The two-summer temperature difference predicted Swiss airborne beech pollen out of sample (1970-2022), and the pollen response to it increased as summers became warmer

Question: does the seed-based 'dT' cue for beech masting (June-July temperature of the previous summer minus that of the summer before) predict an independent measure of flowering, and did the response change as summers warmed? Approach: we used the MeteoSwiss national pollen network (14 stations, manual traps, 470 station-years, 1969-2022) and homogenized MeteoSwiss temperatures. We compared station-fixed-effect models out of sample with complete years held out, and we used six other pollen taxa as comparisons. Results: a dT model fitted on 1970-2002 decreased the 2003-2022 mean squared error of log10 annual beech pollen by 64% relative to the station mean; previous-summer temperature alone gave 4%, and previous-year pollen alone gave 22%. The reverse test gave 38% and leave-one-year-out gave 57%. dT gave no skill for oak, ash, birch, alder or grass; hazel had some skill. The beech slope on dT increased from 0.16 (1969-1989) to 0.34 (1990-2005) and 0.48 log10 per degree C (2006-2022); the decade interaction was +0.089 (SE 0.020). The year-to-year standard deviation was 2.2 times larger in 2006-2022 than in 1969-1989 (95% CI 1.7-2.7). From 1990 to 2022 there was no mean trend (-0.07 +/- 0.14 log10 per decade), as dT predicts, although summers warmed by 0.65 C per decade; low-dT years became lower. This is the opposite of the masting breakdown reported from seed data elsewhere. Automatic monitors (from 2023) gave a positive bias of +0.22 log10 and a compressed response. Because June-July 2026 was about 2 C warmer than 2025, the dT model predicts a strong beech pollen year in 2027 (+0.80 log10; +0.38 with a previous-year term). Limitations: pollen is not seed; early decades have few stations; undocumented changes in counting methods before 2023 cannot be excluded as a cause of the stronger later response.

mast seeding · Fagus sylvatica · airborne pollen · delta-T cue · climate change · Switzerland · MeteoSwiss · out-of-sample forecast · masting breakdown · observation bias · automatic pollen monitoring · phenology