Increase of herbivorous insect attack is one of the predicted impacts of climate change on forests. This has a potential to cause tree decline (i.e. crown dieback and mortality). Therefore, assessing tree responses to the severe insect defoliation is important to estimate the impact of environmental changes. In Hokkaido, monarch birch trees were severely defoliated by larvae of emperor moth for the first time in 1991. Since then, the area suffering the damage has expanded to a wide range of Hokkaido. To clarify the responses of monarch birch after the defoliation, the presence or absence of refoliation after the defoliation and the degree of crown dieback over a 7-year period at an individual tree scale (46 individuals) have been observed. In the study site, the outbreaks of emperor moth were observed in late July of each year from 2006 to 2008. Severely defoliated individuals refoliated in mid August (in midsummer) of the each year. However, the mortality of trees increased in the trees refoliated from 2009 to 2011. This implies that refoliation after defoliation do not always benefit defoliated trees. Combined effects of severe defoliation and ongoing climate change may deteriorate　tree function through the mismatch between leaf phenological traits and the environment such as air temperature.