Earlier spring green-up gave only 0.17 day earlier autumn green-down per day at 133 PhenoCam deciduous sites in 2002–2025, one third of a ground-based carry-over estimate

Some autumn phenology models assume that an earlier spring causes an earlier autumn. Keenan and Richardson (2015) reported 0.52 day per day from ground data and 1.1 day per day from MODIS. We tested this carry-over slope with PhenoCam canopy greenness (gcc_90 transition dates) at 133 deciduous broadleaf camera sites (1,037 site-years, 2002–2025; 119 sites in North America). We compared years within each site, controlled for autumn temperature (NASA POWER; Daymet as a check), and used a bootstrap of 98 location clusters. A 1-day earlier spring green-up (50% of amplitude) came with a 0.17-day earlier autumn green-down (95% interval 0.04 to 0.32). Thus about 83% of a spring advance stayed in the green season length. The upper 95% bound was below 0.52 in 20 of 26 variants and below 1.1 in 25 of 26. A placebo with the next year's spring gave 0.06 (−0.07 to 0.19). The slope depended on the date definition: 0.02 (−0.08 to 0.10) with 10% dates, 0.08 (−0.02 to 0.17) with 25% dates in eastern North America, and 0.36 with our own site-fixed thresholds (0.12 to 0.54 at 25%). A per-year amplitude error does not explain the positive slope. Autumn warmth delayed the green-down by 2.8 days per °C (2.0 to 3.5). In 30-day windows, the temperature effect changed from negative (July, intervals including zero) to positive for windows centred after early August, not at the summer solstice. A joint monthly model gave −1.04 day per °C for July (−1.68 to −0.25), but this July effect was not significant with year fixed effects. Limits: GCC is canopy greenness, not leaf colour or photosynthesis; most sites have 4–10 years; the European subset is small; a cross-camera instrument test in 17 multi-camera clusters was inconclusive. Code, derived tables and retained raw data are included or referenced.

phenology · PhenoCam · autumn senescence · carry-over effect · deciduous forest · growing season length · green chromatic coordinate · summer solstice · placebo test · cluster bootstrap