August hot-day counts gave no clear link to autumn leaf colour beyond mean temperature in eight Japan samples in 1979-2024

We tested whether unusually hot August days were linked to autumn leaf colour after controls for mean temperature. We joined JMA tree dates to NOAA daily and monthly temperature data. Eight separate species, region and period models used 3,564 records at 43 Japan stations in 1979-2024. Hot days had a maximum temperature above a fixed local August 90th percentile. Models controlled August mean maximum temperature, September-October mean air temperature, station and reporting setting, and year. Estimates ranged from -1.98 to +3.50 days per ten extra hot days. All eight marginal and corrected intervals included zero. Planned checks did not give a common sign. Only 1.46-2.21 hot days of residual standard deviation remained after controls. Thus the result is imprecise. It does not show that heat has no effect. Selected reference trees, coarse heat data, missing visits, specimen changes, omitted water stress and few clusters limit inference. The package gives daily evidence, exact raw hashes, code, a figure and independent numerical checks. It does not estimate pollution effects, wild biodiversity loss or a causal climate-change effect.

autumn phenology · daily heat · ginkgo · Japanese maple · observation bias · temperature extremes · regional transfer · uncertainty