Wildfire smoke changed PhenoCam red and blue fractions by about 15% of their seasonal range but GCC by under 2%, and moved transition dates little, at 435 North American sites in 2011-2025

We tested if wildfire smoke changes the canopy colour that PhenoCam cameras record, and if it moves transition dates calculated from camera greenness. We used 564,546 site-days at 435 sites (25-55 N, 130-60 W, March-November 2011-2025), NOAA HMS smoke classes at each site, and NASA POWER cloud and aerosol data. The outcome was the residual from the median of nearby smoke-free days, with site x year x month fixed effects, cloud and rain controls and two-way clustered errors. On heavy smoke days the scene blue fraction decreased by 9.6 x 0.001 (95% interval 8.2-11.1; 15% of the median seasonal range) and the red chromatic coordinate increased by 11.4 x 0.001 (9.6-13.3; 16%). These changes replicated in four samples (East and West of 100 W, 2011-2019 and 2020-2025), increased with smoke density and were absent in placebo years. The green chromatic coordinate GCC90 changed by +0.48 x 0.001 (-0.04 to +1.01; 0.9% of its range). It did not replicate, and smoke-free days before episodes had a residual of the same size. Removal of medium and heavy smoke days gave median transition-date shifts of 0.00-0.01 days and mean shifts of -0.12 to +0.20 days. Large autumn shifts of 3 days or more were only 1.2-2.2 percentage points more frequent than after removal of the same runs of days at other times; this run-preserving null was added after the first results. Limitations: HMS marks smoke only where analysts can see it, so smoke days are biased to clear sky; HMS is a column product; the live PhenoCam archive is provisional; sites are not a random sample. GCC-based phenology is robust to smoke, but red- or blue-based colour indices need smoke screening. Code, aggregate results and hashes are included; source data stay external.