Peak autumn leaf colouring along latitudinal and elevational gradients in Japan evaluated with online phenological data

We evaluated the spatial characteristics of the first day of peak leaf colouring ( PLC start ) and their relationships with air temperature along latitudinal and elevational gradients in Japan from 2015 to 2017. Leaf colouring information collected from more than 740 sites via citizen science was analysed, representing elevations from 0 to 2800 m and latitudes from 32°N to 44°N. We found that locations with earlier PLC start dates (day of year 265–294) displayed steeper slopes in elevation per degree of latitude than locations with later PLC start dates (day of year 295–314). This statistically significant result indicates that the influence of elevation on PLC start (vertical gradient) weakened as the leaf colouring season progressed in Japan. In addition to these spatial characteristics, the PLC start and the warmth index (based on monthly mean air temperature) showed significant linear correlations for latitudinal and elevational gradients. This result suggests that the sensitivity of PLC start to air temperature, as manifested in both latitudinal and elevational gradients, is constant. This study suggests that online phenological data may provide more accurate results for a regional scale (100–1000 km) than the datasets used by previous studies.