Interpopulation variation in leaf out phenology of Fagus crenata along topographic variation associated with the late frost regime in the Hakkoda Mountains, northern Japan

We studied local variations in temperature from winter to spring, and interpopulation variations in leaf out phenology (the day and the temperature sum for budburst and leaf expansion) of Siebold's beech (Fagus crenata) canopy trees. We focused on local topographic variation associated with the late frost regime for the canopy trees using 5-10 years of in situ observation of the temperature at 12 sites and leaf out phenology of two populations inhabiting a basin (basin populations) and 4 populations inhabiting hillside slopes (hillside slope populations) in the Hakkoda Mountains, northern Japan. The late frost regime differed between topographies; late frosts occurred during warmer periods in the basin compared to hillside slopes. The basin populations exhibited a larger temperature sum for leaf out than the hillside slope populations, regardless of altitude, suggesting that local variation in the late frost regime is an abiotic factor causing local variations in leaf out phenology. The six studied populations exhibited earlier days and smaller temperature sum of budburst in years with longer chilling duration. However, the extent of phenotypic plasticity in the temperature sum was small for the basin compared to hillside slopes. The temperature sum of leaf expansion in the hillside slope populations also negatively correlated with chilling duration, whereas that of the basin populations responded in the opposite direction. The safety margin from the last frost differed between the topographies, but not that from the last fatal frost, suggesting local adaptation to the late frost regime in each habitat.