlier than forcing-model forecasts.
- supportsSpring phenology temperature sensitivity at Japanese weather stations did not decrease in 1953–2025 beyond a forcing-only model, which also explains lower cherry sensitivity at warmer stations sci:sha256:a30b18e0b9dbed22d7ae3539df648fe0cae472a7014dbd9770521051485120f2
Same question for Japanese stations; our German result also shows no decrease in sensitivity beyond a forcing-only model, and a peak in sensitivity near the mid-1990s.
- A fixed thermal-time model predicts lower spring temperature sensitivity at warm US lilac and honeysuckle sites in 1956–2009, but not the observed increase in sensitivity over time sci:sha256:c815add42242e8d2f8f213f08dbc384e8ad794e2578349ae8b7b145e55078c11
Related test with US lilac and honeysuckle data; no decrease in sensitivity over time beyond a thermal-time model.
- challengesdoi:10.1038/nature15402
We repeat the 1980–1994 vs 1999–2013 comparison with German data; the decrease (19%) agrees with a forcing-only model and is not present from the 1950s to the 2010s–2020s.
- supportsdoi:10.1111/gcb.15746
A forcing-only model predicts part of the apparent decrease in sensitivity; we test this out of sample.
- usesdoi:10.1007/s00704-020-03388-w
HYRAS gridded temperature dataset (v6-1 used).
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