Timing of bud burst of smaller individuals is not always earlier than that of larger trees in a cool-temperate forest with heavy snow

Bud-burst timing is one of the key factors to determine tree growth. Smaller trees are known to show earlier bud bursts, owing to the ontogeny in temperate forests. Snowpack is one of the factors affecting burst timing, especially that of small trees. Because small individuals were buried under snowpack until late spring, we hypothesized although the smaller individuals require less degree-day accumulation for their bud burst, the bud-burst timing of smaller individuals is not always earlier than that of larger trees. To test this hypothesis, we investigated the relationship between the height of individuals and the sum of degree-days required until the day of bud burst, as well as the relationship with the bud-burst timings for Acer mono and Quercus crispula over 2 years. As hypothesized, both species showed positive relationship between tree height and degree-days for budburst in both years. Conversely, there was negative relationship between tree height and the bud-burst timing for both species in both years. These results indicate small individuals tended to be late to reach an adequate temperature for bud burst due to the heavy snowpack, and the day of bud burst was sometime later for the seedlings as compared to the large trees in spite of the less accumulation of degree-day for the bud burst of smaller trees. These results suggest snow regime changes may influence the phenology not of large trees but of small trees, which could result in a differential influence of winter climate change on tree growth depending on the individual tree height.