We determined the interannual variation of annual litterfall rate in cool-temperate forests [three mixed-forests (Mx1-Mx3), an evergreen coniferous forest (Ec), and a deciduous conifer plantation (Dc)] of northern Hokkaido over a 16-year observation period (1996-2011) and evaluated the effect of meteorological and phenological variables on the annual litterfall production. Total solar radiation during spring (from March to May) positively correlated with the annual litterfall rate in the current year at three mixed forests with statistical significance. A warm spring advanced the day of snow melt and the day of leaf expansion, however, the early leaf expansion did not enhance the annual litter production at any of the studied forests. In conclusion, spring solar radiation was the best explanatory factor among the studied factors that determines the interannual variation of the annual litterfall rate at cool-temperate mixed forests, although the mechanisms behind this relationship remain unknown. The early snowmelt and leaf expansion caused by a warm spring did not directly link to the enhancement of the litterfall rate. This implies that global warming or changing rainfall patterns do not necessarily affect the annual litterfall amount in these forests, at least within the range observed during the 16 years.